Re: Super Reverb trem osc question

Peter Alerich <scooter@xxxxxxxxxxxxx> wrote in
news:m82gk11289pvg7q8qiqjo0k6109dj4cut5@xxxxxxx:

> I have a 1972 SF Super Reverb. I've never really nailed down which
> ABXXX schematic it is - it seemed to have aspects of several when I
> got it and it looked factory to me. I'm not terribly concerned there
> since I've made my own changes along the way. When I upgraded bypass
> caps I pulled out all the white Mallory 25/25 caps and replaced them
> with fresh Sprague Atom 25/50 caps. Measuring the Mallorys on my new
> Fluke 111 they are all running on the high side between 35-40 uf each.
> Dunno what the tolerance is on these items.
>
> I was working from the AB763 schematic. AB763 shows a 25uf bypass cap
> on the cathode of the 2nd half of the trem osc tube. That's what I
> replaced it with. No, I didn't actually bother to look at the
> component beforehand. I was just digging through a box of salvage amp
> parts looking for a bypass cap for a homebrew/hi-fi mod I'm working on
> www.alerich.com/hifi and I found a white Mallory 5/50 cap. On the
> AB1069 schematic I see a 5/25 on the 2nd half of the trem osc so
> evidently that's where it came from.
>
> My question is what do these bypass caps in the trem osc do, besides
> the obvious?

What is obvious to you?

> The signal doesn't appear to actually pass through the
> trem ckt. I've been told that it doesn't really matter what brand of
> tube goes there as long as it is functional. The trem still sounds
> great so I can't discern an audible difference between the 5uf and the
> 25uf although I have not A/B'ed it. I've always thought that bypass
> caps were basically a method of voicing the audio charateristics of
> the ckt.
>
> Why put them on an essentially non-audio portion of the ckt?
>
> Peter
>

The cathode bypass capacitor increases the AC gain of the stage. The size of
the cap affect the frequency at which the gain is affected. A larger cap =
gain increase starts at lower a frequency.

In an audio amp, it will affect the tone by shapping the low frequency
rolloff. But increasing the gain is also usefull in many application.

In a phase shift oscillator, the trems first stage, it helps maintain
oscillation by keeping the closed loop gain above 1.

In a pulse amplifier as in the trem's second stage, it helps to drive the
neon lamp to its turn-on voltage (~60V). This is called pulse shapping.
Evidently, the actual size of the cap is not so important as long as the
initial pulse is high enough to trigger the neon lamp.


.

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