Another heater question... Marshall 2203 &n

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crashtm1
Senior tube assistant
Posts: 124
(4/20/04 10:11 am)
 Another heater question... Marshall 2203
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 I had glowing red tubes after 2 minutes of operation.. But only in Valve 1 & 2... 3&4 were fine... SO I opened up the chassis and checked my heater wires... removed them all on the power tubes,, got new wire,, twisted and ran them high over the sockets,,,, 2 to 2,, and 7 to 7,, through them all... Now, 3&4 glow red,, and 1 & 2 are fine... Am I missing something obvious here??
And what exactly is the best way to measure the current??
(Tubes removed) pin 2 to grnd,, and measure "DC" voltage??
Or across 2 and 7..."DC" voltage???

Thanks,,,,,,,
 
Funkalicousgroove
Junior tube assistant
Posts: 72
(4/20/04 11:44 am)
 Re: Another heater question... Marshall 2203
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 which part of the tubes are glowing?
 
crashtm1
Senior tube assistant
Posts: 125
(4/20/04 1:37 pm)
 heaters
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 the long grey plates....
 
Leevi
Junior tube assistant
Posts: 138
(4/20/04 2:01 pm)
 Re: heaters
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 Have you measured the biasing voltage (pin 5 if your power tubes are EL34)? The voltage should be close to -34V. If it is close to 0V the current flowing through the tube increases and the tube starts to glow orange. The sound may be better but the tube does not last for a long time.
/Leevi
 
crashtm1
Senior tube assistant
Posts: 126
(4/20/04 3:50 pm)
 heaters
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 Running 6L6s..I also changed the bias resistor.. All measurements were with the tubes out.. I've been running my bias at about 32ma..

2...3V
3...482
4...481
5... -46
6...480
 
jimf
Junior tube assistant
Posts: 14
(4/20/04 6:57 pm)
 Re: heaters
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 It doesn't seem like the heaters are the problem.

It would look at the bias voltage. I think glowing red means the tubes are passing too much current, and since idle current is controlled by the bias voltage, that's probably where you need to look.

But just to be sure about the heaters, measure the AC voltage from pins 2 to 7. It should be about 6.3V. If it is, then you're fine.

You give a voltage at pin 5 of -46V, which seems fine, but is that for all 4 tubes? What is the voltage on pin 5 for each of the tubes?

You say the bias is 32ma. How are you measuring the bias current? Do you have a resister from cathode to ground, do you shunt the transformer primary, or...? This is really where you want to look. What is the bias current for each set of tubes.

Jim
 
crashtm1
Senior tube assistant
Posts: 127
(4/20/04 8:58 pm)
 Heaters
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 Ok,,, a clue..:))
I put the tubes back in... they started to show some red, and I measured pin 5...and had -13... Then there was a small "pop" that I heard in my speakers, then the redness went away, and the voltage returned to -46... Once I plugged in my guitar and hit a few power chords,, "pop",, and back to -13...
Could this be the 2 ,, 10uf/100v caps in the bias supply going bad or the 15K resistor tied between them??

 
Leevi
Junior tube assistant
Posts: 139
(4/21/04 3:00 am)
 Re: Heaters
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 Check the circuit (including diode, resistors and caps) that prouduce the minus bias voltage. There can be a bad contact or the caps have to be renewed.
/Leevi
 
 
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