New 6G6-B build, blown OT???

[LarryLarry]

I was playing the Bassman at practice last night and after about ½ hour of sounding great it lost all volume except very very faint sound. I tried different power tubes, switched between SS and tube rectifier and no change. I checked most of my voltages on all my tubes and they all look fine, so I’m suspecting a blown OT. Can you recommend a way to test the OT? I don’t have a variac.

I pulled the power tubes and the rectifier tube and discharged my caps and tested the resistance from B+ to each of the plates of the power tube sockets (blue / brown wires, OT still completely wired up) and they read open. Is this an accurate indicator that the OT is dead?

I also lifted the OT CT and test for resistance from plate to plate (blue to brown) and there was none. There should be some, right?

On the bright side, it sounded awesome while it worked and it looks nice! I am waiting for chassis straps and corners though…

[johnnyreece]

Do you get any resistance from the CT to either of the primaries? I'm no expert, but it sure doesn't sound good for the OT...

[ampbldr2]

If you measure resistance from ct to each plate wire and get open is clear indicator that the primaries are shot. Double check just to be sure.

I do not think you can get a measurement from plate to plate wire. You need to measure from CT to each plate wire and should measure ballpark 100 ohms i think.

[LarryLarry]

Ok, I get about 130 ohms from CT to one side and 118 ohms to the other. Seems like that test passes; not sure what to check next...

[ampbldr2]

Post a internal pic of the amp. Do you still get faint sound out of it? Try chop stick to see if there is intermittent connection. Look for a bad solder joint. Its hard to troubleshoot without a scope and signal generator but with a little patience you will find it.

[LarryLarry]

Yes, I do still get faint sound out of it. I have tried 2 sets of power tubes and switched between SS and GZ34 with no change. I finished the amp about 10 days ago and it has been working fine until last night.

This is a 6G6-B with adjustable fixed bias and switchable rectifier.

This probably where 'non-traditional build' bites me in the booty, but here are some gut shots...

[John_P_WI]

Have you tried a different speaker / cable / and checked the jacks?

[LarryLarry]

I have tried a few different speaker cabs and speaker cables. I also disconnected the 16 ohm OT secondary and ran it straight to the existing speaker jack (shorting type). Do you think I should try a new speaker jack?

[John_P_WI]

Sounds like you have ruled out the speaker and cable.

If you are comfortable working with the amp while powered up and with a speaker connected try a disturbance test. Take your dmm clip one probe to the chassis and take the other lead and touch the power tube grid and PI input grid. You should get a pop sound when touching the the grids. This will verify the output section is working. If you do get a pop doing this continue to move towards the input. When the pop diminishes you are in the area of the problem.

[LarryLarry]

Do I set the DMM to VDC?

[sliberty]

In situations like this, I would want to know where in the amp the sound nosedives. I would use either an audio probe like the one Weber describes in his books, or a scope, and I would begin probing at the input jack. As I move deeper into the amp (V1, V2, etc.) the sound should get progressively louder (assuming that your volume pot is turned up somewhat). One exception is immediately after the tone stack, where it will experience some loss. Once I reach a place in the amp that is dramatically quieter, I will know that I just passed the problem. If this happens before the speaker out, your transformer is OK.

[Don't use a Weber probe after the phase inverter or you may blow up whatever amp the probe is feeding]

[LarryLarry]

I have neither an audio probe or a scope...

[sliberty]

The Weber probe is a capacitor taped to a chop stick, and wired to an alligator clip (ground) and a 1/4 jack (tip). You can make it easily.

[John_P_WI]

IF you are probing the grids you can us VAC, it is just to "disturb" the grid and make a pop. IF you have a signal generator, ipod or other and can inject a signal at the front end, you can measure AFTER the coupling caps at each stage the signal voltage, VAC.

[sliberty]

Probe info:

http://www.diystompboxes.com/pedals/debug.html

Ignore the note about the acap being 16v to 600v. It MUST be 600V since you might be probing at some high voltage points in the amp.