| ThSpeakerDude88 |
Does anyone know where I can get a schematic for the use of a 42 power pentode (from a 30's radio reciever) as a single ended amp?
I also have a 12AT7 I can use for a preamp, along with a slew of 12au6's , 12be6's, etc etc... yes I got them out of an old radio. I also got a nice usable 35c5 out of the small radio. |
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| edl |
Dear ThSpeakerDude88,
Check out this page for 42's datasheet.
http://frank.pocnet.net
With the datas included in the datasheet you will be able to design a simple class-A SE amp with that nice tube.
Regards, |
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| ThSpeakerDude88 |
alright, thanks! I see that this tube is the same as a 6F6 (correct me if I'm wrong :) )
I guess 3 watts would be really great , but it looks like thats at a considerable amount of distortion. Maybe a push-pull AB2 setup would be best? Another thing to consider, If I'm going to use a class A/B setup, would different tubes be better?
I'd just like to order some extras to have around from ebay;) |
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| Miles Prower |
| quote: |
I guess 3 watts would be really great , but it looks like thats at a considerable amount of distortion. Maybe a push-pull AB2 setup would be best? Another thing to consider, If I'm going to use a class A/B setup, would different tubes be better?
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I dropped the 42/6F6 from my "short list" quite some time ago due to the distortion problem. It may not be that big of a deal for a radio, but unacceptable for high end audio. You'd do much better with the 6V6. Similar power output, working voltages, and gain, but with a good deal less distortion. You could run the 6V6's as a class AB1, as pentodes with NFB. Or run them as triodes without NFB.
Either way, they'll sound a whole lot better. |
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| ThSpeakerDude88 |
| ooo triode w/o nfb would be nice, although I would not have much power. I think I'll go with a 6L6 pp combo for 40 watts or a se for 10. What I'm really aiming at is a nice sounding 20 watt duL mono block setup. 20 watts would be easy in pp ab1 mode, but a class A SE 20 watt monoblock would requier a lot of power to run wouldnt it, not to mention be expensive? |
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| tubelab.com |
A 20 watt SE triode amplifier built according to current audiophile criteria (2 X 300B per channel) would definitely be expensive, and would dissipate 80 to 150 watts per channel.
It would be possible to build a 20 WPC SE amplifier for much less money if you use parts that are less "in vogue". 3 X KT88 (triode wired) per channel would be cheaper, or 3 X TV sweep tubes, find cheap ones with sufficient screen grid ratings for triode operation.
I am partial to surplus transmitter tubes myself, or new Chinese 845's, 40 watts per channel, serious bass, the only drawback is the need for 1100 volts!
Diagram on my web site: http://www.tubelab.com/845SE.htm |
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| Brian Donaldson |
| I recently heard a 40W SE with 2 6C33 in parallel. Sounded great with plenty of power. The builder custom made the iron with bifilur primarys. One for each tube. |
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| ThSpeakerDude88 |
ok... I have been sucsessfully scared away from a high powered SE setup because of prices and insanly high voltages needed :xeye:
LOL
I think for my first design, I will try a typical push pull setup. I want something that will be relativly cheap to build, have a very good snr ratio , and obviously sound pretty nice. I may consider a much lower powered SE setup later, unless I can find a cheap tube that will do what I want it to do.
hmm...
how hard would 10 watts be? maybe 5 if that requiers too high of a plate voltage or is too 'spensive. I'm really just designing my first tube amp.. so I want something thats not gonna be expensive, but I dont want it to sound like total crud either! ;) |
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| tubelab.com |
For a 15 Watt push pull amp it would be hard to beat a pair of 6BQ5's/EL84 tubes. This is probably the most common P-P amp design around, and plenty of schematics are on the web. For slightly more power there is the 6V6 P-P amp, also common and good sounding. Either of these would be relatively cheap to build.
If 5 watts is enough, a SE amp could be built with 6L6's, EL34's or KT-88's, one tube per channel. Simpler circuit than P-P, and no crazy voltages required. For a budget SE amp the Hammond 125CSE output transformer is hard to beat at $30. |
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| ThSpeakerDude88 |
sweet! just what my budjet wanted to hear ;)
I think a single 6L6 or kt88 would be nice, I haven't seen an el34 I don't think.. will look up!
would a pp pair of 6L6 monoblocks sound good?
I know the 6V6's in pp make 19 watts, I believe the 6L6's can produce 40 watts, so 20 watts would be at considerably low distortion.
By the way, I have never heard a 'proper' tube amp, just the old radios that I mess with. How does their SNR compare to transistor amps? The chipamp I built has a SNR around 110 db, its dead silent when turned on, even at full volume , signal or no signal :)
can a tube amp come close to that, and if properly designed , will there be any hum? |
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| tubelab.com |
It is possible to build a tube amp with zero hum, if you use a good power supply, good physical isolation between the power supply and the amplifier, and (here is the controversial part) DC on the filaments. In practice all of this is usually not necessary depending on the sensitivity of your speakers. Usually a good power supply is all that is needed with a push pull amp and indirectly heated tubes.
Noise is another thing, it is nearly impossible to get the noise floor low enough to have a SNR in the 110db range, although mid 90's is possible, and generally good enough. My noise measurements are limited by the background noise from the computer which sets the measurement floor around -90db. I see no noise contribution when the amp is switched on. There is also 60, 120 and 180 Hz spikes from the computer that limit hum measurements to -75db. My TubelabSE (all DC filaments) does not raise the hum levels when switched on. |
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| Miles Prower |
"would a pp pair of 6L6 monoblocks sound good?"
Sure they would. However, keep in mind that 6L6s are more often used in guitar amps, which have quite different design criteria. A pair of 807 PP monoblocks would sound better.
"can a tube amp come close to that, and if properly designed , will there be any hum?"
You can certainly come quite close. What you need is a well filtered PS, and use DC on all the heaters (and make certain that you filter the heater supply very well as any residual AC is only going to make matters worse, not better.) and you can get rid of a lot of hum. |
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| tubelab.com |
Miles:
As I just pointed out in another thread the 807 was derived from the 6L6 and is a very similar tube. There were several tubes that were derived from the 6L6:
The 6BG6 was a 6L6 (i believe a 6L6GB) with a plate cap to increase the plate voltage rating so it could be used as a horizontal output tube.
The 807 was used in military applications as RF and audio amplifiers, it is a 6BG6 with a different base.
The 1625 is an 807 with a 12 volt filament and a different base to avoid accidental mix up. It was used in mobile applications.
If you find these tubes from the samp manufacturer and vintage, you will find that they sound similar.
The 6L6 today is primarilarly a guitar amp tube, but in its day it was the king of HiFi until the 6550, EL34, KT-88, 8417 and all of their step children came out. |
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| anatech |
I have some 6L6 based amps that sound great. The EL34 was designed as an inexpensive amp tube so ......
-Chris |
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| Miles Prower |
"As I just pointed out in another thread the 807 was derived from the 6L6 and is a very similar tube. There were several tubes that were derived from the 6L6".
This points out that there are some subtle differences. The 807 was designed primarily for efficiency, and thus uses lower screen voltages. Keeping the screen voltage down has the benefit of causing more electrons to head for the plate, so less power gets wasted in heating up the screen grid. It also has the additional benefit of enhancing linearity. To goose up the output power for consumer electronics, as a cost saving measure, tubes like the 6L6 use higher screen voltages. That gives a heavier plate current for any given Vpk, but also degrades linearity. The spec sheets verify this, in that the max Vsgsg for the 807 is less than half the max Vpp, whereas for the 6L6, it's more like 3/4ths.
If he's right, I would expect there to be some subtle differences, and that the 807 would sound better, especially if you go light on the NFB (which I intend to do). |
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| anatech |
Hi Miles,
Thanks for posting the link. Some mysteries solved for me.
-Chris |
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| tubelab.com |
I read this information, and have seen similar information in the past. This would imply that there were design differences between the individual variants of these tubes of similar construction. This was probably the case for tubes from the big suppliers before WWII. During the high production years of WWII, and the cost cutting era that followed, there were probably some compromises made that resulted in some vendors puting the same guts in all of these tubes.
I base my statements on my experience with my collection of about 100,000 tubes, about half of which is WWII era military surplus. These tubes were given to me (in exchange for a few days work). Unfortunately they were loose tubes (no individual boxes) that were all dumped randomly into large cardboard boxes. Many were broken. I have spent some time examining the insides of many of the broken ones. I had some interest in the 6L6 - 807 - 6BG6 - 1625 connection, so I checked some of these out. This was about a year ago, and I might have missed some details, but at the time I concluded that they were all the same. I am sure that I will find more broken 807's, so I will pay more attention to the screen grid wire spacing, and screen to cathode spacing. There may be an intended difference, that I missed, or there is a bigger variation between different vendors of 807's. There are several different vendors of 807's, many that I have never heard of.
This discussion has prompted me to re-visit this tube, especially since I have about 100 1625's and a good supply of 807's. I will have to check some out electrically, and see what happens. The data sheet says that a 1625 IS a 12 volt 807. |
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| amperex |
| I built PP 6F6GT & they sounded good, but was triode connected. Me thinks the 6V6GT triode connected outplays the EL84 triode connected. EL84 is a good sounding tube, but only GE grey oval plate works for me. The GE embarrashed & handily outgunned Mullard EL84 with square getters. |
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| ThSpeakerDude88 |
wow, the 6f6 in pp triode connected setup has considerably more power than a single tube in triode connection.
Single tube is .85 watts @ 6% THD
PP is 13 watts @ only 2%
how does the power go up so much when still in triode connection as pp?
with the pentode connection it only about doubles. |
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| anatech |
Push pull operation has a much more linear range than SE. That and second harmonics + hum should all but cancel. They had that figured out in the twenties. On top, DC core currents cancel in the transformer leaving more headroom. Much less to worry about.
-Chris |
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| ThSpeakerDude88 |
I read above about running the heaters with AC or DC.
I had thought of that, ac might be more noisy right? I know dc requiers rectification instead of just a resistor to drop the voltage, but if it gets rid of some noise I'd think it would be better. I know in the radios that I work with sometimes you can put your ear up to one or more of the tubes and hear the vibration of the filiment due to AC, when I apply DC to them they don't do that.
oh by the way, a little off topic here, but does anyone know where I can find the schematic for a 12at7 preamped 50c5 or 35c5 amp setup?
(besides the one from dogstar online)
I just really want a simple hookup diagram for the 50c5 and a 12at7.
I have an old radio with a 12AT7 in it I can use, and I have a 50c5 laying around. I ran my guitar through the clean channel on my amp and ran the line out into the radio's amp, it has a very nice overdrive at low volumes so its perfect for practicing at night. On top of that, it can still get pretty dang loud for only 2 watts (through the original 6 inch speaker it has on it)
I tried it with my 8 inch and its louder, but the tone absolutly stinks.. |
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| Miles Prower |
| quote: |
I just really want a simple hookup diagram for the 50c5 and a 12at7.
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Looks like you're gonna have to roll your own. The only ones that I know of are from dogstar, and an old Lafayette Radio design that used the 12AX7A. No reason that a 12AT7A couldn't work as a driver for a 50C5.
One suggestion: Run the 50C5 with a Vpp= 150Vdc, and a Vsgsg= 90Vdc. Getting the screen voltage down improves the linearity of the 50C5 tremendously. |
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| anatech |
Hi ThSpeakerDude88,
It sounds like you are going to use a series heater string. Therefore AC heaters. You could work in DC heaters but I don't see the point. Some of the hum comes from the amp being class A, no cancellation. So beef up the filter section to reduce that.
I find AC heaters with a positive DC bias works very well in a parallel setup. DC heaters have their own set of problems, you may need to use a regulator to get rid of the nasties. Then your filter cap may have a finite life. Lot's of heat doing this.
Now if you used a 6AQ5 or 6BQ5, you would not be using the AC line for your heaters. Now you're into a transformer for your B+ and heaters (much, much safer!). This would be much better as a first project. From the sounds of it, you want a small practice amp?
You are going to have to set a budget. Using an old piece of gear near to what you want and rebuilding / modifying would be less expensive.
-Chris |
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| ThSpeakerDude88 |
hi, thankyou for the replies!:)
Miles Power: wow that sounds good! The 50c5 seems to sound a lot better than the 35c5,even though it only has marginally more output power.
Anatech: Well i have a small practice amp (peavey transtube rage 158) But I really want something small, tube powered, and easily overdriveable without annoying everyone with. I found the 'ol radio has a sweet distortion when I run my clean channel into it with the radio at 3/4 volume. I think it was designed funny, the other radio I had had some sort of a capacitor on the tube side of the opt to cut off the bass frequencies because of the dinky speaker it used and the low power output. This radio does not have that. While it has full range sound, it has too much bass for the 1 watt output tube to handel at any volume without distorting :xeye:
It does have a tone controll circuit, I believe it lowers the high/mid freq.s and leaves the bass turned up, and it doesnt sound very good to me.
Unfortunatly, I don't have the money for a used peice of equipment to modify or I'd have one. Would really like an old bogen to use.
All I have for now is the tubes and parts from this radio to use, as well as a few other peices parts and tubes from the other one.
I do have a ton of caps, resistors, pots, etc etc ... you should see my desk :xeye:
so that stuff shouldn't be a prob |
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| anatech |
Hi ThSpeakerDude88,
LOL, betcha my desk is worst looking - don't take me up on it.
The tone control just cuts the highs, it's a pot in series with a capacitor across the output transformer.
I am concerned that you will end up with an amp with no isolation from the AC line. Your life isn't worth a few bucks. Please use a power transformer.
-Chris |
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| ThSpeakerDude88 |
oh, duh! forgot to mention my plans on that part :D
Yes I am planning on using a transformer for it to be safe, I don't like the idea of 15 amp 120v running around that thing all the time for me to accidentally touch. I have already found out which is the "polarized" plug on the cord (tryed the plug both ways in the wall, which ever way didn't give me a buzz when I touched anything ground was the right way and I marked the negative lead in black)
hmmm... I don't see a cap across the trafo.. Come to think of it I think the primary's are pcb mount on the board and the only wires coming from it are on the secondaries. |
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| anatech |
Hi ThSpeakerDude88,
A 1:1 isolation transformer would work. Then you can simply use the same circuit and series heater string.
Check out Eek Bay.
-Chris |
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| tubelab.com |
Any time that I touch one of those old series string guitar amps I ALWAYS add an isolation transformer and a 3 wire cord. They hum less, and don't shock the #%!* out of you when you touch the guitar and a mic at the same time. I recently resurrected an old Electrolab amp that used a 50C5 output tube. It sounded pretty good since I had BOTH watts working overtime when fed from the ADA preamp!
I usually use a Triad N-68X (Chinese) ($11.20USD from Mouser) or an Allied 6K140HF (Canadian) for about $15 (alliedelec.com)
Need a power transformer for a bigger tube amp, check out "Allied's own tube transformers", Hammond quality for less money.
6K56VG for 10 to 20 watt amps, and 6K7VG for the 50 watt amps. |
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| anatech |
Hi tubelab,
Allied is American, aren't they? ... still looking for those parts on their site. Found it by searching your part #. 50VA isolation would do fine.
-Chris |
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| tubelab.com |
Allied, an electronics distributor that is older than I am, almost dissapeared after being bought up by Tandy in the 80's. I don't know who owns them now but their new 5 pound catalog says "Allied Electronics, An Electrocomponents Company"
I have ordered many of their transformers, they come from a warehouse in Texas. The transformers are "Made in Canada" and look an awful lot like Hammonds. I have been using them for about three years. The only one that ever failed was the one that got seriously wet durring hurricane Francis last year.
They used to have PDF's of their entire catalog on the web site, so you could find what you wanted, even if you don't know what you wanted. Now I have to look in the 4 inch thick book. They will send you one if you fill out the form on the web site. They also have 500 volt Cornell Dubilier electrolytics and other useful parts cheaper than DigiKey.
The Triads that come from Mouser are cheaper, but I like the looks of the Allied transformer better (maybe I am just paranoid), It just looks more robust. The Triads do work OK. |
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| anatech |
Hi tubelab,
Well, that makes sense to me. They could be made under contract by Hammond for Allied.
I actually met Mr. Fred Hammond. Super guy. He passed away a few years ago.
-Chris |
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| ThSpeakerDude88 |
Im running a 13.5vac 5 amp transformer through a 10:1 to step the current back up again, then rectifying that with four 1n4007 diodes, then into the radio (as it is an AC/DC radio) Seems to work great. I added a 680uf cap across the dc lines and it filters out all the hum. It now goes a little louder too. I think however, I am running it on 135v instead of 120 (13.5 x 10:1 x-fmr )
Think it will hurt it any?
By the way, silly question here: Did they still have DC mains back in the 50's in some ultra remote locations, or was the AC/DC radio circuit a cheap design to manufacture?:rolleyes: |
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| tubelab.com |
| I have been using Hammond transformers for years, definitely a good transformer for the price. I will be "building the big one" soon. A 100 WPC SE amp using 1642SE transformers and 833A output tubes. In my quest to save money, I am using a surplus power transformer I got off Ebay. The seller turned out to be another forum member. |
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| ThSpeakerDude88 |
hey heres another off topic question (sorry avout all these,:xeye: )
Has anyone had any experience with the RCA 50?
Its listed as a Tetrode Power Amplifier, used for audio amps.
At 450v plate voltage it makes 4.6 watts of "undistorted power"
They don't give THD specs... |
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| Miles Prower |
"By the way, silly question here: Did they still have DC mains back in the 50's in some ultra remote locations, or was the AC/DC radio circuit a cheap design to manufacture?"
It was just a very cheap, potentially dangerous, way to make a cheap product. Eliminate the power xfmr, and you cut costs considerably. Calling it an "AC/DC" radio sounds better than the truth. |
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| anatech |
Hi Miles,
You forgot to mention a major factor. No power transformer saves a ton on weight. Shipping costs were very important.
And yes, the major push was to cut manufacturing costs.
-Chris |
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| anatech |
Hi ThSpeakerDude88,
| quote: | | At 450v plate voltage it makes 4.6 watts of "undistorted power" |
I don't know. Compared to the typical speaker used, they may be right. It is shown as a triode, directly heated. The RCA data sheet shows plate curves from which you may be able to figure out what the "book" THD figures are. The Cunningham data sheet provides the same info.
-Chris |
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| ThSpeakerDude88 |
So now that I have my 50c5 project working, I'd like to try it with my 42 I have.
Do I set it up basically the same as the 50c5?
I know the pinout will be different , Its a bit confusing.
How would I set up the 42? It would help to have a schematic, just to know where is the input, and the grids.
Also, it seems than all Class AB is is having a driver tube in front of it. Can I use a 50c5 to drive it to slightly more power? |
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| anatech |
Hi ThSpeakerDude88,
The '42 has different parameters, but is used in the same way. I haven't looked, but the output transformer may have a different primary impedance.
Class AB implies push pull, two outputs tubes with some way of providing differential drive. It was done with transformers but it can be done with tube(s) in a phase splitter.
Go ahead and play.
-Chris |
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