[wharfrat]

Hi everyone-

The glow in the power tubes on the left side on my Sonic Frontiers Power 1 are flashing/pulsing blue and red, back and forth.  In other words when one tube is glowing blue, the other is glowing red.  As this happens I get a beating sound in my speakers.

I tried changing tubes (Gold Lion 88's and SED 6550's) and this is still happening.

Any ideas why this is occurring?

Thanks in advance!
David 

[medium jim]

Check the bias!

Jim

[Freo-1]

Sounds like you have a malfunction in your bias circuit.  Sometimes, when a tube goes south, it takes the bias resistor with it.  The red definitely means you are drawing excessive bias current.  I’ve also seen the grid resistor burn up when the screen grid shorts out (which does happen with some of the Russian tubes). 
 
PS: Flashing blue sounds like a screen grid short.

[medium jim]

Sounds like you have a malfunction in your bias circuit.  Sometimes, when a tube goes south, it takes the bias resistor with it.  The red definitely means you are drawing excessive bias current.  I’ve also seen the grid resistor burn up when the screen grid shorts out (which does happen with some of the Russian tubes). 
 
PS: Flashing blue sounds like a screen grid short.

True....though, sometimes, the bias can drift so far that one tube is getting excessive current and the other not enough.   Probably best to have it checked by a professional.   

Jim

[DaveC113]

If you have a multimeter, you could check the values of your bias or cathode resistors/pots (not sure how your amp is biased), as well as the plate resistors with the amp unplugged and the caps drained. If you're not familiar with tubes or high voltage, you could read up on safety precautions and check B+ voltage, as well as voltage across your bias and plate resistors assuming you don't find a bad resistor with the amp off. I'd guess you will find a bad resistor... Also, if you have electrolytic caps in the cathode paralleled to the cathode resistor, check those too... if they are bulged out or have leaked a fluid (failed) this will cause a lot more power to dissipate in the cathode resistor and possibly damage it.

[wharfrat]

Thanks for the advice, guys.  You all basically confirmed what I think has happened--a bias resistor has gone bad.  Next step is to find someone close by, northern NJ, to replace it. 

I'm actually thinking of having the technician replace the resistors and add fuses so that if this happens again it won't be a timely pain in the butt.

You all think this makes sense?

[SteveFord]

While they're in there, have them take a look at the bias pots.  If they're like the VTL ones it'd be foolish not to replace them while the amp is in for service.

[wharfrat]

Good advice Steve!  Thanks.

[Lord Soth]

Check the bias!

Jim

Yes, any "redplating" means that your power tubes are running too hot. The bias has to be adjusted or repaired.

And, maybe your Power Tubes are AMERICAN.....
with the RED WHITE and BLUE ....