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K200B tremolo - "Harmonic Vibrato?" - Fender? [message #26813] Fri, 20 July 2018 14:15 Go to previous message
Saransk is currently offline  Saransk
Messages: 20
Registered: February 2017
Location: Northern Virginia
Junior Member
While trying to troubleshoot a very low out tremolo on a 303 preamp board, I was struck how familiar the circuit looked. Q314 definitely looks like a "Split-Load" phase inverter, just not a 12ax7.

It finally struck me that it looks a lot like the classic Fender "Harmonic Vibrato" found in several "Brownface" amps.
The classic is the 6G13a Vibrasonic.
While not a perfect match, especially since there is no "dry" signal mix, the layout is very similar.

There could be a "dry" signal mix from Q307 which would alternatively be in-phase/out-of-phase with the main Dry signal. (Q314 is driving both a positive and negative phase signal)

And why is there a set of diodes off the Intensity control - they look like a "clipper" set from an overdrive pedal.

IS there a block diagram of how this circuit works? For the 1960's it is pretty sophisticated, especially since 90% of all Solid State tremolos were of the usual amplitude using an opto-isolator.

I find Kustom used some very sophisticated and well designed circuits, as well as well engineered, in their classic amps. (Ever seen the inside of a Thompson VOX solid state amp- Super Beatle - no wonder they blow up). I think a set of block diagrams or such - like Peavey (another "bulletproof" amp) always used to include would be a great addition.

Back to the bench
Michael
 
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