Troubleshooting question

So, I rewired the Earth Revival to the original schematic. When I powered it up, everything seemed OK. I did not check the B+ voltage, which in hindsight I realize I probably should have done first, but I did check the voltage at the grid and it was around -50V or so when the bias pot was adjusted to set it to a minimum, which seemed like a reasonable number to my inexperienced head.

I then put in the power tubes to complete the bias. I initially used the primary shunt method, with a lead clipped to the center tap and the other to one side of the OT primary, and when I turned the amp on, the current started out reading around 80mA and slowly drifted down around 40mA as the amp warmed up. I'm not sure if the drifting is normal, but it seemed reasonable that it would take a minute to "settle in" as all of the capacitors in the amp were new. The current seemed reasonable, as it's a 4 x 6L6 design, so it'd be about 20mA/tube with the bias pot at min.

During this time, I decided to instead measure the voltage at each end of the primary and also at the center tap and do a little math to set the bias, as I already knew the impedance of each side of the primary and the drifting current made me second-guess my measurement. The shunt method also makes me a little nervous, even though I clip the meter leads to the points. I turned off the amp, and moved both of my meter test points to ground + primary, changed the meter to read DC Volts, and turned the amp back on. Shortly after this, the fuse blew. I removed the meter, put a new fuse in, and it immediately blew as soon as I turned the power on. Tried fidgeting with a few other things, replaced the bias pot, since it was about the only thing besides solid-state components I hadn't replaced (all new caps, not just the electrolytes, all new wire as the old was a rat's nest of cheap wire and the filter circuit needed complete rebuilt...).

I've tried all I can think of, but the fuse still blows immediately when I turn on the power. I've triple-checked the grid & screen resistors, the choke resistors, & all of the resistors in the filter circuit, which were all new, and they are all still correct value. I saw no smoke from anywhere other than the fuse when it initially blew.

Any advice/suggestion as to where to begin looking for the problem would be greatly appreciated. The only thing I haven't checked and/or replaced since I started working on the amp are the contact relays & diodes (I think, solid state component w/ 3 legs, labeled "Q" in schematic), mainly because I didn't know how to check them and there were so many other problems I didn't think they were likely to be the source of trouble.

Aaron

 

It was a shorted rectifier diode. I replaced all 4 of them and both of the bias diodes. No more fuse blowing! Now I'm not getting proper current through the OT primary, though. I'm getting 490V at the plates, but I only get 7.5mA of current across each side of the primary, no matter where the bias pot is turned.

 

yes, it's been replaced. even swapped back to the original, then back to the new again, and still the same thing.

 

yes to new tubes, several sets just to make sure, but all new matched tube sets from tubedepot. I'm not sure where you're referring to the -50V reading. I get -68V or so at pin 5 with the tubes out, 490-500 at both pin2 & pin 3. I've replaced every cap in the entire amp, and I've double checked the ones you indicated and they don't short with respect to ground. The schematic I found for the amp doesn't have voltages available, and I'm pretty new at trying to figure out what they should be. It seems like anywhere I measure voltage in the preamp circuit along with at the grids I'm getting right around 70VDC (positive or negative, depending where in the circuit), and at the plates & screens I'm getting around 500VDC. Adjusting the bias pot still does nothing to regulate the grid voltage with the tubes out, which I know is incorrect, so I haven't tried putting the tubes in again to measure voltages that way. Here's the schematic, if it helps.

 

Pin 5: -50V

Plate Voltage does not adjust while turning the bias pot (stays right around 490).

The current over each half of the OT primary (2 tubes) floats, it never stabilizes. It mostly floats between 25 and 35 mA, twice is spiked up to 70mA or so for a portion of a second, and it dipped as low as 10 mA. It is not affected by turning the bias pot.

This is different than earlier in the day, when the primary current was stuck at 7.5mA for each half and the pin 5 voltage was -70. I re-replaced everything in the bias circuit except the resistors, which I pulled out and verified and reinserted. I have tried 3 different bias pots, all of which meter from 0 to 50K with no issues (they are 50K pots, the schematic doesn't specify, but that's what was in there when I opened the amp).

 

Think I forgot to ground the board, will get back to you on this.

 

yep, that did the trick. The ground circuit was wired improperly before, and I remembered to move the standby back into it's proper position, but forgot to insert ground back to that side of the board.

 

Bias is responding fantastically. The power circuit seems to be correct throughout the amp. AC signal not so much. Guitar signal isn't making it to the speakers, but the tube noise definitely is. I have a couple of ideas to try before I beg some more advice, though. Thanks much for the time thus far. I'll post again if I get stuck tracing the AC signal through.