Stereo (left/right panning) tremolo circuit

[Greg]

Hello!
 
Imagine two Fender Champ amplifiers circuit (for simplicity matter) in one unit complete with two speakers and all.

I would like a single LFO to create a left/right tremolo (amplitude) stereo panning between the two amplifiers (one amplifier for each pick-up, tone settings, etc...)

I chose the preamp bias modulation circuit (à la Vibro-Champ). Of course to accomplish this, the circuit had to be radically changed. The LFO signal is splitted in two opposite phases and buffered via a cathode follower. It is then going to each channels second triode's cathode.
 
To control the tremolo effect, I decided to put the intensity control (500k) into the grid circuit of the splitter stage instead of having to install a ''dual ganged'' 25k intensity control into the cathode circuit of the buffer stage like the original champ circuit (single 25k pot).

I didn't build the circuit yet and would like to know if you guys think it could work ''as is'' or if you think it needs some modifications. The partial schematic was reconstructed via the Vibro-Champ schematic archive using MS Paint without the precise voltages. Just for you to have a basic idea of the circuit. The original champ circuit schematic is on the 2nd file.
 
Thanks!
Greg   

[PRR]

That may work. Try it.

You *may* need to fix-bias the 12AU7 grids at +170VDC rather than trust to self-bias. Self-bias is generally better for simple happy amplifiers, but here you want both sides at a rather specific and same DC.

I would avoid 12DW7 in favor of 12AX7. The AX is fine for the cathodyne phase splitter. The DW will be hard to find on a rainy Saturday night in a strange town. The DW actually comes in two forms which swap the high and low gain sections. 12AX7 is ubiquitous and just works.

[kagliostro]

What I don't understand Is the phase inverter out of the tremolo circuit ....

If one signal is positive and the other signal is negative don't they cancel each other at the output of the amp ?

May be this Is a stupid question (here is early morning), but at the moment to my brain seems that this will happen

Please can someone explain better what will happen

Excuse me

Many Thanks

Franco

[pdf64]

I’ve never messed around much with a Vibrochamp so dunno, but I don’t see the need / benefit for the 12AU7 sections?

And conceptually, the effect may sound good if the listener is well placed in the stereo soundstage, but at the back of the room etc, it may effectively become mono and get cancelled out.

[sluckey]

What I don't understand Is the phase inverter out of the tremolo circuit ....

If one signal is positive and the other signal is negative don't they cancel each other at the output of the amp ?

The tremolo signal does not pass through the amp to the output. The tremolo signal simply varies the amplitude of the guitar signal by wiggling the cathode voltage of the second preamp triode.

[Greg]

Thanks PRR, I could certainly make it fixed bias. My coupling caps before the buffer sections are a bit high, I could have made them .01 or .02

Hello kagliostro, I had prepared an answer but Sluckey was faster in his response and it was much simpler than mine!

Thanks pdf64, as for the unnecessary use of the 12au7 buffer, is it implied in your statement that ''if'' the LFO's inverter was going directly into the higher impedance of the grid circuits of the second volt amp stages instead, it  would not then be needed? In any case it made me think... I could also certainly omit the 12au7 buffer to wiggle the power tube bias provided it is cathode-bias push pull. It could then solved the instability/meltdowns associated with the depht pot into the grid circuit and would require no filter to tame the LFO pumping.
 
I'm mainly a living room player now, it started two years ago since you know what. I developed a fondness for that kind of stereo stuff!

Thanks for your answers!

[pdf64]

To rephrase things, as I see it, neither the cathodyne or the cathode follower circuits require more of the triode driving them than a 12AX7 could handle. Unless you’ve got a few spare, there seems no point / benefit in using the 12AU7 sections.

[Greg]

Ok I got you, you are saying it's a matter of tube type. I thought you were saying that the whole buffer circuit was unecessary!  I often saw the au7 favored for this kind of job on many schematics, not wanting to just throw a fancy tube. I understand that for practical reasons, it is an overkill for that application just like the 12DW7 as was mentioned by PRR. For a few months, the 12AU7 was actually 3 times cheaper and more plentiful than a 12AX7 for the reasons that we know until the entry-level PVinsane AX7 arrived on the market

[tubeswell]

I’d just use an ABY connection into couple of amps with trem (which I do already if I want ‘stereo’ trem). But by all means, experiment away!

[pdf64]

... the whole buffer circuit was unecessary!...

That might be the case too 

[Greg]

With an Y box, you can't really syncronize the effect from one speaker to the other with even and smooth cross-over. You end up with a ragged thumping of two mono amps at different rates. Sure, you can put mono tremolo on one amp and lush reverb on the other and have a super stereo sound set-up but you don't end up with a stereo tremolo effect panning from left to right.

I tried a simulation on my daw and it's cool. I can understand however the lack of general interest for this stereo effect. Anyways I'll try it and see! Thanks for chiming in.

[thetragichero]

personally i am intrigued. i have the transformer set from a stereo single-ended reel-to-reel player that i was considering a spin on the ampeg echo twin (8 ohm reverb tank powered off the ot of one of the channels feeding the second) but this might make a nice addition... whenever i get around to it

[Greg]

I'm going to breadboard the circuit and tweak it, until it gives satisfactory results with the less amount of tubes in the circuit. Probably in june when I will have more time. I'll bump the thread with results, volts, etc...

[Greg]

... the whole buffer circuit was unecessary!...

That might be the case too
 

                                                           

I determined that the LFO signal at the output of the cathodyne's plate will be loaded going into 1k5. That's why I figured it needs a buffer. However, the output impedance at the cathodyne's cathode is good enough not to load the LFO signal. Since the amplitude between opposite phases of the LFO signals must be mirrored as much as possible going into 1k5 ohms each, I decided to mirror image the buffer on paper by means of double section 12AU7 or AX7. I admit it doesn't look frugal. We usually see the cathodyne PI as an inverter for p-p grids self-biased power bottles at 270k/470k. But then, the gain of either side of the cathodyne is not affected by any loading and good balance is achieved.

[pdf64]

I was thinking to retain the 68k resistors between the cathodyne outputs and the cathodes being modulated. But yes, it’s probably not so much the 1k5 cathode resistors as their bypass caps; that would probably kill the modulation signal.

[SoundmasterG]

It sounds similar to me to what the Magnatone 280 was doing - a stereo amp with vibrato instead of tremolo.


Greg

[Greg]

It sounds similar to me to what the Magnatone 280 was doing - a stereo amp with vibrato instead of tremolo.

Yes, but I was thinking of the Ga-83S from Gibson (like a dual AC15 ,but, the p-p oscillator circuit injection is reversed in the second channel). When the hi-pass is on the left, you get the lo-pass on the right and so on. For a tremolo effect, you can shunt one side (and the adjacent side on the other channel) with a 0.1 cap.

The problem is that I planned to use two sets of saturated core reactors for vibrato effect and I didn't figure how to switch from vibrato to tremolo from that circuit. So I came up with this idea to modulate the bias by re-assigning the split phase oscillator to the task of bias wiggling either the cathode of the second AF stage (not sure) or the power tube grids (more likely). 

[Ritchie200]

Sounds like you want the attached.  I built it back in '71-'72ish and used it live.  It was very quiet and smooth.  I had it feeding two stacks doing a true ping pong at the end of a song and a lead on another.  I considered it a one trick pony, but thought I was pretty cool because I had just seen Pink Floyd and they were bouncing the sound all around the theater.  I never used it as a trem but it would certainly do just that with a twist of the knobs.  I know it's not tube driven but maybe it will provide some inspiration.  Parts can still be had if all else fails.


Jim


https://www.flickr.com/photos/144782636@N08/sets/72157672622221874/with/30757618121/

[PRR]

Sounds like you want the attached.
https://www.flickr.com/photos/144782636@N08/sets/72157672622221874/with/30757618121/

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[Ritchie200]

My bad. Click on  "3712 Ping Pong"


https://www.paia.com/talk/viewforum.php?f=6

[TIMBO]

Hey guys,
This may not be much help.....
I have a local built SS Overeem amp that has both Tremolo and Vibrato
When I got it neither worked, it didn't want to oscillate so I changed some caps and all was good.
Now I can't remember which was trem and vib but it sounded like a normal tremolo
Switching it to the other freaked me out and started my head spinning.
It had the same effect as a leslie.
On investigation it had it had three LDRs surrounding a single neon.
I have yet to dissect the circuit is see how it works.

[Hywel73]

Hey Greg, did you get this working?
Im thinking of doing the same with a princeton with a stereo output and wiggling the bias on each pair to create the panning effect?