Sound City 120R Adding a Master Volume

[Westrek]

Would be interested in suggestions on adding a master volume to this Sound City 120R. Thanks in advance.

[mozz]

Could i see pictures of the beast? Not going to give any suggestions but 12AT7 driver, i don't see any reason you could not use the typical PPIMV.

[Westrek]

[Helmholtz]

A very unusual circuit using many special solutions ( for presence, DC-coupled LTPI, power stage NFB, reverb..)
Seems the designer dliberately avoided traditional solutions..

Agree, that a post-PI MV would be the way to go.
The power stage uses a kind of fixed presence wiring (i.e. a high frequency boost) in the power amp.
This effect will reduce at lower MV settings.

What kind of reverb tank is used?

[Westrek]

A very unusual circuit using many special solutions ( for presence, DC-coupled LTPI, power stage NFB, reverb..)
Seems the designer dliberately avoided traditional solutions..

Agree, that a post-PI MV would be the way to go.
The power stage uses a kind of fixed presence wiring (i.e. a high frequency boost) in the power amp.
This effect will reduce at lower MV settings.

What kind of reverb tank is used?

The reverb circuitry has all been removed prior. The presence control has very little effect. Also, the schematic I posted is a bit different in the power section. The unit I have has cathode resistors. 22 ohm on one bank and 68 ohm on the other. And it's factory. Why would they do that? Here's the later schematic.

[Helmholtz]

Also, the schematic I posted is a bit different in the power section. The unit I have has cathode resistors. 22 ohm on one bank and 68 ohm on the other. And it's factory. Why would they do that? Here's the later schematic.

Very strange. Maybe intended to compensate some PI imbalance.
But the same effect could be achieved by a slightly different bias setting at both sides.

Traditional remedy is using different value plate resistors in the LTPI.

[The Dude]

......Seems the designer dliberately avoided traditional solutions......

Boy, you aren't kidding there! This amp is just silly, IMO. The different value cathode resistors may be to compensate for PI imbalance, but why bias one set of tubes differently than the other to achieve balance. There are MUCH better ways that make more sense. Rules and traditions are made to be broken, but common sense must apply also.

[Westrek]

After experimenting with several post PI master volume circuits, which none worked well, I used a 100K audio pot before pin 7 grid of V4. Worked perfectly. I'm not well enough schooled to know why the post circuits wouldn't work. The problem with those were there was very little adjustment range, only at the bottom 3rd of the rotation. At that point there was distortion and no full attenuation of the signal. I changed the negative feedback by adding a 100k resistor in series with the 22K off the OT secondary. The amp opened right up and sounded remarkably better. Probably some blocking distortion going on.

[Chuck H]

I used a 100K audio pot before pin 7 grid of V4. Worked perfectly.

If the schematic is accurate to the amp you may have actually solved a circuit error by doing this! As the 6, 7, 8 triode of V4 is a typical cathode biased stage but doesn't appear to have any 0V reference for the grid (pin 7) Assuming the added 100k pot is a voltage divider referenced to ground the circuit you added corrects this flaw. Perhaps improving overall performance as well.?.

[Helmholtz]

If the schematic is accurate to the amp you may have actually solved a circuit error by doing this! As the 6, 7, 8 triode of V4 is a typical cathode biased stage but doesn't appear to have any 0V reference for the grid (pin 7) Assuming the added 100k pot is a voltage divider referenced to ground the circuit you added corrects this flaw. Perhaps improving overall performance as well.?.

I also noticed the missing ground reference for V4 pin 7. Also there's no bypass for the dry (without reverb) signal. Wondered if there's some additional circuitry on a separate reverb board.
But the second schematic with the EL34 cathode resistors might be ok.

[Helmholtz]

After experimenting with several post PI master volume circuits, which none worked well, I used a 100K audio pot before pin 7 grid of V4. Worked perfectly. I'm not well enough schooled to know why the post circuits wouldn't work. The problem with those were there was very little adjustment range, only at the bottom 3rd of the rotation. At that point there was distortion and no full attenuation of the signal.

Curious which types of post-PI master volumes you tried.

[Chuck H]

Something you might try...

Connect a 1M fixed resistor to ground where your 100k master is now and then retry the post PI master circuits. It's possible the circuit didn't sound good because of the lack of grid load on that triode and will behave better now with the post PI master.

[Westrek]

Curious which types of post-PI master volumes you tried.

Here's 2 types I tried. They sounded fine full up, but there was no real range of adjustment until 2/3rds of the way down and then there was distortion and no full attenuation. I experimented with 250K and 1 meg dual pots and the results were similar.

[Helmholtz]

Here's 2 types I tried. They sounded fine full up, but there was no real range of adjustment until 2/3rds of the way down and then there was distortion and no full attenuation. I experimented with 250K and 1 meg dual pots and the results were similar.

Thanks.

I don't like the first circuit at all. It uses reverse wired pots and works by loading down the PI, thereby introducing unnatural (IMO) PI distortion.

Second one often works ok.

General problem with a post-PIMV and an amp using global NFB is that the NFB counteracts a reduction in gain/volume for a considerable part of the pot's travel.
I would have recommended a pre-PI MV, but was confused by the first schematic you posted.