Music Man RD50 bias question

[xtian]

This RD50 arrived with burned open 470R screen resistor on one tube socket. I replaced screen and grid resistors on both sockets and cleaned up the carbon from the burned resistor. Next I noticed that bias voltage at pins 5 was zero--the bias circuit had failed. Traced the issue to D9, a 30v Zener, had to order, but I put in a 27v Zener, and everything is working.

My question is about how the bias operates on this odd dog.

bias-question.jpg

On the left is the RD50 scheme: 80v cathode, 30v control grid (this is the bias voltage, and yes, it is positive), 350v screen, 600v plate.

On the right is a typical fixed bias arrangement: 0v cathode, -30v control grid bias voltage, 380v screen, 400v plate.

For the typical fixed bias, I know that if the bias voltage fails and drops to 0v, the tube's current will run away and red plate.

For the RD50, if the bias voltage fails to 0v, will the current increase or decrease?

Musicman-RD50A-Schematic.pdf

[sluckey]

As drawn you have 30 - 80 = -50v bias. If grid decreases to 0 and the cathode remains at 80, you will then have -80v bias, so tube current goes down (a bunch).

If grid remains at 30 and the cathode decreases to 0 you will have some well done tubes!

[FUCHSAUDIO]

This amp has a solid state driver circuit. There’s a transistor in the cathode circuit that blew. The tube should be changed, the driver transistor, the chip (use same part number) as well. Also new filters if not donealready, and low voltage zener diodes which may have drifted by now.

[xtian]

Thanks, Andy. I replaced the Zener and the amp sounds good and is already back with client. I’ll appreciate your comments even more if this amp comes back again!

[FUCHSAUDIO]

Excellent !