Gibson GA-8t (EA-35T) Question

[zzallin]

I have a gibson ga-8t that keeps burning up a 1k rectifier cathode resistor (circled in red). Anyone have any idea what could be causing this? It stats burning with the output tubes removed. Could it be a bad rectifier?


[img:750:402]http://img512.imageshack.us/img512/6997/ga8vx2.jpg[/img]

Thanks,
Zack

[Firestorm]

That's not a cathode resistor for the rectifier. That's a dropping resistor in the B+ rail between the plate supply (to the center tap) and the screen supply. A bad screen supply filter cap could be shorting and pulling big current through the resistor. A screen short in the tube would do the same thing (depending on where it shorted).

How old are the caps? Tubes?

{addendum} Oh, I just saw that is getting hot with no tubes, too. Besides the cap, make sure there are no shorts, arc traces, etc. on the sockets. Also the obvious question: how big a resistor (wattage) are you using?

[bluefireamps]

Have you changed that resistor and it burns up the new ones too? If it's the stock one that keeps giving you trouble, replace it with a resistor that has a minimum of a 1 watt rating. The stock one is 1/2 watt and it's pretty common for these to be burned.
Dave

[zzallin]

I re-flowed the solder on the filter caps and it seemed to clear up the problem. I also replaced the resistor with a higher wattage replacement. Thanks a lot.

Zack

[Phil_S]

How old are the filter caps? Have you checked them? The stock R should not burn out. I think maybe you should replace the caps and not rely on a higher wattage rated R.

[zzallin]

The filter caps are brand new. The resistor was still smoking when I upped the wattage. But it seems fine now. The plate voltages are a little high, 300 volts on the 6bq5s as opposed to 270v in the schematic. Would that cause that resistor to over heat?

Zack

[Phil_S]

The filter caps are brand new. The resistor was still smoking when I upped the wattage. But it seems fine now. The plate voltages are a little high, 300 volts on the 6bq5s as opposed to 270v in the schematic. Would that cause that resistor to over heat?
Zack

I don't think that's it. Expect voltages to be high. Wall supply is about 5% higher these days. Also, you can allow +/- 10% just on odds 'n' ends. I don't think 300v is the problem unless the caps are rated 300v or less.

I see 310-315v or so in my Gibson Minuteman (pair of 6BQ5's) and it works fine, although that might be the reason the interstage transformer burnt out. Somehow that seems unlikely.

It's possible that the 1/2W rating is just too wimpy. OTOH, I might change back to it to see if a 1/2W will hold up now that you reflowed solder. I would want to know that the amp is OK.

Hard to know without seeing the amp.

[jjman]

The scematic shows a 13v drop across the 1k.

1000=13/amps
amps=.013

watts = volts x amps
watts = 13 x .013
watts = 0.17

This is after the filters charge. Initial load could be more.

[Firestorm]

You should measure the voltage across the 1K and see how much it really is dropping. If it is significantly more than 13V (anything over 22V will start to toast a 1/2 watt 1K), then check for a current draw downstream of the screen supply. All of the power used in the amp (except the output tube plates and the heaters) is going through that 1K.
One place to look would be the .01 and .02 caps in the tremolo circuit (especially the .02 on the oscillator tube's plate). These things do get leaky and might be pulling enough current to stress the 1K. Does the tremolo work, btw?

[zzallin]

I turned it on today and there were purple bolts arcing in the rectifier tube, so I guess that's no good. I replaced that resistor with a 1 watt. Everything was working last night including the tremolo.

Thanks,
Zack

[Firestorm]

Hopefully, it's okay now. But you still should check the voltages up and down the B+ rail (especially around that 1K). If there is still an excessive current draw, that could eventually take out the new rectifier.

[zzallin]

I got a new rectifier tube and it seems to be working. There's a 11 volt drop across the 1k resistor so a .25 watt resistor should do the trick. I'm having a problem with the volume pot. I replaced the old 500k pot with a 500k alpha pot twice and both of them failed. Any idea what could be wrong? Do I need a higher wattage pot?

Thanks,
Zack

[Firestorm]

What do you mean "failed"? I doubt you need a higher wattage pot. There's really not much current flowing there unless the coupling caps have gone leaky. Disconnect the "cold" side of the .01 and see if there's any DC on it with the amp on. If so, replace it and the .0005, too. That one's probably not rated for much voltage since it's protected by the coupling cap.