Vibro Champ Rebuild - Plate voltage high

[jescher356]

Tom,
Sorry for the delay, I had some other business to attend to...

I just disconnected the feedback wire and it got quieter. The squeal is gone. So it sounds as though the transformer is wired backwards. I will rewire and re-measure voltage and get back with you. Sounds like we are making progress.

Tom,
I just reversed the OT yellow and black wire and it sounds great, and what do you know the voltage is now correct!

Thanks you so much I never would have thought to check that because of the fender diagram. Why did it affect the voltage (out of curiosity?)

[Enzo]

Possibly the amp was oscillating at very high frequency and the elevated voltages were due to the huge signal you couldn;t hear. Just a notion.

[Tom Phillips]

I'm glad the problem is solved. This is a good lesson to troubleshoot based on the clues observed. So often we miss things because we believe "it couldn't be that because..." (I already changed the part OR I know it's wired correctly per the schematic OR ...)

As to the high voltage readings you were measuring it may have been a misinterpretation by your meter. As Enzo suggested, the circuit could have been oscillating at a high frequency due to the positive feedback. Your meter may have been "confused" by the presence of a large amplitude high frequency signal on top of the DC voltage. It is possible that the AC voltage could be rectified inside your meter and added to the DC voltage being measured, thus changing the accuracy of the reading. What model meter were you using?

Again, I'm glad the amp is now sounding good. Per the photos, it looks like you did a nice rebuild job.

Cheers,
Tom

[jescher356]

I'm glad the problem is solved. This is a good lesson to troubleshoot based on the clues observed. So often we miss things because we believe "it couldn't be that because..." (I already changed the part OR I know it's wired correctly per the schematic OR ...)

As to the high voltage readings you were measuring it may have been a misinterpretation by your meter. As Enzo suggested, the circuit could have been oscillating at a high frequency due to the positive feedback. Your meter may have been "confused" by the presence of a large amplitude high frequency signal on top of the DC voltage. It is possible that the AC voltage could be rectified inside your meter and added to the DC voltage being measured, thus changing the accuracy of the reading. What model meter were you using?

Again, I'm glad the amp is now sounding good. Per the photos, it looks like you did a nice rebuild job.

Cheers,
Tom

Thanks again for your help Tom.

The meter is a Radio Shack unit, one of their better digital meters back 10-15 years ago. If you think there is a better unit out there that I should have, let me know...

Also, one more question; the bass response is almost not noticeable when you turn the knob. Would I increase the value of the .1 cap or reduce the value of the .047 cap to increase the effect?

Justin

[Tom Phillips]

…The meter is a Radio Shack unit, one of their better digital meters back 10-15 years ago. If you think there is a better unit out there that I should have, let me know...

There is always something better. My comment about the meter is just a theory so I’m not suggesting that you actually need a better unit. Meters can easily be a whole new discussion topic and you can spend really big bucks on a high end meter if you want to.

…Also, one more question; the bass response is almost not noticeable when you turn the knob. Would I increase the value of the .1 cap or reduce the value of the .047 cap to increase the effect?

To increase the effect I would lower the value of the .047 cap. However, more than one characteristic will be affected. In this case, the mid notch will move up in frequency. I suggest you download a copy of the tone stack calculator available at http://www.duncanamps.com/tsc/ . Using it you will be able to see the effect of different values of each component in the tone stack. Then you can experiment with different values in your amp and hear the result for yourself. Great fun!

Regards,
Tom

[jescher356]

Thanks for the info... I replaced the .047 with a .022 and it sounds really good.
Thanks for your help on this one. FYI, I am going to post another question about a second Vibro Champ that I home built form parts and leftovers from the amp I restored, just letting you know so you don't think it's the same one.

Thanks again for your help.