Goofy Voltages on New Tweed Harvard

[ellum68]

I spent all weekend wiring up a Tweed Harvard only to be let down on my very first check. The amp is built using the original Fender layout with the exception of my attempt at an adjustable bias, standby switch, and a twisted heater filament pair with a virtual center tap. Transformers are Mercury on the output and power. I'm getting ~190v on the plates of the 6V6s and around 90 millivolts across my 1-ohm resistor when checking bias current! I'm not sure where to go from here. I'd rather not cook my tubes. Oh, I was around ~350v on my rectifier.

[Jazz P Bass]

If the rectifier is putting out 350 Vdc & then it runs through your OPT center tapped winding to the plates, why are the plates at 190Vdc?

[ellum68]

why are the plates at 190Vdc?

That's what I don't understand. Lord knows I'm not on the level of knowledge as many members of this form. Pin 8 on the rectifier does contain deviation from the original layout. Normally a yellow wire from the PT hits pin 8 along with the center tap of the PT and the B+ for the filter caps. I wanted a standby option of of the amp. So, I used a DPDT swith to do this. Both positions of the switch let line voltage hit the transformer, except one position cuts off the connection to the filter caps and OT center tap.



The center red wire on the switch drops straight down to Pin 8 of the rectifier. The back lug is the center tap for the PT and my B+.



Pin 8 is the one with the single yellow and red wire. The red goes up my standby switch

[ellum68]

I think it's a bias problem. I removed what I assume is 56k bias resistor in the upper left of the board and replaced with a 10k pot and 50k resistor. So, I can swing from 50k-60k to adjust my bias. I tried 50k and 60k with no change on my bias reading. If you look on the layout, directly up from the 12AX7 at the junction of the 220k resistors, you can see a bias voltage spec of -27v. I have POSTIVE 14v. Do I have stinking diode in backwards?! I'm guessing that I'm sucking so much current that it's causing my plate voltage to fall. I would like to think I would at least see SOME reading change with my 10k of swing to play with.

Is it safe to pull out my power tubes and take a few more readings. I know they'll be higher, but is it safe?

[Tom Phillips]

...Do I have stinking diode in backwards?! ...

I think you do. In the first photo you posted it looks backwards for a bias circuit.

[ellum68]

Alright, in the ballpark now. We have GOOD negative voltage on my check point. However, the two power tube sockets are behaving differently from one another. The idle current is around 35ma and one and 19ma on another. These are a set of EH 6V6s that I robbed from my Deluxe Reverb that were supposed to be a matched set. Also, there is a slight hum that goes away when i take a reading on the 19ma tube.

[ellum68]

Bias voltage to each 6v6 are almost identical. If I measure on the OT from the center tap each leg, I get 336-ohm on the 35ma socket and 373-ohm on the 19ma socket.


***********************PROBLEM RESOLVED**************************************
Bad solder joint on my 1-ohm resistor to ground on pin 8. I can not believe that happened as I'm OCD on building things. A little fine tune of the bias when my new power tubes show up and it's a happy ending for me!