twin reverb bias question

[davebolden44]

maybe you guys can tell me if I am doing something wrong here. I have a 71 silverface twin reverb here that belongs to a friend and I was just going over it to get it ready for a sale. I converted it to ab763 specs about 2 years ago and it has always been a great sounding amplifier. However, when I measure the plate voltage I only get about 330 volts. I am using the shunt method, half the ot, ct to pin 3- .030 amps. From everything else I see on the web people are getting over 400 volts. Funny thing is the damn thing sounds great. Just wondering if there is something about biasing that I am not quite understanding?

[tubeswell]

SF AA769 or a AA270 amps are fairly cold-biased in the stock configuration. The tapped 10k pot they employ is a bias balance control rather than a bias range control. When you hot them up to AB763 bias, it does drag the plate voltage down a bit. But 330 sounds low. Does the amp have an international voltage PT installed?

[davebolden44]

No sir, all transformers are stock and the usuall you would find in a U.S. model

[davebolden44]

I checked the screen voltage and it is about 336 dc, the plates measure the same. I measured across the secondary before rectification and got around 624 v ac which I think is low and the culprit. The schematic for a 763 says 340 0 340. Part number is 022756 and classictones diagram for that part designation states 640 v total sooo 320 0 320?. I think its low nonetheless. Am I missing something or perhaps misinformed?

[sluckey]

Pull the output tubes. How much plate voltage do you have now?

[martin manning]

The AC at the secondary is about right, but you should have a bit over 400V on the plates. Have you replaced the diodes and reservoir capacitors? What happens to the voltage if you remove the power tubes?

[davebolden44]

without power tubes the plate voltage is 442 dcv. never changed the diodes but all the filter caps were changed. I cant remember if I did the work or not but all the PS caps are the cheapest money can buy. I ordered some nos mil spec russian tubes the other day, was hoping to put them in my dumble but it looks like they are going in here. my lafayette tube checker said these were full of life.

[martin manning]

The 442V doesn't sound bad. I'd try disconnecting the HV secondary and checking each diode string to rule out an open diode. If the 30mA per tube you measured is accurate the voltage drop does seem excessive.

[davebolden44]

ok Martin, I have disconnected the secondary, I get continuity on each individual diode, but not across the 3 that makes the string of 3 on either side, shouldnt I?

[martin manning]

If you are just using the continuity check function the voltage drop may be too big. Getting continuity across each one sounds good, though. If you reverse the leads you should get "open." Assuming about 0.6V per junction, a diode check across all three (if you meter has that) should show about 1.8V.

[davebolden44]

Yeah sorry continuity was just the wrong word. the diode check function on my meter reads between .575 to .6 on all diodes. do you think that this is more than a tube issue?

[davebolden44]

1.66 across each string. Just want to bend your ear while I have it, I have never had the luxury of a competent person looking over my shoulder. Am I understanding this correctly, meter hooked up in mA mode from CT to pin 3 is half the total current draw? 30 Ma is the total for both tubes on that side of the OT meaning roughly 15 Ma per tube? at least thats what I am understanding from reading the trainwreck pages and TUT. I am aside from this forum and youtube self taught from books so I tend to second guess myself.

[martin manning]

The large voltage drop is an indication of high current draw somewhere. Without tubes you are close to the schematic, with them you are 70V low. You said you measured 30 mA using the shunt method, which would mean 30 mA on that side, which seems low, and might not be accurate. What about the other side? Is it similar? Another way to measure bias current is to measure the DC resistance of each side of the primary with power off, and then measure the voltage drop across each side with power on, and calculate current.

[davebolden44]

well these tubes never were matched, in their current configuration i am getting 31 ma/ 39 ma

[davebolden44]

well I think I am getting on the right track, maybe you can help my confusion though. I am probing under the doghouse for voltages and I get 0 volts on one of the 70 uf filters. the first one. I assume the voltage is going through the 220 k bleeder resistor to the second filter and this thing is limping on half a leg