Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ed_Chambley on November 25, 2014, 11:38:09 am
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Guys, I have been messing around with the Brown Face Tonestack and of course the BlackFace. I read where builders speak of tone stack loss and the Bassman having a different style.
How does one know bu looking at a schematic how "lossy" the tone stack is. Also, I find the EQ much more responsive in the Black Face and Brown Face amps whereas the Marshall/Bassman tonestack doesn't seem to change the tone as much as the Black Face. Just wondering why this is and if there are some measures that can be taken to make the Bassman type of tonestack to react more like a Black Face Super for instance.
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In playing with the Duncan tone stack calculator, using a TB Fender type tone stack, I noted that the mid resistor seemed to have the most effect on losses. going from 10K to 20K had very little change on the EQ curve, but allowed the curve to rise overall on the chart, meaning it would sound very close to the same, but with lower losses. I used the layout for TMB, but set the mid slider on max for all comparisons
I ended up with 500K TB pots, a 250p Treble cap, .1 bass cap, and .02 Mid (I think that is each cap's purpose). Then the 20k resistor on the bottom of the stack allowed the whole curve to rise. The EQ came out very close to flat with T & B both at minimum setting, and turning either upward leans toward a mid dipped condition. For me, this made the whole range of TB settings useful tone wise.
If I wanted to measure total losses, I'd check resistance from Treble wiper to ground, assuming that the greater the resistance overall, the lower the losses of the stack. Otherwise you'd have to measure the AC impedance from input to ground. The caps would prevent any meaningful DC resistance (to ground) measurement at input.
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> How does one know bu looking at a schematic how "lossy" the tone stack is.
Assume all big caps are short and all small caps are open. What is left? What is its loss? That's roughly the mid-band loss.
This does take some experience and discretion. (And a solid grasp of voltage-dividers.)
> the mid resistor seemed to have the most effect
Right. Using Duncan's notation, first everything is whacked-down by R4/R4, 10K/100K, or about 10;1. This is diddleable with the MID pot. Then in the top-right pot-set, TREB and BASS can be added-back up to nearly the input level.
It is not that simple. C3 R4 and C2 R3 form two poles which interact and cause a notch much deeper than two separate poles could cause. This is perhaps the magic of the Blackface tonestack.
> much more responsive in the Black Face
The Blackface tonestack is a classic. If you need a BIG hit of EQ, don't fool around, steal the Blackface verbatim (including drive and load). If you need a touch of EQ, other plans have less loss and less extreme settings.
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The BF tone stack is after one stage in the amp so it will have more noticeable tonal change than the '59 Bassman tone stack since that is after two stages, a volume control, and a CF. As you move effects later in the amp they have less effect. Fender's BF trem mixes in after several stages also and if you compare that to another roach trem system, the Silvertone 1484 and 1485 which mix that in after oen stage....the trem is much more intense on the Silvertone.
Greg