delta blues 100hz on output

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techoff2
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delta blues 100hz on output

Post by techoff2 » Sun May 29, 2022 11:25 pm

I have 8 volts of 100 Hz ripple at positive end of c39 and c40 and this is being feed onto the plate DC
Have changed both capacitors with no effect.
Ripple is minimal with any el84 removed.
Have checked all wire jumpers and the earths are ok at the bottom of each cap.
The primary of the output transformer reads 96.9 ohms and 127 ohms, not sure whether they should be equal and therefore any ripple should be cancelled out across each winding.
There is no loss of audio output but the 100 Hz causes the sound to warble.
Any help would be appreciated.

Thanks

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JamesPaul
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Re: delta blues 100hz on output

Post by JamesPaul » Sun Sep 04, 2022 7:22 pm

techoff2 wrote:
Sun May 29, 2022 11:25 pm
I have 8 volts of 100 Hz ripple at positive end of c39 and c40 and this is being feed onto the plate DC
Have changed both capacitors with no effect.
Ripple is minimal with any el84 removed.
Have checked all wire jumpers and the earths are ok at the bottom of each cap.
The primary of the output transformer reads 96.9 ohms and 127 ohms, not sure whether they should be equal and therefore any ripple should be cancelled out across each winding.
There is no loss of audio output but the 100 Hz causes the sound to warble.
Any help would be appreciated.

Thanks
Is your power 50Hz? How do the Screen, B++, and B+ look? Do they have the ripple and is their voltage approximately correct?


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j0nsnell
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Re: delta blues 100hz on output

Post by j0nsnell » Wed Dec 28, 2022 5:24 am

I would ensure the EL84s are matched, check the bias voltage is 14volts or a touch more. Ensure pin 9 of all EL84s has HT (screen voltage).
I suspect if those three criteria are met, the hum will stop and the valves will draw less quiescent current.
It is not uncommon, for the coupling capacitors (C32 & 35) to fail, after many years of use/storage.

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