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Discussion in 'Technical Side of Things' started by Free, Jun 11, 2021.
This is the Bass channel treble pot
This is the normal channel pots
Got a couple questions on rewiring the amp.
1. How do I rewire the first two filter caps so they are not in series any longer?
I want to make them like the original 6g6B schematic which looks like 2x 20u 600 (I’ll be using 22u 500 to match my other caps) wired in parallel. Right now they are in series with one reversed and from what I understand it’s supposed to have to 270 K resistors but it looks like mine doesn’t have those although there are two pieces of foam on the Board that I suppose could be covering them up.
2. when I bias, if I don’t do the adjustable bias mod, does a 27k resistor make the bias hotter or colder than 32k? I need to order resistors to test what I need for the bias, what range of resistors should I pickup? maybe a 25k, 27k, 29k, 32k? Or do I need more in between resistors
3. The normal Chanel bass and treble pots are wired differently, I’m wondering if this changes the tone. Pic below
I was looking at the bias mod you did for the Bassman. You have a 15k res. and a 10k pot. My Bassman is currently biased with a 32k resistor. Would I be correct in replacing the 15k in you mod with a 24k so that I can get up to 34k? I still am not sure if a larger resistor makes a hotter bias or the other way around, I've read multiple posts that have seemed to say it both ways. Thanks
I think that you talk about this, @Free :
My mod is based on the required bias voltage that needed the 6G6-B of my friend. For your amp, I can't really tell you, in fact...
With the new power transformer (export model), I determined that the suitable bias voltage was circa -47VDC. I made a calculation with a regular B10K bias pot and centered on -47V to have a range of +/-10V IIRC, and I found those R values.
Your 32K resistor has certainly drifted from 27K original value (check the marking and the value disconnected).
Personally, I would stick with the values that I used, since the adjustment range is large enough in plus or minus the nominal value that I found, that is to say -37 to -57V, since it can reach the original -54V.
But of course, you have to check by measurements to confirm if it is correct for your amp ! Since it is quite widely adjustable, it should work, but... Your amp isn't on my bench, right ?
You also may search and experiment with other values, since your amp may present a different operating point at the power stage than my friend's 6G6-B : there can be a significative variation from yours to my friend's !
I repost below the complete schematic of the 6G6-B of my pal Clément, where you will find all voltages to compare with yours :
Well, you'll need to measure and tweak a bit!