Ampeg GS-15R Gemini VI

[mtlbasslad]

Yes, tube cathodes to each end of the pot and wiper to ground. It's basically a variable/dual cathode resistor to match tube current. I'm curious, though, what is there now (cathodes to ground)? You must have something or tubes would not be conducting.

Cathodes jumpered.

[mtlbasslad]

What kind of wattage for the pot?

No idea - it's supposed to be a Fender pot from Solo guitars...

[g1]

No idea - it's supposed to be a Fender pot from Solo guitars...

If it's the standard Fender hum balance pot, I think it should be good for at least 2W. I see them being called anywhere from 2,3, or 5 watts, but nothing less than 2.

[g1]

i think the so-called 'matched tubes' are not at all.
Tube idle current measure is beyond my current knowledge...

OT voltage/resistance method is non-invasive, just a couple measurements and some math (volts divided by ohms).
You measure the resistance of the OT primary halves, then with unit running you measure voltage at same OT primary halves. Write down the 4 numbers then divide the voltage by the resistance for each half. So for example, you might measure 3VDC across one half that had measured 145 ohms. 3/145 equals .0206, or about 21mA current.

For your ampeg, if the OT wire colors match the schematic, it makes it easier. The primary CT wire is red, and blue and brown go to the power tube plates (pin3).
The red wire (A), comes off the rectifier diodes.
So you would measure resistance of red to blue, and red to brown. Then with amp running measure volts at same places. Then do the calculations for each half.

If it were an amp that used 4 power tubes, you would further divide those numbers by 2, as each OT primary half would be running 2 tubes.

[mtlbasslad]

OT voltage/resistance method is non-invasive, just a couple measurements and some math (volts divided by ohms).
You measure the resistance of the OT primary halves, then with unit running you measure voltage at same OT primary halves. Write down the 4 numbers then divide the voltage by the resistance for each half. So for example, you might measure 3VDC across one half that had measured 145 ohms. 3/145 equals .0206, or about 21mA current.

For your ampeg, if the OT wire colors match the schematic, it makes it easier. The primary CT wire is red, and blue and brown go to the power tube plates (pin3).
The red wire (A), comes off the rectifier diodes.
So you would measure resistance of red to blue, and red to brown. Then with amp running measure volts at same places. Then do the calculations for each half.

If it were an amp that used 4 power tubes, you would further divide those numbers by 2, as each OT primary half would be running 2 tubes.

So, first installed 'hum balance' pot - a nice genuine Fender part marked 'CTS' - Chicago Telephone Supply?

OT primarys measured from junction of rec diodes to Pin 3 are both 133.5 ohms.
Voltage 2.1 & 5.0 - the 5.0 on the tube that still wants to redplate a bit. This calculates to .16ma & .37ma I believe... now what?

[Enzo]

Tubes do what they do based upon the relationships between elements.

So did you first try to adjust balance control for minimal hum? And if the plate voltages are even, are the screen voltages the same? With no screen resistors they should be, but... And more important, the control grid voltages. Are they both the same?

[Helmholtz]

Now use the pot to balance the plate currents.
In your case with equal DCRs on both sides you can just connect your voltmeter leads to the plates and adjust pot for zero differential voltage.

[mtlbasslad]

Tubes do what they do based upon the relationships between elements.

So did you first try to adjust balance control for minimal hum? And if the plate voltages are even, are the screen voltages the same? With no screen resistors they should be, but... And more important, the control grid voltages. Are they both the same?

No I did not touch the balance control yet. Now Enzo, after almost 15 years of you trying to educate me you must know I'm still an amateur. So plate or screen or control grid voltages? I need you to walk me through this, as g1 did in post #49. Really it's not fun to be so ignorant...

[mtlbasslad]

Now use the pot to balance the plate currents.
In your case with equal DCRs on both sides you can just connect your voltmeter leads to the plates and adjust pot for zero differential voltage.

Um, what is a DCR?

[Enzo]

DC resistance - the amount on your ohm meter.

[Helmholtz]

Um, what is a DCR?

DCR means DC resistance as measured with an Ohmmeter.

EDit: Missed post above.

[Helmholtz]

Really it's not fun to be so ignorant...

So why not change that?

15 years could have been used to read some books and learn the basics.

[mtlbasslad]

So why not change that?

15 years could have been used to read some books and learn the basics.

I've been trying, but I'm not that bright. I've been reading but I'm a slow learner. Your comment doesn't really encourage me...

[g1]

For the Fender amps that have the 'output matching' pot, the instructions are just to adjust the pot for minimum hum, by ear.
I think if you did that, then checked again the idle currents for the power tubes, they would be fairly close to balanced. (or close to zero differential DC volts from plate to plate, like Helmoltz said)

[mtlbasslad]

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