Problem Biasing my Voodoo modded 6505+

[chemh0ser]

Hey guys,

I recently bought a used 6505+ that was modded by voodoo amps. The first thing I did was check the bias and it was at 21mA per tube, which is cold. So I crank it up to 28mA per tube which should be totally safe for the RUBY 6L6GC STRs in there. After playing for about 20 mins, My amp loses all bass and the tubes are redplating. What am I doing wrong?

 

[BerkleeBill]

Sounds like you're not calculating/measuring the bias correctly.

 

[chemh0ser]

I calculate it like this watts per tube / plate voltage x .70 for 70% max dissipation.
So: 30/469*.7= 44.7 mA which is way too high. I am am using a weber bias rite

 

[Anonymous]

I use this as my guide....I flunked math....

http://www.webervst.com/tubes1/calcbias.htm

According to this...your 44.7 is more in the ballpark....

 

[chemh0ser]

Exactly. If it should be biased at 44.7 mA, how the hell are the tubes redplating at 28 mA? Anyone? Maybe Voodoo amps could chime in?

 

[Anonymous]

Exactly. If it should be biased at 44.7 mA, how the hell are the tubes redplating at 28 mA? Anyone? Maybe Voodoo amps could chime in?

all the power tubes are red plating??

 

[chemh0ser]

Yes they were all redplating and they are fairly new.

 

[voodooamps]

Due to liability issues I cannot tell you how to properly bias your amp. It's presently Saturday morning, please feel free to call the shop on Monday and ask to speak with me.

Let me know exactly how are you biasing the amp (IE: what method you are using) as based on what you are describing it appear that you may be reading the negative voltage at the bias splitters verses monitoring the current through the power tubes cathode or plate. As a side note, if the tubes have been red plating then you may very well have to replace them (just a heads up).


I look forward to talking with you and have a great weekend;
Trace