You Make the Call

[TheAmpNerd]

Okay y'all.

In keeping with the spirit of TGP and knowing how some of you have
a fit about gutting an amp and rebuilding it.

We can do this a cap at a time!

You make the call!

The amp is a 1966 Ampeg Model J-12D Jet.
The amp hums like crazy and the tremolo doesn't work.

So far I've done the following:
+ Replaced the mutisection cap
+ Replaced the bypass caps
+ Replaced the tone pot (original was coming apart).
+ Cleaned and lubed all the pots
+ Cleanded and tightened all sockets
+ Cleaned and tested tubes and tube pins
+ Cleaned the input jacks, including ground and switch

The voltages and specs are as follows:
(I'll post them when I find my lab book)
I have three and the one with these measurements
is hiding from me.

Here is a pic of circuit:

That's right, I'll label all the coupling caps from left to right
and include my measurements (when I find the lab book).

 

[Blue Strat]

I wouldn't do anything else to it unless it doesn't sound right.

 

[WaltC]

Is this one of the 7868 ones? I just worked on one and had to replace two defective plate load resistors (270K in the one I worked on). I put in higher value resistors to tame some of the difference in modern day wall current (330K in one and 470K in the other) and got the volume up and the tremelo working fine now. Took some time signal tracing and checking voltages in the amp to find the bad (one drifted *way* off, three and a half megs, and the other, the tremelo one, "shorted" to infinity) resistors, but once replace, the amp works and sound very good now.

 

[TheAmpNerd]

I wouldn't do anything else to it unless it doesn't sound right.

Trem doesn't work and it still hums. Output is weak
and almost spitty sounding.

I think most all the caps are leaky.
I still need to find my lab book with my notes on it too.

Is this one of the 7868 ones? I just worked on one and had to replace two defective plate load resistors (270K in the one I worked on). I put in higher value resistors to tame some of the difference in modern day wall current (330K in one and 470K in the other) and got the volume up and the tremelo working fine now. Took some time signal tracing and checking voltages in the amp to find the bad (one drifted *way* off, three and a half megs, and the other, the tremelo one, "shorted" to infinity) resistors, but once replace, the amp works and sound very good now.

That is up next, checking the resistors. Thinking all the ones I did check were in spec.
Walt, this one has the 6Bk11s and the 7591s.

 

[rhinson]

you need to replace all the caps in the trem section---i've worked on a couple of these before and they are leaky and a problem. they'll bring down down the power section voltage leaking to ground. just replace them and go from there---i bet voltages and volume pop back to normal when you do, assuming you don't have something else smoked--then you can better assess the rest of the amp. rh

 

[slider313]

And replace the 6BK11 as I believe it is the tremolo tube. If you can't find a 6BK11 I think a 6C10 will work in it's place.

 

[WaltC]

Trem doesn't work and it still hums. Output is weak
and almost spitty sounding.

I think most all the caps are leaky.
I still need to find my lab book with my notes on it too.



That is up next, checking the resistors. Thinking all the ones I did check were in spec.
Walt, this one has the 6Bk11s and the 7591s.

the one I was working on had two 6BK11s and two 7868s. Sounded good when I got done, but was real weak and noisy when I started. Not much break up in there though <G>.

Walt

 

[roknfnrol]

You ever get this amp up-and-running? If so how does it sound?

 

[Dana-L]

The amp hums like crazy and the tremolo doesn't work.

So far I've done the following:
+ Replaced the mutisection cap
+ Replaced the bypass caps
+ Replaced the tone pot (original was coming apart).
+ Cleaned and lubed all the pots
+ Cleanded and tightened all sockets
+ Cleaned and tested tubes and tube pins
+ Cleaned the input jacks, including ground and switch

Did you ever find the root cause(s) and get this amp working again?

-Dana

 

[TheAmpNerd]

I set it aside.

I'll get back to it eventually.

I did replace all the caps in the vibrato section
along with most of the other coupling caps.

Most everything elese was in spec, i.e., ressitors, pots,
and voltages.