Gibson GA-25RVT no sound

[flintrock]

I have a Gibson Hawk GA-25RVT on the bench. Replaced the caps, rebuilt the bias circuit to spec, it didnt work before it came in. There was a 5k ceramic resistor that fell off the board. I replaced that exactly where it goes according to the schematic. When I powered it up that resistor got very hot very quick. If I disconnect the wires supplying power to the reverb and driver transformer it stays cool. Testing the various leads from the driver trans to case ground, I'm getting about 50ohms on the red power supply lead and on one of the power tube leads when measured to the trans case ground. I assume its bad? I did testing with no tubes installed.

[xtian]

I had a shorted reverb transformer in my Princeton Reverb build that was sucking current just like you describe. And I also noticed because my first dropping resistor started to smoke.

Can't say for sure this is your problem, but sounds very similar. Even after I replaced my reverb tranny (which fixed the problem), the old, bad one's resistances measured OK, but it was failing in the circuit.

[flintrock]

Just measured the reverb transformer, input has high readings but the output side reads full continuity to ground. Not sure what that means.

[xtian]

but the output side reads full continuity to ground. Not sure what that means.

Means nothing if connected in circuit.

[Stevem]

The only thing that would make that resistor smoke is a short in that drive transformer, the problem will be finding a replacement!
What resistance do you read on the input to the pan?

[martin manning]

If indeed it's shorted, Mercury GA-RV, I think.

[flintrock]

It gets hot with only the interstage transformer hooked up, reverb tranny disconnected. I'll check the pan and the reverb tranny in case that went south too. I see Mercury has both.

[flintrock]

With just the reverb transformer hooked up it is fine. Looks like its the interstage tranny is bad. Hard to believe that thing is $65 at Mercury.

[Phil_S]

With just the reverb transformer hooked up it is fine. Looks like its the interstage tranny is bad. Hard to believe that thing is $65 at Mercury.

Yeah, I don't like the pricing on it either, but they seem to be the only one who actually has spot-on replacement. The turns ratio on that one is 1:2.4 if I remember correctly. You can find them 1:2 and 1:3, but not in between, and there must be other characteristics that make that little tx what it is. I tried a close Hammond replacement in my Minuteman and it just wasn't right -- making it a wast of money to get the wrong part. Bite the bullet is my best advice.

OTOH, I think this little tx has approximately (very close) the correct turns ratio (if I didn't get it backwards) and for the price is very attractive: https://www.edcorusa.com/xpp10-6-8k They make several in this line with 40% screen taps. You may be able to figure which one and save some money. Or, it may be all wrong ;-/

[flintrock]

Mercury Magnetics came through, transplanted the new interstage transformer and its up and running! One of the tubes was just starting to redplate, so shut it down and changed the bias tail resistor from 47K to 68K and it cooled it down quite a bit. But the problem I have now is it just doesn't sound too good. I know these amps are known for being tinny, it may be a signal not coming through? I had to replace the 5w 5K resistor but could only find a 5.6K 5W. It seems to be way too hot. Should i throw a 10W in there or could something else be wrong? I'm getting 405V plate, 394 screen, -19 bias, and about 4v on the 150 ohm OT using the shunt method (dont have the exact shunt measurements but thats real close). Numberswise, it seems to be good, those 7591's how they are wired confuses me a little on where to put a 1 ohm bias current resistor. But that resistor is definitely too hot.

[martin manning]

You can measure idle current directly using the OT shunt method, or calculate it using the DC resistance of the OT primary on each side, and the voltage drop at idle. Sounds like you are confusing those two methods.

Get voltages from V4A and V7B, and also the voltage across the 5k6 resistor, and compare them to the schematic values.

[Phil_S]

You can measure idle current directly using the OT shunt method, or calculate it using the DC resistance of the OT primary on each side, and the voltage drop at idle...

I rather dislike using a meter set to measure amps. The discomfort is because you put the meter in series with the circuit, creating a high danger factor. That is why the meter has a fuse in line when set to measure current. The simple truth is, this is safe to do by those who have a steady hand, a cool head, and enough experience. For someone like me, no, and I suspect many others.

I will measure the DCR of the primary and write what I find on the chassis or the tx with a Sharpie. Making the calculation is simple, and doesn't give me the willies like metering directly for current.

I"m glad to hear that MM tx worked out!

[flintrock]

I am not getting the two methods confused, rather my wording confused you. I normally use the 1 ohm resistor method or bias probe, but currently my bias probe is not working. So I used the shunt method to bias instead of the afordementioned ones. I calculated the circuit, and the voltage across the 5.6k and it should be at least a 7w instead of a 5w for heat dissipation reasons. I could only find a 4.7k 10w in town which should work, as the original value is suppossed to be 5k. The added resistance from the 5.6k could be generating more heat as well. Thanks for everyones help so far, this is a weird amp, and the first I have worked on utilizing that interstage tranny.

[flintrock]

Smooth sailing with the new resistor installed. Just warm now. I have to tweak the bias a hair, 47k too hot, 68k too cold. Not sure if I'll just use a 56k or a 47k with a 10k pot. Then..I can play it!

[flintrock]

Did the bias pot mod and I have it dialed in at 70%. Sounds pretty good to me. I drilled two 1" holes in the chassis cover to dissipate any interior heat. That chassis all sealed up is not a great design. Customer called, this one is going home!