1974 Superlead Weird Crackle Sizzle Sound

[matttornado]

My amp is 1959 Superlead 1974: Yes it's old but I take very good care of it.

After a note (open A) for example, would be played and then allowed to sustain till the note ends, I would hear a crackle / sizzle kind of sound.

Amp sounds fine otherwise.

Any ideas or suggestions would be greatly appreciated.

[neikeel]

Ok I will start it off and see what others suggest too:

First have you subbed in some replacement glassware that you know is good? One may have become microphonic?
Next thing with this sound might be a failing plate resistor on V1 or V2 (the one on the socket).
Worth having a look inside, checking and measuring that all the B+ resistors are in spec.
I also remember that someone had a tone stack cap fail on a clone with very similar symptoms but cannot recall the thread.

On a Marshall (apart from pre 64 prototypes) there are hardly any carbon comps they are nearly all carbon film made by Iskra (Yugoslavia) and Piher (Spain). You typically see AB carbon comps onFenders and they eventually fail with the symptoms you describe.
Yours probably has Lemco tubular ceramic tone stake cap ad these rarely fail, you cannot really test them in circuit and would have to sub it out.
The Pihers are very reliable and hardlly ever fail. The Iskras are pretty good but I have had a few duff ones on amps left standing for a while. The only carbon comps in your amp, as a 74, maybe in the B+ line - the big 10ks but pics would help.

Do the tube thing first

[matttornado]

Thanks, Neil. I'll try tubes first.

[matttornado]

So I checked the tubes, cleaned the tube sockets & pins & checked all of the 100k plate resistors. The note still falls apart during the end of the note decay. Cuts in out and makes a static & squeal sound. I think the squeal is because the chassis is out of the head cab. though.

What next? Filter Caps?

[neikeel]

So I checked the tubes, cleaned the tube sockets & pins & checked all of the 100k plate resistors. The note still falls apart during the end of the note decay. Cuts in out and makes a static & squeal sound. I think the squeal is because the chassis is out of the head cab. though.

What next? Filter Caps?

Ok, have you tried swapping the preamp tubes for known good ones, or even just swapped them around if you do not have spares? Presume you do not see arcing or popping inside the output tubes and you bias measures correctly.

It is unlikely to be the filter caps causing this exact problem (they may be the originals but keep that thought in reserve for later, sort this first). Usually you get excessive background hum (ac ripple) coming through.

The squealing will be noise out of the cab.

Have you tried different settings on the tone/volume and presence to see if they affect the frequency or volume of the noise?

Can you post a couple of gut shots of the amp too?

Cheers

[matttornado]

I checked the bias & all is good. I need to check at different volume / tone settings. It does it on all channels / inputs.
At first I thought it was a bad 100k plate R on V1 because it was only measuring a few k. The other ones all measured ~100K but after I replaced it, the problem was still there.

The amps sounds fantastic except at the end of a decaying note. I'll try different tone & volume settings & report back tomorrow.

Thanks!

[matttornado]

And I'll try swapping preamp tubes around!

[matttornado]

So I was swapping preamp tubes around & suddenly the amp stopped making any sound - no fuses blown. Then I walked away from it for a few days. I put a different preamp tube in V1 and it came alive with no problems. Then I cleaned all of the input & output jacks. Works fine now but the 4 Ruby EL34s are starting to sound tired.

I guess it was a poor contact(s) somewhere in one of the preamp tubes? Could that cause the problem I was having?

So many possibilities.... UGH

[neikeel]

Yes!

First rule with a valve amp playing up is to check the tubes and always have a spare set of known good working ones (even if they do not sound particularly good) that just work properly.

Glad you sorted it.

What replacement outputs are you going to go for?

[matttornado]

Either Ruby el34 BSTRs again or maybe Tungsol? I had bad luck with most current production tubes (Russian Made) but the Chinese Rubys seem to be of best quality IMO.

What do you like, use or suggest? my amp is 1959 superlead from 1974. Man is it old!

I buy from Valve Queen. She does the best testing prior to selling as far as I know.

[matttornado]

Thanks Neil!

[neikeel]

http://www.ebay.com/bhp/rft-el34

These are best value nowadays.