Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on February 04, 2019, 02:35:38 pm

Title: Hoffman AB763 SR tremolo thump
Post by: mresistor on February 04, 2019, 02:35:38 pm

I have a Hoffman AB763 board in my Super Reverb..    I have the red led mod installed, and 150K resistor instead of 470K and replaced one of the .1uf caps with a .047uf I think its C31 - it's the one closer to the led.


The trem is much stronger now, before it was weak.  But after about 3 1/2 - 4 on the intensity dial I am getting some fairly audible thumping. 


Would the diode across the intensity pot reduce this thumping? Any other ways to reduce it?   


It's biased around 65%.

Title: Re: Hoffman AB763 SR tremolo thump
Post by: sluckey on February 04, 2019, 02:59:29 pm

The diode across the INT pot is supposed to address the thumping.

Title: Re: Hoffman AB763 SR tremolo thump
Post by: Joel on February 05, 2019, 04:33:17 am

I had exactly this problem.

The diode across the Intensity pot did not help.

I found my solution here: http://el34world.com/Forum/index.php?topic=23529.msg253086#msg253086

Essentially, lower the Trem oscillator voltage to 300V.  It's a little bit weaker, but there is no tick/thump.

Title: Re: Hoffman AB763 SR tremolo thump
Post by: mresistor on February 05, 2019, 09:03:21 am

Thanks guys..     I put a brand new JJ 803S in the trem and that helped but still some audible thumping. I jumpered in the diode across the int pot and the trem is now very quiet, deep and lush.
So I can say the diode mod worked in my case. My V5 plates are 425v.



Joel - how did you go about lowering the plate voltage to V5?