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oldmanStrat 19th June 2009 03:39 PM

Simple SE static
 
My 6 week old amp has started doing something different...

I now get random static at a fairly low level. It comes and it goes. It literally sounds like an old radio tuner in between stations. It also seems to be getting progressively worse over time.

I've tightened up every connection possible, disconnected the input (still there) and tried different output tubes.

I'm thinking that maybe my input tube is going bad?

thoughts?

robertaudio 19th June 2009 04:31 PM

Hello oldmanStrat,
Check all coupling caps and signal caps. If one starts to go bad this can give random spikes.

Happy Diying,
Robert

oldmanStrat 19th June 2009 05:53 PM

The static is in both channels, so I don't think it's the auricaps.

I can't put a scope on B+ since it only goes to 100v. How do you suggest I "check" the caps?

robertaudio 20th June 2009 04:40 AM

If this is in both channels, check power supply filter caps. best way to check them is to replace all if you have a spare or one by one till it goes away. If you can show me schematic, it can help to figure this out. The problem seems to be common to both channels, thus any circuit that is common to both is suspect.

Cheers,
Robertaudio

robertaudio 20th June 2009 04:40 AM

As for your o-scope just you a simple voltage divider, if the culprit is in the pwr sup you might be able to see it.

Robertaudio

rknize 20th June 2009 05:31 AM

I agree it is most likely the power supply. Does wiggling the power switch do anything? Tickling the fuse? Those green phoenix connectors have a reputation for working a bit loose after a bunch of heat-cycling. Go over them and torque them all down.

I can't remember if you have a switch on the SS rectifier, but that is another possibility as is a switch for UL/triode modes. If no dice, start easy by disconnecting the external cap.

Ty_Bower 20th June 2009 12:11 PM

Re: Simple SE static
 
Quote:

Originally posted by oldmanStrat
I've tightened up every connection possible, disconnected the input (still there) and tried different output tubes.

I'm thinking that maybe my input tube is going bad?

Simply disconnecting the inputs does not make a valid test. You need to short the inputs to ground. Leave them floating, and they might pick up noise which then gets into the amp.

Replacing the 12AT7 is simple enough to try, unless you don't have a spare.

oldmanStrat 20th June 2009 01:29 PM

Ok - I don't have spares of any of the caps - but I can remove the the big oil can to see if it might be a problem. Seems unlikely since it's way over rated to begin with and brand new.

I did tap on the 12at7 (lightly of course) and I was able to affect the static... still leaning towards the tube - of course I don't have a spare one...

oldmanStrat 20th June 2009 09:54 PM

replaced the volume pot today... it was on the list to do anyway, the RS pot was not exactly balanced between channels.

That wasn't it either.

Taking a closer look at the 12AT7 - I can barely see the heaters glowing. Is that normal for these tubes?

Next I'll try removing the big cap, and then I'll see if I can measure the B+ rail with my scope. Any recommended values for the divider? I was thinking a divide by 10 would make easy math, but I don't want to use values so high that the scope's capacitance becomes a factor.

rknize 20th June 2009 10:01 PM

I guess it depends on the impedance of your scope. :) Mine is simply 90k + 9k + 1k 1/2W to give x10 and x100. Not the best for sniffing around high impedance stuff, but it works fine for PSU work and other poking around. Was going to build a 900k+90k+10k for that.

The heaters should be fairly visible. Who's AT7 is that? Check your solder joints on that noval socket.


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