adding tremolo to SS amp

[timeisthefourthdemension]

Hello,

I am looking to install a tremolo  in a simple solidstate amp circuit, inserting it between the preamp section and the power amp. My question is how to power the tremolo circuit. The Amp has 16vac off the transformers secondary voltage, and both +-23vdc and +-15vdc rails.
Assuming the tremolo circuit does not consume much, could I power it off the +15vdc with a 7809 regulator circuit or is there a more common solution to this?

Here is the Schematic for the amp: http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Frontman-15G-Schematic.pdf

Thanks,
Rodrigo

[bluebunny]

I would be inclined to find (or adapt) a tremolo circuit that works with +/-15V, rather than trying to shoe-horn it into the 9V sitting north of 0V.  Then you don't have to worry about coupling, biasing, headroom, ...  Search for amp circuits with tremolos!

[amz-fx]

The Frontman is an interesting design that anyone who likes to look at solid state amp schematics should review. Among the interesting items is the high pass filter (with gain) on the input, the Normal Volume, Q1 and IC3A.

You could power a tremolo circuit directly from the +23v rail, if you like. Or use a 220 ohm/9v zener to reduce the voltage. A simple tremolo like the Schaller could be added in the front before the input 15k.

If you want to get more adventurous, then R6 sets the gain of the input HPF and an opto-coupler in parallel with R6 can make a trem effect.

There are plenty of ways to do it, but an external pedal might be easier. 

Best regards, Jack

[R.G.]

Tossing in my $0.02, I agree, but more emphatically.

There are plenty of ways to do it, but an external pedal might WILL be easier.

[PRR]

Using 1970 thinking: while you still can, put a photoresistor across R15. Shine an LED at it. Build a small-power LFO to drive the LED.

Agree whole-heart: put a pedal on front.

[timeisthefourthdemension]

Thanks for your thoughts.

I ordered an EA tremolo circuit and wish to implement it in the frontman circuit. I am more interested in figuring stuff out with these projects, than keeping it easy, so to learn more about how things work. My plan is to eliminate  the distortion channel and use its controls to work with the tremolo.

So I have 2 more questions: 
-Is there a reason I should power it off the +23v rail rather than the +15v?

-My thinking was to try first to insert the tremolo after the preamp stage rather than before it, assuming it would produce a cleaner signal. Would there be any any problems associated to that?

thanks

[Mark Hammer]

Using 1970 thinking: while you still can, put a photoresistor across R15. Shine an LED at it. Build a small-power LFO to drive the LED.

Agree whole-heart: put a pedal on front.

I agree with the R15 suggestion wholeheartedly.  R15 sets that op-amp stage at unity gain, given the 220k values of the clean and overdrive channels feeding it, such that varying its value will alter gain no matter what channel you are using.

The wrinkle is that modulating the volume by reducing it below unity gain will result in an average signal amplitude that will be below the non-tremolo level.  If one wishes to engage the tremolo for a solo, that compromises the "solo" quality.

So here would be the solution.  One sticks a 47k-100k resistor in series with R15 to provide a little extra boost to the signal.  A SPDT switch (stomp or toggle) hasits common connected to the junction of R15 and that additional resistor.  An LDR is placed in parallel with R15.  In one position, the switch bridges the extra resistor and lifts the connection to one end of the LDR.  In the other position, the extra resistor goes unbridged and the switch common gets connected to the LDR.   So you either have no tremolo with unity gain, or tremolo with a little extra oomph to compensate for the modulated volume drop. 

Keep in mind that since the extra series resistor would be there, the gain could never drop to zero for strong "chop" effects.  If the extra resistor were, say 39k, then the lowest gain would be 39k/220k (input resistor) =.177 plus whatever the combined parallel resistance of R15 and  the LDR might be.  If, for example, the LDR could be brought down to 5k, that would yield a parallel resistance of 4.88k, added to 39k, giving 43.88k, which provides a gain of .199 or essentially an 80% drop in maximum level.  So. not dead quiet, but enough of a level drop that some degree of throb is attainable.

That still leaves the matter of how to power an LFO and which type to use, in addition to what sort of LDR would be optimal.  But if I was going to retrofit a tremolo in that amp, Paul has picked the ideal spot for it.