Gibson GA-79RV Troubles

[earache]

Hello All

I am looking at an early 60's Gibson GA-79RV (no tremolo) that has a really really microphonic chassis. Turn it on and once it warms up it has a really crackly hum that is heard despite all of the controls being turned off, and spikes loudly when touching anywhere on the chassis. I can't seem to pinpoint the cause because touching anywhere in or on the chassis with the chopstick makes the noise spike. I don't want to leave it on too long when in this state as I don't know if there is further harm being caused.

Some other info about the amp -
All 6EU7 preamp tubes are original Gibson
6BQ5 Output tubes are newer (Groove Tubes)
Phase Inverter is JAN Phillips 5814A (not 12EU7 as listed on schematic)
Tried a different Phase Inverter (12AU7) - no difference
There is no measurable DC across the speakers
The separate power chassis, which lives in the bottom of the cab, has new looking Sprague Blue Atom filter caps in it



Wondering how to proceed without causing further harm..


Thanks In Advance
Earache

[g1]

Pull all the tubes except the phase splitter and power tubes. See if it still has the problem.
The only real hazard I can think of is power tube redplating, so keep an eye on the power tubes and shut down if you see any arcing or redplating.

[earache]

Hi g-one

I pulled all of the preamp tubes except the phase inverter and fired up the amp....now all I get is a hum, no crackle at all.
The hum doesn't vary if I slap the amp or touch any of the components.

[52 Bill]

Reinstall the preamp tubes one at a time and see when the noise comes back. I'd start with V2 and then V1.

[earache]

Adding back the individual preamp tubes, alone or together, yields the same hum as without them UNTIL the addition of V4, which is when the crackly static returns

[g1]

V4 is for the reverb. Does killing the reverb with the footswitch have any effect on the crackle?

[earache]

g-one...you pointed me in the right direction, certainly! Turns out that the plug to the reverb tank had fallen out of the jack, and was pressed against the body of the tank, causing the god-awful crackle. I couldn't see it because it was the upper plug, and was hidden behind the chassis. Plugged it in, and the crackle is gone.

Now - the amp passes guitar signal on both channels but there is a low-level hum evident all the time. It sounds the same note as the florescent lights in my basement, just louder. The guitar signal is not that loud, and when you turn up the volume the amp sounds as though it has tremolo but it doesn't. When you turn up the reverb pot (the reverb doesn't work, by the way) the amp either motorboats or squeals.

The output tubes seem excessively HOT but that may be normal for all I know.

Additional question..if I have to get a new reverb tank, how do I source it? How will I determine what the input and output impedance of the tank is?

[earache]

...also wondering if it is a valid troubleshooting technique with this amp (as it is a stereo with two output stages) to remove the output tubes from one side or the other to see if the hum stops...don't have access to the amp this moment to try it.

[audiopete]

Are the filter caps original or new? You need to be able to recognize what kind of hum you have to help you fix it. 60Hz? 120Hz? The oscillations and hum lead me to think bad e-caps.

[earache]

Audiopete: the filter caps in the bottom power chassis seem new as they are Sprague Blue Atoms.
I replaced the (2) 20uf e-caps in the upper chassis...but there's always the possibility that I may have messed up something there.
I dont know how to differentiat between 120 and 60 Hz hum. The hum from the amp sounds like the hum I hear coming from the fluorescent lights in the shop, except louder.

[earache]

Audiopete: I found a YouTube video that played a 60 Hz hum
60 Hz Hum - YouTube
and the amp sounds more like 60 Hz than 120 Hz
120 Hz Sine Wave Sound Frequency Tone - YouTube

I believe that the main filter caps are relatively new, being Sprague Blue Atoms, but I can't say how old they are as I didn't install them myself.

Sooo..how about the caps that are in the preamp circuit? Could they cause the hum? The appear to be electrolytics that have polarity. Can you help me identify them? See the picture -



The black axial caps are marked with a "+" at one end and have either "20-6 D.C." or "20-12 D.C." marked on them. Does that make them 20uF x 6Vand/ or 20uf x 12V ?

Here's another question - the colored markings on the other caps, what values do they represent? The colors go from top to bottom - SILVER (end) GREEN BLACK BROWN BLACK

[earache]

bump

[earache]

Bump!

[Randall]

The black axial caps are marked with a "+" at one end and have either "20-6 D.C." or "20-12 D.C." marked on them. Does that make them 20uF x 6Vand/ or 20uf x 12V ?

Yes. They probably aren't inducing hum, but if it were me.... I'd replace those puppies as well as put in a proper grounded AC cord, and disconnect the polarity switch. Then, I'd go looking for hum. YMMV.

[52 Bill]

I believe that the main filter caps are relatively new, being Sprague Blue Atoms, but I can't say how old they are as I didn't install them myself.

Just because something looks new, doesn't mean that the part is still working correctly.

Sooo..how about the caps that are in the preamp circuit? Could they cause the hum? The appear to be electrolytics that have polarity. Can you help me identify them?

The black axial caps are marked with a "+" at one end and have either "20-6 D.C." or "20-12 D.C." marked on them. Does that make them 20uF x 6Vand/ or 20uf x 12V ?

Yes, they are electrolytic caps, 20uF @ 6vdc and 20uF @ 12vdc. They can and do go bad, but will not cause the amp to hum. Most likely they are the cathode caps for the preamp tubes.

Here's another question - the colored markings on the other caps, what values do they represent? The colors go from top to bottom - SILVER (end) GREEN BLACK BROWN BLACK

Based on the color codes silver is the temperature rating, the color stripes are the value and tolerance. Green (5)-black (0)-brown(1)+ 500pF 20% tolerance.

As for your hum, it could be a filter cap problem or a ground problem or a bad part somewhere. Try and isolate it to a smaller and smaller section of the circuit, just like you did with the microphonic problem.