• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tube Bias question

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Hello, just completed the bias on my tube amp, is it ok if one of the tubes have a very slight glow in the middle of it? You can only see the glow if the lights are off in the room (totaly dark). As well the amp sounds fine, and it takes about 10-15 minutes for this to happen. If the lights in the room are on, can't see the glow. The tubes are 6550's if that matters.


Rino Odorico
No, I haven't checked the current (don't actually know how to), I have the service manual for this amp (Luxman MQ-360), what it calls for is to check the DC voltage and adjust between 5.3 ~ 5.9 volts. My voltage readings are a bit high...6.4 is the lowest I can get it, don't know why...changed all of the caps, trans. are good...even replaced the bias pots (same value) and only changed for the better about 1/2 volt.

The glow I am explaining is a sight redness in the middle of the tube...only shows with the lights off. Also tubes are a Matched quad from Sovetek 6550's.

BTW, plate volatage is 450VDC

Thanks Rino Odorico
diyAudio Senior Member
Joined 2002

I don't feel you have a problem to loose sleep over.
Still if you want to isolate the problem I suggest you swap the tubes's position around.
You would know for sure if the prob. lies with the tube or not.
Seems to me the matching was only so-so.

One last question Frank

Hello Frank,

Thanks for all of your help. I just have one last question, so what it sounds like, having .5 to 1 volt difference on the bias is not that bad of a thing...so I shouldn't loose sleep over it then. As long as the amp doesn't blow fuses and sound like Jimi Hendrix has his guitar plugged into it, it's fine?

Rino Odorico
I am willing to help you wid the bias problem... Check the value of the resistors connected to pin 8 and 1 to ground. Using ohms law V=I R , You can calculate the bias current by this method. Using voltmeter, measure the voltage on this resistor... example, if it is 100 ohm, then a 0.5 volt would correspond to a bias of of 50 milliampere.

Good Luck,

Thanks John

Great, thanks John. I'll try that this weekend!

The amp still has a small glow to the tubes (I thought that it was just the one, but it's all of them...4-6550's). Strange, the glow is very slight and doesn't effect the sound of the amp. If anything it sounds like it has a little better bass. As well, I felt brave and had the amp on for 8 hours...the glow never increased, or decreased for that matter.


Rino Odorico
You need to check the plate dissipation. You mentioned the Voltage is 450VDC (You measured this, right?)

For a 6550, the plate dissipation should be around 35W. Any higher and you will shorten the life of the tubes. The sound will be good... for a while. Could get expensive!

This means you should have a plate current of ~80 milliamps or less. If the amps are push pull, you can bias them colder than this. I assume you have some 6550 datasheets?

Did this only happen with the Sovtek tubes? They may not have a very high power handling....
My 6550 have a glow in the middle. I think you can just see all the way to the cathode.

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