Traynor YBA-1A 6550 Bias Question

[plumcrazyfx]

I've done some work on a 1972 Traynor YBA-1A (caps, replaced dropping resistors, etc.) that I would like to start using 6550s for reliability due to the high plate/screen voltage these run at.  The Schematic (Jan 69 dated) shows the bias circuit as a 150K/2W before the diode, then a 15K with a 10K pot + 47K to ground going to the 100Ks bias feed resistors. 


Right now it is at 570v at 70ma at the lowest setting of the bias pot.  Any recommendations for what changes to get the bias more in control?  I do not have any fear for the power transformer as it is dang-near the size of my head.

[sluckey]

First... PULL THOSE OUTPUT TUBES AND LAY THEM ASIDE. When you can put -50v to -65v on pin 5 of the empty sockets, then you can put the tubes in and start checking the tube cathode (bias) current.

OK, now replace the 10 K bias pot with a 50K bias pot and adjust for max negative voltage on pin 5 of the output sockets. . That may be the only mod you need to do. If still not enough negative voltage on pin 5 of the empty sockets then lower that 15K resistor. I'd start with 10K, then 5K, then experiment to go up/down as needed.

[thetragichero]

besides above, i have also read it suggested to reduce the bias feed resistors to 150k so i also do that

[sluckey]

Don't know how I missed that. That's the first thing to do.

[plumcrazyfx]

The 50K pot brought it in control.  At 570v at 37ma.  Do I need to change the 100K bias feed to 150K?


This thing is blistering loud.

[thetragichero]

100k should be fine. there are a number of revisions of the yba-1(a) so the ones that have higher bias feed resistors would need to be lowered
and yes, these amps are crazy loud. and can usually be had for a very nice price (i drove across the state to pick up one from a pawn shop for $300 out the door)

[plumcrazyfx]

Thanks for the help.