Grid and plate are swapped in the output of that schematic.
You also have a strong possibility of dc bias thorugh the speaker.
The only other OTL i've seen discussed lately was based on 6AS7, which while a dual triode does not qualify as a small tube.
Actually, there are several errors in the schematic. bias reisstors for one tube not ten in the output stage. 700V supply for triodes designed for 250V max, -213 V on the grid (currently shown as plate) of output with -13 on the cathode ....
Hopefully someone else can point you to a schematic of an amp that meets your requirements, and works.
You also have a strong possibility of dc bias thorugh the speaker.
The only other OTL i've seen discussed lately was based on 6AS7, which while a dual triode does not qualify as a small tube.
Actually, there are several errors in the schematic. bias reisstors for one tube not ten in the output stage. 700V supply for triodes designed for 250V max, -213 V on the grid (currently shown as plate) of output with -13 on the cathode ....
Hopefully someone else can point you to a schematic of an amp that meets your requirements, and works.
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Looks bizarre, but it's right...
The circuit looks quite strange, but actually it is what is intended. The design, by Steve Bench, runs the output tubes in "inverted triode" mode. More details can be found at Inverted Tube Operation, and other articles by Steve Bench at Steve's Tube Pages. I think this is a mirror of his original, now defunct, website. He seems to like exploring slightly unusual circuit designs!
Chris
The circuit looks quite strange, but actually it is what is intended. The design, by Steve Bench, runs the output tubes in "inverted triode" mode. More details can be found at Inverted Tube Operation, and other articles by Steve Bench at Steve's Tube Pages. I think this is a mirror of his original, now defunct, website. He seems to like exploring slightly unusual circuit designs!
Chris
Simple answer: either use a metric sh**load of tubes in parallel, or forget it. OTL is a fundamentally flawed concept around vacuum tubes. The impedances just aren't right.
If you insist, I recall there are some Russian types with very low plate resistance, analogous to 1/2 6AS7 in a 9-pin bottle. I don't remember the number.
Tim
If you insist, I recall there are some Russian types with very low plate resistance, analogous to 1/2 6AS7 in a 9-pin bottle. I don't remember the number.
Tim
6H13C ( with europeen letter is 6N13S ) but this is in 8 pins socket
direct equivalent of 6AS7G
of course , but you need a power plant to lite up 10 pieces of that tube for a decent low output impedance !
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As posted by fellow user that schematics really work !
But no need to say that are lot of other OTL schematics better that that :
- circlotron OTL
- futterman OTL
- technics OTL variant of futterman
- ciufolli OTL variant with 6C33C
- atmasphere variant (Nice AT M60 schematics will you find on this forum !) with 6H13C
- and of course old Philips OTL with capacitor on output (like an SRPP but with pentode )
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An example of a more conventional simple OTL is the design by Tim Mellow, which appeared in the February issue of Audio Express. It's 25W per channel. Admittedly the output tubes (a pair of 6C33C's per channel) could hardly be called "small" (unless you compare them to transmitting tubes or something like that!), but the design is about as simple as they get. I have built one, and it sounds great. Another nice design, which I've also built, is the one by Hans Beijner, again using 6C33C's. Both of these are essentially modified Futterman designs. I've built a circlotron too, using a design by Alan Kimmel, using EL509S tubes in the output. This one is a bit more complicated, because of the need for two separate floating output power supplies per channel.
Interestingly enough, it is really rather easy to get a low output impedance with an OTL, particularly with the use of negative feedback. A fraction of an ohm output impedance is quite routine for such amps.
Chris
Interestingly enough, it is really rather easy to get a low output impedance with an OTL, particularly with the use of negative feedback. A fraction of an ohm output impedance is quite routine for such amps.
Chris
I stand corrected. I never heard of inverted triode mode before.
That is just plain bizar, in a neat way.
Lots of interesting stuff on inverted triode operation on Steve's web site.
That is just plain bizar, in a neat way.
Lots of interesting stuff on inverted triode operation on Steve's web site.
The smallest output valves (non inverted operation?) I have found is this futterman one attatched.
Ciuffoli says he hasn't found any problems yet in futterman's approach
Althought it is not clear to me if it uses 4 tubes per channel or 8 tubes per channel.
(schematic removed by moderators for safety reasons)
Ciuffoli says he hasn't found any problems yet in futterman's approach
Althought it is not clear to me if it uses 4 tubes per channel or 8 tubes per channel.
(schematic removed by moderators for safety reasons)
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Another point I was always wondered about, if I use a single valve at the final of an OTL and obtain a great impedance mismatch,what will then hapen?
Will there be only a huge dectease in power or also any other effects?
You see... you could be able to get some usefull power even if the mismatch is high, if you have sensitive speakers. But does this impedance mismatch only affect the sound volume or also the sound quality?
Will there be only a huge dectease in power or also any other effects?
You see... you could be able to get some usefull power even if the mismatch is high, if you have sensitive speakers. But does this impedance mismatch only affect the sound volume or also the sound quality?
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That schematic, derived from Futterman's 12B4 circuit (an amp I used to own) shows an amp that is DEADLY DANGEROUS. Death is not supposed to be part of diy.
And if you constructed a similar circuit, but in such a way as to not kill you or some innocent person who happened to touch it when it was plugged in, you'd find that the power was limited and the distortion was high with anything but the highest impedance loads and at power above minuscule.
And if you constructed a similar circuit, but in such a way as to not kill you or some innocent person who happened to touch it when it was plugged in, you'd find that the power was limited and the distortion was high with anything but the highest impedance loads and at power above minuscule.
Is it possible to let us know what circuit you refer to that is DEADLY DANGEROUS? I'd like to avoid it if possible. 😱 Thanks
"Death by DIY" - sounds like a mystery novel🙂 In the early days of my OTL prototyping, I was actually getting up to 5 Watts out of a single 6AS7G into 8 ohms if I drove it hard enough into AB2. And with a combination of negative voltage and positive current FB I was able to get a quite respectable DF with that single 6AS7G, too.
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I know which one he's talking about. It was an old RCA design that used a DC power supply that did not include a power xfmr. Running a half wave diode straight off the AC mains without isolation is really asking for trouble.
At least with Russian Roulette, you have 5 out of six attempts to get it right. With these types of power supplies, it's just one out of two. There really is no excuse for doing that, and there really never was. The most expensive power xfmr is a good deal less pricey than the cheapest funeral.They also tend to destroy other equipment (your computer sound card won't stand up to 120Vrms).
Back in the "good ol' days", these power xfmrless designs caused a lot of electrocutions and near electrocutions. They're really bad news. They also tended to hum like crazy (60Hz is twice as hard to filter as 120Hz) and let in every bit of nastiness riding in on the AC mains.
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Deadly dangerous
Thanks, I know the one you mean, and I agree a TRANSFORMERLESS POWER SUPPLY IS ASKING FOR TROUBLE
. I have a Zenith SE amp using 7695s that was originally that way, so I put in two 120:22vac [dirt cheap] back to back for isolation. For those of you new to this game - always put in an isolation tranny - just make sure you have enough VA, and for the b-t-b scheme, the secondary doesn't really matter as long as they are equal.
Thanks, I know the one you mean, and I agree a TRANSFORMERLESS POWER SUPPLY IS ASKING FOR TROUBLE
. I have a Zenith SE amp using 7695s that was originally that way, so I put in two 120:22vac [dirt cheap] back to back for isolation. For those of you new to this game - always put in an isolation tranny - just make sure you have enough VA, and for the b-t-b scheme, the secondary doesn't really matter as long as they are equal.- Status
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