Some theoretical questions about tubes

[POTL]

Hello
I had a few trivial questions about tube preamps. I understand that the forum is dedicated to stompboxes, but I am sure that competent people will help me.
I looked at the scheme of the preamp EVH 5150 III, which is a modernized version of the Peavey 5150/6505.
I have a few questions
1) In most preamplifier circuits, there is a voltage divider in front of the triode; however, before the V5-A, the r66 + r67 design resembles the input circuit of vintage amplifiers, what is called such a connection, what are its pros and cons, how can the influence of resistors on the volume drop be calculated (in the divider voltage can always be calculated on how many decibels volume falls).
2) V6-A is Triode with Local Negative Feedback, as far as I understand the gain is 1, and the bandwidth of the frequencies is very high, the input impedance is high and the output is low. A logical question arises: what advantage does this construction have with a classic buffer? Buffer is easier to perform and requires less detail, just so no one would complicate the scheme.
3) V6-B is a buffer, why is it needed if its functions are performed by the previous stage?



4) Do resistors R7 and R8 affect the gain of V2-A or the nominal does not play a big role?
For example, if I change the value of 270 to 470, the gain will be less?

5) Do the resistor values ​​in the voltage divider affect the gain?
For example, a pair of R47K + R47K and a pair of R470K + R470K will give the same voltage drop of 6dB or will the resistance value further affect the overall gain?
6) In Mesa Boogie Mark amplifiers, the output stage looks very interesting, there are several voltage dividers at once, which give a strong signal drop, what is this for?

[Phoenix]

1) In most preamplifier circuits, there is a voltage divider in front of the triode; however, before the V5-A, the r66 + r67 design resembles the input circuit of vintage amplifiers, what is called such a connection, what are its pros and cons, how can the influence of resistors on the volume drop be calculated (in the divider voltage can always be calculated on how many decibels volume falls).

R66 and R67 do not form a voltage divider, rather, R67 is a grid stopper resistor. It, along with the Miller capacitance of the following gainstage, forms a low-pass filter. Grid stoppers help suppress radio frequency interference by limiting the bandwidth, this is why they are almost universally seen on input stages. But here, its most important function is to reduce blocking distortion, which is where continuous overdrive results in a bias shift by charging the coupling capacitor (C38), pushing the V5b into cutoff. As this is a high gain amp, large overdrive is to be expected.

2) V6-A is Triode with Local Negative Feedback, as far as I understand the gain is 1, and the bandwidth of the frequencies is very high, the input impedance is high and the output is low. A logical question arises: what advantage does this construction have with a classic buffer? Buffer is easier to perform and requires less detail, just so no one would complicate the scheme.

You've correctly identified that this is a stage with local negative feedback, however, it does not have unity gain, and its output impedance is not particularly low. The stage still has gain, and while the output impedance is reduced, it does not have the low output impedance of a cathode follower.

3) V6-B is a buffer, why is it needed if its functions are performed by the previous stage?

See above, but also, DC coupled cathode followers like this one have a particular flavor of distortion, or what most people would call compression, due to being biased "hot" and drawing grid current, which squishes the positive-going side of the waveform. The added series grid resistance (voltage divider/level shifter) of R74 will accentuate this effect.

4) Do resistors R7 and R8 affect the gain of V2-A or the nominal does not play a big role?
For example, if I change the value of 270 to 470, the gain will be less?

R7 and R8 are again, grid stoppers and do not effect the gain, as they are in series only with the effectively infinite input impedance of the tube. But here, the job they perform is actually as passive mixer resistors. Imagine if R7 and R8 were not present, now turn down Volume I, turn up Volume II. Plugged into Channel II, even though the volume is turned up, you will not get any sound unless Volume I is also turned up. R7 and R8 fix this issue. See if you can figure out why.

5) Do the resistor values ​​in the voltage divider affect the gain?
For example, a pair of R47K + R47K and a pair of R470K + R470K will give the same voltage drop of 6dB or will the resistance value further affect the overall gain?

Remember that ALL stages have an output impedance, op amps have vanishingly low output impedances, but vacuum tubes have very high output impedances. This will form a voltage divider with the load impedance, so heavily loading a stage will always reduce output of the stage - barely noticeable with an op amp (until it runs out of output current), but very noticeable for tubes.

6) In Mesa Boogie Mark amplifiers, the output stage looks very interesting, there are several voltage dividers at once, which give a strong signal drop, what is this for?

This is not the output stage, but just the effects loop and pre phase inverter master volumes and a lot of switching. There really aren't any intentional voltage dividers in there at all, rather just isolating and mixing resistors. It's all to stop all the needlessly complicated switching from interacting with each other/muting each other.

[PRR]

2) V6-A is Triode with Local Negative Feedback, as far as I understand the gain is 1, ...

You've correctly identified that this is a stage with local negative feedback, however, it does not have unity gain, and its output impedance is not particularly low. The stage still has gain, and while the output impedance is reduced, it does not have the low output impedance of a cathode follower.

Nit picking: equal resistors R71 R72 set gain near unity. The voltages noted are 24V in 22V out, near unity. A hair short, like the amplifier gain is 9. R42 eats half the AX7's gain, R73 sucks some, I think the noted numbers are correct.

The symmetry of C28 C37 suggests no bass rolloff until far below the audio band, like 1Hz. This may preserve the "thud" of asymmetric clipping in earlier stages.

The output impedance is likely some under 10k. Very low compared to R74, the input of an EQ network. If V6-A were dropped and R74 loaded V5-B, the EQ values would want to be different.

However all-in-all I'm not sure every part here is "essential". I suspect the design brief was "Use as many tubes as possible!", so the potential buyer would say "WOW! Lookit all dem tubes!!"

[Phoenix]

Nit picking: equal resistors R71 R72 set gain near unity. The voltages noted are 24V in 22V out, near unity. A hair short, like the amplifier gain is 9. R42 eats half the AX7's gain, R73 sucks some, I think the noted numbers are correct.

Yes, quite correct, I wasn't paying close enough attention, didn't notice the feedback potential divider (R71/R72). Don't often see local negative feedback in guitar amps, and when you do Rin is very often just Zo of the previous stage, and so small enough to basically ignore compared to the Rf/Rg potential divider (with typical Rf values of 1M+ anyway).

[Rob Strand]

Don't often see local negative feedback in guitar amps

Not very common.  IIRC the Dumble preamp had a feedback resistor on one of the stages (not unity gain though).

[amptramp]

2) V6-A is Triode with Local Negative Feedback, as far as I understand the gain is 1, and the bandwidth of the frequencies is very high, the input impedance is high and the output is low. A logical question arises: what advantage does this construction have with a classic buffer? Buffer is easier to perform and requires less detail, just so no one would complicate the scheme.

The input impedance is not high.  As the input pulls the grid up, the plate voltage declines and tends to pull the input voltage down.  This requires more current to overcome the negative feedback, so the input impedance is lower than the 1 megohm it would be without it.

[PRR]

> The input impedance is not high.  ... so the input impedance is lower than the 1 megohm it would be

If the tube had infinite gain then Zin would be exactly R71 1meg. Penciling finite gain and the to-ground resistor, here probably 1.1Meg. Strikingly high against the ~~50pFd of grid capacitance (not ~~100p because 12AX7 gain is lamed by unbypassed RK). NFB is falling by 3KHz, though total response is maintained to 20KHz.

The overload performance should not matter: being unity-gain it is likely the stages before and after will be slammed before this stage shows much distress. The large and near-equal charge/discharge resistances mean it won't tend to grid-block.