Static with low notes (definitely NOT a speaker issue - vibrolux reverb)

bbill335

At band practice on tuesday, I noticed the amp was making a white noise/static sound when I hit a low note/chord. Anything between low E up to B seems to do it, but it doesn't do it when no note is played. Valve swaps haven't helped. I guessed that meant there was a dodgy connection inside that was affected by certain vibration frequencies, and that it was time to go to a tech and open up my wallet again.

One of the speakers also has quite an annoying cone cry so I was gonna try inverting it to see if it helped, which meant inverting both speakers (because the leads between them aren't long enough for a diagonal connection) which meant pulling the chassis to get to the bolt behind the mains transformer.

With the chassis out, I noticed the black lead off the reverb transformer was loose inside. Great, that's probably what's making that horrible static noise.  It only goes to ground and it had been grounded through the footswitch jack before coming loose. Feeling gung-ho and hoping to avoid a tech trip, I resoldered the connection, tested continuity and put it back together.

Well that ain't fixed it (and the cone cry is just as bad as before). Am I missing something obvious or is someone going to have to go through looking for a dry joint inside? 

jacksparrow

Try ceaning your valve sockets with servisol. Worth a punt.

jpfamps

Could also be a damaged speaker.

DJH83004

Have you tried to isolate the fault at all? e.g. is it the same through the normal channel as through the vib channel? reverb on / off, different EQ settings etc.etc.  

ICBM

Does it still do it with the reverb tank disconnected and the reverb drive valve (V3) pulled?

bbill335

jacksparrow said:

Try ceaning your valve sockets with servisol. Worth a punt.

tried, no good

jpfamps said:

Could also be a damaged speaker.

it definitely sounds electrical (if that makes any kind of sense) and seems to be coming from both speakers (whereas the cone cry is obviously the one speaker, although both problems have arisen simultaneously )

DJH83004 said:

Have you tried to isolate the fault at all? e.g. is it the same through the normal channel as through the vib channel? reverb on / off, different EQ settings etc.etc.  

Does it on both channels, with or without reverb, once the volume is up around 6

ICBM said:

Does it still do it with the reverb tank disconnected and the reverb drive valve (V3) pulled?

will try this out.

Thanks for the suggestions everyone, will report back

digitalkettle

I had a static-like problem once with low notes and any reasonable volume. Turned out to be a push-pull pot with vibrating contacts!

bbill335

digitalkettle said:

I had a static-like problem once with low notes and any reasonable volume. Turned out to be a push-pull pot with vibrating contacts!

It's a mostly-original 73/74 so no switching pots BUT the solder tags on the valve bases (inside the chassis) seem rather lose and potentially rattly when carefully prodded with a pencil eraser end.

DJH83004

As it is common to both channels and not affected by the reverb, have you tried plugging into another cab (using the main speaker output not the extension), as it does sound like a speaker issue.  

bbill335

I don't have another  cabinet around to try that. 

ICBM said:

Does it still do it with the reverb tank disconnected and the reverb drive valve (V3) pulled?

Yeah it's still there. 

Looks like it's tech time

bbill335

I do reckon it *is* a speaker issue (haven't yet had a chance to test it through a different cab or get over to a tech) as it seems to be only affecting the speaker with the potentially 45-year-old cone (the other is dated '98).

I will get around to confirming it definitively but does anyone have any recommendations for how to get a 10" CTS alnico reconed?

Bygone_Tones

You need to get the speaker out of the cab/amp and test it in isolation. My blog post here should help:

http://www.bygonetones.com/common-speaker-problems-how-to-check-for-them.html

If it definitely needs reconing try wembley speakers. http://www.wembleyloudspeaker.com/ However it is always worth attempting a repair first before opting for a recone in my opinion. You have nothing to lose if you are going to trash the cone anyway.

Ddigger

Bygone Tones - thanks for posting, great reference.

Ddig

bbill335

I finally got a chance to play it through another cab (my girlfriend's trace bass cab) and it still made the static sound when I hit a low E.

So it's NOT the amp's speakers, it's something else.  

TimmyO

I hd an odd similar sounding issue once - turned out to be dry solder joint on a ground connection - the tech re-flowed all the ground bus joints and rejigged the physical support of the ground bus (as he suspected it had been vibrating and putting stress on some of the joints) and all was well.

Apparently he diagnosed it by having a signal going through the the amp while pressing solder joints in turn with a chopstick! 

bbill335

TimmyO said:

I hd an odd similar sounding issue once - turned out to be dry solder joint on a ground connection - the tech re-flowed all the ground bus joints and rejigged the physical support of the ground bus (as he suspected it had been vibrating and putting stress on some of the joints) and all was well.

Apparently he diagnosed it by having a signal going through the the amp while pressing solder joints in turn with a chopstick! 

that sounds like the kind of job that's tedious for the amp tech and expensive for the amp owner!