Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Effects => Topic started by: luthierwnc on May 03, 2023, 12:52:48 pm

Title: tweaks to tube-driven effect
Post by: luthierwnc on May 03, 2023, 12:52:48 pm

Hi everyone.  I completed the enclosed project in a discarded Dumbleator chassis a year ago and am finally getting around to building the pedalboard for a '56 Bassman variant.  It is based on an assortment of schematics but isn't very complicated.  The plan was for this to drive a Fairfield Barbershop on the pedalboard but is too dark -- even with the tone control dimed.  That and the volume is way too hot.  I'll put a 56k resistor across the lugs to tame that down.  Otherwise it is quiet and does what I hoped.  The big box sits on the amp and the pedal is controlled with the 6v supply.

The preliminary plan is to knock the coupling cap on the second stage down to .0047 or even .0022.  My question is that the design I borrowed has 100k and 220k supply resistors to the plates but 1k5 cathode resistors on both.  On the first half of the tube that's a pleasing 1:66 ratio.  Not so much on the second.  Is there a better K resistor I should try?  I'm not married to the hi and lo cuts either.  A clean boost sounds a lot better after the Barbershop so pulling this from the line-up won't be traumatic.  The gizmo does sound really good with a much brighter amp so it will find a home.

Thanks for looking, Skip

Title: Re: tweaks to tube-driven effect
Post by: luthierwnc on May 05, 2023, 10:43:37 am

More information:  The second stage is really overloading.  On the scope I've got unity output between on and off on the volume pot at 3.5k to ground.  With all knobs at noon the same sized waveform is .1 volts bypassed and .5v engaged.  It's fairly distorted too.
I changed the second coupler to .01 but might go lower after taming the volume.  The plan is to put a voltage divider after R6 with in the neighborhood of 150k to ground -- might use a trimmer.  Spending more time looking at the back half I'm not sure I get the tone circuit.  The tone pot is basically a volume pot loading another volume pot with virtually no effect on frequencies.  Something more like the attached would be cleaner. 

Any ideas would be great.  Skip