Heres a strange one for you

[iknowjohnny]

Maybe someone can tell me what could cause this because it makes no sense to me. First of all, note that i use a tung sol 12Ax7 in V1 tho not in any other position. This is a home brew marshall jcm800 single channel master volume type amp with 6V6 and 18 watt transformer. I built it long ago so it's proven to be reliable. But i have 10k dropping resistors and pretty love voltage on the preamp tubes, about 150v at the tung sol's plate. So i had the bright idea to change the dropping resistors to smaller and i first put a 2.2k from the screen node to the PI node. Then the next one upstream i used another 2.2 then the final one to the tun sol's node a 8.2k. I don't recall what the voltage was but it was less than a jcm according to the voltage charts i have seen for those amps. Well under i think. Anyways, i found that i was getting what sounded like blocking distortion when i turn the gain to 10. I never do that usually, it rarely goes past 3:00 and if i want more i have a clean boost pedal. But with the different feel/sound i wanted to see how it sounded on 10 and at 10 it sounded like blocking so i put the 10k droppers back except the first one at the PI node and left that at 2.2. Then i replaced the tung sol with another one when that didn't cure the issue and the issue did not go away. So i put the remaining dropper back to 10k and the problem persisted.

Eventually i put a 3rd tung sol in and it sounds fine now. I do hear about these tubes go bad easily, but i've been using these for a long while and they've been fine. I can see how the voltages the first one was getting when i changed all the droppers may have been too much even tho they were lower than many maps, but maybe these tubes really have a problem with higher voltages. But unless i got my timeline wrong when doing this, i'm pretty sure i put the second tube in AFTER i put all but the first dropper back to 10k. I can't imagine only changing the PI node to 2.2k would cause the TS in V1 to go back, as the voltages would be well under 200v unless these tube are extremely sensitive and can't handle even middle range voltages for a 12AX7. then again i tried one in the loop ciruit for a few minutes too and i'm not sure if i did that while i had the low value droppers in or not.

I suppose thats rather confusing to follow, but you get the gist of it. Two TS reissue tubes were damaged by voltages well under what a 12ax7 should be able to handle. I think the highest with all droppers changed was 245 or there abouts. I tried both tubes since all droppers were changed back several time and A/B'd them with a known good TS and the damaged ones definitely are because at 10 that blocking sound happens and as you turn it down just on notch or so you hear the nasty distortion and smooth treble and thin sound change to smooth top and the lows come back in. Put the known good TS in and it works normal.

Can anyone comment on the TS reissues and if it is common for them to go bad with anything over very low voltages?

[frankdrebin]

they go bad easily,especially on cathode followers.

[iknowjohnny]

yeah, i know that; but I never use them in the CF slot because of that.