As I tinker with my el-cheapo guitar amp, I've been thinking about what I build next. A few guys I know need a new amp, and any project where I make money is a good project in my book.
There are oodles of very nice tubes on eBay. However, output transformers - good ones - are rare indeed. New OPTs, courtsey of Hammond, will cost at least triple the cost of the tubes in the amplifier.
So, output transformers are expensive. (No surprise here.)
Why not ditch it?
Using various combinations of tubes, Phillips made some pretty terrific OTL amplifiers during the 40's and 50's. While these amplifiers did use 600 ohm speaker drivers, I figure that I'll be safe if I triple up the output tubes and use a 192 ohm load. This should also give a reasonable amount of output power.
http://people.cs.uu.nl/gerard/RadioCorner/Articles/SerBalOut.htm
Then, there's also the issue of high-power tubes used in Russian fighter jets. I very much like the idea of using something intended for killing people instead to play Jimmi Hendrix, and the price is right. It appears that many of these amplifiers' efficiencies are limited by output impeadance; in the case of a guitar amp, 32-ohm drivers are not uncommon, and I can use four in series.
A few examples:
http://www.tubebuilder.com/schematic3.html
There's also the guys at Tubecad, who have a similar project with 12AS7s.
http://www.tubecad.com/2004/blog0018.htm
Furthermore, power transformers are also expensive. (Also, water is wet.) To keep down costs, I'd like to instead use a pair of rectifier tubes to form a bridge rectifier. Is there a reason why this is a bad idea?
There are oodles of very nice tubes on eBay. However, output transformers - good ones - are rare indeed. New OPTs, courtsey of Hammond, will cost at least triple the cost of the tubes in the amplifier.
So, output transformers are expensive. (No surprise here.)
Why not ditch it?
Using various combinations of tubes, Phillips made some pretty terrific OTL amplifiers during the 40's and 50's. While these amplifiers did use 600 ohm speaker drivers, I figure that I'll be safe if I triple up the output tubes and use a 192 ohm load. This should also give a reasonable amount of output power.
http://people.cs.uu.nl/gerard/RadioCorner/Articles/SerBalOut.htm
Then, there's also the issue of high-power tubes used in Russian fighter jets. I very much like the idea of using something intended for killing people instead to play Jimmi Hendrix, and the price is right. It appears that many of these amplifiers' efficiencies are limited by output impeadance; in the case of a guitar amp, 32-ohm drivers are not uncommon, and I can use four in series.
A few examples:
http://www.tubebuilder.com/schematic3.html
There's also the guys at Tubecad, who have a similar project with 12AS7s.
http://www.tubecad.com/2004/blog0018.htm
Furthermore, power transformers are also expensive. (Also, water is wet.) To keep down costs, I'd like to instead use a pair of rectifier tubes to form a bridge rectifier. Is there a reason why this is a bad idea?
Sounds like an interesting project, I can think of many reasons why it might be ill-advised, but regardless if you are curious you should try it. Read my missive on my ill-fated OTL project on my site if you have a moment, there are some pitfalls you can avoid.
One thing to bear in mind is that most otl designs are not intended to deliver high levels of output current on a continuous basis, given the trend towards heavily overdriven and saturated tones this might be a serious issue in an otl guitar amplifier.
No reason not to use a bunch of rectifier tubes in a bridge if you are so inclined, just remember to choose ones that can handle the load current. It would also be good if they have a long enough time delay so that no special warm up precautions need to be taken to protect the output tube cathodes. (6C33 will arc if presented with 170V on the plates with not fully warmed filaments...
)
Certainly in this application a 4 speaker stack would be your friend if wired in series, you could even go with 16 ohm drivers for a whopping 64 ohm load - this is definitely otl friendly territory.
Avoid the temptation to over complicate, an inverted futterman or futterman is easier to get working that most circlotrons.
I would recommend staying away from the 6C33 unless you have the time and energy to test a lot of tubes after a lengthy burn - in. 6080 are very cheap, rugged and have more modest filament requirements, a quartet of them should put you in about the same power range as a pair of 6C33.
One thing to bear in mind is that most otl designs are not intended to deliver high levels of output current on a continuous basis, given the trend towards heavily overdriven and saturated tones this might be a serious issue in an otl guitar amplifier.
No reason not to use a bunch of rectifier tubes in a bridge if you are so inclined, just remember to choose ones that can handle the load current. It would also be good if they have a long enough time delay so that no special warm up precautions need to be taken to protect the output tube cathodes. (6C33 will arc if presented with 170V on the plates with not fully warmed filaments...
)Certainly in this application a 4 speaker stack would be your friend if wired in series, you could even go with 16 ohm drivers for a whopping 64 ohm load - this is definitely otl friendly territory.
Avoid the temptation to over complicate, an inverted futterman or futterman is easier to get working that most circlotrons.
I would recommend staying away from the 6C33 unless you have the time and energy to test a lot of tubes after a lengthy burn - in. 6080 are very cheap, rugged and have more modest filament requirements, a quartet of them should put you in about the same power range as a pair of 6C33.
Even four 8 ohm drivers in series would be pretty OTL friendly.SY said:
Actually, I figured I'd just use some nice, big isolation transformers - likely, two isolation transformers to give me 240v with a center tap. The point of using the two rectifiers is to get me double the wattage.
kevinkr said:
Fair enough. I was'nt aware of the issues of output current - but, then again, this is a guitar amplifier. Distortion is not necessarily a bad thing, nor is odd characteristics.
kevinkr said:No reason not to use a bunch of rectifier tubes in a bridge if you are so inclined, just remember to choose ones that can handle the load current. It would also be good if they have a long enough time delay so that no special warm up precautions need to be taken to protect the output tube cathodes. (6C33 will arc if presented with 170V on the plates with not fully warmed filaments...
So, why not just add a delay? I figure I'll be using a seperate transformer for the heaters, so I can add some sort of timed delay circuit with a relay.
kevinkr said:
Actually, Jensen makes a variety of 32 ohm drivers. I figure that by using a 128 ohm cabinet, I can improve efficiency by a large margin.
kevinkr said:
6080 it is, then! I'm all in favor of a simple, reliable schematic. And I'm really, really in favor of cheap tubes.
and make good use of them...
I've never seen that and I've built quite a bit with this tube. Though every circuit has had a choke, resistor or relay controlled current inrush limiter in the power supply.
As far as your complaints against the 6c33c most are valid, I've also bought a box of them and found them to be quite mis-matched and they do change after burn-in. Though 6as7's, and pretty much any high Gm/ low Rp tube can also suffer from this. 6c33c's can still be good for some situations. In a circumstance like your 6 output tube Circlotron, the 6c33c could be quite difficult (It's a very impressive looking peice though). When used in fewer pairs and in variations of futterman there not a bad tube at all. Futterman spinoffs are a lot less prone to biasing drift causing DC offset on the output when you leave the center tap of the power transformer floating.
1 pair of 6c33c's into 32+ ohm load would probably produce really good results. It's not too hard to match 1 pair.
SY said:
Jeb-D. said:
Hi SY,
I had a lot of issues with section to section mismatch. I burned in several at a time and selected the ones with the best section to section match by running one filament at a time and measuring the plate current at two plate voltages and three different grid voltages - very tedious and time consuming, and of about 32 I purchased I ended up with about six that were pretty closely matched, say within 5%, another six that were within 10% and a number that were so far apart as to be useless. I also had some failures in the test jig.
Hi Jeb-D,
I had no time delay and no current limiting and the original Russian language 6C33 specification is very emphatic about not applying plate voltage until the filaments have fully warmed up. I violated this rule and had repeated arc overs, in the first instance it destroyed a woofer. My fault totally of course for not heeding the warning. I think in your case there may be just enough of a delay to avoid this issue. Others have also mentioned that internal contamination might account for these arc overs and that a long burn in may eliminate this as an issue.
Re: Use the chinese 6N13P instead of 6080
One thing I should specify is that I'm not very good at using tubes. The only reason I'm building an OTL amp is that there is no way I can afford decent components to build anything else. If someone could provide a schematic capable of working with a 200-ohm input impeadance, that would be most dandy.
I personally like the simplicity of this one:
http://webpages.charter.net/dawill/tmoranwms/Circuits/6AS7_Futterman_OTL.gif
However, I'm not sure if I can reduce the number of output tubes and/or increase the voltage. This was (I think) designed for an 8-ohm load, and I'm working with a load at least eight times that.
Also, I'm not sure if it will work with the 6N13P.
Here's
Wacky source impeadance is one of the more noted sources of "Guitar sound." That said, I'll try adding a tube rectifier after the amp already works on a silicon rectifier.
More durable AND cheaper? Bada-Bing!
At $6 a tube, I can afford to use multiple pairs in the output stage. 50W into 64 or 128 ohms is the target for this amp, which from what I've read, is a much more OTL-friendly load than 8 ohms.
The power switch on my amp will actually be connected to the 12v transformer. Once the amp is powered on, a simple capacitor-based timer will then turn on a relay connected to the power transformer. Hence, no risk of arcing - the tubes will get a good thirty seconds of warm-up time before any voltage is applied.
One thing I should specify is that I'm not very good at using tubes. The only reason I'm building an OTL amp is that there is no way I can afford decent components to build anything else. If someone could provide a schematic capable of working with a 200-ohm input impeadance, that would be most dandy.
I personally like the simplicity of this one:
http://webpages.charter.net/dawill/tmoranwms/Circuits/6AS7_Futterman_OTL.gif
However, I'm not sure if I can reduce the number of output tubes and/or increase the voltage. This was (I think) designed for an 8-ohm load, and I'm working with a load at least eight times that.
Also, I'm not sure if it will work with the 6N13P.
Here's
SY said:
Wacky source impeadance is one of the more noted sources of "Guitar sound." That said, I'll try adding a tube rectifier after the amp already works on a silicon rectifier.
LineArrayNut said:
More durable AND cheaper? Bada-Bing!
At $6 a tube, I can afford to use multiple pairs in the output stage. 50W into 64 or 128 ohms is the target for this amp, which from what I've read, is a much more OTL-friendly load than 8 ohms.
SY said:
The power switch on my amp will actually be connected to the 12v transformer. Once the amp is powered on, a simple capacitor-based timer will then turn on a relay connected to the power transformer. Hence, no risk of arcing - the tubes will get a good thirty seconds of warm-up time before any voltage is applied.
Well, I'm not the first to do this, as it turns out.
http://www.renditionaudio.com/evolution.html
I'm going to use a simpler preamp, though. I have yet to meet a musician who would rather have his controls on his amp (pain in the *** to get to mid-song) than attached to a handy-dandy pedal.
However, there's a rather nifty doohickey that they overlooked: 32 ohm drivers - Jensen's MOD10-50-32. Their MOD series has a reputation for high "bang for the buck", and at 32 ohms, two in series gives me a very OTL-friendly 64 ohm load. And if the guitar amp works well, I might turn it into a 4-driver cabinet - 128 ohms!
http://www.andrewsamplab.com/mods_series.htm
http://www.renditionaudio.com/evolution.html
I'm going to use a simpler preamp, though. I have yet to meet a musician who would rather have his controls on his amp (pain in the *** to get to mid-song) than attached to a handy-dandy pedal.
However, there's a rather nifty doohickey that they overlooked: 32 ohm drivers - Jensen's MOD10-50-32. Their MOD series has a reputation for high "bang for the buck", and at 32 ohms, two in series gives me a very OTL-friendly 64 ohm load. And if the guitar amp works well, I might turn it into a 4-driver cabinet - 128 ohms!
http://www.andrewsamplab.com/mods_series.htm
Surely you could afford transformers from Edcor?
http://www.edcorusa.com/products/class_x.html
Their "high end" models still cost <$50 each. The closest to that is Hammond which is near $100.
http://www.edcorusa.com/products/class_x.html
Their "high end" models still cost <$50 each. The closest to that is Hammond which is near $100.
astouffer said:
Well, maybe. The point of this is to build a really, really, really cheap guitar amplifier capable of producing 30 watts or more.
I already have a bunch of caps and resistors, the transformers (120v->240v step-up units, or a pair of isolation transformers) are dirt cheap, and the tubes are $7 each.
No, that is not really correct, they are quite different which you can see if you compare the specs like here http://www.tubetvr.com/favtube.html they are similar but that you can say about almost all regulator triodes.
My experience of 6336 is very bad, they are not built very well and can't withstand almost any over current for any time however short, one reason why I gave up on OTL's for a long time was due to this experience, (and the price of 6336)
For long life and good performance it is important to burn in 6C33C in the correct way and it is mandatory to switch on heater at least 30 seconds before the anode voltage. With correct burn in the specs of these tubes are very similar, I bought a bunch of 100 from GS tubes in Russia and at the same grid voltage and at 200V anode voltage I didn't get bigger variation of anode current than +- 10%, I think that is quite good. Viktor Khomenko at BAT have reported that they get a failure rate of new unused tubes of about 1% which is not to bad either.
If using 6C33C in parallel it is important to derate current, (there are values for this in the data sheet) and use cathode resistors for each tube, it is a major difference to build an OTL with just 2 tubes or to use multiple parallell ones, the difficulties mutiplies even faster.
Regards Hans
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.