Help me understand an Ampeg SVT schematic

[SoulFetish]

I'm guessing but the directly coupled cathode followers could be there to provide power tube grid current to take into class AB2 for more power output.

The diodes could be a limiter to prevent the power amp being driven too hard.

Is can’t operate in AB2 due to the cllamping action caused by the 47k grid stoppers. My guess is the direct coupling is to eliminate the RC HPF, and preventing blocking/crossover at low frequencies.

[Dave H]

Is can’t operate in AB2 due to the cllamping action caused by the 47k grid stoppers.

I thought of that but the cathode followers should be able to supply a couple of mA of grid current through the 47k stoppers.

[SoulFetish]

I thought of that but the cathode followers should be able to supply a couple of mA of grid current through the 47k stoppers.

The problem is, 1mA across 47k would cause a voltage drop of 47 volts. I don’t see how the grids could be driven into positive grid voltage

[Dave H]

The problem is, 1mA across 47k would cause a voltage drop of 47 volts. I don’t see how the grids could be driven into positive grid voltage

I agree, the grids won't be driven far positive with a only few mA of current.

As you can see I don't know much about AB2 It was just the first thing that came into my head when I saw the cathode followers. They must be there to buffer the previous stage from the 47k grid leak resistor so as not to reduce the voltage swing available for the power tube grids.

[g1]

I think the 'why cathode followers' and 'why DC coupled cathode followers' are really 2 different questions.

[Helmholtz]

And I would like to add the question "why these extremely large grid stoppers?"
Does anybody know if previous versions had lower values?

[Dave H]

I think the 'why cathode followers' and 'why DC coupled cathode followers' are really 2 different questions.

You'll have to enlighten me

[g1]

I can't, but just commenting on the answers I'm reading.
I'm fairly sure the plain old 'cathode follower' part of the question is mostly due to the drive loss when dealing with many power tubes (lets say more than 4). We see this with SVT, Hiwatt 400, etc. whereas Fender uses it for Super Twin, but interstage drive transformer for PS400.
The 'DC coupled' part is the question I'm hoping to hear answered.

[Bloomfield]

Actually i don't anderstand the schematic of these switches. Not what purpose they serve. What kind of switch have 2x4poles with 3 positions? What kind of switch (hardware) can be used there?

Here's one such switch that came to mind:

https://www.stewmac.com/Pickups_and_..._Switches.html

Likely also available from the usual sources.

[SoulFetish]

And I would like to add the question "why these extremely large grid stoppers?"
Does anybody know if previous versions had lower values?

I’ll have to double check, but I believe the early ones had 47k ( or similarity large) grid stoppers as well.

[Helmholtz]

The 'DC coupled' part is the question I'm hoping to hear answered.

DC coupling eliminates the risk of blocking distortion.

[tepsamps]

Here's one such switch that came to mind:

https://www.stewmac.com/Pickups_and_..._Switches.html

Likely also available from the usual sources.

Good ! But no ! It could be but the ampeg switch has a center shaft that is a contact actually and connects side 1 to side 2.

[loudthud]

If the RCA tube manual RC-28 is to be believed, the maximum grid #1 circuit resistance for fixed bias operation is (not a typo) 0.05 MegOhm per tube. This is especially important at high plate Voltage to prevent thermal runaway. With three tubes in parallel, this would create an impossible to drive grid circuit impedance unless a direct coupled cathode follower is used. The direct coupled CF allows a higher much easier to drive impedance on the CF's grid.

[g1]

Good ! But no ! It could be but the ampeg switch has a center shaft that is a contact actually and connects side 1 to side 2.

I see at least one typo on the schematic in that area. The schematic shows the 2 sides of the switch connected, but have you verified that with your meter? I'm a bit surprised they would use a different switch there than the other positions.

[Dave H]

I see at least one typo on the schematic in that area. The schematic shows the 2 sides of the switch connected, but have you verified that with your meter? I'm a bit surprised they would use a different switch there than the other positions.

When I was trying to figure out how they worked I assumed that the broken line between the wipers of the two sides indicated that they were ganged not electrically connected. I thought they would operate like the 2 row, 4 contact switches in post #24, i.e. type #1232.



Edit: The diagonal connection between top row contact 3 and bottom row contact 2 is a wire link.