express died

[garycs]

Hi:

I finished my Express build and adjusted the bias voltage to 32v, fired it up and it sounded great. The best sound I have heard in a long time. I decided that I would adjust the bias by putting my meter in parrallel with half of the OT so I connected one side to the OT center tap and the other to pin 3 of an EL34. To bring the ma down to the lower 40s I had to run the bias voltage up. It seemed OK so I plugged in my guitar. As I was thinking about why I had to run the pot to one end there was a small pop and then the very low hiss in the speakers died. I checked the bias voltage and found that it was about 50v. Did I toast the OT? Any thoughts will be appreciated as to where to start.

Thanks:

Gary

[Firestorm]

Just to be clear -- when you say the bias voltage was 32V you DO mean -32V don't you? If you had 395V on the output tube plates, -32V bias should have you well down into the 40ma range to start with.

What power transformer? Got any pics?

[garycs]

Yes, you are correct it was -32v. I am using toneslut trannies. I played a gig with it and then thought I should probably set the bias correctly by measuring the current at the tube. When I first measured across one side of the OT it was above 50ma. To bring it down to 40ma I brought the bias voltage to -50. I did not directly measure the bias voltage until after things went south.

[Firestorm]

Nothing you've described should be able to kill an OT or even a tube. 50ma is far from ideal at idle, but still shouldn't even be able to redplate an EL34. When you were shunting the OT, did you accidentally short something? You obviously still have power if you can read -50 bias volts. What's the B+? Pictures would help.

[Bob-I]

I agree, you didn't do anything that would blow an xformer.

While you're in there, get some 1 ohm 1% 2 W resistors and install them from cathode to ground. Measure mv across these for mv through the tube.

Good luck with the troubleshooting.

[garycs]

I turned the amp on again today and it now has sound, but it is poor. The voltages are as follows.

403v before the 1K resistor
392v after the resistor
294v B3
277v B4
263v B5
32v Bias

Could it be a bad cap?

Thanks for all of the help.
Gary

[Allynmey]

Gary, do a voltage chart...at the tube socket by pin. It looks like you are taking voltages from the dropping string and not the socket.

[jelle]

What resistances do you find from the CT to the plates of the OT primaries? I had a partially shorted OT that sounded just like you described. The amp, a bandmaster BF, was mistreated and the shielding mesh of the cabinet had touched the poweramp.....

Jelle

[garycs]

Hi:

Thanks for the responses.
THe OT has about 47 ohms between the CT and the legs while still soldered in.
The pin voltages are as follows.

EL34
pin 3 400
pin 6 390

PI
pin 1 189
pin 6 200

B4
260

B5
pin 1 204
pin 6 182

If I should measure differently please let me know. I am defininitely a newb.
Thanks for all of your help.

Gary

[Phil_S]

What resistances do you find from the CT to the plates of the OT primaries? I had a partially shorted OT that sounded just like you described. The amp, a bandmaster BF, was mistreated and the shielding mesh of the cabinet had touched the poweramp.....
Jelle

+1 on that. While what you say you did should not have caused a problem, there is no guarantee you observed the precipitating event. Pop and poor sound is reason to check your OT. Pull the tubes and check for ohms between plate and CT. If not about equal, then lift all three wires (desolder) and check again to be sure. If not equal, you have a problem with your OT and it needs to be replaced.

Checking bias by the shunt method is problematic for the inexperienced (me among them). If you must, determine the ohms on each half of the winding prior to installation, then read the voltage drop between the CT and the plate. Use Ohm's law to solve for current. I make a chart in Excel to help with instant gratification.

Surely, though, it is more than adequate to use the 1 ohm resistor method. If you want to be a stickler about it, subtract the assumed mA for the screen grids to get the plate current.

[garycs]

I removed the tubes and measured both legs of the OT. They were 42 and 47 ohms. Do the voltages that I previously measured seem reasonable? The amp is not noisy or unstable but it does not have the volume that it did before things changed. At one point cranking up the treble increased the volume quite a bit. I cannot repeat that event. It seems as though something may be changing as it heats up. Any ideas will sure be appreciated.

Thanks:

Gary