Ampeg GS-15R Gemini VI

[g1]

Also, the 470R shown on schematic between node A and B has been changed to a 1K 5watt.

[The Dude]

Good catch, g1!

[mtlbasslad]

Well I've looked high & low for that 10D6 diode, no can find. Would the circuit function without it?
Hiding under the 'copper' cap (100uf/100V) is a 56K resistor.
Here is a pic of a suspect mod, original res jumped with something in rainbow colors...



That's the bias 15K pot upper right

[g1]

The diode must be there or you would not have the negative voltage at the power tube grids. One end of it will connect to one end of the 56K. Maybe it's under that brown cap next to the copper one.
The resistor added across the 39K is just a tweak of the bias range (will allow more negative voltage at grids). The 39K is shown as 33K at the bias pot in schematic. It will measure something lower now as it has parallel resistance added. Looks like maybe a 68K, which would make the new value measure around 25K ?

[Enzo]

DIode MUST be there. But it doesn't have to be the EXACT part they used 50 years ago. It is just a rectifier to make the bias supply. And since the bias supply never needs much current, even a small diode is fine. I would just grab a 1N4007 from my drawer and use that

[mtlbasslad]

OK, I'm very slowly learning - I see that the bias supply is coming off the PT before the 1/2 wave rectifier, so it's still AC. Now I'm familiar with 1/2 wave (2 diodes) & full wave (4 diodes) but it's new to me that 1 diode can do what it's doing here. I must have been away that day in the electronics school that I never went to LOL.
So I'll give up looking for that diode (10D6 is rare now but 1N4007 works fine) because it must be there, even if my old eyes can't find it.
I'll just migrate the wires from the EL84 to the 7591 & have joy? Keeping the extra resistors?

[Helmholtz]

Now I'm familiar with 1/2 wave (2 diodes) & full wave (4 diodes) but it's new to me that 1 diode can do what it's doing here.

A rectifier with 2 diodes typically is a full wave rectifier (using center-tapped AC input).
A half-wave rectifier only needs 1 diode.

[Enzo]

Now I'm familiar with 1/2 wave (2 diodes) & full wave (4 diodes) but it's new to me that 1 diode can do what it's doing here.

the two diodes with a center tapped transformer winding is full wave. Four diodes in a bridge is ALSO full wave. One diode is half wave. AC has alternating polarities, and the two or four diode arrangements pass from both phases. A single diode passes one phase but not the other.

Half wave is less smooth for a supply where current is drawn, but in a bias supply, the only current being drawn is by the voltage dividing resistor themselves.

https://byjus.com/physics/half-wave-rectifier/

[g1]

I'll just migrate the wires from the EL84 to the 7591 & have joy? Keeping the extra resistors?

Should be fine with the extra screen resistors. If you find the bias pot will not get you into the desired idle current, you may need to remove that added resistor in the bias circuit.

[mtlbasslad]

OK, I've been away for a while sorry - domestic chores got in the way. Then there was the 'derecho' - near hurricane winds no power for 3 days no internet for 4 days...
So thank you all for the education about rectifiers/diodes etc. - I did go back to the 40 year old textbook (which I should have done before posting ignorant questions) - I plead old age...

I have managed to unsolder all the connections to the EL84 sockets (& labeled them all) removed the 9 pin sockets & installed 8 pin sockets - not without some misery as new 8 pin sockets mounting holes did not line up with 1960's ones, Dremel to the rescue.

Of course with the different pinouts between EL84 & 7591 most of the jumpers don't 'jump' that well, boo-hoo. I can deal with that.
My new problem is understanding the G2 connections - EL84 seems to be internally connected but 7591 is not. Schematic shows G2 as pin 4,8 - does that mean I need to jumper from 4 to 8?

Why do I take on projects that are obviously beyond my skills? I'll be asking my therapist about that LOL

[The Dude]

Both pins of the 7591 are G2 and already internally connected. You don't need a jumper. Just connect to either pin.

[Enzo]

No, it means the screen grid is connected to both 4 and 8, so you can use either one. Or both. For example you could run a wire into pin 4 on one tube, and to get to the next tube it might be more convenient to run a wire from pin 8 of the first tube to pin 4 or 8 of the second. Look at the base diagram here:
https://www.radiomuseum.org/tubes/tube_7591.html

The screen grid in the EL84 is NOT internally connected. The third grid -- called the suppressor grid - is internally connected to the cathode. In fact same as in the 7591.

In both tubes the screen grid, which is g2, has its own pin. Pin 9 on EL84, and pins 4 and 8 on the 7591.

[mtlbasslad]

Both pins of the 7591 are G2 and already internally connected. You don't need a jumper. Just connect to either pin.

Thanks Dude - onward we go & hope to avoid any 'magic smoke' LOL

[mtlbasslad]

No, it means the screen grid is connected to both 4 and 8, so you can use either one. Or both. For example you could run a wire into pin 4 on one tube, and to get to the next tube it might be more convenient to run a wire from pin 8 of the first tube to pin 4 or 8 of the second. Look at the base diagram here:
https://www.radiomuseum.org/tubes/tube_7591.html

The screen grid in the EL84 is NOT internally connected. The third grid -- called the suppressor grid - is internally connected to the cathode. In fact same as in the 7591.

In both tubes the screen grid, which is g2, has its own pin. Pin 9 on EL84, and pins 4 and 8 on the 7591.

Thanks Enzo - and I think the weather channel said the near hurricane we got came from your neck of the woods LOL

[Enzo]

Well we don't speak French, so it left here.