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So I just finished de-modifying a DR201 and it has low, distorted output. I am in a little bit over my head now. My first question has to do with the bias. I put in 1ohm resistors from cathode to ground on the output tubes, and I am not reading any voltage across the resistor when the amp is on? The plate voltage is about 717 volts and the bias supply is -37 volts. I don't know anything about about the health of the tubes, but they are 70s GE and Sylvania 6CA7s. Any other debugging hints are appreciated.

I just verified the transformer resistance is ~20 ohms on each side and the voltage drop was a few millivolts, almost no current on each side. Also, no voltage drop across the screen resistor.

Ok, with standby on the HT2/HT3 voltage is at 410, when I switch standby off, it trickles down to 205V and stabilizes. When I swtich it back I can watch it go back up to 410V?

Something is very wrong with that HT2 voltage. Also, there should be some voltage across 1 ohm resistor. Check if there is a short circuit somewhere (maybe in the output tubes even).

Is the HT fuse ok? Is there any voltage on either of the HT fuse terminals?

--mark

The fuse is in tact, but there is no voltage at the post from the board to the fuse terminal. The wire had been resoldered (very sloppy) at some point, but it appears that none of the other connections have been messed with.

After checking connections and scratching my head all day, I put it back to gether and check the voltages with the power tubes pulled. No voltage drain, I guess one is shorted. I don't have spares around right now, so I guess I will put them in a pair at a time to narrow it down. Out of time for today though.

YESSSSSS!!! Got it, output is clear and noise free! Gotta go play.

theSeeker wrote:YESSSSSS!!! Got it, output is clear and noise free!

Great news !

Dave