The splitter schematic was wrong. 33k versus 3k.
The explanation of 27k and 33k was correct, there was no real Long Tailed Pair phase splitter.
The reason for the unmatched plate loads was to reduce/eliminate 2nd harmonic distortion of the phase splitter.
There are many ways to fix the 2nd harmonic distortion of the phase splitter, and the thread gave most or all of those.
2nd Harmonic distortion was a sub-category of this thread, was it not?
I am sorry for the digression from the original post.
The explanation of 27k and 33k was correct, there was no real Long Tailed Pair phase splitter.
The reason for the unmatched plate loads was to reduce/eliminate 2nd harmonic distortion of the phase splitter.
There are many ways to fix the 2nd harmonic distortion of the phase splitter, and the thread gave most or all of those.
2nd Harmonic distortion was a sub-category of this thread, was it not?
I am sorry for the digression from the original post.
No , the reason for different values is to get the same amplitude signals out .
Read about long tail pair inverter if you never build / measured one .
Read about long tail pair inverter if you never build / measured one .
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The 30k is a standard 5% EIA E24 resistor value. And you would want a better tolerance than that
for a phase splitter anyway.
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I use a Constant Current Sink to the parallel cathodes, and matched plate loads in all of my cathode coupled phase inverters. The amplitudes are intrinsically matched (unless there is any grid current).
No adjustment of plate loads, and no adjustment rheostats required here.
And . . . this configuration intrinsically cancels the phase splitter's 2nd harmonic distortion.
Using the other circuit, such as was shown in this thread . . .
2nd order and other complex distortions may be produced by a less than perfect 'almost long tailed' pair.
First of all, with unmatched plate loads of such a stage, the DC plate voltages are usually not the same.
The circuit works, but the performance can vary from tube to tube.
The current of each triode is shared by the other triode, and the 'almost long tailed' resistor.
The above are just my opinions.
No adjustment of plate loads, and no adjustment rheostats required here.
And . . . this configuration intrinsically cancels the phase splitter's 2nd harmonic distortion.
Using the other circuit, such as was shown in this thread . . .
2nd order and other complex distortions may be produced by a less than perfect 'almost long tailed' pair.
First of all, with unmatched plate loads of such a stage, the DC plate voltages are usually not the same.
The circuit works, but the performance can vary from tube to tube.
The current of each triode is shared by the other triode, and the 'almost long tailed' resistor.
The above are just my opinions.
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Tolerance in this case doesn't mean too much , ideally you would want another 27K + 4,7K pot or something like that ... so you can measure and adjust for perfectly equal output voltages .
another schematic
Just to confirm the 12AU7 plate resistors are indeed 27K and 30K with another schematic, thanks to PRR. And he's right that never trust Sams too far!
Just to confirm the 12AU7 plate resistors are indeed 27K and 30K with another schematic, thanks to PRR. And he's right that never trust Sams too far!
But this stage does not have to be EXACT. There is yet another stage with a long tail to improve balance.
The difficulty of using other-name or home-brew drivers with Mac output transformers is that the Mac is half cathode follower. The power tube grids need HUNDREDS of volts of drive, not just dozens.
Ok, here's a dumb question. Why use 12AX7's to drive the output tubes when the preceding stage (12BH7's) configured as cathode followers would be a better driver stage?
jeff
jeff
As PRR said, the imperfect balance of the phase inverter is improved by the next stage.
. . . Consider the action of R32 and R17.
Both the phase inverter and 12BH7 stages are required.
The balance provided by R17 goes away if you use the cathodes of the 12BH7 to drive the output stage grids.
And, do you want to replace the 12AX7 cathode followers with a second 12BH7 which can drive Lots of grid current into the output tubes?
This is a synergistic design, requiring special attention to detail, and at least one special part per channel.
I can not confirm or deny the performance level, and sound signature/lack of signature of the MC30 amplifier.
But it is not my favorite design. The need to Bootstrap driver and cathode follower tubes is not my favorite thing (and that is positive feedback).
And, there are the legendary almost Un-Obtanium output transformers that you have to find a source for.
. . . Consider the action of R32 and R17.
Both the phase inverter and 12BH7 stages are required.
The balance provided by R17 goes away if you use the cathodes of the 12BH7 to drive the output stage grids.
And, do you want to replace the 12AX7 cathode followers with a second 12BH7 which can drive Lots of grid current into the output tubes?
This is a synergistic design, requiring special attention to detail, and at least one special part per channel.
I can not confirm or deny the performance level, and sound signature/lack of signature of the MC30 amplifier.
But it is not my favorite design. The need to Bootstrap driver and cathode follower tubes is not my favorite thing (and that is positive feedback).
And, there are the legendary almost Un-Obtanium output transformers that you have to find a source for.
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In the late 1950s Norman Crowhurst authored three articles for publication covering a McIntosh clone that used two OPT's. One OPT in the Plate cct where we would usually find it, the other in the cathode cct. The signal coupling is accomplished by paralleling the OPT secondaries & capacitively coupling the plate & cathode signals.
I built two of these, one using PP 6LU8's clips at ~35 watts. In addition to Crowhurst's cct I added some mods of my own. The PS is SS with a FET follower set at ~400V. And a Zenered -150 rail for the pull downs.
And the front end & driver can run full differential.
The other is PP 6V6's, about 14 watts running Class AB.
Both are experimental, many possibilities explored.🙂
And the sound as always depends on the source material & the reproducing equipment & environment. A glass of wein helps.
I built two of these, one using PP 6LU8's clips at ~35 watts. In addition to Crowhurst's cct I added some mods of my own. The PS is SS with a FET follower set at ~400V. And a Zenered -150 rail for the pull downs.
And the front end & driver can run full differential.
The other is PP 6V6's, about 14 watts running Class AB.
Both are experimental, many possibilities explored.🙂
And the sound as always depends on the source material & the reproducing equipment & environment. A glass of wein helps.
The 12AX7 should become a 12AT7, just like the 275/75 used. Also, keeping the fixed, fixed bias is asking for trouble; install adjusters so bias can be adjusted to spec.
There is other stuff that is useful; resistor value changes to tune a 12AY7 in place of the 12AX7 for instance. Getting rid of the positive FB is also useful... 🙂
It is a commercial amp; of course it can be improved.
cheers,
Douglas
There is other stuff that is useful; resistor value changes to tune a 12AY7 in place of the 12AX7 for instance. Getting rid of the positive FB is also useful... 🙂
It is a commercial amp; of course it can be improved.
cheers,
Douglas
No, I would get rid of the 12AX7 stage altogether.
Yes, with so many stages, you'd have to know what you're doing.🙂
jeff
We do not want a "better driver". The grid current limits on 1614 (6L6) are not specified. Yes, the 807 has B2 conditions: 4mA ought to be ample. That's about all you can get from a 12AX7 at these conditons. Any more might be risky? (I dunno the price of 1614.)
Are you worried about the AX7 tainting the tone? A 1V bias tube working cathode follower is about the least nonlinear single device possible.
Maybe helpful, all Mc amps of the era, Mc225, Mc240, Mc275, and mono versions used the same first several stages, and 27K/30K Ohm plate loads.
A split-load inverter here would need a very large supply B+ voltage to allow enough swing into the 12BH7 stage. It's easy and common to try to second-guess GG's design, but it's harder to make something better. Everybody needs to do their homework first.
The issue of a 12AX7 as cathode follower has a clue left in the anode-cathode voltage that these gals sit at. 435 to 470 VDC design center for half a century. You'll often see the original Telefunken, marked McIntosh, valves working fine today. There's method in their madness.
All good fortune,
Chris
A split-load inverter here would need a very large supply B+ voltage to allow enough swing into the 12BH7 stage. It's easy and common to try to second-guess GG's design, but it's harder to make something better. Everybody needs to do their homework first.
The issue of a 12AX7 as cathode follower has a clue left in the anode-cathode voltage that these gals sit at. 435 to 470 VDC design center for half a century. You'll often see the original Telefunken, marked McIntosh, valves working fine today. There's method in their madness.
All good fortune,
Chris
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vinylkid58,
You said:
"No, I would get rid of the 12AX7 stage altogether."
"Yes, with so many stages, you'd have to know what you're doing."
I agree, I just did not want to rain on somebody's favorite parade.
The old signal sources did not put out 3V peak like today's CD players do.
Amplifiers needed more gain. And Unity Coupling needed tremendous drive voltages, used positive feedback (boot strapping) to get enough plate voltage, etc.
It seems that any difficult problem introduced by a special output transformer, can be overcome by more complex circuitry ahead of all that.
I prefer simpler amplifiers.
"You should make things as simple as possible, but no simpler" - Albert Einstein.
You said:
"No, I would get rid of the 12AX7 stage altogether."
"Yes, with so many stages, you'd have to know what you're doing."
I agree, I just did not want to rain on somebody's favorite parade.
The old signal sources did not put out 3V peak like today's CD players do.
Amplifiers needed more gain. And Unity Coupling needed tremendous drive voltages, used positive feedback (boot strapping) to get enough plate voltage, etc.
It seems that any difficult problem introduced by a special output transformer, can be overcome by more complex circuitry ahead of all that.
I prefer simpler amplifiers.
"You should make things as simple as possible, but no simpler" - Albert Einstein.
The positive FB was a side effect of getting large enough drive voltage headroom. This brings me to the next bit that ought to go; those 12k plate loads in favour of solid CCS-es...still hooked up to bootstrap the supply, but w/o the FB.
cheers,
Douglas
cheers,
Douglas