Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mark on May 30, 2024, 11:59:57 pm

Title: Noise in Silver Face MV Twin Reverb amp.
Post by: Mark on May 30, 2024, 11:59:57 pm

I have a 70’s Twin Reverb to fix with the boost switch. The initial problem was the power transformer was blown, that was rewound and the amp worked. I then noticed the tremolo/vibrato oscillator was leaking into the audio path. This was due to a conductive fibreboard, I lifted the components off the board and that came into place.


I then pulled the boost switch to make sure that was okay, there was hum with the master volume on “10”.
I replaced the filter caps as they were the original caps and they needed replacing.


I noticed if I remove the boost wire to the reverb transformer secondary there is no hum, so the wire isn’t picking up hum. If I remove the valve the volume of the hum goes down by about half. If I short the input to the reverb 12AT7 (in circuit) to ground the hum is unchanged. If I lift the earth to the reverb secondary the hum goes. I have tried moving the earth for the secondary around but I still get hum. The AC voltage on the rails of the amp is stellar with about 0.1vac hum present.


The hum isn’t audible if the master volume is below “5”, and most noticeable on “10”.


Any thoughts on this?


I have heard people allude to improving the earthing on these Silver Face amps but I’m not aware of what these changes are. Any thoughts?


Thanks for your assistance.


Mark


https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_100_schem.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_100_schem.pdf)

Title: Re: Noise in Silver Face MV Twin Reverb amp.
Post by: Merlin on May 31, 2024, 06:33:08 am

From your description it sounds like the hum could be magnetic radiation from the power transformer being picked up by the reverb input coil. Does the hum disappear if you short out the reverb transformer secondary?

Quote

The AC voltage on the rails of the amp is stellar with about 0.1vac hum present.

Is that true for the node supplying the reverb driver?

Title: Re: Noise in Silver Face MV Twin Reverb amp.
Post by: Mark on May 31, 2024, 10:47:45 am

Hi Merlin,


 Thanks for your reply to my question. Yes the hum goes when the secondary of the reverb transformer is shorted out. I have a 10 ohm resistor on an RCA plug to simulate the reverb tank, this has no effect on the hum whatsoever.

The ac on the reverb node has a 0.1vac ripple on it.

The hum is reduced when I remove the power to the primary of the reverb transformer.


The transformer is quite a distance from the power transformer but that might not be enough .


Regards


Mark