Quote from: Roly on July 03, 2013, 07:20:46 AMHi, thanks for the reply
Hi and welcome.
Let's dispose of one thing at the outset - your overheating output FET's have nothing to do with your microphonic valve problem.Quote from: belleraphon88the amount of volts that gets sent to the 12AT7 valve in these randall G3 amps
The supply voltage is given on the circuit as 375V. From looking at the RetroValve specs they specify 120-400V, so it would appear that they are intended to work in amps circuits just like this one, and if you get the correct RetroValve to replace a 12AT7 (gain wise) then I can see no obvious reason why it shouldn't work.
I do however share JM's curiosity about your string of microphonic valves. All valves tend to be a bit microphonic to some small extent, even brand new, but it sounds like you have either had a run of bad luck, or this particular circuit is sensitive to slightly microphonic valves, 'tho I can't see any obvious reason why that should be.
To be quite specific; a "microphonic" valve produces a glassy tinkling sound a bit like wind chimes when tapped with a chopstick or similar, not crackles or loud blurts, these normally being cause by poor socket contacts - are we addressing the right problem here? Apart from the character of the sound produced it should also be very localised to the valve. There are other components (and dodgy connections) that can also produce strange sounds, but they are normally of a different character and not localised to tapping (gently) on the valve glass.
If indeed your problem is microphonic valves then a solid state replacement would seem to be a good idea to try, and this RetroValve product seems suitable.Quote from: belleraphon88would it be possible to modify the amp power amp so that i can get rid of the valve.
It is, but is a different order of complexity. Even as a tech I would be inclined to try the RetroValve path first as the simplest solution to your problem.
The RetroValve page does make a point about their product changing the tonality of an amp, mainly because FET's don't have an identical transfer function to the valve they replace, however they are still similar, but it's something to keep in mind - it may not "crunch" or distort in quite the same way you have become used to with a real valve.
If you do decide to try these out I'm sure we would all be interested in your report on how you find the replacement.
Postscript - a "popping" sound isn't microphonics; sounds more like a base connection problem. As JM says, a sound sample of the problem would be helpful. Also, does wiggling the valve in its socket help, make the problem go away for a while?
maybe popping wasnt the best way to describe the sound.
i have recorded a short clip, i hope it helps, not he best sound, but hopefully it gives you an idea
http://www.youtube.com/watch?v=Onolh61F_Ao&feature=youtu.be
Wiggling the valve doesn't change anything
I will order the retrovalve tonight, i will certainly let you know how the the retrovalve worked out for me
Andrei