Re: 1969 Super Reverb Vibrato Circuit Problem



JimIslander wrote:
>
> Folks,
>
> Vibrato circuit arrived dead on my 69 SR. No good 12AX7 on hand, so
> pulled reverb 12AT7 and put in trem circuit. Interesting result.
> Vibrato pulsed at about 90 hertz (by ear), very weak. I think more is
> wrong than a bad tube (these are all original caps/components). I know
> the 12AT7 has less gain, but would that give me a week, very slow
> vibrato frequency?
>

Take the 12AX7 from the first stage and lap it in the vib hole.

You can't do anything without the right tube in the circuit to begin with.

If you have the neon bulb flashing, the vib circuit is working.

30 to 50% of full signal is normal to see on the scope on the output of the amp, due to variation of
the cells.

Less than 30%, replace the cell.



If the bulb is not flashing, look for around 280 on the first plate and 390 on the second.

First check to see if the bulb leads are OK.

Next, check plate and kathode resistors for value, and REPLACE the kathode caps.

Now, if it doesn't come up, replace the 2) .01 uf and 1) .02 uf caps.

That's the usual problem spots.

Good Luck.



> Real questions:
> 1. I have a good multimeter and know electronics safety. Years of
> electronics repair experience, including 6 years military (all types of
> gear from old analog to digital). No experience troubleshooting guitar
> amps. Can I take some simple voltage/resistance/capacitance
> measurements to identify a likely culprit?
>
> 2. The "roach", how can I check it? Should I?

Watch the scope for less than 30% fluctuation of the sine wave.


> 3. If it were you, how would you troubleshoot?

Without a scope, you are going to be lost.


Regards,

Rich Koerner,
Time Electronics.
http://www.timeelect.com

Specialists in Live Sound FOH Engineering,
Music & Studio Production,
Vintage Instruments, and Tube Amplifiers
.

Relevant Pages

  • Re: Suggestions for aspiring hobbyist
    ... I find myself wanting an expert electronics guy around to ... what's going on inside a circuit. ... effects of which you can then view on the scope. ... you can directly compare the input and output signals - at any two points in ...
    (sci.electronics.basics)
  • Re: 1969 Super Reverb Vibrato Circuit Problem
    ... > pulled reverb 12AT7 and put in trem circuit. ... > Vibrato pulsed at about 90 hertz, ... well be the caps in the plate to grid circuit. ... and figuring out how to check that dumb roach. ...
    (alt.guitar.amps)
  • Re: The Plans, A Bit Disappointing
    ... These modulate the *amplitude* of the signal to give ... a tremolo effect, not the pitch as required for vibrato. ... with building this circuit (Rod Elliott is good value, ...
    (rec.music.makers.builders)
  • Re: The Plans, A Bit Disappointing
    ... You might get tremolo, but vibrato is pitch-shift. ... circuit found at: ... in series with a sustain circuit I had found (actually I found two good ... if I use that last knob as an on/off switch. ...
    (rec.music.makers.builders)
  • Re: First post: best place for this?
    ... Here is the complete posting. ... This FAQ is a tourist's guide to the various electronics newsgroups. ... be a Residual Current Circuit Breaker across the Atlantic. ...
    (sci.electronics.design)