Anybody mod their VC to tweed specs?

[steel1953]

I have a SFVC that's okay, but not particulary inspiring. I read in my Kendrick book about modding it to tweed specs ( or at least closer ). Seems easy enough. I just wonder if anyone has already done it, and if they liked the results? If so, what kind of mic do you use?

Brian

[bob p]

I have a SFVC that's okay, but not particulary inspiring. I read in my Kendrick book about modding it to tweed specs ( or at least closer ). Seems easy enough. I just wonder if anyone has already done it, and if they liked the results? If so, what kind of mic do you use?

Brian
As you probably know, there's a huge amount of gain that gets lost when you replace the coupling cap in the Tweed Champ circuit with the FMV tonestack in the Vibrochamp. Its easy enough to put a switch in there to select between the two configurations if that's what you're after.

If you look at the schems there are some other differences related to bias and feedback.

IMO, making those changes isn't enough. My SFVC has a really high B+ around 410 volts. That B+ really makes the SFVC sound different than the Tweed Champ, where the B+ was about 360 or so. Maybe you could try using Zener diodes at the PT center tap to bring the Hi Voltage secondary down to tweed specs to "brown out" the tone. Since this is a pure Class A amp, you should be able to reach the same result with an appropriate power resistor on the center tap.

[MWJB]

For harp I wouldn't necessarily drop the B+. I would however rebias with an 800-1K cathode resistor.

If you convert directly to 5F1 spec your champ will feedback more and put out less volume, but if you add an additional cap to ground from the RH tab of the volume control (.say .01 or .02uf) this will bleed some of the feedback inducing highs to ground and make the amp more manageable. Experiment with removing the V1 pin 3 cathode bypass cap also, may be preferable/maybe not?

[bob p]

you're right -- i don't have the experience with harp to say whether or not the tweedifying mods would sound good in that application. i was just offering tech notes on how to effect the mod.