early 60s Epiphone Comet works but super loud crackling frying sound

[Toxophilite]

A friend brought over an Epiphone Comet which I believe there is a Gibson version of the same unit
It works and is quite loud but has a super loud crackling/popping noise
The trem works as does the reverb
It was mostly recapped with sprague orange drops about 10 years back and a couple of the smaller electrolytics on the 'board' have been replaced. It still seem to have the original filter can cap
It doesn't seem to have a lot of hum though, if any
You can pull both the 6eu7 preamp tubes and the crackling continues
if you pull the 12au7 it stops entirely
I did try swapping in a different 12au7 and power tubes with no result, I don't however have any extra 6eu7s
I also tried poking at all the components and solder joins with a chopstick with no discernable result


It's an unusual amp that uses a transformer as a PI I believe


i was going to get some new de-oxit and clean the tube sockets next


Any other ideas?

[Willabe]

It works and is quite loud but has a super loud crackling/popping noise
You can pull both the 6eu7 preamp tubes and the crackling continues
if you pull the 12au7 it stops entirely

Change the 9 pin tubes plate R's.

They all probably be changed, there's only 3, 2x100K and 1x220K.

Since pulling the 12AU7 (V3) stops the crackling you could start with that 1 and see if that nails it.

And yes clean the tube sockets, that's been posted as the fix a good # of times here. 

[sluckey]

Here's a better quality schematic...

     http://el34world.com/charts/Schematics/files/epiphone/EPIPHONE_EA-32RVT.pdf

[PRR]

> super loud crackling/popping noise... if you pull the 12au7 it stops entirely

That is strange, because the 12AU7 is *only* the reverb driver. Sure that can crackle, but you don't say that the crackle is reverberated?

[2deaf]

There is a possibility that there is a disturbance on the HT supply that is only there with the 12AU7 in place.  It looks like the disturbance would almost have to be coupled to the output via the HT supply.  It could conceivably be coupled through a series of failures including the X1 footswitch, but it seems rather remote.

Going with the HT hunch, the primary of the interstage transformer could be inducing the same disturbance on the secondary even with one end open when the 6EU7 has been removed.  Just for fun, try alligatoring both ends of the primary together with the 6EU7 removed and see what happens.  For some more fun, turn the depth control clear off, also.

Or you could disconnect the cap. can (which is real suspect, anyway) and alligator in some good cap.'s then see what happens.  I mean, if it is coupling through the HT, how come the cap.'s aren't preventing that? 

[Toxophilite]

OKay dokey smoky
Something is rotten in the state of Denmark!
First thanks for the suggestions and the schematic, This amp seems to be definitely the EA32RVT. it says it on the front of the chassis and has the same tube compliment and transformers


However I was looking at the 12AU7 socket as my first culprit
and it's not like the schematic
pin:
1 - attached directly to the PI transformer, nothing in between the transformer lead is connected right to pin one!(dark blue lead)
2 - connects to the junction of a 100K and a 470 resistor, the 100k goes to the wiper of the loudness(volume) pot (not the reverb loudness!!)
3 - has a cathode bypass situation 1 k resitor in paralell with a 10uf cap to ground
6 - directly to the reverb transformer  (light blue lead)
7 - junction of a 220k res and a .0047 cap
8 - another cathode bypass exactly the same as pin 3 (both cathode caps are newer 'replacements' soldered onto the leads of older ones??


the recap work etc seems to be relatively neatly done and was allegedly done by a tube pre guy.

(other things I tried)
The crackling happens even with all the pots turned to zero and it is VERY LOUD!
I tried disconnecting teh reverb too, same crackle
In the pictures the 12AU7 socket (V3) is the second from the left


Am I totally missing something here??
oy yoy yoy!

[Willabe]

Am I totally missing something here??

Yes!

Divide and concur.

Try cleaning the 9 pin tube sockets 1st and then changing the 9 pin tube plate R's. Then go from there.     

[Toxophilite]

do you mean 'divide and conquer' ?


is the 12au7 socket wired correctly. It has NO resistors attached to the plates (Pin 1 and 6)


the 6eu7s have plate resistors
v1 pin 6 -220k (and .047)
    pin 7 -220k (and .0047)


v2- pin 6 -100k +.0047(schematic says hooked straight to interstage...I think this point should be actually hooked to the 12au7??)
      pin7 - 150k = .047 (like the schematic)

[Willabe]

....is the 12au7 socket wired correctly. It has NO resistors attached to the plates (Pin 1 and 6)

If I'm reading the schemo right, V3a has a plate R but V3b has the verb OT B+ as the B+ R., just like a Fender verb amp.   

[Toxophilite]

Should I be expecting this amp to be hugely different from the schematic??   Check out the pictures because it is NOT like the schematic and it is an epiphone EA32RVT


Comparing V2 and V3 on the schematic with V2 and V3 in the amp it would seem that


the plate. grid and cathode of v2A (6EU7 pins 6,5,4 )has been swapped with the plate grid and cathode of V3A(12au7 pins 1,2,3)


As well the Plate of V3B is hooked Straight to the reverb transformer input when it should be hooked to the power side of the PI tranny!!
the grid of V#B seems to be in the right place but the cathode (pin 8 )has a resistor and cap to ground when it should be hooked to a 3.3k resistor which meets at a junction with a 10k res to ground, a 220k res and a 10uf cap straight to the reverb tranny!!!!!!


Aiiieeeeee I'm going to upchuck
Should I bother trying to set this straight??


Or is it even wrong??

[Toxophilite]

Compared to the actual schematic here is how V3A and V2A are currently wired on this amp


And there are more things that have been connected up wrong!

[sluckey]

That's Gibson for you! I want to see one good pic that shows the entire inside of the chassis and one good pic that shows the entire topside (tubes, transformers, etc.).

[sluckey]

Ha! Guess what? There are two versions of this amp! One version matches the schematic. It has a cap driven reverb tank.

The other version has a transformer driven reverb tank like the one you have. I cannot find a schematic for this version.

I have found pictures of both versions. So, you're not crazy. And no one has done a butcher job on the amp. I've unraveled the schematic to show what you have based on your pics and description. This should make much more sense.

[Toxophilite]

It's embarrassing that I didn't notice that the schematic only had 3 transformers on the chassis and not 4
Also there's a choke in the power supply
It's identical to this particular Gibson GA-15 RVT explorer i found online though (as you probably noticed)there doesn't seem to be an original schematic.
thanks for the one you put together, There's a few other differences too
i guess I'll stick to replacing what plate resistors I can and see what I get

[sluckey]

So, back to the original problem... Those orange drops are prone to have a lead come loose at the body of the cap. Get your pencil or chop stick and start tapping the ODs. You may find one that has a broken lead that may be causing this problem. Filter caps are also suspect. If those two multi section firecracker caps are original, they need to be replaced.

[shooter]

I had similar *sounds* on a Gibson GA5-T and ga-8, the 5 had a bad PI tranny which wasn't obvious, dcr reading were close, but whacking at it while injecting a signal caused all manner of cracklin.  After installing a new one it hummed and crackled less, the socket for v1 was arcing over at the filaments. the 8 had  an arcing cap, but only under load, and if you where touching it, you arc'd also! 

[Toxophilite]

To avoid further confusion here's a couple of pictures of the actual amp I'm looking at

[2deaf]

Obviously, you can ignore my previous post now that the true function of the 12AU7 has been determined.  Also the part about the cap. can since it has already been replaced.

This thing looks like a 32RVT with some of the circuit from a GA-20RVT or GA-35RVT.

[Toxophilite]

Well I replaced the plate resistors on the 6EU7s (the 12au7 has none)

All the other resistors in the amp seem to test within spec beyond that it's just replace them all


The tube sockets have been cleaned with contact cleaner as have the pots
Could this be a leaky coupling cap??


Just ti reiterate, this is a LOUD crackling/popping
It comes on as soon as the amp does (with the audio) and though it passes signal and apparently well
The crackle is so loud as to make the amp unusable..it annoys the *&^*(& out of me when I'm trying to poke around

[shooter]

with the audio

so at *idle* nothing plugged in it's quiet?

[Toxophilite]

Oh No at idle it was hugely loud, didn't even need a guitar plugged in
i think I found the culprit though
As the noise seem to go away when the 12AU7 was unplugged
I tried unhooking the grids of the 12au7 one by one with the 12au7 back in
The noise stopped when I unhooked pin 7 (the plate of which goes to the reverb transformer)
on the board it attaches to a 220 resistor going to ground annnd a .0047 coupling cap which attaches to the plate of the 6EU7 preceding it
So I replaced the cap and the 220K resistor, cause I was there and the flameproof ones are better anyway (IMHO)
And the crackle went away..for a good 30 min or so

OOOOPs spoke too soon. still crackles occasionally, just not constantly, nor quite as loud


Now I can stop it by pulling out V1 (6EU7) then I plug it back in and it stays away. but thn come back again a little after a while...oy vey
I'm beginning to suspect the 6EU7s
I have it cooking in the other room and I'm keeping an ear out for it
It is much better than before..but the inconsistent thing bugs me
It's making the odd fizz/pop noise now but not like before

[punkykatt]

My guess its the PI tranny.  Worked on a Ga25RVT bout  a month  ago with the same symptoms.  Take your listing amp device and start at the input jacks and work your way back to the OPT.  You will find exactly where the noise is entering the circuit.  Punky

[shooter]

I'm beginning to suspect the 6EU7s

the 5T I worked on used a 6EU7, same problem AFTER replacing the PI tranny, talked with owner and wound up changing and re-wiring V1 for a 12au7, problem gone, only problem after that - owner said it was REALY  loud  when he turned it up 

[Toxophilite]

hmm
I'm not entirely sure what's wrong!! (obviously!)
Though, at least now it doesn't have super loud snap crackle pop anymore and at most does it intermitently
Happily my friend is a laid back guy and as it's a huge improvement he's happy to have it playable (he wants it for home use)
I told him to pick up some more 6EU7 so we could at least try to eliminate that as a possibility or perhaps fix the problem as one of the 6EU7 is pretty microphonic. (If you tap it lightly you can hear it pretty loudly in the amp).
Anyway it makes it a little harder to troubleshoot if you can't eliminate all the easy things first.
I might revisit it later if he buys some new(or NOS) tubes

Sluckey was right about the old Spragues, replacing one that had a loose leg got rid of most of the super loud bacon frying.


Just to make sure I did reflow pretty well every solder joint, cleaned the pots and the tube sockets (even tightened the power tube sockets up a little)