Screen glow when pushed

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whittele
I only work on tube amps
Posts: 478
(2/6/04 11:28 pm)
 Screen glow when pushed
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 Good to back from sea. I finally washed the sub smell off and sleep in a real bed. Anyway...
I constructed a 50 watt plexi using only the bright channel (single volume only using half of V1 and a single "mixing" resistor) per the hoffman layout. Used Doug's tranny and mostly his parts. Here is the issue:

When I push the volume and I am not talking about pushing it hard say about 11 o'clock I get a red glow from inside of the plates. But NOT the plates. I am biased at 37.2 / 38mA at 461VDC (Sylvanias) and have the 5.6K grid resistors on the tube sockets.

This occurs with good Mullards, NOS sylvanias, JJ's, and EH EL34/6CA7.

The amp is otherwise very quiet and sound great.

What the heck is going on? Should I be concerned over amp health here? Thanks in advance. whittele
 
Dbltlk44
Junior tube assistant
Posts: 4
(2/6/04 11:41 pm)
 Re: Screen glow when pushed
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Can't help ya with the problem, but just wanted to say welcome back! Buddy of mine is a Sub driver, and always glad to see you guys make it back safe and sound..............
 
GroundhogKen
Forum Moderator
Posts: 2241
(2/7/04 9:22 am)
 Re: Screen glow when pushed
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 Since you used the Hoffman layout, may I assume that you have 1K screen resistors on pin 4 of the power tubes?

It may be OK. I've heard that when the screen intercept rate is high ( tubes are being pushed hard ) the screens of cheap tubes will light up white. Red may be OK.

Install your least expensive set of tubes and wail on the thing for a while. If nothing melts, its probably fine. How's that for a scientific assessment?


Welcome back to dry land.

Ken
 
whittele
I only work on tube amps
Posts: 479
(2/9/04 7:33 pm)
 Re: Screen glow when pushed
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 GHK,

I sure did use the 1K's. Nothing sounds out of sort and actually sounds pretty darn good. I just have not seen this before. And these are not cheap tubes - that is what bothers me. I will try to melt down the EH's. Fire up the lab....keep the extinguisher handy.
 
 
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