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Re: Fender Brownface "Harmonic Vibrato" (6G12A)

10/16/2000 1:58 AM
Re: Fender Brownface "Harmonic Vibrato" (6G12A)
on the slower issue - I found that with higher B+ (I think I went out to a touch over 400Va) I got better range out of the circuit with stock components. A better engineering solution would be just to replace the osc with a function generator (for ex, and XR8038?) and have more range than you'd ever need. But the stock osc is also a bit lopsided and I can't help thinking that's part of the charm.
10/16/2000 3:52 PM
Scott Swartz

I think you mean Intersil ICL8038 or Exar XR2206, both function generators chips, just add a timing cap and a few resistors. I've used the ICL8038 and it works well, and it has a CV in port for hookup to a speed pedal. These chips generate a wave that is a fairly small percentage of the rail voltage (max +/- 15V), so you will need some additional voltage gain.  
I submitted a tube based Univibe for publication to Vacuum Tube Valley magazine and they said it would be published in 2000. I need to email them and check on it. Magazines have a very long delay between when the article is accepted and when its actually out, as much as a year or more! Its the same thing at Glass Audio. Anyway, when its out you guys might be interested in checking it out....
10/16/2000 4:44 PM
An even better arrangement for the LFO is to use a ring-counter generator for making the LFO signal. This uses a digital oscillator to clock a several-stages digital counter, and adding together the outputs with mixing resistors makes the output waveform. You can get good-purity sine waves, triangle waves, quadrature sines, sawtooths, and exponential peaking waves just by how you clock the ring and by diddling the values of the mixing resistors. The digital oscillator can have a *huge* range compared to a linear phase shift oscillator, and the mixing circuit always mixes to what you wanted for an output LFO waveform in a frequency independent way. You can easily make the waveform be unusually shaped any way you like.  
I just put an LFO article up at GEO describing the basics. If you make the logic out of CMOS, it lives quite naturally inside a tube amp. The power needed for CMOS is so low that a simple resistor/zener/capacitor combination can supply all the current that the CMOS needs from B+. The output of the LFO can be buffered or amplified by high voltage MOS transistors, or it can be used to drive an LED or lamp source for modulating the rest of the amp.

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