75' Deluxe Reverb Vibrato speed mod (switchable caps?)

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75' Deluxe Reverb Vibrato speed mod (switchable caps?)

Postby beedoola on Fri Feb 26, 2016 12:01 am

I've seen some information saying the vib speed on most (all?) fender amps can be slowed down by replacing 3 caps. Would it be safely - not electrocution- possible to have both the stock and replacement caps wired to a switch so that I could have both the original and slower speed ranges?

I don't use input 2 on the vib channel and I figured I'd remove the input jack and use input 2's hole for the added switch.
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Re: 75' Deluxe Reverb Vibrato speed mod (switchable caps?)

Postby pdf64 on Fri Feb 26, 2016 9:30 am

The trem oscillator frequency can be halved just by doubling the value of any one of the 3 phase shift caps in the plate-to-grid circuit, eg 0.01uF to 0.022uF; or just switch in another 0.01uF in parallel with the stock one.
The oscillator may be working with big signals; fitting a switch dealing with that circuit immediately adjacent to a sensitive high impedance input doesn't seem to be good practice, as it may result in unwanted coupling / interference.
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Re: 75' Deluxe Reverb Vibrato speed mod (switchable caps?)

Postby stratele52 on Fri Feb 26, 2016 11:10 am

Switch and wires could add some noise , you may need shielded wires.

Normal Speed pot is not slow down enough for you ?
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Re: 75' Deluxe Reverb Vibrato speed mod (switchable caps?)

Postby stratele52 on Sat Feb 27, 2016 10:29 am

If normal slow speed is not enough slow for you, amp may have a tremolo issue .

Normal slow speed is so slow I don't understand how it can be slower.
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Re: 75' Deluxe Reverb Vibrato speed mod (switchable caps?)

Postby Mitch Roberts on Sun Feb 28, 2016 2:23 pm

I have owned or worked on many vintage Fender amps, and I now almost routinely change one of the .01uf caps to .02. That always seems to have just the right slowing down of the slowest speed, and doesn't noticeably alter the fastest tremolo frequency. It never even occurred to me to make a switchable setup. I agree that would likely introduce issues.

I have done this mod so many times, I have a hand full of the replaced .01 caps in my parts box.
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Re: 75' Deluxe Reverb Vibrato speed mod (switchable caps?)

Postby beedoola on Sun Feb 28, 2016 7:42 pm

stratele52 wrote:If normal slow speed is not enough slow for you, amp may have a tremolo issue .

Normal slow speed is so slow I don't understand how it can be slower.


Hmm, maybe there is an issue then. Could it be a tube issue? If I open it up, what should I look for to see if anything is out of the ordinary? I'll upload a short video showing the range.


Image
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Re: 75' Deluxe Reverb Vibrato speed mod (switchable caps?)

Postby pdf64 on Mon Feb 29, 2016 4:01 pm

It seems fairly normal.
The caps used have a poor tolerance and temperature co-efficient spec; so the speed may change as the amp warms up!
You may as well check the resistor values for the trem oscillator if you are going in there; but they tend to drift up, which would send the speed lower.
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Re: 75' Deluxe Reverb Vibrato speed mod (switchable caps?)

Postby beedoola on Mon Feb 29, 2016 6:47 pm

Is there any way to mod it - like changing the Speed pot value?
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Re: 75' Deluxe Reverb Vibrato speed mod (switchable caps?)

Postby pdf64 on Tue Mar 01, 2016 12:05 pm

The speed pots are specialized parts.
Manufacturers could make you a higher value to increase the range (eg 6M reverse audio), but would require you to order a minimum batch size, maybe 200 pieces.
If you want to shift you trem range of control a bit slower, then double up one of the cap values; unless you use it on 10 currently, that should give the useful range of control you would like.
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