Gibson GA200

[pontiacpete]

This amp is blowing fuses intermittently, sort of. I'm not sure if my question is more theoretical or is there a problem with this amp. It seems that I can get it to blow a fuse when turning the amp off and within 10 to 15 seconds turn on.
If you can see this in the schematic the standby does not cut the B+ from the power tubes. So the power/standby switch works like this: It turns heaters and B+ on and the standby(which is the one of the switches at the top right of the schematic) turns on the signal. The power switch is right below that.
I put an ammeter across the fuse and found that it idles at about 1 amp. So I'm think that a 3 or 4 amp fuse would do.
I realize it's never a good idea to turn on an amp and then off and back on, especially with an amp that has B+ close to 600vdc,
my question is: Is it to be expected that it will blow fuses with no B+ cancelling standby in an amp this size?

As you can see i did a complete cap job. The amp was really, really dirty. It had a wasp nest in the pre amp. So I've been thinking that maybe the dirt is the problem. And I'm wondering about the old style waffer power switch. I tried jumping across the power part of that switch and it still blows fuse at will. This is an old pitcher before I re-wired it. I got rid of the poorly insulted wires that go to the plug socket in the lower left corner of the power amp.

[52 Bill]

The insulation on the wire that connects the wafer switch to the power supply chassis is brittle and crumbling. You must change that wire before doing anything else. In fact that might be reason for the intermittent fuse blowing as well.

[pontiacpete]

I was still editing my post. This picture is prior to the changing of the wires. Still blowing fuses.

[pontiacpete]

I'm back at it here with some small steps toward getting this to work right.
Changed all the connecting wires between the pre amp board and the power amp. But the problem of blowing fuses continued, so I decided to change the rectifier tube socket since it was the rectifier that was arcing inside the glass and blows the fuse.
So far this is working!

But now I'm not convinced the amp is working at it's best. On the output I am getting 25w. With 2 6550's shouldn't there be more 30 to 50w of power at the output?

Also does anyone know what the 6BJ8 is doing in this circuit? I'm not familiar with this tube, it has 2 diodes and 1 triode: http://www.tubebooks.org/tubedata/hb...art_1/6BJ8.PDF
I'm guessing that there should not be any voltage on those plates of the diode, no?

There are the 2 switches that are called 'compression' which in the amp it looks definitely to be original. But it an 8 position rotary switch.
I"m only seeing 2 switches at the 'S2'. On the panel it says just on and off.

any ideas?

[trobbins]

I know this thread is old, and amp is likely no longer 'on the bench', but I can't find any other technical discussion on the GA200, and especially on the function of the 6BJ8, so I'll use this thread as a start on that discussion.

The 6BJ8 is fed from the output stage 300V regulated screen voltage via a 2.7k resistor and via pin 3 of a 5 pin plug. The schematic indicates there is a DC current path through the 6BJ8 diodes in series and then into circuitry that would be modulating the B+ supply to the 12AY7 cathodyne splitter. But the circuitry around the 6BJ8 triode doesn't make sense, so I'm guessing there is an intended schematic error by Gibson.

I'm guessing the lowest diode common cathode node should be the B+ feed to the 12AY7 cathodyne (and not connect through to the triode common cathode). The series diodes would then be dropping some voltage off the 300V supply (which would also be lower due to the 2.7k dropper), but their voltage drop would be modulated by capacitive coupled signal from the 6BJ8 triode push-pull circuitry. But the 6BJ8 diode voltage drop curve seems to be quite a low resistance (250 ohm).

Has anyone looked at this amp?

[Enzo]

New threads don't cost anything, no need to find an old one about someone else's amp to tack onto.

so I'm guessing there is an intended schematic error by Gibson.

INTENDED error? They change circuits so often as it is, I hardly think they have any incentive to draw it wrong on purpose. ANy competitor looking to steal the circuit would not be fooled. ANyone who has worked on a lot of Gibsons and Epis knows the schematic OFTEN does not match the circuit.

I don't see a B+ connection to the diodes. The lower pair shares a cathode with the triodes. The 0.05 caps block any DC from the diodes. Pin 5 of hte connector goes to the grids of the driver tubes. The only B+ I see feeds the two 100k triode plate resistors.

is this a compressor? I see the switch turn on and feeds the phase inverter output into the two triodes, which then send that amplified signal into the diodes, which rectify it and control bias on the grids of V6,V7.

[trobbins]

Arrgh, I misread the 5-pin plug. The top of the diode bridge (negative end) connects to the common grid-leak of 6SK7 push-pull driver via pin 5. The positive end of the bridge is biased by the triode common cathode voltage. So the 6SK7 grid leak voltage can be modulated by peak signals from either swing excursion.

Arrgh part 2, I missed the 300V supply connection node to the 100k supplies of the 6BJ8 triodes.

Yes I think its supposedly a compressor function to suppress unintended feedback overload.

Thanx for another set of eyes.

[Enzo]

As foggy as I have been the last month, you just hit me on a good day. I have never seen such a circuit before, I'll say.

I spent some time trying to mentally mate the 5 pinner with the 6 pinner for a while.