SD Convertible 100W too midrangey

[sonicmojo]

Aloha,

I've been playing with a Seymour Duncan Convertible (100W) and this amp has a pretty unique overdrive sound but overall the midrange is very high and not very adjustable with the mid control, on Channel 1 or Channel 2. The tone stack is placed right after the first tube module and has Fender type values but really doesn't work like a Fender. I've experimented with different modules and making my own modules also but the inherent "too much mids" remains there at all times. The amp sounds cool for a Brian May type lead sound but I can't get and good cleans or crunch rhythm to happen in this amp so far.

I'm looking to do some small mods to tone down the massive mid spike. I think this problem may have to do with the additional coupling cap (C1) that comes before the tone stack or possibly the 2.2M resistors to ground off the treble caps. There is some information out there and some folks have moved the tone stack to behind all the modules but I really want to try something simple first. Any suggestions to scoop the mids a bit based on what you see on this schematic?

Thanks for any suggestions!

[xtian]

It appears you have two identical tone stacks. Why not change the values in one to match Fender Bassman? Here's a great explainer by Rob Rob:

https://robrobinette.com/How_The_TMB_To ... _Works.htm

[xtian]

I think this problem may have to do with the additional coupling cap (C1) that comes before the tone stack or possibly the 2.2M resistors to ground off the treble caps.

C1 lets all frequencies thru. You can cut bass by reducing C1 to 0.01uF or so.

I'm not positive about the function of the 2M2 resistors. But on Rob's page, he suggests it's a high-pass filter. And plugging in the values gives me 3732Hz.

[sonicmojo]

It appears you have two identical tone stacks. Why not change the values in one to match Fender Bassman? Here's a great explainer by Rob Rob:

https://robrobinette.com/How_The_TMB_To ... _Works.htm

I've been studying things there. I think changing to that style would possibly add more mids. What I am looking for is the mid cut that is prevalent with the Fender stack. I think the coupling cap (C1) and the additional 1M to ground (R3) are doing something here that I don't quite understand. Normally you get high voltage to the tone caps but C1 is preventing that. I am guessing that SD didn't want high voltage running through the tiny wires going to to the front panel pots PCB. Do you think that extra cap prior to the tone stack is affecting the overall sweep of things?

[xtian]

The three caps in the tone stack block DC. C1 should not have any effect on the tone stack. I explained the function of the 2M2 resistor.

Don't you consider Fender Bassman a "Fender" tone stack? Which Fender tone stack do you want to emulate? Showman?

[sonicmojo]

Looking for AB763 Twin Reverb type activity here and the values are virtually identical. I removed the 2M2 resistors and upped the slopes to 150K. The slope change makes the mid pot more responsive but there is something about this amp that is giving it a mid hump still. I'll keep experimenting.

[xtian]

Looking for AB763 Twin Reverb type activity here and the values are virtually identical. I removed the 2M2 resistors and upped the slopes to 150K. The slope change makes the mid pot more responsive but there is something about this amp that is giving it a mid hump still. I'll keep experimenting.

Good job. Have you tried playing thru a different speaker/cabinet?

[sonicmojo]

Yes, It is a combo with a newly reconed EVM 12L. It does sound better to me going into Marshall voiced speakers. The filter caps are 31 years old so I should probably replace them too. It is a weird amp but fun to switch the modules and I've made a bunch of them with various tweaks. Now if I can just get the tone stack where I want it, I'll be happier.

sdmodules.jpg

sd2000.jpg

[sonicmojo]

So I feel a bit of a dope. I never checked the post module preamp tubes that came in this amp. This amp has 3 tubes after the preamp for the variable damping output, power scaling control, and phase inverter. So a lot of stuff happens after the module preamp.

V1 had a 12AU7 instead of 12AX7 and V2 had a 12AX7 instead of 12AU7. V3 (phase inverter) had a 12AU7 instead of a 12AX7 but I am pretty sure this is not correct even though the schematic shows 12AU7. Or maybe it is? The earlier non-model 2000 has a 12AX7 in this position. Anyhow, I am attaching both schematics. They are confusingly notated differently but the circuit is the same. I am assuming the tube order should be 12AX7->12AU7->12AX7 like on the pre-Model 2000 schematic. I have a 1988 Model 2000. Anyhow, the amp sounds a lot better this way and I think the incorrect tubes were the source of my weird mid hump "flappiness". I'm going to leave the slope resistors at 150K for now though and thoroughly check the rest of the amp out.

Conv2000-PowerAmp.pdf

seymourduncan_convertible_100justpower.pdf

EDIT: I just took the chassis out and on my amp, V2 is the first and V1 is the middle tube, seem to be swapped from the earlier model so forget most everything I was saying here except I still think a 12AX7 in the PI position works and sounds a lot better.