Intermittent amp hiss and distortion

[pushover]

My Peavey Bravo has started making intermittent hissing and distortion noises when I play it. It only happens at intermittent intervals and sometimes I can go for a complete session with no problem at all, but it's there and will show up, usually when I least can afford it too. I've been through this drill before and it's almost always the tubes that are the cause, but in this case the tubes aren't that old and don't have many hours on them.

 

I kept the original set of tubes that came with the amp when I bought it. There's nothing wrong with them, except that I thought they sounded harsh, so I put them back in the amp, and sure enough there has been no problems since then (except that the amp sounds harsh again..). The intermittent nature of the problem means this may not be conclusive proof, but it sure seems to support my hunch that it's the tubes.

 

I'd just as soon not completely re-tube the amp again so soon, so I'm wondering if there is any way to tell whether the problem is with one tube, or perhaps at least narrow it down to whether the problem is with preamp or power tubes? Other than trial and error replacement is there any way to identify a bad tube? As I mentioned above, the problem was only intermittent so trial and error may take a while to figure out if it's one tube, let along maybe more than one. Or should I just bite the bullet and buy a complete new set?

 

Any suggestions from the enlightened?

[mars_hall]

Pencil test and the noisy one is often the first in the chain, typically V1. Could be the socket as well if the original tube pins are slightly larger or there is any oxidation going on.

Don't know if the Bravo is tube output, but these are usually the first to go. Watch them during the noise for arcing.

[Hfan]

You could reinstall the tubes you just removed, then swap in the old / known to be good tubes one at a time until your problem returns. If it does not return, you may have just needed to reseat the tube in question.

 

I'm no expert but I would think it is not a power tube causing this so I would try the pre amp tubes 1st.

[H]

Just had the same issue in my 5E3. Pencil test on V1 reveals it's microphonic. It's got about 50 hours on it.

 

It seems that it doesn't matter how new they are...

[Hfan]

Remember when doing a pencil test, even a good tube will transmit the "tap" but there shoudn't be any funky hisses etc. Keep your best pre amp tube for the 1st stage preamp gain position. In a Fender thats V1, 1st on the right. For a two channel Fender V1 is for the normal channel and V2 is for the vibrato reverb channel. If you have no spares, you can take a same type pre amp tube from another position and swap into the pre amp gain position. I would try reseating them 1st though. Really though, spare tubes are mandatory.

 

Also, the female tube sockets can get loose and can be re tensioned with a small screwdriver. Oxidation can occur to the sockets and tube pins. De oxit works well for this. All this should be done with the amp powered off, I beleive (someone correct me if I'm wrong) you can drain the stored charge on the power tubes by pulling the plug with the amp powered on and a guitar plugged in. If I was going into the chassis I would always check with a meter.

[DetroitBlues]

I think KBP810 can add a little input into to this too. I would agree though it appears to be bad tubes. This was the point of me bringing up the tubes threads. I've heard this very often about tubes being bad right out of the box or ones that were barely used going bad. Knowing what manufactures to trust and where to get reliable tubes is very difficult.