Ampeg GS-15R Gemini VI

[mtlbasslad]

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.

By 'far out' you mean the price of tubes? Get used to it, thanks Mr Putin...
I checked the balance pot before installation & middle setting gave close to 50 ohms each side.
Screen grids are both at 483V - I really think I have 1 bad tube, last time I swapped them the redplating followed the tube.
As adjusted now are 38 & 35mV - close enough?

Sorry for the delay getting back, I've been doing a lot of volunteer work at the local Church camp to get it ready for opening. Best excuse ever for my tardiness?

[g1]

With the pot set roughly middle and only 3mA difference between tubes, that is a pretty good match. How is the hum volume?

[mtlbasslad]

The pot is nowhere near middle, as I said in post #69 I could get one thing or another, so I went for close plate current. Using only a signal generator, not a guitar, negligible hum.

[Helmholtz]

. Using only a signal generator, not a guitar, negligible hum.

You can't blame the amp for additional hum when connecting a guitar.

Why did you connect a signal generator?

No more redplating?

[mtlbasslad]

I use a signal generator because it's easier than moving a guitar to the shop. No hum complaints. No more redplating, I think I have one bad tube.

[Helmholtz]

No hum complaints. No more redplating, I think I have one bad tube.

Why a bad tube if everything works fine now?
I see no signs of a bad tube. The previously redplating tube probably just has more gain (actually transconductance Gm) than the other one.

As power tube balancing and hum assessment don't require a signal, did you use the signal generator to measure power output?

[mtlbasslad]

I did not use the signal generator to measure power output - measured plate current. I don't really understand your question... how does a sig gen measure power uotput?

[Enzo]

To measure power output, there needs to be a signal being amplified. A signal generator is often used for such a signal.

[mtlbasslad]

To measure power output, there needs to be a signal being amplified. A signal generator is often used for such a signal.

That makes sense, but how am I measuring power output?

[Helmholtz]

- measured plate current.

You don't want a signal when measuring plate idle current.

[Enzo]

I think we are talking past one another. You need a signal to measure power, but if you are not measuring power, then you don't Idle current is just that, at idle, NO signal. Idle current and output power are not the same tests.

[Helmholtz]

That makes sense, but how am I measuring power output?

To measure power output you connect a sine generator to the input and a dummy load of rated resistance and power to the output and scope the output.
With all amp controls fully up, input signal level is adjusted until the output signal just starts to clip.
Output RMS voltage squared divided by load resistance gives output power.

[mtlbasslad]

To measure power output you connect a sine generator to the input and a dummy load of rated resistance to the output and scope the output.
With all amp controls fully up, input signal level is adjusted until the output signal just starts to clip.
Output RMS voltage squared divided by load resistance gives output power.

No dummy load available here, just a speaker.

[mtlbasslad]

OK, time to close out this post. I ordered new TungSol tubes of which one was giving off fireworks... got a new set and everything is good - 'balance' pot is near the middle & I calculate (thanks for that g1) plate current at 32mA.
No redplating & with a really good guitarist playing the amp sounds really fine.
So I would like to thank all who helped me here, & I will be contributing to the forum again.
Until next time - think I'll have a new post about the famous (infamous?) Peavey Classic 30...