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Valve vibrato circuit
Valve vibrato circuit
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Old 3rd October 2019, 08:08 PM   #1
El Sid is offline El Sid  South Africa
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Default Valve vibrato circuit

I was left with a chassis of an old Philips console valve radio that I bought for spares when I restored another one. The power transformer, an output transformer (about 3W, for EL84) and a whole lot of other useful bits such as pots, switches and magic eyes remain.

So I had the idea of converting it into a guitar amp, retaining the chassis, fascia and controls. I have designed and built a few hifi valve amps, so know what needs to be done. (In fact I first learnt about valves from Uncle Doug so am familiar with guitar amps). Another old Philips console I have lying around has a reverb tank in it (bizarrely!), so I'm going to add that.

I was going to add a tremolo circuit using the AM tuning cap of the radio to set the speed. But my interest has been piqued in vibrato rather than tremolo as it seems to me to sound nicer. However, I have really struggled to find a valve-based vibrato circuit - even here. There are plenty of sockets on the chassis and ECC83s are cheap...

Can anyone point me in the right direction?

PS - I know there's some interchangeability/confusion between the terms tremolo and vibrato. I use them in the sense of vibrato = frequency shifting, tremolo = amplitude shifting.

(I would have posted this in the valve amp section where I normally lurk but they get tetchy over there when anyone mentions guitar amps )

Last edited by El Sid; 3rd October 2019 at 08:10 PM.
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Old 3rd October 2019, 08:17 PM   #2
knockbill is offline knockbill  United States
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Hers an interesting Uncle Doug video about a vibrato amp... maybe copy the schem?


YouTube
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Old 3rd October 2019, 08:25 PM   #3
El Sid is offline El Sid  South Africa
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Aha! Thanks John.

I should have known Uncle Doug covered it!
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Old 3rd October 2019, 10:04 PM   #4
PRR is offline PRR  United States
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Quote:
Originally Posted by El Sid View Post
...old Philips ...radio ...going to add a tremolo circuit using the AM tuning cap of the radio to set the speed. But my interest has been piqued in vibrato rather than tremolo as it seems to...
Disregarding the tremolo/vibrato issue--

Both use an oscillator around 10Hz. The radio tuner was 1MHz, 100,000 times higher. In principle we could use a coil 10,000,000 times bigger. In practice we can't find a 10,000 Henry coil, can't wind one with low enough losses.

H-P made an R-C oscillator go 20Hz with 500pFd caps (huge). The smaller caps in an AM radio suggest 50Meg-100Meg resistances, which are very awkward, and also prone to pick up hum modulation.

True pitch-shift modulation requires a delay. Fender Harmonic tremolo is an attempt which soothes the ear but is very complex. One of the other amp companies had a phase-shift modulator, more what we want, but not simple and based on now-obsolete varistors.

You are really barking against the wind. Good luck!

Last edited by PRR; 3rd October 2019 at 10:10 PM.
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Old 6th October 2019, 01:55 AM   #5
trobbins is online now trobbins  Australia
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An article mainly on the Magnatone amp vibrato technique, but also the Wurlitzer as used in Vox AC30 and many others.
https://www.dalmura.com.au/static/Ma...o%20design.pdf
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Old 6th October 2019, 03:45 AM   #6
Bill Coltrane is offline Bill Coltrane  Netherlands
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The magnatone 280 had true stereo vibrato, but it's a complicated schematic with obsolete tubes.
I have the schematics somewhere.
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Old 6th October 2019, 03:55 AM   #7
PRR is offline PRR  United States
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Quote:
Originally Posted by Bill Coltrane View Post
The magnatone 280 had true stereo vibrato, but it's a complicated schematic with obsolete tubes. I have the schematics somewhere.
MagnatoneAmps.com - 280
https://www.magnatoneamps.com/schema...atone_280a.pdf
https://www.magnatoneamps.com/schema...atone_280B.png

Not counting output bottles: The Magnatone and Wurlitzer used 12AX7 preamps and LFO, 12AU7/6CG7/6SN7 cathodynes and buffers. Yes, "obsolete", but readily available New, and fairly available NOS if you change to 12V heaters. (The 280_ outputs, 6CZ5, is I believe a re-bottled 6V6?)

EDIT: I missed the 12AH7. This seems to be a "lame" 6SN7/12AU7. It does not seem to be used in anything else? I wonder if 20 years later Maggie found a crate of 12AH7 on the close-out table and used in in the minor stages to shave a buck. Today the rest of that crate is still with us, selling $3-$8.

Last edited by PRR; 6th October 2019 at 04:04 AM.
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Old 6th October 2019, 04:18 AM   #8
Bill Coltrane is offline Bill Coltrane  Netherlands
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Don't forget the varistors, they are essential.

Thanks for posting the schematics.
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Old 6th October 2019, 04:21 AM   #9
PRR is offline PRR  United States
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Yes, sorry, the VARISTORs are the unobtainable obsolete parts. (Other varistors exist, but not with the soft curves of the old ones.

And the complexity is hard to argue, except against the fanciest Fender, which still was not stereo.
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Old 6th October 2019, 02:04 PM   #10
El Sid is offline El Sid  South Africa
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Thanks all for the input. This certainly seems to be a tricky thing! Certainly the Magnatone approach is simple by parts count, but the varistors look to be unobtainium. I found a reference to the Littelfuse V68ZA2P which someone says worked in their circuit.

Anyway, this is still some way off as I have a few other projects before that. I'll keep everyone posted.
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