Ampeg GS-15R Gemini VI

[mtlbasslad]

Well here we go again with another Ampeg - I seem to attract them...

Ampeg-GS15R-Gemini-IV-Amp-Schematic.pdf

[mtlbasslad]

OK - now this may seem foolish as the amp is working fine, but was converted from original 7591's to EL84's & the owner wants it back to original.
I have all (I think) parts on hand - new 8 pin sockets etc. But I'm having trouble looking at the board vs the schematic: can't find the 'hum balance' pot, nor the diode below V1 on the schematic.
Amazingly all the original (I think) caps are good - no hum.

Some pics



All suggestions welcome

[mtlbasslad]

The original Ampeg schematic -

GS-15R Schematics.pdf

[The Dude]

I can't see everything from the pics, but my bet is that when it was modified for EL84's, the bias rectifier diode was removed and the amp is now cathode biased. What value are your cathode resistor(s)? Probably the same for the output tubes matching pot in the cathode circuit (removed for the modification). FWIW: I generally think of hum balance pots as being in the filament circuit. The schematic actually shows a tubes matching pot.

[mtlbasslad]

Hi Dude,
I'm a little lost in here (as usual) Pin2 has 1K resistor... and there is a jumper from Pin6 (unused on EL84) to Pin9 with a1K...

[The Dude]

EL84 pinout for your reference.



The resistor or resistors I was asking about are the cathode resistor(s) and would be tied to pin 3. Also, is there a negative bias voltage on pin 2 (control grid). If there is not, the amp must be cathode biased.

Edit: If you do find a negative bias voltage on the grids, trace it back through the resistors and it will lead you to the location of the bias supply rectifier.

[mtlbasslad]

EL84 pinout for your reference.



The resistor or resistors I was asking about are the cathode resistor(s) and would be tied to pin 3. Also, is there a negative bias voltage on pin 2 (control grid). If there is not, the amp must be cathode biased.

Edit: If you do find a negative bias voltage on the grids, trace it back through the resistors and it will lead you to the location of the bias supply rectifier.

Dude, thanks for the pinout of the EL84 - I had both that & for the 7591 on hand.
I find a 1K to pin 2...
Checking with my trusty Fluke 115 pin2 on standby is -36V, power on -20V pin3 is 0V & pin9 is +480V. For reasons I don't understand there is a 1K between pin6 & pin9... ?? Pin says NC - no connection?

I seem to have opened a can of worms here...

P.S. - sorry for the delay, doctor appointments & other annoyances get in the way sometimes... more often at my age, 'sigh'

[The Dude]

They are just using the unused socket pin as a solder terminal. It's common practice.

So you have negative bias. That says there has to be a bias rectifier (the diode you were looking for).Follow the wiper of the bias pot and it will take you to the diode. It may be hidden under a cap or something.

[mtlbasslad]

I've looked until I'm cross-eyed & can't find that diode. I know about that trick of using unused pins, but what is that 1K doing?
And -20V bias?

[g1]

Yes, the bias diode could be hidden under that copper coloured electrolytic.
The extra individual 1K screen resistors are a modification.

[mtlbasslad]

Yes, the bias diode could be hidden under that copper coloured electrolytic.
The extra individual 1K screen resistors are a modification.

Hi g1 - I'll look again but... still don't understand that 1K connection, sheesh - I've so much to learn...

[g1]

There is a 'common' screen resistor for both power tubes, the 470R between nodes A and B of the supply. Node B feeds both screen grids. It's a good idea to also have individual screen resistors for each tube, so the mod adds 1K at each screen grid.

[mtlbasslad]

Nodes you say? Ah crap, drowning here. But I don't see the 1K connected to anything...

[g1]

I'm looking at the factory schematic. You can see the various nodes (points) of the supply labelled as A, B, F, etc.
The power tube screen pins are connected to 'B'. The mod adds 1K between 'B' and each power tube's screen pin (pin9).

[The Dude]

In your first picture in post #2 I see the node connection from the filter cap going to the 1K resistors (red wire and jumpered to the other socket with white wire). They go to pin 6, which is an unused socket pin. This pin is used only to connect the wires to one end of the resistors. Then the resistors go to pin 9 of the tubes, which is the screen grid.

Edit: I'll add, none of it is really relevant anyway if the customer wants it back to original. Just remove all the mods and rewire it back to match the schematic.