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Possible for tubes to cause unpleasant feedback?

guitarjb44

Silver Supporting Member
Messages
817
Is it possible for power tubes to cause unpleasant and uncontrollable feedback?

I installed new Valve Art Shuguang/Chinese EL34s in my Bray-modded '69 50-watt Marshall last week. They seem ok, but since then I am experiencing harsh, unpleasant and uncontrollable feedback. This occurs when I have the amp volume at live-gig levels, or at full-band rehearsal level (loud enough to hear over the drums). Before the new tubes I was always able to get great-sounding and controllable feedback. Now it is uncontrollable, harsh, and unpleasant.

I have assumed the problem is that my bridge pickup went on the fritz and is in need of being re-potted, just coincidentally when I installed the new tubes. But now I am thinking maybe not- maybe I have a microphonic power tube?

I am sure I will figure this out the next time I get to the rehearsal space; I have not yet had a chance to try the amp with a different guitar or pickup. But in the mean time, I just wanted to throw this out there, see if anyone has had a similar experience.

-Joe
 

Blue Strat

Member
Messages
30,190
More often than not preamp tubes, not power tubes, cause the symptoms you've described but anything is possible. Whenever a problem like this pops up the first thing to do is change everything back to the way you had it previously. That would mean reinstalling the original power tubes. It's the only way to confirm that the new ones are a problem.

Any time you change power tubes you have to rebias.
 

guitarjb44

Silver Supporting Member
Messages
817
I finally had a chance to check out my amp to see what's going on with the annoying/unpleasant feedback.

Upon initial inspection, I noticed that the power tubes were not seated all the way into their sockets. Once I corrected that, the unpleasant feedback went away.

The first time I noticed this feedback problem was the last time I gigged, which was a few weeks ago. I assume that when the amp was being transported to the venue, it was jostled about too much and the power tubes got loose in their sockets. The amp is a '69 50-watt Marshall; if it ever had power tube retainer springs, they were removed somewhere along the way on the amp's journey to me.

Anyway- looks like problem solved, and lesson learned.

-Joe
 

TimmyP

Member
Messages
2,488
Had that problem with two different sets of JJ 6L6s in a Fender Tone Master. Put in Sovteks and all is well.
 




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