Proper way to test a Power Transformer after an amp meltdown?

[Jonny toetags]

So here are my updated findings:
-no voltage at pin 5 of the powertubes ( reads 0.00v)
-pin 2 to 7 show 6.6v, so proper heater voltage.
-I put some tubes in to check the bias with my bias meter and the minute i take the amp off standby, the bias shows 80mA, and quickly keeps going up.
At this point i quickly turn the amp off.

[Jazz P Bass]

"-no voltage at pin 5 of the powertubes ( reads 0.00v)
-pin 5 to 7 show 6.6v, so proper heater voltage."

#1: you do not have any bias voltage on pin #5 (so why do you have the power tubes installed?)
#2: I hope that the heater pins is a misprint: pin #2 & pin #7 are your heaters.

dualshowmanreverbaa270 Bias circuit.pdf

[Jonny toetags]

..yes, #1 was a typo.

i just put the old tubes because i wanted to use the bias meter quickly.
so how would i go about troubleshooting the bias?

I'm checking all the parts related to the bias circuit, i found the 80uF/75V cap that goes from the balance pot to ground as faulty with my Peak meter. I'm not too familiar with the balance pots on these amps, but I'm guessing changing it couldn't hurt.
* i just changed that cap and I'm reading -48V at the cap AND on pin 5 of the powertube sockets

[Chuck H]

"-no voltage at pin 5 of the powertubes ( reads 0.00v)
-pin 5 to 7 show 6.6v, so proper heater voltage."

#1: you do not have any bias voltage on pin #5 (so why do you have the power tubes installed?)
#2: I hope that the heater pins is a misprint: pin #2 & pin #7 are your heaters.

[ATTACH]34826[/ATTACH]

I think he meant 'Measured from pin 2 to pin 7.'

EDIT: I see that post was edited. Never mind.

[Chuck H]

I'm checking all the parts related to the bias circuit, i found the 80uF/75V cap that goes from the balance pot to ground as faulty with my Peak meter. I'm not too familiar with the balance pots on these amps, but I'm guessing changing it couldn't hurt.
* i just changed that cap and I'm reading -48V at the cap AND on pin 5 of the powertube sockets

I would have expected a little more -V from the bias supply. If that is with your current limiter in place then you're probably alright. Otherwise you may have drifted resistor values.

If that IS with the current limiter in place then remove the current limiter and check again. If you get something more like -52 or more ON EVERY POWER TUBE SOCKET PIN #5 I would say to go ahead and plug in some tubes. But don't use your old tubes anymore. If it's a bad tube that caused your failure in the first place then you don't want to plug it back in.

[Jonny toetags]

I would have expected a little more -V from the bias supply. If that is with your current limiter in place then you're probably alright. Otherwise you may have drifted resistor values.

If that IS with the current limiter in place then remove the current limiter and check again. If you get something more like -52 or more ON EVERY POWER TUBE SOCKET PIN #5 I would say to go ahead and plug in some tubes. But don't use your old tubes anymore. If it's a bad tube that caused your failure in the first place then you don't want to plug it back in.

That's without the limiter. I haven't used it at all in today's testing.
I get the same -48.8V reading on every tube socket on pin #5. I will check the resistors. Would it be a good idea to increase them a little while I'm at it instead of just putting in the same value? The two 68K's right?
I have new tubes coming in tomorrow.

[Jazz P Bass]

Seeing that you now have a reasonable bias voltage, it's time for tubes.

-48.8 Vdc is fine for now.

[Chuck H]

I don't know which DSR you have. Some schems show plate voltages in the 420 range and spec a bias voltage of -52. Then I saw the schem Jazz posted. Anyway, Jazz has it. -48.8 is fine. I still wouldn't plug in the old tubes though. Just because they didn't hurt your Super Six doesn't mean thy didn't break your DSR. They're old, bad tubes and one or more may even be failed.

[g1]

In the version Jazz posted, and several other versions, there are 2 bias filter caps, on either side of a resistor that appears to be 3.3K?. Make sure you change them both, and probably a good idea to use 100V caps for these.

[Jonny toetags]

Here's the latest.
-I changed those drifted 68K resistors anyways and now am getting -49.5
-I received a fresh quad of tubes and put them in. I used my bias meter and am getting a bias of 19.51mA.
-I left the amp fully running for about 20min, with the tubes in/bias meter connected and the bias pretty much hovered around that reading the entire time and the fuse didn't blow.

What would be the next step? Should I just plug in and see what happens?
If the OT is also shot, I'm guessing once I introduce a signal (guitar), the fuse should pop, saving the new tubes?

[Chuck H]

Sometimes new tubes still need some burn in time that can cause the bias to drift a little, but the bias voltage should get to a steady state in 20 min. Not "hovering around". You should replace the other bias filter cap before continuing. If you don't, and it's going bad as the one you replaced did, then it's at your own risk.

I guess I can't suggest plugging in then. But it sounds like you're ready to go there otherwise.

Whether a bad OT causes a fuse to blow would depend on the nature and severity of the failure. an OT can be compromised and NOT blow the fuse. In this case it's not likely a tube will be damaged, but that's not a certainty by any stretch. If the voltages on the tube plates look right and there's no excessive hum then the OT is probably fine.

The fuse isn't there to protect the tubes. It's there to prevent a fire. Don't expect any tube protection from the fuse. If you're genuinely concerned about the condition of the OT you can test it as per the link I posted in #9. And FWIW my recent experience is that not all new tubes are good and may suffer a premature failure due to a defect. Hopefully you bought them from a reputable source that offers a warranty. IMHE acquiring new tubes is a real "buyer beware" adventure anymore. I think Ruby has the best rep and I haven't had as much trouble with their product.

[Jonny toetags]

I replaced pretty much all the parts in the bias (including the two 0.1 caps), except for the balance pot.
I'm going to do a few tests on the output transformer as per that link and see how it goes.
If it seem ok, I will plug in and see how she holds up.

[Chuck H]

I replaced pretty much all the parts in the bias (including the two 0.1 caps)

There are no 0.1 caps in the bias supply that I can see. And "pretty much all the parts" doesn't convince me that you replaced the second bias filter cap. The bias supply has two electrolytic (can type) capacitors in reverse polarity (+ terminal to ground). You only mentioned replacing one of those previously. If you didn't replace two caps like this then you still have one old one in there. These caps can go bad with age and fail (as you've noticed ). If one was bad, both should be replaced. Your post above sounds like a pale excuse not to.

EDIT: I just need to reiterate the importance of replacing that second bias filter cap. Look at it this way... It's VERY likely a tube failure caused an over current condition that took out the PT. That tube likely failed because it didn't have bias voltage. There was no bias voltage because that bias supply filter failed. Ergo, that bias supply filter cap failure caused the expensive and time consuming repair in that amp. There are two such caps in there operating in the same circuit. With the same age and ware. If you haven't replaced both of them, I can't even imagine why you wouldn't before using the amp.

[Jonny toetags]

Oh, sorry. Yes, both these caps have been replaced. I put 100uF since I didn't have any 80's on hand.
By the 0.1's I meant the first two on the board. One looked 'cooked/melted' so I changed them both out.

[Jonny toetags]

Regarding the OT, I took some readings from where the OT connects to the B+ to the powertube sockets.
B+ to:
pin1: 150K and constantly rises
pin2: 15M and constantly lowers
pin3: 38 ohm
pin4: .607K
pin5: 150K and constantly rises
pin6: 96 ohm
pin7: 15M and constantly lowers

also...just measured the output jack. It's supposed to be 4ohm, but I'm reading 0,7ohm.