Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Effects => Topic started by: Jay Omega on January 30, 2009, 09:35:56 pm
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I was looking into building a Torevibe type unit. Looking at the vibrochamp and the torevibe, it seems as if there is some unnecessary filtering going on with the bias modulated gain stage. It makes sense why the attenuator is there, but I can't make sense of the cascading high pass filters. Anybody have an explanation? Here's a link to the schematic: http://www.triple-t.no/torevibe/images/torevibe-schem.jpg. I think the oscillator will work as it is in the champ, there just needs to be a stage that passes audio to be modulated. (This is the stage that has the unnecessary filters on the output) This is then followed by a cathode follower to lower the output impedance for the amplifier input.
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> unnecessary filtering going on
No. There's ~~100V of 8Hz wobble at V4A plate. WAY more LF wobble than signal. If that got into the main amp input, it would go nuts.
C13-C15 R18-R20 form a 3-pole ~~70Hz high-pass, so guitar gets through but 8Hz wobble is seriously whacked.
> Looking at the vibrochamp
VibroChamp has 0.02u-220K hi-pass at ~~40Hz, 25u cathode-cap about 30Hz, and that dinky little OT which doesn't pass an honest 80Hz. This is still not as good as the Torevibe... the Champ is a cheap box, the Torevibe is deluxe stuff.
It's $4 of parts. Put them in. Bypass them. See which you like better.
The really-serious way to do tremolo is with a push-pull stage. You can rig it so most of the raw wobble doesn't get out, only the side-effect (gain change). But to do that well needs 6 tubes, or 4 tubes and a transformer. The box is already crowded enough.
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Question from out of the blue:
> But a push-pull tremolo?
No "but" about it. The STANDARD big-amplifier tremolo. The audio is push-pull, like most big power amplifiers. Each side out-of-phase. The low wobble tone is injected in-phase both sides.
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