The McIntosh MC30 monoblock tube amp uses a 12AU7 as long tail pair for phase splitter but the two plate resistors, R15=27K and R17=3K, are so dissimilar that I am scratching my head why such discrepancy. I usually see them identical or off by about 10%. Any thoughts on its balance? The plate voltages are identical, according to the schematic below. And what's the use of R12, R13, C12?
One grid is driven, the other is not. So the outputs are inherently unbalanced, and the plate resistors
being made unequal is an attempt to correct that. Those other parts are a hum reduction scheme.
Yes, the 3k is a typo, should be 30k. Sams often had mistakes like that.
being made unequal is an attempt to correct that. Those other parts are a hum reduction scheme.
Yes, the 3k is a typo, should be 30k. Sams often had mistakes like that.
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I bet "3K" should be 30K.
Mac thinks so too.
https://www.thetubestore.com/lib/th...tosh/McIntosh-MC30-Owner-Manual-Schematic.pdf
Never trust a Sams too far.
Mac thinks so too.
https://www.thetubestore.com/lib/th...tosh/McIntosh-MC30-Owner-Manual-Schematic.pdf
Never trust a Sams too far.
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That is a classic long-tail pair phase inverter , 27K and 33K are usual values if you want equal amplitude signal on both outputs ( for pefect balance pots are needed ) . Only using a CCS in the tail those values can be equal.
Depanatoru,
Or . . .
Do not use equal plate loads, and do not use negative feedback . . .
That will give you Dominant 2nd Harmonic distortion, even if the Push Pull outputs are precisely balanced.
(a little like some Single Ended amplifiers).
Hmmm, how does that sound?
Pun intended.
Or . . .
Do not use equal plate loads, and do not use negative feedback . . .
That will give you Dominant 2nd Harmonic distortion, even if the Push Pull outputs are precisely balanced.
(a little like some Single Ended amplifiers).
Hmmm, how does that sound?
Pun intended.
A negative rail of 50 to 150V can fix that. With resisters , BJTs or FETS in the AU7 & BH7 cathodes. But depending on when that vers of the McI was built, if no low cost silicon rectifiers were available may not have been cost effective.
An even greater -ve supply is an effective pull down for the AX7 driver cathodes. But there needs to be a stopper circuit to avoid H-K failure.
With Si rectifiers all is easily possible.🙂
An even greater -ve supply is an effective pull down for the AX7 driver cathodes. But there needs to be a stopper circuit to avoid H-K failure.
With Si rectifiers all is easily possible.🙂
6A3sUMMER , for hi-fi I don't think you would want unequal drive , for guitar amplifiers everything is possible 😀
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Depanatoru,
Have you ever heard a single ended Hi Fi amplifier?
Try it sometime.
A very large percentage of single ended amplifiers have lots of 2nd harmonic distortion, and very low 3rd harmonic distortion.
That is called Dominant 2nd harmonic distortion.
Please tell single ended amplifier fans that their amplifier is not Hi Fi.
That will start quite a discussion.
By the way, loudspeakers have 2nd harmonic distortion. Cancel some of that 2nd harmonic distortion by connecting a single ended amplifier so the phase of the 2nd harmonic distortion of the amplifier and the 2nd harmonic distortion of the loudspeaker are in opposite phase.
That is somewhat like the 2nd harmonic cancellation of a push pull output stage.
Do not take the hobby to seriously; relax and listen to the music.
To each his own.
To get 2nd harmonic out of a cathode coupled phase splitter, do one of the following:
1. Use a CCS in the cathode circuit, and Un-matched plate load resistances.
Or
2. Use a resistor to provide the cathode coupled current, and do not use a negative supply (a very bad'LTP' = no Long Tail), and use Matched plate load resistances.
Of course, the McIntosh MC30 Does have Global Negative Feedback, and that covers over a lot of 2nd harmonic distortion that might come out of any cathode coupled phase splitter.
Have you ever heard a single ended Hi Fi amplifier?
Try it sometime.
A very large percentage of single ended amplifiers have lots of 2nd harmonic distortion, and very low 3rd harmonic distortion.
That is called Dominant 2nd harmonic distortion.
Please tell single ended amplifier fans that their amplifier is not Hi Fi.
That will start quite a discussion.
By the way, loudspeakers have 2nd harmonic distortion. Cancel some of that 2nd harmonic distortion by connecting a single ended amplifier so the phase of the 2nd harmonic distortion of the amplifier and the 2nd harmonic distortion of the loudspeaker are in opposite phase.
That is somewhat like the 2nd harmonic cancellation of a push pull output stage.
Do not take the hobby to seriously; relax and listen to the music.
To each his own.
To get 2nd harmonic out of a cathode coupled phase splitter, do one of the following:
1. Use a CCS in the cathode circuit, and Un-matched plate load resistances.
Or
2. Use a resistor to provide the cathode coupled current, and do not use a negative supply (a very bad'LTP' = no Long Tail), and use Matched plate load resistances.
Of course, the McIntosh MC30 Does have Global Negative Feedback, and that covers over a lot of 2nd harmonic distortion that might come out of any cathode coupled phase splitter.
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I have had a few MC30's and they are indeed 27 and 30k plate loads. If the 18k tail load is 'improved', preferably with a CCS is used, they can go to 27k. I would also suggest improving the heater and running a 6CG7 instead.
cheers,
Douglas
cheers,
Douglas
1.Yes he did.
2.Really!?
3.+4. I bet you didn't try that with a McIntosh yet!That GNFB is really global there.Work on its phase splitter and you'll get tons of nonlinear distortions.
By the way ,loudspeaker's own nonlinear distortions are the ones dissipating loudspeaker's thd, including 2nd harmonic, but amp's nonlinear distortions are simply reproduced by the speakers while partly produced by amp-speaker interaction!!
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R17 = 30K
Thank you all for the correction of the value of R17 resistor. My friend sent me a picture of the circuit board and it is indeed 30K.
Just a thought exercise, is it possible to replace the entire 12AU7 long tail pair to be a cathodyne/concertina/split-load phase inverter by paralleling 12AU7 and use 18K cathode and plate resistors, to keep same current draw, but keeping everything else the same? I know the amp will lose gain but most Mac amps have a ton of gain anyway and coupled with a preamp gain shouldn't be a problem. Not that I would butcher a classic piece but if I ever acquired just the output transformer and build a clone it might be an alternative. I always like the sound and simplicity of split-load circuit. It got turned into a kind of a Williamson circuit with cathode follower. Should other parameters be changed, say feedback resistor and increase input tube plate resistor value to increase gain?
Thank you all for the correction of the value of R17 resistor. My friend sent me a picture of the circuit board and it is indeed 30K.
Just a thought exercise, is it possible to replace the entire 12AU7 long tail pair to be a cathodyne/concertina/split-load phase inverter by paralleling 12AU7 and use 18K cathode and plate resistors, to keep same current draw, but keeping everything else the same? I know the amp will lose gain but most Mac amps have a ton of gain anyway and coupled with a preamp gain shouldn't be a problem. Not that I would butcher a classic piece but if I ever acquired just the output transformer and build a clone it might be an alternative. I always like the sound and simplicity of split-load circuit. It got turned into a kind of a Williamson circuit with cathode follower. Should other parameters be changed, say feedback resistor and increase input tube plate resistor value to increase gain?
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Having a 12bh7 splitter before a 12ax7 driver would make me very reluctant to modify the original McIntosh design simply because i don't know all the factors....
As far as I know tube McIntosh amplifiers were the best performers of the tube world and that must have been for a reason or two...
As far as I know tube McIntosh amplifiers were the best performers of the tube world and that must have been for a reason or two...
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Could be done and many more changes , but if you have those special output transformers normaly you would want a McIntosh not a butchered schematic 😛
A Wiliamson can be made with many , many output transformers , a McIntosh not .
Depanatoru,
A test of the effects of a single ended 2A3 amplifier (no negative feedback), and a Spica TC-50 was conducted:
The test was of the 2nd harmonic distortion interaction of the 2A3 amplifier and the Spica TC-50.
The amount of the Acoustic 2nd harmonic distortion was changed when the phase of the speaker cable connections from the amplifier output to the speaker input was reversed.
A test of the effects of a single ended 2A3 amplifier (no negative feedback), and a Spica TC-50 was conducted:
The test was of the 2nd harmonic distortion interaction of the 2A3 amplifier and the Spica TC-50.
The amount of the Acoustic 2nd harmonic distortion was changed when the phase of the speaker cable connections from the amplifier output to the speaker input was reversed.
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Why not? I am not copying the Williamson exactly, just the concept. The drivers and cathode followers are not altered and all the Schade style feedbacks are not changed. Only the phase splitter is changed from long tail pair to split-load. I don't see why it can't be done.
McIntosh amplifiers are outstanding performers not by chance but by design.
1) Local CFB in the output stage is about 12 db. The resulting DF before other NFB is 4 to 5
2) The output tubes are direct driven by CFs to avoid blocking
3) The impedance to common of the OPT primary windings is 1/4 the usual🙂
The Electrovoice vers of the Circlotron does about the same.
But two B+ power supplies.
The topic is about a push-pull ... I have no idea what is the connection with your 2A3 single ended amp and second harmonics 😀 .