63 Pro OT HELP!

[dmartn149]

Okay, I got my new output transformer, but before I put it in, I wanted to make sure that everything else is okay so I don't ruin it too. The folks on this forum who have been helping me with this amp may remember that the amp was working until I changed all the e-caps. Then the output was real low and distorted. I checked the resistance of the OT and found that the resistance between one end of the primary and the center was only 40ohms. On further checking, I found that the lead supplying bias voltage to the tubes had come loose when I changed it's cap. I am assuming that is what ruined the OT. BTW it looks like it's out of an Ampeg.

So now I am getting 47.2v on pin 5, both tubes
498v on pin 3 of the tube with low resistance through the OT
493v on pin 3 of the other tube
current from pin 3 to the center OT lead is .117A on the "bad" side
and .084A on the other.

So, can I go ahead and install the new OT? Do these measurements sound reasonable? or is there anything else I should check?

Thanks, Dan

[Wilder Amplification]

Okay, I got my new output transformer, but before I put it in, I wanted to make sure that everything else is okay so I don't ruin it too. The folks on this forum who have been helping me with this amp may remember that the amp was working until I changed all the e-caps. Then the output was real low and distorted. I checked the resistance of the OT and found that the resistance between one end of the primary and the center was only 40ohms. On further checking, I found that the lead supplying bias voltage to the tubes had come loose when I changed it's cap. I am assuming that is what ruined the OT. BTW it looks like it's out of an Ampeg.

So now I am getting 47.2v on pin 5, both tubes
498v on pin 3 of the tube with low resistance through the OT
493v on pin 3 of the other tube
current from pin 3 to the center OT lead is .117A on the "bad" side
and .084A on the other.

So, can I go ahead and install the new OT? Do these measurements sound reasonable? or is there anything else I should check?

Thanks, Dan

Your current readings are WAY hot for 2 x 6L6s @ 495V. Maximum you should see would be about 42mA for a 70% dissipation @ 495V.

Also, -47.2 is a bit low. For 6L6s running that hot I'd expect to see somewhere in the neighborhood of about -55V @ pin 5 give or take.

Another thing too...there seems to be quite a bit of DC offset (i.e. idle imbalance) between the two sides. Should be seeing idle DC mA per side within about 2-5mA offset. Are those tubes matched?

Honestly I'd fix these issues before installing that new OT. Who knows? This could be your whole issue and you may not even need the new OT.

One question I have for you is on your 40 ohm measurement...are you seeing roughly 40 ohms from center tap to both plate wires? Or is one plate wire reading vastly different than the other relative to the CT?

[MWJB]

I'd want to see more like -55 to -60vdc at pin 5 of the power tubes before installing the tubes & fine tuning the bias. Have you added a bias pot (if not, I'm a bit concerned as to how you are seeing -47v)?

If the old OT has some shorted turns then your plate currents won't match up.

[dmartn149]

Thanks for the replies. The resistance to the center of the OT is 130ohms on one side and 40 on the other. Thats why i figured it was bad.
Should I replace the resistor on the bias/rect. board with one of these?http://cgi.ebay.com/Fender-Amp-10K-l...item2c53b26227
A higher neg bias voltage will bring the current down right?
Please forgive me if I ask a stupid question. I am learning while trying to fix this amp.
(I know about the lethal voltages, caps etc)
Here is my schem.
http://www.ampwares.com/schematics/pro_6g5.pdf
Thanks again for all your help

[dmartn149]

Okay those links don't seem to be working. It's just a Fender 10k linear bias pot.

[Wilder Amplification]

Thanks for the replies. The resistance to the center of the OT is 130ohms on one side and 40 on the other. Thats why i figured it was bad.
Should I replace the resistor on the bias/rect. board with one of these?http://cgi.ebay.com/Fender-Amp-10K-l...item2c53b26227
A higher neg bias voltage will bring the current down right?
Please forgive me if I ask a stupid question. I am learning while trying to fix this amp.
(I know about the lethal voltages, caps etc)
Here is my schem.
http://www.ampwares.com/schematics/pro_6g5.pdf
Thanks again for all your help

Yes, more negative voltage will drop the plate current. Increasing the resistor value on the back of the bias pot will push your adjustment range more negative and vice versa.

Also, if your DC resistance readings are that far offset from each other I'd say it's definitely bad.

[dmartn149]

Okay, so the 10k pot replaces the 10k resistor that is between the negative ends of the 8uf 150v caps on the bias/recto. board? The negative end of the diode and first cap go to an outside tab on the pot, the wiper goes to the other cap,the 56k resistor and the lead to the main board, and the other end of the pot goes through a resistor to ground. Does that sound right?
Thanks

[dmartn149]

[Wilder Amplification]

Actually, most Fender circuits with a 10K bias pot only use one filter cap (25uF) in the bias supply. They usually have a 470 ohm resistor upstream of the bias rectifier, filter cap after the rectifier, outside leg of 10K pot, other outside leg of 10K pot goes to a 27K resistor, other end of 27K goes to ground. Then the wiper connects to the junction of the 220K grid load resistors at the power tubes.

This is assuming that your bias supply is coming off of the 70 volt bias tap.

I suppose you could wire up the 10K pot in place of the 10K resistor, but the wiper would go to the junction of the 220K grid load resistors while the outer legs would wire up between the two filter caps in your circuit. This may provide too much of an adjustment range and make the pot really sensitive though, which wouldn't be a good idea.

This is the layout from a AA165 Pro Reverb schematic -

[dmartn149]

Thanks Jon. All of the diagrams I've found on the internet have been pretty much how you describe, but I really don't want to rewire my bias board. Is there any reason I can't just replace the resistor with a pot? Also, I just noticed that on my amp I have a 27k resistor where the schem. shows a 10k. Changed by a previous owner to run the tubes hotter?

[Wilder Amplification]

Thanks Jon. All of the diagrams I've found on the internet have been pretty much how you describe, but I really don't want to rewire my bias board. Is there any reason I can't just replace the resistor with a pot? Also, I just noticed that on my amp I have a 27k resistor where the schem. shows a 10k. Changed by a previous owner to run the tubes hotter?

That would definitely decrease negative voltage/increase plate current.

If you do replace that resistor with the pot, check the bias range from one extreme to the other and make sure you have a range of no more than 10 volts from one extreme to the other. I'd say about -50 to -60 should be what you're after.

[MWJB]

The best way to achieve adjustable fixed bias with this circuit is to replace the 56K bias circuit load resistor (to ground) with a 39K-47K fixed resistor (grounded), wired in series with a 50K cermet trim pot wired as a variable resistor (using the wiper & one other terminal).

You could also use a 10K pot in place of the 10K fixed dropping resistor, again wired as a varaible resistor. A small, single turn PC style pot like Doug Hoffman sells will fit nicely in the existing eyelets.

A 10K pot in the load side of the circuit won't present you with a useable sweep of adjustment.

Brush up on the function of "voltage dividers", you probably have around -68v coming off the bias diode, you need a divider that gets that down to -55 to -60, like Jon says.

I'm also surprised that a '63 pro still has the bias circuit with the 2x8uf caps, usually these have a 1K resistor off the bias tap, followed by the diode, then a 27K to ground, one electrolytic cap?

[dmartn149]

I think it probably is a 62. I thought at first it was a 63, now I am just used to calling it that. Anyway, it has the bias arrangement of the 6g5 schem. but it has an extension speaker jack like the 6g5A

[dmartn149]

I went ahead and installed a 10k fixed resistor on the bias board, and now I'm getting about -62v. that brought the current down to .032a on one side and .048a on the other. The .048a is on the side with the low resistance on the OT. Voltage (B+?) is now 518.

Safe to change the OT now?

[MWJB]

"Safe to change the OT now?" Yes, 40ohms is way low for a Pro OT primary. Even a BF twin is around 70ohms...& I'd expect twice that for a Pro/Bandmaster etc.