Screeching tubes

[9am53]

Hi, I converted a bogen PA to a guitar amp last year and got the most amazing tone out of it, I am not kidding, the only problem is thatI blew up both my ecl86 output tubes....since then I have bought new ones and got a crazy loud screaching noise...when you turn the amp on, with or without a guitar plugged in you get no signal through, and a screaching slowly gets louder and louder till its unbearable and I turn it off...
I thought that the tubes were microphonic or whatever so I bought another set, and got the same result...is this a magor coincidence and I need a nother set up tubes or another preamp tube...or could it be something else...the exploding tubes I believe is from my doubling the plate voltage to the 6eu7 preamp tube...but the gain is unreal!
thank you for looking

[Geek]

Sounds like something is wrong in the circuit. Time to grab your voltmeter and poke around, particularily at cathode resistors and coupling caps.

If it took you a year to go through output tubes in a PA-pushed-into-guitar-duty, that's great!

[sreten]

Hi,

As a PA amplifier it might have overall feedback.

Guitar amplifiers AFAIK generally do not have overall feedback.

Might be with overall feedback you have too much loop gain causing instability.
In this case removing the feedback (giving you more gain) might help.
(could be your new tubes have more gain than the old ones)

/sreten.

[9am53]

hmm, if this feedback you speak ofis so intense that you literally cant hear anything but screaching, and it begins upon startup then maybe...I have a feeling that I blew something (the PA is from '66) and that casued the tubes to go, and changing the tubes wont fix the problem...if I can find what is blowing up and replace it for a few hours of sweet sweet tone its worth it, I think I will poke around with the voltmeter asd earlier mentioned.thanks guiys