Newbie with a bias question, 1989 .50 Caliber

[fenderonly]

Hi to all! I have a .50 caliber in with heavy crossover distortion. The tubes in it were Groove tubes. The pair look and test excellent on my Hickock Mutual Trans tester. They are running about 5 ma at idle! Put in a pair of JJ Telsa 6L6GC (and I know they always run a bit cold) and got up to 17ma and still a lot of crossover there but improved.

Normal?? Would Mesa tubes run any hotter? Is a bias adjust mod really possible? This is a strange bias setup compared to any other tube amp I have seen and worked on. I am not ready to jump in and change the world here just yet!!

Seems odd that they take the cathodes of the inverter tube to the bias voltage via a 15K resistor. That make a bias adjust a bot tricky for sure! The schematic show -1.8 volts at that point. My amp is at +1.8 Volts at that point. My amp also does not have a 47K after the series diode. It has a 33k after the two parallel resistors (68K and 82K). In fact the 15K resistor and 220uf filter come after the two paralleled resistors. My amp dates 10-89 and the schematic I have is dated 4-6-89. Perhaps there were some changes made.

Any input appreciated!

 

[Monsta-Tone]

The drawing I have dates 4-6-89 also, so it is probably the same one.
You are looking at point 'F.' The Bias is inserted at point 'D,' -47 volts.
These types of Bias circuits are a huge pain in the ***. Especially when they are tied to the Cathodes of the Phase Inverter tube.

The easiest thing to do is to call Mesa or Eurotubes and get tubes that fit in the bias range needed. Eurotubes can get you a pair that is biased a bit hotter, the Mesa ones will be a bit cold.


You can make the amp Cathode Biased, but I'm not sure what that will do to the 1.8v on the Cathodes without trying it and seeing for myself.

Sorry to not be of much help,
Andy

 

[fenderonly]

I think I will call Eurotube and get a set biased hotter. This circuit is a pain as you say!! Designed to sell Mesa tubes.

 

[fenderonly]

Had the customer pick up a set of Mesa tubes form the local Mesa dealer. Sent them back!! One running at 32 ma the the at 17ma/ So much for their computer matching!! Nothing wrong with the amp. Swapped the tubes around and the results were the same for the tubes again!

May be back to Bob at Euro tubes soon for a hot pair of JJ tubes.