Ampeg GS-15R Gemini VI

[g1]

Adjusting the pot makes no difference to plate voltages, hmm. I'll take readings on all pins of both tubes & post...

Do you hear an audible difference in the hum? Did you remove the jumpers from power tube cathodes to ground?

[mtlbasslad]

I can't hear any difference in the hum. Jumpers removed...
When I installed the 'balance' pot I tested it & set it at the midpoint, about 50 ohms each side. Now that it's installed I read 50 ohms 1 side & 1 ohm on the redplating side... hmm.

Voltages for the 2 7591's:

V1 (redplating) V2

Pin1 492v 492
2 6v 6
3 490v 493
4 486v 488
5 0v 1
6 -26v -26
7 6.2v 6.2

V1 pin 5? Bad tube?

[Helmholtz]

Is there an additional wire connecting to V1 pin 5? The cathodes should only connect to the outer lugs of the balance pot.
Looks as if V1 pin 5 is somehow grounded.

The balance pot wiper (middle lug) must connect only to ground - not to any of the outer pot ends.

[mtlbasslad]

Is there an additional wire connecting to V1 pin 5? The cathodes should only connect to the outer lugs of the balance pot.
Looks as if V1 pin 5 is somehow grounded.

The balance pot wiper (middle lug) must connect only to ground - not to any of the outer pot ends.

All connected as should be - cathodes (Pin 5) to outer lugs & wiper to ground.

[Helmholtz]

Cathodes jumpered.

How were the cathodes grounded when cathodes were jumpered?

If you did not remove that old ground path, that could explain your strange voltage and pot resistance readings as well as the redplating.

[mtlbasslad]

Maybe some confusion here - only connection to cathodes (Pin 5) was a jumper between V1 & V2. That has been replaced with 100 ohm pot, outer lugs connected to Pin 5 & center to ground. Remember, the redplating followed the tube when I swapped them.

[Helmholtz]

- only connection to cathodes (Pin 5) was a jumper between V1 & V2.

There MUST have been a connection to ground from one of the cathodes, otherwise no tube current is possible.

I'm trying to find an explanation for your resistance and (low) cathode voltage readings.
With a 100R pot both sides must always add up to 100R - except if there's some external bypass/short.

[g1]

Now that it's installed I read 50 ohms 1 side & 1 ohm on the redplating side... hmm.

Pull the power tubes and see if that 1 ohm reading changes.

[mtlbasslad]

OK, minor breakthrough here - when I migrated the wiring from the EL84 to 7591 I made careful notes about what went where. And that included a jumper from the 100uF/100V cap (near the bias, the copper colored one in the pics) to EL84 Pin3 - 7591 Pin5. This was a mistake, as studying the schematic it's not there - the mod to EL84's did that. So I removed that jumper & things changed - no more redplating.
However, trying to measure as g1 instructed, & playing with the 'balance' pot, I can only have 2 results:
Voltage at Pin5 both at 1.5V, V1 Pin3 is 5.5V V2 is 2.8V OR moving the pot to the other near extreme where V1 Pin3 is 4.1 & V2 3.9 then V1 Pin5 is 3.6 & V2 0.01. So can't seem to have my cake & eat it LOL
I guess I must have made another bad assumption when migrating...
Always remember that when I first swapped tubes the redplating follwed the tube...
I apologize for not reading the schematic right - only connection between cathodes (Pin5) is the balance pot...
Now what? Set the balance pot for plate current or?

[mtlbasslad]

Pull the power tubes and see if that 1 ohm reading changes.

I didn't do that because of my 'AHA' as post #69 describes. May be the first time I have ignored any of your suggestions...

[Helmholtz]

With unmatched tubes you can't have equal idle currents AND equal cathode voltages.
Goal is equal plate currents. Adjust the pot to get them close.
Easiest way is the method I described earlier - or adjust for minimum hum.
If perfect current matching isn't possible, your tubes are too different - or there's a wiring error.

[mtlbasslad]

With unmatched tubes you can't have equal idle currents AND equal cathode voltages.
Goal is equal plate currents. Adjust the pot to get them close.
Easiest way is the method I described earlier - or adjust for minimum hum.
If perfect current matching isn't possible, your tubes are too different - or there's a wiring error.

These are supposed to be matched tubes, but I have my suspicions. I'll go with equal plate currents, I guess - but I'd love to try another set of tubes. Only option these days are Electro-Harmonix at $75 each... or TungSol at $83... plus tax plus shipping... of course in my case a wiring error is also possible...

[Helmholtz]

https://www.tubesandmore.com/product...jj-electronics

[mtlbasslad]

https://www.tubesandmore.com/product...jj-electronics

"Currently out of stock" JJ in the Slovak Republic is about the only tube factory still working. Shugang in China has shut down to move I think (they make dozens of brands) & anything from Russia is not happening - also many brands.

[g1]

For matched tubes that seems a bit far out. Check with meter that resistance of balance pot is roughly same on each side when set to middle.
Also recheck that screen grid voltages (pin4 & 8) are same for both tubes.
Then swap tubes around and see if readings follow tubes.