Duncan Tone Stack Calculator

[Diablo]

Can anyone tell me what the Z-values and loads should be for a Fender 5F6A (Tweed Bassman), and the 5E8A (Tweed Twin)? I'd like to model those tone stacks using the TSC. I played around with it last night and found different values of Z make a huge difference in the frequency response curve. Maybe it doesn't matter much, but I use a 12AX7 for the cathode follower on the 5F6a, and a 12AY7 for the cathode follower on the 5E8A.

Also, while playing with the TSC on an E-series stack (5E8A), I found that decreasing the treble pot value creates a midrange scoop. So, I was thinking about putting a resistor (120K) across the treble pot (on a switch) in order to go from Tweed to Blackface. Do you think this will work?

http://www.schematicheaven.com/fende...5e8a_schem.pdf

http://www.schematicheaven.com/fende...5f6a_schem.pdf

[Albert Kreuzer]

Zsrc is the same as in the Marshall tone stack with cathode follower: 1.3k.
Output load is roughly 1M (PI input resistor).
Your treble resistor trick will work somewhat. Won't be a blackface, but a step in that direction.

HTH,
Albert

[Diablo]

Thanks Albert for the info. I tried the resistor across the treble pot on a switch (the unused ground switch), but all I got out of it was hum and no guitar signal. I could understand a little hum, but not having any guitar come through the speakers had me scratching my head. Perhaps it was the long run of wires from the pot to the switch picking up AC. Anyway, I desoldered my mod and now it makes guitar noise again...

[Albert Kreuzer]

Yes, the long wires account for the hum. You need shielded wires here (connect the shield to ground on one side only).
No signal - maybe you miswired something? Temporarily solder the resistor directly to the pot and listen if it works. Or make that a push-pull pot.

Cheers,
Albert

[Diablo]

Thanks Albert. I was thinking to try the same thing, and solder the resistor directly to the pot terminals. All I know for sure is that the 5E8A tone stack is fairly nutty in comparison to the 5F6A, when I look at the frequency response curves with the TSC. What's really whacky is when you crank the bass up from 10 to 12 on the dial, the treble drops by 10-20dB. That's just the way my amp sounds too.

[Albert Kreuzer]

Talk about interactive

And watch the response of the bass control between 0 and 1%

[Diablo]

Well, I went back into the amp and did it the right way this time with grounded cables between the treble pot and the ground switch.....and it works. No hum, and it definately changes the voicing of the amp and the operation of the tone stack - it's got midrange scoop now. I used a 150K resistor. Flipping in the resistor does knock some volume off the amp, but that's not a problem for me. I can flip the switch in and out while powered up and there's no unwanted pops either. I'm starting to feel like Dumble, I could use a few more switches on my amp.....
Thanks Albert.

[dad love mom]

Well, I went back into the amp and did it the right way this time with grounded cables between the treble pot and the ground switch.....and it works. No hum, and it definately changes the voicing of the amp and the operation of the tone stack - it's got midrange scoop now. I used a 150K resistor. Flipping in the resistor does knock some volume off the amp, but that's not a problem for me. I can flip the switch in and out while powered up and there's no unwanted pops either. I'm starting to feel like Dumble, I could use a few more switches on my amp.....
Thanks Albert.

Diablo - Hello! I'm a newbie here. I signed up based on your post here. I, too, have a 5e8-a circuit. I am looking to get more scoop from the midrange as you once did. Could you tell me if this 150K resistor ended up working out for the duration of your ownership with the amp?

Thanks!