Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bruno on November 27, 2011, 04:28:05 am

Title: Any other ways around a bias tremolo with no depth, other than decreasing bias?
Post by: bruno on November 27, 2011, 04:28:05 am

I've tried changing the horizontal resistor to 150k, added a 22uf cap across the 4.7k resistor, changed the left 0.1uf cap to a 0.047uf. These did help.

I also biased cold, which made the tremolo sound awesome, with lots of depth, I like the amp's overall sound even with these bias levels, however the owner, claims the amp sounds sterile. I told him, it's a compromise, for the tremolo to work properly.

Is there any other way around this, in order to raise the bias current and still get the tremolo to have enough depth?

thanks

Title: Re: Any other ways around a bias tremolo with no depth, other than decreasing bias?
Post by: stratele52 on November 27, 2011, 05:00:54 am

Hi Bruno , what is the amp ? Schematic

Horizontal resistor ????

Title: Re: Any other ways around a bias tremolo with no depth, other than decreasing bias?
Post by: jjasilli on November 27, 2011, 05:15:16 am

You may need a larger value for the depth pot.  This will prevent too much trem voltage from bleeding to ground.  Restore proper bias.  Then try putting a pot wired as a variable resistor, or a resistor substitution box, between the depth pot & ground.  Of course during this procedure the tremolo depth cannot be dialed fully OFF.  Increase the resistance until you get sufficient trem depth.  Then replace with a suitable value pot.  E.g., if a 500K resistor works under a 250K pot, but 1M is too strong, then you need a 750K pot.  Use a 1M pot with a fixed resistor across it in parallel to get 750K.  

Then test for redplating.  It might be good to use a dummy load.  Run a strong signal through the amp with the tremolo at various settings.  This is to make sure sure that the bias voltage swing caused by the trem + signal voltage swing does not cause the power tubes to redplate.

Title: Re: Any other ways around a bias tremolo with no depth, other than decreasing bias?
Post by: sluckey on November 27, 2011, 06:11:52 am

Quote

I told him, it's a compromise, for the tremolo to work properly.

Is there any other way around this, in order to raise the bias current and still get the tremolo to have enough depth?

Sounds like you've done all the right things. Replacing the depth pot with a 500K pot 'may' give you a bit more trem. As you say, bias vary trem is a compromise.

Fender has an easy fix for this. Convert to the roach trem circuit.   :icon_biggrin:

Title: Re: Any other ways around a bias tremolo with no depth, other than decreasing bias?
Post by: tubeswell on November 28, 2011, 12:08:30 am

Aside from cooler bias, the stuff you can do to increase the trem output voltage swing:

a) change the voltage divider ratio at the trem output,

b) (for bias-vary trem) put a buffer stage between the LFO stage and the output tube grids. I have successfully used an IRF820 source follower for this in a number of amps now where I didn't have a spare triode.