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Plate glowing red


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10/7/2004 2:41 PM
Jeffrey
Plate glowing red
Hello,  
 
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?!  
Jeffrey
 
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10/7/2004 4:02 PM
Brutus
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

Hi Jeffrey  
 
What's your B+ voltage?  
Double-check all solderjoints between V2 ( driver/splitter ) and OT to be sure.  
 
Chris
 
10/8/2004 1:54 AM
Jeffrey

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).  
Jeffrey
 

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