"patrick-turner" wrote in message
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- show quoted text -
Me again folks. I wrote a new page about using a differential input pair
of triodes for a PP amp. See my page at
http://www.turneraudio.com.au/basic-tube-5.html
Brick bats, buckets of poop, poisoned arrows, drone delivered ordinance
all tolerated, but returned to sender with gusto.
Patrick Turner.
Alex:
I skimmed this page.
Noticed an error in the MJE340 current source circuit in the first
schematic. (Correct in the others.)
Noticed a typo "oft3en" in the first line of the section "DRIVER STAGE
DIFFERENTIAL AMP".
OK, I will check again.
Was pleased that the term "differential stage" is widely used, and that
arcane "long tail pair" is only occasionally -- sort of being phased out.
Did not understand the purpose of long and distracting
calculations/speculations about mu and gain... Was it all to point out that
the actual mu of 6CG7 is 22, not 20 as quoted in the datasheets?
The purpose of calculations was to not only point out the µ is NOT always the 20you expect to find as data sheets suggest, but to also point out that Ra is more than the usual 8k0 usually given for where Ia = 10mAdc. What varies most in small triodes is Ra and Gm, and their product = µ.
So while comtemplating the use of any small triode the µ, Ra and gm used for gain equations must be correct for the Ia condition and Ea condition.
The other thing I wanted to point out was that where you have Va of each triode about equal or less than the highest Vin signal, then the anode load load of the tubes becomes quite different, so the Ea change of each triode becomes very different yet the Ia change is the same, where there is a CCS for commoned cathodes.
There would be a limit for how much Vin you could have,and I have not measured what it is. But suppose you had 100.1Vac at V1 input grid, and 100.00Vac at V2 input grid, then differential input signal is 0.1Vac so expect say +/-1.7Vac at the anodes, providing the input signals have the same phase and wave shape. The signal that is common to both grids would be 100Vac. So because there is virtually no Ik change anywhere, you only get the amplified diffential signal.
But a slight amount of finite resistance or capacitance between cathodes and -ve rail or 0V will produce a common mode output signal at each anode.
The CCS shown could be better, with higher finite resistance. And such an input stage can be used for the balanced outputs of a floating transformer secondary of a line level transformer where there may be a high common mode noise signal.
There were no explanation why the tubes with mu around 20 are preferred.
With medium µ triodes you get a useful amount of gain, reasonably low Ra, and low THD&IMD,and sound is usually excellent. This is a general idea that is valid for all tube amps.
Obviously, 12AX7 is not a good candidate because it can not work with a
decent current, but 12AT7, 6BQ7, ECC85 seem even better than 6CG7 because of
higher mu, larger Gm and about the same current.
But diff gain may be excessive in a power amp where you want 2 stages with fairly equal gain and tube types for simplicity and effectiveness and easy re-tubing in 10 years.
I much favour the use of 6CG7 in diff mode driving a pair of trioded EL84 as a balanced amp with highish value "tail" resistance, maybe at least 2Ra of one of the triodes. The two stages have overall gain = say 17 x 17 perhaps = 289.
If the OP stage needs say 180Vac grid to grid if it uses a large amount of local CFB, then you need 0.62Vac grid to grid at diff input pair.
If the GNFB is say 10dB, the VFB will be about 1.24Vac, and Vin becomes 1.86Vac, and this is quite OK, because most of the error correction is done in the OP stage where the beam tetrode Ra is reduced to say less than 1/2 that of the tubes connected as triodes. But a good input pair can be made using 6922/6DJ8, without any change to R&C parts and open loop gain will rise by +3dB.
Using less local output stage NFB means more GNFB can be applied and you end up with an amp like the Williamson with 2Vac sensitivity for 16Watts with trioded KT66, and 20dB GNFB.
I have used 12AT7/6AQ8 which will run with Ia = 4mA per triode OK. ECC85 and 6SL7 may also be used, 12AY7, and also 12AX7, if the drive voltage to the next stage isn't very high. The following stage Miller C becomes less of a problem causing slewing as voltage drive is reduced. I have often used 2 paralleled triodes for each side of input diff amp, or use small pentode as triodes, 6BX6, 6EJ7, 6AU6, and small power pentodes like 6AR5 etc, etc, etc, etc. And there really isn't any limit to what might be used - but what we end up with can *ALWAYS* be better than some dopey input amp from 1955 that was designed by a shareholder or bean counter.
Again, I did not read the whole article attentively. Perhaps there are some
other minor errors...
I sure ain't perfect, I'm jist a retired amp-worker.
A pair of j-fets as a diff pair can be quite good. The ARC Classic 30 with 2 x 6550 output tubes is a perfect example. But one might use a pair of darlington connected bjts, maybe with emitter R on each side, lower RLc, and that can approximate the same goodness of a 6CG7.
I'd rather let tubes handle all the signal if there are output tubes in an amp.
KR Audio had a SET 30+30 stereo amp with 2 x KRT100 triodes, like 845, but 5V heater and twice gm. But they used an all solid state driver stage. It don't make sense that one but ppl spent $19,000 here to buy one of those amps.
Patrick Turner.