Ampeg Gemini 2 standby switch

[fradenigris]

hello,
first post here!
I just restored a 65 Ampeg Gemini 2 (it's the 7591, fixed bias type): did a cap job, redid the tone sections with discrete components (one channel was humming badly and the other had way more gain than the other), new screen and grid resistors and new standby wiring besides other maintenance duties.

the schematics glued to the back of the amp has a standby switch that works by lifting the secondary center tap from ground. Is that safe? Any counter indication?

Voltages are higher than those on the schematics but within norm, only voltage that seems bad is the screen one, that appears to be a couple of volts higher than the plate one (448 vs. 446-7).
the output transformer appears not original and shows an impedance of about 9k Ohm.
could this be the cause of that voltages?
to reduce screen voltage to 5 volts less than plate voltage I need to raise screen resistor from 470 to 1500 Ohms, but it seems that sound suffers a bit.
any suggestion?
thanks

Francesco

[soundguruman]

The CT is lifted, that's correct. Lots of amps have that.
The line voltage was 110 when the amp was built. Now it's 120 or higher. No wonder the voltage is a bit higher.
We see the same thing in most vintage amps.
No, you don't have to install 1500 instead of 470. All you really need is to bias the output tubes correctly, and it should probably work OK.

[fradenigris]

The CT is lifted, that's correct. Lots of amps have that.
The line voltage was 110 when the amp was built. Now it's 120 or higher. No wonder the voltage is a bit higher.
We see the same thing in most vintage amps.
No, you don't have to install 1500 instead of 470. All you really need is to bias the output tubes correctly, and it should probably work OK.

Tubes are a pair of EH7591A, biased at about 30 mA for a plate voltage of 445 V (bias voltage it turns out to be -22 V as per schematics). I calculated that with the usual conservative bias of 70% of maximum plate dissipation, assuming a max power handling for the 7591s of 19 W.
The relation between plate and screen voltage does not appear to change, regardless of the bias voltage.

I don't get how screen voltage can be higher than screen voltage if screen voltage is taken after the 470 Ohms dropping resistor.
Could you please shed some light?

Thanks for your reply.

F

[Jazz P Bass]

Per member 'Loudthud", don't fret about it.
The screen voltage is very steady whereas the plate voltage is rippling.
Hense the screen 'appears' higher.
Per member "Steve Conner', the screen voltage will drop under signal.
So, not to worry.
http://music-electronics-forum.com/t25161/

[fradenigris]

thanks,
even if I must point out that on my VM I can see screen voltage change when signal is present. it doesn't remain steady as I understand it should.

[Tom Phillips]

…I don't get how screen voltage can be higher than screen [edit PLATE] voltage if screen voltage is taken after the 470 Ohms dropping resistor…

This is because of plate supply looses voltage due to the IR drop as it passes thru the output transformer primary winding. Even though the winding resistance is usually smaller that the screen resistor the plate current at idle is much higher than the screen current at idle. Therefore, the screen voltage can measure higher than plate voltage. If you check the voltage at the center tap of the output transformer primary winding I expect that you will find it to be a little higher than the screen voltage. As pointed out in the other referenced thread, the conditions change when the amp starts to produce power.

Regards,
Tom

[Tom Phillips]

… I must point out that on my VM I can see screen voltage change when signal is present. it doesn't remain steady as I understand it should.…

Many guitar amps are designed to let the screen voltage drop as power output rises.

Screen current rises as the amp starts to produce power. This is not a linear rise with respect to output power. In fact the screen current starts to increase really steeply as you approach full power output and when you overdrive the output section. The screen resistors are there to drop some screen voltage as the current rises.

There are some amplifiers designed to hold the screen voltage steady. Your Ampeg guitar amp isn’t one of them so the voltages you are measuring are OK for your amp.

Cheers,
Tom

[fradenigris]

thanks Tom