http://www.bonavolta.ch/hobby/en/audio/6as7_2.htm
Few questions:
1) Is the input circuitry sufficient to drive the otl stage to full output = can I change the R1 grid resistor with a volume pot and drive the output stage into clipping?
2) What always turned me off on this circuit is the 4-section input-driver-phase splitter circuitry (><KISS); when taking a closer look the second stage (two sections of 6sn7) looks very odd to me; is that stage part of the phase-splitter or what else does it do?
3) Any suggestions for simplifying/improving the circuitry preceding the output stage? (I'm aware of the fact that a redesign probably isn't a beginner's task due to the massive gNfb involved)
Thanks -- Simon
Few questions:
1) Is the input circuitry sufficient to drive the otl stage to full output = can I change the R1 grid resistor with a volume pot and drive the output stage into clipping?
2) What always turned me off on this circuit is the 4-section input-driver-phase splitter circuitry (><KISS); when taking a closer look the second stage (two sections of 6sn7) looks very odd to me; is that stage part of the phase-splitter or what else does it do?
3) Any suggestions for simplifying/improving the circuitry preceding the output stage? (I'm aware of the fact that a redesign probably isn't a beginner's task due to the massive gNfb involved)
Thanks -- Simon
Klimon,
This should easily drive the output to full clip.
2nd 6SN7 stage uses two triodes to give high Z input, reasonable gain, NO phase inversion. This is necessary to return global nfb to the cathode of the input tube.
The phase splitter circuitry to drive the output tube is accreddited to Julius Futterman, and is something of a legend in OTL designs. This rides the driver off the output rail, while delivering equal, antiphase drive to the two sets of triodes. It's very, very neat, and elegant beyond most circuits you see.
Hugh
This should easily drive the output to full clip.
2nd 6SN7 stage uses two triodes to give high Z input, reasonable gain, NO phase inversion. This is necessary to return global nfb to the cathode of the input tube.
The phase splitter circuitry to drive the output tube is accreddited to Julius Futterman, and is something of a legend in OTL designs. This rides the driver off the output rail, while delivering equal, antiphase drive to the two sets of triodes. It's very, very neat, and elegant beyond most circuits you see.
Hugh
Klimon said:
It isn't so odd. V2 forms a noninverting differential voltage amp. It gives the advantage of presenting the driving stage a friendlier load that's less capacitive to improve slew rate. At least it would if he hadn't screwed that up. R6 needs a bypass capacitor connected across it to make the input section of the 6SN7 act more like a cathode follower that doesn't have the Cmiller problem.
Otherwise, it's a conventional Futterman output stage.
I did the same thing in a design I'm working on to make a 6BX7 verticle deflection dual triode present a friendlier load. These may not have much gain, but what they do have makes for a nasty Cmiller.
I am thinking about trying this version as I have tons of NOS NIB 6AS7's. I have 2 questions if you guys can help.
1. Is the 3rd stage 6SN7 splitter using only half the tube or is it paralleling the 2 halfs of the 6sn7?
2. What are the bias voltages? seems like -70 for the top and -220 to -240 for the bottom. Can someone enlighten me.
Thx in advance
If you have tons of NOS 6AS7s, I would suggest forgetting all about this particular design. It SUX. Instead, use the 6AS7s to build a Circlotron OTL: similar level of complexity, much better balance since you're not forcing balance.
Just keep in mind that NOS 6AS7s can't be pushed quite as hard as the newer versions.
I hear you but building for multi testing purposes so not as big an issue but thx. With that said two things.
1. Do you have the information to help me answer my questions?
2. Have you seen BR's el509 OTL from his book? If so what is your opinion on that design (splitting to the output is done with seperate triode sections).
Thx
The way I'm reading that schemo, both the cathodyne driver and the first preamp are using one section of the 12AX7 and 6SN7 respectively.
Given the Vpp= 150Vdc for the finals, a Vgk= -65 to -70V looks about right.
I've seen other OTL designs that use different types. However, I haven't found anything that works better than the 6AS7. When Ralph designed the Atmasphere line of OTLs, he knew what he was doing.
I still don't like the design here at all. If you insist on going this route, I'd look into the Inverted Futterman topology instead. It avoids that positive feedback to force a cathode follower to look more like a grounded plate amp, but it still has a potentially nasty DC offset problem that can poof speeks. It's still a problem with the Circlotron design, but far less of a potential problem.
Given the Vpp= 150Vdc for the finals, a Vgk= -65 to -70V looks about right.
I've seen other OTL designs that use different types. However, I haven't found anything that works better than the 6AS7. When Ralph designed the Atmasphere line of OTLs, he knew what he was doing.
I still don't like the design here at all. If you insist on going this route, I'd look into the Inverted Futterman topology instead. It avoids that positive feedback to force a cathode follower to look more like a grounded plate amp, but it still has a potentially nasty DC offset problem that can poof speeks. It's still a problem with the Circlotron design, but far less of a potential problem.
Thx so much. So you think the -240 is also correct as I suggest?
Also I should clarify that I am using 6080's for the design test not 6as7's sorry got to generic on you there.
I own several NYAL OTL3's and have owned Futterman H3's in the past. Are you saying you would rather go with the Futterman front end instead? Please clarify. THX!!!
Let's assume you get it all working, Futterman, Circlotron, or whatever.
The magic of tubes is best heard with no feedback.
The OTL deals with the intrinsic high Z of tubes by implementing huge amounts of feedback (for tubes at least), typically 35-50dB. This gives a sufficiently low Z (around 1ohm) to drive an 8R speaker. A 600R speaker would be better, but this is what we are stuck with, and there is a determination to avoid the output trafo.
Suggestion: If we use lashes of interstage fb to force the OTL to be load tolerant (and ramp up the number of tubes to give decent current), aren't we dialling out those special qualities of tubes by reducing their colorations with feedback?
IOW, force the issue with tubes to make a reasonable OTL, and have it sound like a very good Class A SS amp.
I don't want to start arguments here, but what's the point? Wouldn't it be a better idea to use a great tube front end, no feedback, for that wonderful 'tube sound', then strap it to a very good Class A output stage, mebbe even single ended a la NP?
A LOT simpler, cheaper, and safer for the speaker too, and D-A, you already have lots of OTLs, why not try something different?
Hugh
The magic of tubes is best heard with no feedback.
The OTL deals with the intrinsic high Z of tubes by implementing huge amounts of feedback (for tubes at least), typically 35-50dB. This gives a sufficiently low Z (around 1ohm) to drive an 8R speaker. A 600R speaker would be better, but this is what we are stuck with, and there is a determination to avoid the output trafo.
Suggestion: If we use lashes of interstage fb to force the OTL to be load tolerant (and ramp up the number of tubes to give decent current), aren't we dialling out those special qualities of tubes by reducing their colorations with feedback?
IOW, force the issue with tubes to make a reasonable OTL, and have it sound like a very good Class A SS amp.
I don't want to start arguments here, but what's the point? Wouldn't it be a better idea to use a great tube front end, no feedback, for that wonderful 'tube sound', then strap it to a very good Class A output stage, mebbe even single ended a la NP?
A LOT simpler, cheaper, and safer for the speaker too, and D-A, you already have lots of OTLs, why not try something different?
Hugh
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With -150Vdc on the cathode, you'll need -220Vdc on the control grid to give Vgk= -70Vdc.
See Here for a description of the Inverted Futterman. If not Circlotron, then this.
I hear you Hugh,
I have had the best hybrids made by the man himself George Kaye. So no need, as in general I liked those amps, but not to the extent I like OTL's. Especially as that style amp (OTL's) sounds with my Quad 57's, 63's and 989's. And from time to time the few other speaker sets I own.
My interest in the hobby at this point after some 30+ years tends to remains viable solely based on my want to try circuits I have interest in and ones that I have yet to hear. If i did not do that I would stop altogether, buy the best solid state amp money and my ear could buy/hear and hang up the rest and never look back. So I keep doing this kind of thing to stay interested because I have to tell you, that I personally have never heard any other type of tube amp sound better then an OTL tube amp to my ears and I have had and or built them all with cheap parts and the best parts money can buy and I stand buy that. Now my experience with OTL is long but all with Futtermans by design. So I want to try others since I have yet to build many of those. Take care.
Fair call, D-A, we all have our preferences.
I urge you to read Hans descriptions, linked by Miles, as they are well done, simple, and to the point. He gravitates to the 6C33B, a good tube, but one which operates at high temperatures, and with small pins, is inclined to corrosion, particularly on the filament pins. If pins are regularly cleaned, these are an excellent tube, with rp of just 92R, very low and highly suited to OTLs. The 6080 has rp of 280r, quite a bit higher, of course.
D-A., you might consider the imminent danger to speakers if one of the (many!) output tubes should fail. This is the real issue with most of these topologies, in my view. You can completely remove this threat by designing with a single, 320V rail, and cap coupling the output to the speaker. Although this is not cool, the cap, if carefully chosen (3000uF is fine), adds almost nothing to the sound and affords very good protection. A integrating DC voltage sense at the output can then be set up to switch off B+ in the event of the output rising above 170V or below 150V. This would make the OTL quite safe for expensive speakers and very simply cope with any tube failures.
And there can be no doubt tube amps go best with the Quads, though a few SS amps (including my own!) drive them with relative ease.
Thank you for your reply,
Hugh
I urge you to read Hans descriptions, linked by Miles, as they are well done, simple, and to the point. He gravitates to the 6C33B, a good tube, but one which operates at high temperatures, and with small pins, is inclined to corrosion, particularly on the filament pins. If pins are regularly cleaned, these are an excellent tube, with rp of just 92R, very low and highly suited to OTLs. The 6080 has rp of 280r, quite a bit higher, of course.
D-A., you might consider the imminent danger to speakers if one of the (many!) output tubes should fail. This is the real issue with most of these topologies, in my view. You can completely remove this threat by designing with a single, 320V rail, and cap coupling the output to the speaker. Although this is not cool, the cap, if carefully chosen (3000uF is fine), adds almost nothing to the sound and affords very good protection. A integrating DC voltage sense at the output can then be set up to switch off B+ in the event of the output rising above 170V or below 150V. This would make the OTL quite safe for expensive speakers and very simply cope with any tube failures.
And there can be no doubt tube amps go best with the Quads, though a few SS amps (including my own!) drive them with relative ease.
Thank you for your reply,
Hugh
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You should check out Bruce's OTL Patent. The schematic on the Patent uses the 6C33 tube, which may be his first Production OTL T4? The schematic is also very close to his T8 that uses EL509. The Patent Schematic is missing the some resistor in the output stage, but it easy to figure the missing resistor. I've scratched built his T8 from his book, and it is one of the best amp I've heard. I've also owned a Futterman H3aa and was later moded by G. Kayes.
Transformerless output vacuum tube ... - Google Patent Search
Transformerless output vacuum tube ... - Google Patent Search
Well just finished breadboarding BR's T8 150w version of this amp. I have been listening now for about 15 hours. She sounds great! Just abot as good as my rebuilt NYAL OTL3's. I used 6LF6's instead on the EL509's shown in BR"s book Audio Reality. The T8 is the phase of the 6AS7 version discssed in this thread. I ended up building this version as opposed to the 6AS7 version. I will post some pic's of the breadboard later today and I should have the first channel in a case by the end of the day.
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