| gmilitano |
When I go to a thrift store, there is almost always an old Tube Stereo Console available for $7 to $25 CDN.
The stereo consoles usually have a radio and phono. The most common brands are Viking (from Eatons) and Fleetwood. I have attached a picture to give a general idea of what these things look like.
Now, to my questiond. Is it worth while picking one of these up for the parts inside?
Typically there are a couple radio, preamp, amp and rectifier tubes. Given how old these consoles are and assuming that they were used a lot, are the tubes of any appreciable value or would they be too worn out and not suitable for a DIY project?
The power transformer is typically enclosed and very large. It must have several different voltages for all the different tube types.
The output transformers are typically for 8ohm speakers and very small.
I am interested in building a preamp, are these consoles a good source for parts?
Cheers,
Gio. |
|
|
| Duo |
That there is a Grundig Mandello, a unit which I have pride in. I wouldn't recommend taking it apart if that's the one you're thinking of.
Might I ask where you got this picture? I have one of these myself and I use it on a regular basis. It's one of the best radios I've ever used and the FM quality is outstanding.
It has the obligatory european oval speakers, reminiscent of the american 6x9, but bigger and really nice. On the sides of the unit, underneath the little plastic grilles, are small electrostatic tweeters.
Perhaps you already know all about this unit and I'm wasting my time. But I had to put my comment in on it ;) |
|
|
| heater |
>> 7$ - 25$
Good grief. Here in Helsinki I almost never find any kind of old tube radios/consoles in "thrift stores", second hand shops, junk stores, call them what you will. When I do they are seriously expensive. For example:
One huge radio/gramaphone player from 50s/60s, make unknown, not such good condition, not operational 250 Euro.
One small "3D sound" radio/gramaphone from 50s/60s, make unknown, terrible condition, not working 100 Euro.
One AGA radio from 1947, model 1741, no case only chassis, not working, 18 Euro. I snapped that up for parts but now have challenged myself to get it working as I found the schematic on the net.
One Phillips "Panos" (I think they are called) AM/FM mono radio with stereo amp from 1961, 78 Euro. I also snapped that up as well. Amazingly it's in mint, factory fresh condition and works fine and sounds brilliant so it will remain on show in my living room for many years to come.
Not sure what I mean to say here but sometimes I miss the days back in England in the 70s/80s when mountains of this stuff were kicking around for free. |
|
|
| planet10 |
Consoles are a good source for parts -- either to use or to profit from. German ones... like the one pictured, Nordmendes, Grundigs, etc are almost always worth buying, especially if they are as cheap as you say.
1/ they come with nice speakers -- the Nordmendes in particular are a rich source for vintage alnico Isophons (older Noresco speakers are also a good source for isophons)
2/ they tended to be run conservatively. I've gotten 40 year old Telefunken EL84s that test as new. You'll also run across an occasional ECC83. I'd like to find a good way to use ECC85s in amps & such. Every once in awhile you'll come across an EF86 (althou these are more common in old reel-to-reel tape decks). You'll also see some odd-balls such as ELL80s and ECLL800s. The ELL80 is essentially 2 EL95s in a single bottle... if you have enuff drive (ie a foreplay or such) they make a neat spud amp. I've run across a single example of a real beefy PP EL84 Telefunken.
 .
The ECLL800 is an entire push-pull power amp in a single tube.
3/ most of the times the iron will be small but well made... but every once in awhile you score one of the higher end, and that iron is really nice.
Little Nordmende vrs big Nordmende iron
usually the iron is SE, but occasionally you'll run across a PP. The little SE iron will make an amp that doesn't have a whole lotta bottom end, but conversly this makes them quite good you use as tweeter amps (one of the constraints of SE iron is that to get them to go low you have to scrifice top). If you find 2 sets of iron you can also do the parafeed thing... use the primary of one OPT for a parafeed choke, and another for the OPT -- the friend who built the spud above eveloved the Decware ZEN idea into parafeed monobloks (EF86 into EL84, both triode strapped) that are quite stunning. He used Grundig trafo, OPTs, hammond choke, and RS filament trafo. I've also run across a couple RTF consoles with SE EL84 OPTs that are monsters (think ST70 OPTs)
Canadian consoles are usually a little less exciting. In my experience often more expensive -- because the woodwork is better (a good source for Robertsen screws). Newer ones are typically SE EL84 or PP ECL82/6BM8 or PP ECL86 (pictured). Sometimes you'll run into a 50EH5 amp -- these make good spuds.
Older ones often have more interesting looking iron, but are most often mono. These -- at least in my sample set -- are most often SE 6V6
The iron is probably Hammond, and not to the same build level as the Euro stuff.
Speakers are invariably Radio Speakers of Canada. Not the level of finish as say the isophons but often quite decent sonically. RSC OEmed for Jensen, RCA, and Isophon. These -- if no better than regular stock -- carry a resale premium associated with the name.
I've had one BayCrest that was made in Canada with imported Telefunken electronics & english Sonotone speakers. Sometimes you will also find Tesla speakers.
American consoles in Canada are a bit more scarce, but there were some nice RCAs (one 3 channel unit that had a really nice bass OPT -- 2 of those consoles and you have a pr of them -- i shipped my sole example of that off to Montreal to make a pair in exchange for a power supply the fellow had given me). You may also run across the famed Magnavox PP amp that Gabe used as a model for one of his production amps, or a Fisher buried inside something with a different name on the outside.
My samples haven't included anything from the UK, but i have run into a few Philips. My samples all had 800 ohm iron & speakers in them. There is a nice example sitting in a 2nd hand store with ECL86 SE and pristine alnico AD2800 (?) and a little alnico 3 or 4" i've never seen before -- she wants $300 for the thing thou. I expect it will sit in her window for a long time.
So in short, the answer is yes, scoop them up. What you can't use yourself or give to your friends is worth cash on eBay -- either for pizza, beer, other parts, or like me, a way to help stay alive in times of under employment.
Reel-to-reels are also a good source for bits. Here is a pic of some sizeable EL84 SE OPTs from a Sony portable R-R (sitting on top of a pair of Nordmende power trafos). They also have a 600 ohm secondary which could be run out to RCAs and used to drive a SS woofer amp and add the tube amps flavour too, or even use as a real ballsy pre-amp. (i'm guessing the OPTs are made by Tamura). The tape deck these came out of also came with a pr of a Coral version of the FE103A)
 .
dave |
|
|
| Sherman |
| quote: | Originally posted by gmilitano
When I go to a thrift store, there is almost always an old Tube Stereo Console available for $7 to $25 CDN ...
Now, to my questiond. Is it worth while picking one of these up for the parts inside? ....
I am interested in building a preamp, are these consoles a good source for parts?
Cheers,
Gio. |
Gio,
They are worth picking up for the whole amplifier!
I have purchased 3 such consoles over the past 18 months. One was a mono unit with a broken turntable. I removed the amp and the 12"!!! and 3" speakers. I then built a new cabinet to install the speakers and amp added a couple of jacks and my oldest son uses it as a guitar amp. It sounds great.
The second one is in my younger son's room, console and all. It is a large stereo unit with turntable and tuner. After replacing a couple caps and cleaning the pots it also has a very nice sound.
The amps from the third are sitting on a shelf waiting for inspiration to strike.
If you find one that doesn't work it can often be repaired by replacing the caps and sometimes the resistors. Of course having a tube tester is also handy. Those that don't work can be cannibalized for parts. Often the tubes are fine as are the transformers. At the price it is hard to go wrong. |
|
|
| gmilitano |
| quote: | Originally posted by Duo
Might I ask where you got this picture? |
Sorry, I found the pic on the net. It was ment to show people what what a console kinda looks like. The consoles I see are typically Viking or Fleetwood.
| quote: | Originally posted by planet10
So in short, the answer is yes, scoop them up. What you can't use yourself or give to your friends is worth cash on eBay -- either for pizza, beer, other parts, or like me, a way to help stay alive in times of under employment.
|
I will give one a try tommorow (there is no football!). From what I saw, the power transformer is much bigger than the pictures you showed but the output transformers where quite a bit smaller than the ones you have shown.
Typically, the consoles I have seen have a tube power supply, tube radio, tube preamp and tube amp all on one chasis. I assume that is the reason for the large power transformer.
| quote: | Originally posted by Sherman
Often the tubes are fine as are the transformers. At the price it is hard to go wrong. |
I may have to consider getting a tube tester. What do these normally cost?
It is worth a shot, I could easily use the power transformer and preamp tubes to build a simple phono preamp from scratch.
Thanks for the info guys. I am always impressed with how friendly and knowledgable the people are in the forums.
Cheers,
Gio. |
|
|
| Sherman |
| quote: | Originally posted by GG
...I may have to consider getting a tube tester. What do these normally cost?...
Cheers,
Gio. |
Like anything else it depends on the model and where you get it. I picked up mine, which is a portable field technician's unit at an "antique" store for about $25 US. It works great and came with a "Tube Substitution Handbook" as well as the original manual. |
|
|
| planet10 |
My tube tester (Heath) turned out to be broken (good for testing shorts thou). I have another i haven't checked out yet (so old i need to check everything over before plugging it in). I also have a couple articles on making your own tube tester. I haven't pursued any of these because there is a local fellow (Pacific TV) with a whole whack of really good testers. He charges me a loon a tube (sometimes i swap him parts). The real value is that the guy is a wealth of info about tubes, tube kit etc. I always leave with my tested tubes with way more value in information and stories than the cost of the testing :)
dave |
|
|
| planet10 |
Here is one of the articles (snipped from a Glass Audio )
dave |
|
|
| GG |
Well, I got the bug, so I purchased two consoles today and took them apart. Here are the details:
Viking $13CDN
Power Iron EEA3 24-80035-04 (enclosed)
Output Iron EEA3 24-10034-01
(2) 6GW8 / ECL86 - amp
(1) 6C84 - rectifier
Radio tubes (one each) ECC85/6AQ8, ECH81/6AJ8, 6BA6, EABC80/6AK8
Fleetwood $7CDN
Power Iron ANB8 71F055 60CY (enclosed)
Output Iron AT-18J (marked 8 ohms)
(2) 50EH5 - amp
(1) 12AX7A - preamp ?
Radio tubes (one each) 19EA8, 12BE6, 17EW8
(three) 12BA6
I did not have to look hard for something to do with the 50EH5 tubes:
http://www.t-linespeakers.org/tubes/SEP_50EH5.html
Mine would have to be a $7 amp! :D
How do I figure out what type of output tranformers these are?
Also, anyone have any project suggestions.
Cheers,
Gio. |
|
|
| planet10 |
Can you take pics... the 50EH5 amp may be the same as a pr of amps i have for the mark II (BTW, my $5 amp cost more than $5 --- just the name given to it by Allen Wright.
The ECL86 is a candidate for an RH circuit.
dave |
|
|
| GG |
| Here is a pic of the Fleetwood with the 50EH5 tubes. |
|
|
| GG |
For interests sake, here is the Viking with ECL86
BTW - what is an RH Circuit?
Gio. |
|
|
| planet10 |
| quote: | Originally posted by GG
Here is a pic of the Fleetwood with the 50EH5 tubes. |
Yes, that is the one i have 2 of. They have much, much nicer OPTs than the usual 50EH5 faire and they come with power transformers to boot... i've got chassis started for 2 stereo amps. I was going to see how few parts i could get away with in the amp (ie 3Rs).
dave |
|
|
| planet10 |
No mention of the speakers in these puppies....
I know i was able to sell the one's out of the Vikings. If you remind me what they are i can tell you for how much.
I also note that your Fleetwood has the same stream of transformer numbers as my last Viking with the PP ECL86 amp -- same construction style -- means Fleetwood was buying amps from the same place Viking was (come to think of it, aren't Fleetwood & Viking both Eatons house brands?). That console had RSC speakers with numbers that indicated they were OEMed for Jensen (the Jensen name makes them no better, but does make them more valuable if resold)
dave |
|
|
| audiousername |
| quote: | Originally posted by GG
BTW - what is an RH Circuit? |
It's a name given by Alex Kitic to a circuit which uses shunt feedback around the output stage between anode and control grid, and operates the output valve in single ended fixed screen (pentode) mode. The shunt feedback lowers the input impedance of the output stage, so you need a driver stage capable of swinging some current, preferably with a high output impedance as well, as this makes the feedback more effective.
EDIT: Wasn't your diyAudio moniker gmilitano or something before? |
|
|
| planet10 |
| quote: | Originally posted by audiousername
EDIT: Wasn't your diyAudio moniker gmilitano or something before? |
He had me change it today...
dave |
|
|
| GG |
| quote: | Originally posted by planet10
i've got chassis started for 2 stereo amps. I was going to see how few parts i could get away with in the amp (ie 3Rs).
|
Do keep us posted. Simple is the way I like things. Were you planning to use the same iron from the console?
| quote: | Originally posted by planet10
No mention of the speakers in these puppies....
|
I did not pay close attention to the speakers. The Fleetwood were 6 or 8", one was badly damaged, the other had a small hole in the cone so I did not pull them. The Viking were 6X9 stamped 3W max, but one was damaged, so I did not bother with them. There were also paper cone tweeters, three pairs that I pulled out. They do not look too exciting.
Gio. |
|
|
| planet10 |
| quote: | Originally posted by GG
Do keep us posted. Simple is the way I like things. Were you planning to use the same iron from the console? |
Will do... your find has inspired me to dig one out and at least trace the thing.
| quote: | I did not pay close attention to the speakers. The Fleetwood were 6 or 8", one was badly damaged, the other had a small hole in the cone so I did not pull them. The Viking were 6X9 stamped 3W max, but one was damaged, so I did not bother with them. There were also paper cone tweeters, three pairs that I pulled out. They do not look too exciting.
|
Even so -- worth pulling the singles... you nevr know when the opportubity to mate them will come... the alnico makes good fridge magnets on the damaged ones... feed me the numbers and maybe i can come up with some info for future reference.
dave |
|
|
| planet10 |
| quote: | Originally posted by GG
Were you planning to use the same iron from the console?
|
Yes
dave |
|
|
| planet10 |
Turns out my Fleetwoods differ slightly. The biggest difference is that one has way bigger OPTs -- and a bit of weird circuitry on the input of the power tube. The one with the smaller OPTs -- still substantial for 50EH5s -- is a Baycrest. This one has the same OPTs as Giovani's.
The amp is actually Japanese, and much more neatly constructed than the typical North American or German units.
dave |
|
|
| planet10 |
Here is the essentials of the circuitry... with only a requirement for 2.5V to reach full power the 12AX7 is way overkill (and may partially explain the large gridstopper). The grid leak on the power tube is one of the tone control pots.
dave |
|
|
| planet10 |
And here is the minimalist design i'm hoping to get away with... no grid stopper, wired in triode (lower power, lower output resisitance), no cathode bypass (some degeneration, higher output resistance). If i can get away with it, then the only cap in the signal path is the last power supply cap.,,, with only a requirement for 200V i can put a nice poly there.
dave |
|
|
| GG |
| quote: | Originally posted by planet10
And here is the minimalist design i'm hoping to get away with... no grid stopper, wired in triode (lower power, lower output resisitance), no cathode bypass (some degeneration, higher output resistance). If i can get away with it, then the only cap in the signal path is the last power supply cap.,,, with only a requirement for 200V i can put a nice poly there.
dave |
This looks great. I'd be willing to try this out too. When you say poly, are you talking polypropylene or is polyester good too?
Is that 6CA4 tube of any use in a power supply.
I am also interested in a preamp. Can the ECL86 tubes be used for that?
Cheers,
Gio. |
|
|
| planet10 |
| quote: | Originally posted by GG
This looks great. I'd be willing to try this out too. When you say poly, are you talking polypropylene or is polyester good too? |
In general polypropylene is better.... If i don't have something suitable in my collection, i'll probably just get a Solen.
| quote: | | Is that 6CA4 tube of any use in a power supply. |
Yes... we have been having good results with EZ81 -- been using them in hybrid bridges to replace SS bridges,
| quote: | I am also interested in a preamp. Can the ECL86 tubes be used for that?
|
There is a thread somewhere here on this forum about just such a beast.
dave |
|
|
| GG |
Just for fun, I decided to power up the two old consoles. A little to my suprise, they still worked! Some of the capacitors must be dead as you get clicks and pops. I did get a kick playing an mp3 player through them. The radio sounds very good!
Other than the audio tubes and iron, what is worth salvaging off of these consoles? Are the radio tubes worthwhile to hang on to? I imagine the electrolytic caps will be close to dead. Are the carbon comp. resitsors worthwhile to pull, they are mostly silver tolerance?
These capacitors caught my attention .. they look like some sort of poly.
Gio. |
|
|
| planet10 |
| quote: | Originally posted by GG
Other than the audio tubes and iron, what is worth salvaging off of these consoles? Are the radio tubes worthwhile to hang on to? I imagine the electrolytic caps will be close to dead. Are the carbon comp. resitsors worthwhile to pull, they are mostly silver tolerance? |
You never know... i save everything i can strip out that isn't obviously pooched... i'm often surprised at what is useful (or saleable).... pull those drivers -- even if it is only for me to see a pic :)
dave |
|
|
| GG |
Hi Dave,
Any thoughts on those capacitors? The outside feels like ceramic?
Sorry, with only one good driver I did not bother pull the speakers. I will take pictures next time.
I think I may go hunting for more consoles this weekend. I'd still like to get some more parts for a phono amp as I am not sure about an ECL86 phono amp. Do these consoles ever contain any sizeable chokes?
As for the 50EH5 tubes, I'd like to try a small wattage amp which I would use for the office since I have to play music very quietly.
I have been playing around with PSU Designer II...very nice, I can't wait to build a powersupply using an EZ81. Is there something similar available that would help me design an amp or preamp?
Regards,
Gio. |
|
|
| planet10 |
| quote: | Originally posted by GG
Any thoughts on those capacitors? The outside feels like ceramic? |
I'm not quite sure what those are... Aerovox is a good brand name thou.
I don't think i've ever seen a choke in a console hifi.... organs thou are often quite rich for power supply bits.
dave |
|
|
| GG |
| quote: | Originally posted by GG
I have been playing around with PSU Designer II...very nice, I can't wait to build a powersupply using an EZ81. Is there something similar available that would help me design an amp or preamp?
|
I took a look at SPICE, but it looks too complicated. Can someone suggest any other tube design software? Is TubeCAD the only other choice? The price sure looks ok if it does what it claims to do.
Regards,
Gio. |
|
|
| planet10 |
| quote: | Originally posted by GG
Is TubeCAD the only other choice? |
TubeCAD is good. I'd buy it in a heartbeat if he ported it to my flavour of UNIX.
dave |
|
|
| GG |
| quote: | Originally posted by planet10
.... pull those drivers -- even if it is only for me to see a pic :)
dave |
I have come across another console. This one from a buddies grandma.
Philips console, tube compliment, EZ81, 2X ECL86 and 2X 12AX7.
The 12AX7 are marked Rogers, Made in Holland
The ECL86 are GE, made in Japan
The EZ81 marked Rogers, made in Great Britan.
The power iron is very large. The OP transformers are much bigger (twice the size) of the previous ECL86 OPT that I pulled.
The console had 4 ~6.5" alnico, all paper, full range speakers. The motor is small and the frame is light. However, they are in great condition for their age, but I do not find them too exciting.
This time I took pictures! :) |
|
|
| GG |
The amp worked. Naturally the pots are very dirty and it looks like some caps have leaked.
Now I have way too many consoles and will have to start to part with them.
Another picture if the drivers. |
|
|
| planet10 |
| quote: | Originally posted by GG
The 12AX7 are marked Rogers, Made in Holland |
Philips, made in the same factory as Miniwatts & Valvos -- quite good as far as 12AX7s go.
| quote: | | The ECL86 are GE, made in Japan |
Those likely aren't original
| quote: | | The EZ81 marked Rogers, made in Great Britan. |
Mullard (ie Philips but made in England)
| quote: | | The power iron is very large. The OP transformers are much bigger (twice the size) of the previous ECL86 OPT that I pulled. |
Very likely Hammond iron -- you have the bits there for a nice little SE amp (either reuse the ECL86 or go to EL84)
| quote: | The console had 4 ~6.5" alnico, all paper, full range speakers. The motor is small and the frame is light. However, they are in great condition for their age, but I do not find them too exciting.
|
Don't let the tiny magnets fool you, those are quite sought after for open baffles...
I just scored a relative of your console -- similar Philips speakers (different basket, otherwise quite close), not as many ECC83, and the ECL86s were the original Rogers labeled Mullards.
Because of import duties, some parts were shipped from Europe and the units were assembed in Canada with a certain amount of Canadian content.
dave |
|
|
| GG |
| quote: | Originally posted by planet10
Very likely Hammond iron -- you have the bits there for a nice little SE amp (either reuse the ECL86 or go to EL84)
dave |
Yeah, the OPTs on this console are respectible, so I will try an SE amp. I also want to try a simple 12AX7 phono preamp.
Is there a way to measure the OPT to determine if it is also suitable for an EL84?
Gio. |
|
|
| planet10 |
| quote: | Originally posted by GG
Is there a way to measure the OPT to determine if it is also suitable for an EL84?
|
An ECL86 can generally be considered half of a 12AX7 & an EL84 in the same bottle (a bit less dissapation than an EL84) and uses the same range of OPTs as an EL84 -- given the economics of manufacturing, it is likely that the same OPT was also used for EL84/6V6/6AQ5 applications.
To measure the turns ratio, i connect my Variac to the primary (set to 10V) and then measure the secondary. Divide 10V by the measured voltage on the seconday and square it... that is your turns ratio. Multiply that by the nominal impedance of your speakers and you have the reflected impedance of the OPT. For an EL84, 5-8K is typical.
An example can be seen here (my guess is that these OPTs are similar to yours)
http://cgi.ebay.com/ws/eBayISAPI.dl...item=5764712827
dave |
|
|
| rickl |
I picked up an EL84 SE console last weekend. the name on the brass plate says Stereophonic by Century.
I have cleaned it up and slowly brought it up on a varaic.
I connected a CD player to the amp section and it worked!
Tubes for the amp include:
2 GE el84/6BQ5
2 GE 12ax7
1 Amperex "Buggle Boy" EZ81/6ac4
AM/FM section tubes includes 2 12at7 (plus 3 6BA6, 6AU6A, 6BE6, 6AL5 7 pinners). ALL GEs.
I don't know who mades the iron but the audio transformer has the number TA-8006-606-2-47. It looks like the Hammond iron on the ebay link above.
I did trace out the schematic but basically it is:
input -> volume -> 12ax7 -> bass/treble -> 12ax7 -> balance -> el84 -> output transformer.
The plan is to build Alex's EL84 amp (see: this DIYAudio Thread )
This design uses 1/2 of a 12at7 plus an el84 per per channel. I now have these tubes and a PS :-).
The console enclosure was a mess so I junked it.
I think consoles are great sources of tube parts and allow DIY guys get a taste for different tubes and designs.
rick |
|
|
| planet10 |
| quote: | Originally posted by rickl
The console enclosure was a mess so I junked it. |
Speakers?
dave |
|
|
| rickl |
I kept the speakers. I wired them to the output transfomers when I brought the amp up. Each channel includes a 8" woofer and two 3" tweeters in parallel and a 6uF 'crossover'.
No foam rot like some other speakers I've seen! I was going to throw them away but after reading a bit more, I may try to use them. Gee-tar cabinet?
rick |
|
|
| planet10 |
Any chance you can take pics?
Try throwing the 8s on a baffle (even just a big piece of cardboard... you might be surprised :)
dave |
|
|
| rickl |
| quote: | | Try throwing the 8s on a baffle (even just a big piece of cardboard... you might be surprised |
I spent all Saturday listening to the el84 SE amp using the original speakers. I *am* surprised. They are currently on the exsiting baffle but now I need to make an enclosure.
I'm currently rebuilding the amp similar to this thread.
1/2 of a 12at7 driver into a el84. I only had to buy two 470k resistors :-).
rick |
|
|
| Sherman |
| quote: | Originally posted by rickl
...listening to the el84 SE amp using the original speakers. I *am* surprised. They are currently on the exsiting baffle but now I need to make an enclosure.
...
rick |
My first thrift store tube amp was a mono unit from an old console record player. I too was surprised at the quality of the sound with the 15" woofer and 3" tweeter. Very natural and detailed midrange, decent bass (but without "punch") and an OK high end which got better when I replaced the cap in the XO.
However when I tried to put the speaker in an enclosure it killed the sound. I know these old drivers were designed for use in an enclosure without a back but I was surprised at just how terrible the sound became when enclosed, regardless of the size of port I tried.
I ended up building a cabinet with an open back quite a bit larger than the original and added a 1/4" jack and an RCA connector for my oldest son to use as a guitar amp. It sounds great with a CD player connected to the RCA and pretty darn good with his guitars.
Just be careful with the enclosure and you should end up with pretty decent sound. |
|
|
| rickl |
Sherman,
Thanks for the note on enclosures. I'm currently rebuilding the amp and once I get that done, I'll focus on the speakers. My guess I'll just build only back 'boxes' and set them on the shelf.
Thanks again,
rick |
|
|
| GG |
Greetings,
I need some advice and pointers to finish up a 50EH5 "spud" amp. Underside picture attached.
The datasheet for the 50EH5 can be found at:
http://www.mif.pg.gda.pl/homepages/...093/5/50EH5.pdf
Dave mapped the console that the parts came from:
http://www.diyaudio.com/forums/atta...tamp=1107083701
I am generally following this circuit:
http://www.mif.pg.gda.pl/homepages/...001/5/50EH5.gif
I will not use a DC blocking cap. The heaters are 50VAC. Going to DC heaters is possible.
I need some help with grounding. I have made this with a 3 prong outlet, so I have taken the ground from the outlet to the chassis. The AC from the mains transformer is to the heaters and to a 4XUF4006 bridge into a 70ohm, 220uF, 220ohm 470uF, 22ohm, 470uF supply.
Should the speaker negative and RCA negative go to the chassis?
What about the DC negative ... should that also go to the chassis?
Regards,
Gio. |
|
|
| planet10 |
| quote: | Originally posted by GG
Should the speaker negative and RCA negative go to the chassis?
What about the DC negative ... should that also go to the chassis? |
If you aren't using any feedback you can just float the speaker outputs. As to wiring schemes... do a serach on star grounding... to much material to go into here, but as a simplification all the signal grounds should go to one point, all the PS grounds to a different point, then these 2 should be wired together & to the chassis (often thru a very small cap)
dave |
|
|
| GG |
| quote: | Originally posted by planet10
If you aren't using any feedback you can just float the speaker outputs. As to wiring schemes... do a serach on star grounding... to much material to go into here, but as a simplification all the signal grounds should go to one point, all the PS grounds to a different point, then these 2 should be wired together & to the chassis (often thru a very small cap)
dave |
Thanks .. I was a little worried about connecting the AC ground to the DC negative. I will worry about snubber, coupling and bypass caps later.
Dave, in your proposed minimalistic 50EH5 SE amp, you have deleted the capacitor which is parallel to the 62ohm resistor. What is the purpose of this capacitor? Typically I have seen a low voltage 100uF electroylitc there. |
|
|
| planet10 |
| quote: | Originally posted by GG
in your proposed minimalistic 50EH5 SE amp, you have deleted the capacitor which is parallel to the 62ohm resistor. What is the purpose of this capacitor? Typically I have seen a low voltage 100uF electroylitc there. |
That is called the cathode bypass. It shunts AC around the DC bias resistor. With it removed the signal modulates the bias giving what is called degenerative feedback. Distrotion is theoretically lowered, but stage gain will go down and the plate impedance will go up (the later can be compensated for somewhat by attaching higher impedance speakers -- the gain requires more grunt going in). I don't know whether it will work out in this case, but the appeal of using as a few parts as possible is driving the concept.
dave |
|
|
| wa2ise |
| Those capacitors in the ceramic tubes are usually just wax paper caps. They tend to absorb moisture from the air and get leaky. So they are not as good as they look :whazzat: However, if you use them across low value resistors (say below 50K) the leakage resistance would be swamped out. DO NOT use them to couple from a plate circuit to a grid circuit, or else that grid will get pulled positive and make for orange plates and overloaded output transformers and power supplies and other undesired disasters....:eek: |
|
|
| GG |
| quote: | Originally posted by planet10
That is called the cathode bypass. It shunts AC around the DC bias resistor. With it removed the signal modulates the bias giving what is called degenerative feedback. Distrotion is theoretically lowered, but stage gain will go down and the plate impedance will go up (the later can be compensated for somewhat by attaching higher impedance speakers -- the gain requires more grunt going in). I don't know whether it will work out in this case, but the appeal of using as a few parts as possible is driving the concept.
dave |
OK. It looks like you can get away without that capacitor. Right now I have it going with only the 62ohm on the cathode.
I checked voltages... 52VAC on the heaters, I think that is close enough. The plate and grid voltage is ~118 to 123V .. that is about 6-8V more than I was shooting for. Should I drop the voltage down or is this fine?
The sound ... very good. There is more bass than what I was expecting from those little OTs, but you do not get full bass extension. The midrange and highs are very strong...I am rather impressed. Power as expected is limited, a little over 1 watt. This looks like it will be a good canidate for a head amp or as part of a bi-amp system driving a high efficiency supertweeter.
Also to my suprise is that there is no audible hum. I was concerned that the 50VAC heaters would be a problem ... they are not, so the heaters will stay AC.
I will try capacitors on the cathode and see if that makes a difference....I prefer not to use a capacitor if possible. I also will try a resistor between the input and ground.
:D |
|
|
| planet10 |
| quote: | Originally posted by GG
I checked voltages... 52VAC on the heaters, I think that is close enough. The plate and grid voltage is ~118 to 123V .. that is about 6-8V more than I was shooting for. Should I drop the voltage down or is this fine? |
I wouldn't woory too much
| quote: | I also will try a resistor between the input and ground.
|
You go have a grid leak resistor there already don't you?
dave |
|
|
| GG |
| quote: | Originally posted by planet10
You go have a grid leak resistor there already don't you?
dave |
No. All I have is the 62ohm resistor. What is the purpose of a resistor between the line in and ground? |
|
|
| planet10 |
| quote: | Originally posted by GG
No. All I have is the 62ohm resistor. What is the purpose of a resistor between the line in and ground? |
The grid leak ties the grid to ground (since almost no current flows thru it). Without it you aren't developing the proper bias... the last time i forgot the grid leak i quickly let the smoke out of a few things. I'm surprised the amp works without it -- or have you put a pot on the input (which would then be your grid-leak)?
dave |
|
|
| GG |
| quote: | Originally posted by planet10
The grid leak ties the grid to ground (since almost no current flows thru it). Without it you aren't developing the proper bias... the last time i forgot the grid leak i quickly let the smoke out of a few things. I'm surprised the amp works without it -- or have you put a pot on the input (which would then be your grid-leak)?
dave |
Yes, I had a pot on. What sort of damage could the lack of a grid leak cause? |
|
|
| planet10 |
| quote: | Originally posted by GG
Yes, I had a pot on. What sort of damage could the lack of a grid leak cause? |
The pot acts as the grid leak -- it is a resistor from grid to ground. You probably have a 100k, higher would be better.
dave |
|
|
| GG |
| quote: | Originally posted by planet10
That is called the cathode bypass. It shunts AC around the DC bias resistor. With it removed the signal modulates the bias giving what is called degenerative feedback. Distrotion is theoretically lowered, but stage gain will go down and the plate impedance will go up (the later can be compensated for somewhat by attaching higher impedance speakers -- the gain requires more grunt going in). I don't know whether it will work out in this case, but the appeal of using as a few parts as possible is driving the concept.
dave |
I have experimented with and without a cathode bypass. Without the bypass, there is a clear loss of power. Connected to a Radio Shack 40-1271 on an open baffle, I measure 80dB with a 100uF bypass and only 74dB without a bypass at a distance of 1m. The 40-1271 are rated at 88dB, so my CDP is not providing enough drive to get the maximum output of about 1.3W.
In addition to the clear loss of power without the cathode bypass, there is a loss of depth to bass and mid-bass. However, the mid-range and high frequency response seem to be unaffected. This was confirmed by driving only a tweeter. I will try this again when I get my hands on a nice super tweeter.
If the intended use of the amp is only to drive a midrange/tweeter, you can likely get away without a cathode bypass.
I also experimented with the size of the cathode bypass. 82uF does not quite get you there and there is no advantage in going larger than 100uF. Unfortunately, electrolytic seems to be the only practical choice at 100uF.
I also experimented with the grid leak. When connected to a 100k pot, there is no noticable differnence with additional resistance.
Gio. |
|
|
| planet10 |
| quote: | Originally posted by GG
I have experimented with and without a cathode bypass. Without the bypass, there is a clear loss of power. Connected to a Radio Shack 40-1271 on an open baffle, I measure 80dB with a 100uF bypass and only 74dB without a bypass at a distance of 1m. The 40-1271 are rated at 88dB, so my CDP is not providing enough drive to get the maximum output of about 1.3W. |
This could just be a loss of gain without the bypass... it possibly just needs more drive than you can give it.
dave |
|
|
| GG |
| quote: | Originally posted by planet10
This could just be a loss of gain without the bypass... it possibly just needs more drive than you can give it.
dave |
This could be. I will have to experiment with some more drive. However, it sounds to me like there is a lack of depth to bass without the bypass. |
|
|
| planet10 |
| quote: | Originally posted by GG
This could be. I will have to experiment with some more drive. However, it sounds to me like there is a lack of depth to bass without the bypass. |
It could well be the case... an in the word's of a great audio hero....
"In all things audio, the ear is the final arbiter."
Harry Olson
dave |
|
|
| kevinkr |
Leaving out the cathode bypass on the output tube raises the effective RP (plate resistance) driving the transformer which in a nutshell means that the due to the limited inductance of the output transformer the bass roll off occurs sooner. Think of the tube as a voltage source with a series resistance driving the frequency dependent inductive reactance at the primary of the transformer, changing RP effectively changes the ratio of voltage division between the tube RP and the transformer reactance. Distortion into the non optimum load impedance reflected by the transformer in this case may be higher, sensitivity will be lower due to the local feedback, and the low frequency response will be compromised. You could attempt fixed bias by determining the voltage across your cathode resistor and either substituting nicads or nimhs in the cathode circuit (be VERY careful - bias current should not be higher than the recommended float charge current of the battery or <10% of rated capacity. Positive terminal of battery(ies) connects to cathode.) Alternately ground cathode and use fixed negative grid bias using batteries. Sometimes this is inconvenient if the bias voltage is a bit odd.
Kevin |
|
|
| GG |
| quote: | Originally posted by kevinkr
Leaving out the cathode bypass on the output tube raises the effective RP (plate resistance) driving the transformer which in a nutshell means that the due to the limited inductance of the output transformer the bass roll off occurs sooner.
|
Well, this confirms what I am hearing without the bypass. I should put more faith in my hearing. I also noticed that the lack of a cathode bypass does not affect the quality of the midrange and high frequency responce, with the exception that there is a loss of power. Is this consistent with your experience?
| quote: | Originally posted by kevinkr
Think of the tube as a voltage source with a series resistance driving the frequency dependent inductive reactance at the primary of the transformer, changing RP effectively changes the ratio of voltage division between the tube RP and the transformer reactance. Distortion into the non optimum load impedance reflected by the transformer in this case may be higher, sensitivity will be lower due to the local feedback, and the low frequency response will be compromised. You could attempt fixed bias by determining the voltage across your cathode resistor and either substituting nicads or nimhs in the cathode circuit (be VERY careful - bias current should not be higher than the recommended float charge current of the battery or <10% of rated capacity. Positive terminal of battery(ies) connects to cathode.) Alternately ground cathode and use fixed negative grid bias using batteries. Sometimes this is inconvenient if the bias voltage is a bit odd.
Kevin |
That is very well explained. I have seen the use of batteries on the cathode in circuits, but never an explanation. Now, would the batteries have to be charged up on occasion, or do they pick up enough power from the circuit? Having to charge up the batteries would not be desireble for me.
Right now, I plan on just listening to this little guy as an amp. However, with the limited bass extension, my long term plans are to use this as a tweeter amp. When I do that, I will simply remove the cathode bypass as it does not sound like it affects the mid-range and high frequency. However, there is a significant loss of power. What would be the effect of remove the cathode resistor Rp and going straight to the ground? Could this damage the tubes?
I have posted a couple of pics of the finished amp in the Photo section.
http://www.diyaudio.com/forums/show...2095#post642095
I am very pleased with the results of this low cost project (<$15US) and I have put togther a webpage. (see my signature).
Thank you to all who helped out with this project and esspecially Dave who provided a lot of input.
Cheers,
Gio. |
|
|
| kevinkr |
You can't remove the cathode resistor unless you replace it with some form of fixed bias as in batteries or a small negative supply to the control grid. The cathode resistor provides the operating bias for the tube and without bias the tube will quickly die due to excessive dissipation.
Note that eliminating the cathode bypass capacitor will degrade linearity in the midrange and treble as well as reduce the sensitivity.
Given the quality of the output transformer in this design it is unlikely that you can't find a far better quality even electrolytic capacitor... If you are really that phobic about the quality of that capacitor use an inexpensive blackgate and/or try bypassing with a good quality film type.
If you insert nicad/nimh batteries in the cathode circuit normal operating currents will keep them fully charged. Under no circumstances should you ever use an alkaline or conventional carbon drycell as they will leak and quite possibly explode. I can't emphasize this enough.
If you choose to use a negative bias on the grid regular batteries may be employed and these will periodically have to be replaced. (every few years) |
|
|
| GG |
I finally got around to measuring my 50EH5 project. Attached is the frequency response from 40Hz to 20kHz into 8.2 ohms.
Basically, the amp is -3dB @ 100Hz and about +1dB @ 20kHz.
The response is about what I expected from these tiny OPTs. The plan for this amp is to power a radio shack super tweeter.
Is there any way to measure the response of an output transformer without having it connected to an amp?
Cheers,
Gio. |
|
|
| Sherman |
| quote: | Originally posted by GG
I finally got around to measuring my 50EH5 project...
Basically, the amp is -3dB @ 100Hz and about +1dB @ 20kHz.
...The plan for this amp is to power a radio shack super tweeter.
...
Cheers,
Gio. |
Gio,
It looks like it would also be good as a "main" amp connected to a subwoofer to supply the low end. |
|
|
| GG |
| quote: | Originally posted by Sherman
It looks like it would also be good as a "main" amp connected to a subwoofer to supply the low end. |
It would be ok, the problem is I don't have a suitable (efficient) 8 ohm speaker. With the 8ohm 2.5k OPT, this amp sounds horrible into 4 ohms and even worse into 16ohms!
Check out the attached graph.
The volume control will let me adjust the tweeter seperately from the preamp and into an 8 ohm supertweet @ 97dB, the low power is not an issue.
Cheers,
Gio. |
|
|
| planet10 |
| quote: | Originally posted by GG
about +1dB @ 20kHz. |
The rise in the top may well be a sign of some ringing in the OPT... adding an appropriately sized (small) cap shunting the primaries is a tactic often used to counter this.
dave |
|
|
| Geek |
| quote: | Originally posted by GG
When I go to a thrift store, there is almost always an old Tube Stereo Console available for $7 to $25 CDN. |
You are sooooo lucky!!!!! :bigeyes:
The thrift stores in Abbotsford/Chilliwack *know* what they have and price accordingly :mad: |
|
|
|
