Tube Amps / Music Electronics
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|10/7/2004 2:41 PM|
||Plate glowing red|
after a major revamp of almost all the solder joints and replacing the coupling caps in my 5E3 clone to (successfully) reduce the "popp" noise i mentiones earlier I have to face a new problem:
I've put in a well matched pair of vintage Sylvania 6V6 GTA's that aren't NOS but still test better than new on my Hickok 800a. They sounded great, but after a while teh amp made strange noises (even without a cable in the input jack) and I noticed the plate of one tube glowing red. I switched off immediately, swapped the tubes from left to right and after applying B+ the plate of the same tube began to glow, now in the other socket.
I checked the voltages at the plates, but everything was well, just a bit below the figures Adam from Adam Amps provided as guidline.
I then put in a pair of vintage RCA's, also not NOS, yet testing very strong. After a while of playing the same thing happened.
I've tryed with new production EH 6V6GT's and there was no glow (but I didn't play that long with them) and no noise.
Is it just the tubes that are defective or is it possible there is a bug in the amp that only becomes evident with older tubes?!
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|10/7/2004 4:02 PM|
Pull out your DVM and measure the bias voltage. Is it too hot?
Also, if you used a PS tranny with higher voltage then the original 5E3, you will have similar prolems. To resolve, simply bias colder by changing the cathode resisitor to a slightly higher value.
|10/7/2004 4:58 PM|
|Chris @ CMW amps
What's your B+ voltage?
Double-check all solderjoints between V2 ( driver/splitter ) and OT to be sure.
|10/8/2004 1:54 AM|
Thanks, I'll check again. B+ at the power tubes is 329V, I don't remember what bias voltage I had, I'll check that as well.
I use a Mojo PT (with taps for European 220V).
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