Adjustable cathode bias with balance pot? (6550 amp schematic question) - now with voltage chart

[yello]

I have an old military amp on my bench and am trying to understand the way it is biased. Appears to be cathode biased but the main bias resistor splits to individual resistors then to a balance pot before ground. turning the pot changes the 6550 grid voltages up/down relative to each other. Anyone seen this before, or can explain what is going on?



Extra question, there is also a "balance test" button/switch between the phase inverter and coupling caps that sends both halves of the signal thru a 3.9m resistor then to the 5ar4 rectifier plate at pin 5. What the heck is it, and how do I use it? Seems to be related to the above balance pot, at least in name.

Insight appreciated, I've never come across either before!

[Bieworm]

Given the value of 3.9M leads me to believe it is an anti pop resistor for the switch

[yello]

Given the value of 3.9M leads me to believe it is an anti pop resistor for the switch

Any idea what the test switch tests or how to implement it or measure something?

How about the cathode bias arrangement with extra resistors and balance pot? Seems to be related to the above balance pot, at least in name.

[JMPGuitars]

Given the value of 3.9M leads me to believe it is an anti pop resistor for the switch

Any idea what the test switch tests or how to implement it or measure something?

How about the cathode bias arrangement with extra resistors and balance pot? Seems to be related to the above balance pot, at least in name.

Let's follow a little together:

cathode bias balance.jpg

Cathode biasing is done via resistance to ground. It doesn't matter how many resistors there are in series. Series resistance is just R + R + R + R etc. Following the power tubes from either cathode to ground you get: Cathode -> 160Ω + 47Ω + 100Ω variable. With the Sweep (center pin of a pot) going to ground on the balance adjustment pot, either side can see anywhere from roughly 0Ω to about 100Ω.
You would start with the balance pot in the middle, and adjust to get equal bias when comparing tubes.

Look closely at the switch now. In normal position, the PI is connected to the power tube grids. In the test position, the PI is disconnected, and C405 and C406 are connected to each other. This is the path to the tube grids (inputs).

I don't know the purpose of R423 connecting to the rectifier plate/transformer secondary at pin 5. However, my guess, is that it's feeding AC voltage to both of the power tubes to make balancing possible. My assumption about the 3.9M value being high is to reduce the voltage to whatever the test voltage is supposed to be.

Thanks,
Josh

[yello]

I appreciate the input and discussion, I understand it much better now!

Here is a voltage chart I made for this amp (I have three identical amps on my bench at the same time). First is the voltage chart from the schematic showing expected voltages at 115vac. Then I show each of the three amps tested at the schematic 115vac as well as my typical wall voltage of 122vac.

It looks like all three test within range of each other, but with much higher voltages than the schematic, even when tested at 115vac. I used the same tube set for tests (vintage 5ar4, modern Sovtek 6550 pair, vintage 7199). All amps are re-capped with resistors testing within spec range and wiring confirmed.

I may end up moving to a 5u4 (or 5r4) rectifier to lower the voltages, given the PT supplies 3A of 5v, along with a standby switch to protect caps at startup with a directly heated rectifier. Open to thoughts on the voltages I am observing:

[JMPGuitars]

Check your voltages at C408a and C408b with both wall voltages. That's what we typically refer to as B+ nodes A and B.

If you want to use the schematic voltages as a reference, then you need to also correct your heater voltage. Correcting B+ and the heater voltages might get you in the ballpark.

Note that the schematic shows all the tube heaters below the normal 6.3VAC. Your voltages show heater voltages ~normal (not reduced like schematic) at 115VAC, and slightly elevated at 122VAC.

Go ahead and try other rectifiers with higher voltage drops, but you'll still probably need to adjust your heaters if you want to match the schem.

[yello]

Check your voltages at C408a and C408b with both wall voltages. That's what we typically refer to as B+ nodes A and B.

If you want to use the schematic voltages as a reference, then you need to also correct your heater voltage. Correcting B+ and the heater voltages might get you in the ballpark.

Note that the schematic shows all the tube heaters below the normal 6.3VAC. Your voltages show heater voltages ~normal (not reduced like schematic) at 115VAC, and slightly elevated at 122VAC.

Go ahead and try other rectifiers with higher voltage drops, but you'll still probably need to adjust your heaters if you want to match the schem.

Just looked at the obvious (per above pic), which is the stock PT in all three amps on my bench is a 380-0-380, so the voltages I see in real life match them as built, and not necessarily the schematic which may be from a different year.

One option could be to run these amps at a lower voltage like 115vac, but given they might be run at modern wall voltage by the end user, I'm wondering:

1) How to address heater voltage best? Is it a huge deal to run them at 6.6vac? I'm not arbitrarily wanting to match the schematic, just want them to run well. I've never lowered just heater voltage but I assume there is some series resistors or something to slightly lower it?
2) Is the stock screen voltage on the 6550's too high? Are screen stoppers appropriate here, even with a different rectifier to lower B+?
3) Any other considerations if these are to be run at 122vac?

[JMPGuitars]

Arbitrarily following a random schematic's voltages doesn't make a lot of sense to me, but I gave you the info in case you want to. The schematic you want to match should probably also match the hardware you're working with.

If you're wondering about tube voltages, look at the tube datasheets. If the tubes are within safe ranges, then you have to answer the same question you would for any amp...

How does it sound?

[yello]

Arbitrarily following a random schematic's voltages doesn't make a lot of sense to me, but I gave you the info in case you want to. The schematic you want to match should probably also match the hardware you're working with.

If you're wondering about tube voltages, look at the tube datasheets. If the tubes are within safe ranges, then you have to answer the same question you would for any amp...

How does it sound?

This is the only known schematic for this amp model, which was produced for the military. I don't have a way to know if there is a slightly different schematic for different years, but it is accurate other than some voltage differences.

Amps sound great - they are power amps so depends what you feed it. Sounds cool with a tube preamp, or being fed a line out from another tube amp.

[JMPGuitars]

This is the only known schematic for this amp model, which was produced for the military. I don't have a way to know if there is a slightly different schematic for different years, but it is accurate other than some voltage differences.

Amps sound great - they are power amps so depends what you feed it. Sounds cool with a tube preamp, or being fed a line out from another tube amp.

I would ignore those voltages then. Decide (if you can) which of the 3 amps you have sounds better, and compare voltages from there.