Help with '76 Ampeg VT40 with Previous Mods

[syscokid]

This amp belongs to a friend of mine who just recently acquired it. The couple of mods are minor, but I don't understand them. One half of the phase inverter tube had its original grid to cathode cap (.15/250v, c202) removed. How does this affect the PI's circuit?

Ampeg_vt22_vt40_V2_V4_Distortion Schematic_1.jpeg

The original 10/100v bias filtering caps were replaced. But the new pair were installed on the flipside of the circuit board. Escape the heat?

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The amp needs some good work...

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[xtian]

C202:
• Reduced gain at high frequencies
• Improved stability
• Suppression of parasitic oscillation
This is especially useful in circuits using 6AN8A triode-pentode drivers, which were commonly used in hi-fi power amps and can oscillate if the layout or output stage feedback gets aggressive.

Bias caps installed on reverse: probably just easier, from the repairperson's POV.

Yup, cap job needed.

"Missing" components are likely not needed in this model.

[Helmholtz]

C202 is not part of the PI.
It's used to smooth the screen voltage of the pentode.
Without it the gain will be lower because of screen NFB.

[Stevem]

While we are talking caps. Be careful around C301 because its case is way above ground.

When I replace that can I slit its cardboard cover down one side to remove it and then silicone glue it back on the new can and then for added protection put a big ID section of shrink tubing over it.

In the last 4 years I have had two V4s in to work on and after completing the rebuild both still had a high level of hiss I had to track down.

Turns out it was a resistor on the front panel switch board which is a crazy PIA to remove and work on.

It’s good to be proactive with these amps and replace all the plate load resistors and any of the multi colored stripped coupling caps that might be in it because aging from heat tends to crack them open.

If I have time today I will see if I noted which resistor it was on my schematic and post up which one it was for you in case you have this issue.

Also make sure that heater hum balance pot is not on the edge of falling .

[syscokid]

It’s good to be proactive with these amps and replace all the plate load resistors and any of the multi colored stripped coupling caps that might be in it because aging from heat tends to crack them open.

All three of those tropical fishy’s are cracked. Two of them are severely cracked. About five of the half watt CF’s are noticeably cracking or flaking. They’re all working but three have tested with about 20 to 30% drift from their intended values. I still need to measure all the other CF’s. Definitely will be replacing all the plate resistors… Thanks!

Also make sure that heater hum balance pot is not on the edge of falling .

What should I be looking for exactly?

[Stevem]

For the hum balance just test that it does not go open intermittently when turning and does not make any mechanical scratchy sounds.

[syscokid]

For the hum balance just test that it does not go open intermittently when turning and does not make any mechanical scratchy sounds.

Thanks, Stevem!

[syscokid]

For the hum balance just test that it does not go open intermittently when turning and does not make any mechanical scratchy sounds.

That was a good call. Couple crazy points of intermittence and the turning motion was really rough. Look what I found inside:

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[syscokid]

Since I'm planning to replace all six of the amps 2-watt CC resistors, can I use 2-watt metal film resistors instead of metal oxides? I really like the size of the Vishay PR series. Following marked up schem shows the six locations:

Ampeg_vt22_vt40_V2_V4_Distortion Schematic_1 - Copy.jpeg

Vishay PR datasheet:

r-pr0x.pdf

[SoulFetish]

One half of the phase inverter tube had its original grid to cathode cap (.15/250v, c202) removed. How does this affect the PI's circuit?

It's like Helmholtz says. It acts as a bypass cap, with the value setting a HPF by shunting degenerative feedback in the screen grid. Ampeg commonly used Pentode-Triode tubes to leverage the gain of a pentode as an independent gain stage and using triode as a cathodyne phase inverter to drive the output tubes.

The original 10/100v bias filtering caps were replaced. But the new pair were installed on the flipside of the circuit board. Escape the heat?

I will often mount my repairs on this side of the PCB in the V series of amps. There is plenty of space on the board as well as much more height room in the chassis oriented that way, but the best case for doing mounting the components on the solder side of the board here is that the board acts as a insulating barrier from the tube heat and allows plenty of space for heat dissipation.
If I have to replace filter caps in these, I wont even use replacement can caps unless the customer specifically requests that he wants them. I've used 700V Vishay Film caps to rebuild the filter supply, as well as Axial Electrolytics. I wish I took pictures last time I did it because it gave me the opportunity to make the grounding much more efficient and it preformed really well.
Stevem right on about the hum balance pots in these amps. Common component failure in these.
Oh, one other thing I got to try out recently in another ampeg model was the PC boards to replace can caps with modern radial components, and would definitely use them again. I think I got a couple from grainger amps. I only mention that because I wont use CE replacement can caps. They are trash. I've seen too many of what I would consider way premature failures in them.