Unusual Tremolo circuit

[Rhodesplyr]

I've traced out the circuit schematic of an early Flot-A-Tone amp that someone brought to me for refurbishing. No schematics appear to be available for these amps, so I had to sketch it out myself. It's only the second guitar amp I've seen where a volume control precedes the first gain stage, but it's generally an unremarkable amp: a triode gain stage into a paraphrase inverter driving a cathode-biased output stage. The first 6SC7 was completely dead (no emission) except for the filament, pointing out the downside of trying to provide a control grid ground-reference through old, scratchy pots.

What interested me was that I've never seen a tremolo circuit remotely like this one. With tremolo switched on (via a footswitch) it links the cathodes of the two 6SC7s via a 1,000 Ohm wirewound pot. I'm guessing that the idea is to set up a subsonic oscillation between the 6SC7s. The 1k pot is on the amp's control panel, but its function isn't indicated. The amp's new owner thought it might be a speed/depth control, but I was thinking that it might be there simply to allow you to fine tune the circuit to get an oscillation. We'll see how it works when I get it back up and running.

Any comments? Was this a clever way to get tremolo or a really bad idea? I think this company's primary design goal was low parts count.

I was thinking that, for contemporary use, it might be better to have a volume control after the first gain stage, but then that would likely interfere with the tremolo arrangement....

Flot-A-Tone.pdf

[tubeswell]

Well I know that when you hook two cascaded stages up to a common cathode resistor, you get oscillation. And at a guess I'd say that the 1k pot affects the strength/intensity of that? JM2CW

[EFK]

A good number of old valco amps use half a triode as an oscillator and then implement various methods to link the cathode of that oscillator to the cathode of another preamp tube. Works very well. Speed controls are straightforward and simple, but what is difficult is to get a useful intensity control into the equation with only the half triode being used as the oscillator. Just guessing without seeing a schematic or anything but the 1K pot is probably the intensity control; I've seen this before in 50s Valco amps and used it myself - it is placed in the line between the two cathodes, and works as a very crude "blend" control. Not the greatest but it works. Anything over 1K just creates a longer sweep of uselessness; even w/ 1K, most of the adjustment is over only around half the pot's sweep. The other common method of intensity that Valco standardized by the late 50s was using a 250K or 500K linear pot in conjunction with a smaller plate resistor on the oscillator (yes, they put 300-350V across the pot); this works quite well but when I've done this, I use those PEC 2W pots.

Edit - I just looked at your schematic. Interesting! That isn;t done the way Valco did it as you have no dedicated oscillator. I'll be interested to hear about how it works or if it works when you fire it up! I'm guessing that 1K is still a crude 'intensity' type control that will control how much of a thump you hear, and the speed I guess is what it is - whatever speed it oscillates at when the two are linked?

[Rhodesplyr]

Interesting! That isn;t done the way Valco did it as you have no dedicated oscillator. I'll be interested to hear about how it works or if it works when you fire it up! I'm guessing that 1K is still a crude 'intensity' type control that will control how much of a thump you hear, and the speed I guess is what it is - whatever speed it oscillates at when the two are linked?

Yes, that's what I meant when I said I'd never seen a tremolo circuit like this one before. Every other one I've seen has used some type of dedicated oscillator. I was a bit puzzled when I looked under the chassis because there weren't as many capacitors as I thought there would be. There's a version of this amp on eBay at the moment that appears exactly the same except that it has no tremolo, so this was apparently an attempt to get tremolo on the cheap. With two dual triodes, I'd expected to find a dedicated oscillator, but then, with the 6SC7 common cathode, that might be very tricky to do.

Instead, you get two inputs, one of which is probably never going to get used.

[d95err]

Tremolo circuit with a parts count of one! Cool!

I guess the volume control needs to be before the first gain stages, or the tremolo wouldn't work. I guess the same goes for the tone control, which also is in an unusual position.

Thanks for sharing this!

[Rhodesplyr]

I finished most of the restoration of this Flot-A-Tone amp this evening and fired it up. It's a sweet-sounding amp overall, but the cheap 'n dirty tremolo circuit is pretty disappointing. It works, insofar as it does set up a subsonic oscillation that you can tailor slightly, but it ends up sounding like what it is: an amp with a subsonic oscillation problem.

As a cousin of mine once said, "They don't build 'em like that any more, and there's good reasons why they don't."

[gumbyguit]

Flot-A-Tone tremolo

I finished most of the restoration of this Flot-A-Tone amp this evening and fired it up. It's a sweet-sounding amp overall, but the cheap 'n dirty tremolo circuit is pretty disappointing. It works, insofar as it does set up a subsonic oscillation that you can tailor slightly, but it ends up sounding like what it is: an amp with a subsonic oscillation problem.

As a cousin of mine once said, "They don't build 'em like that any more, and there's good reasons why they don't."

Thanks for all the info you have posted about this Flot-A-Tone circuit. I have a Ducatone that is exactly the same circuit, except there doesn't appear to be a footswitch jack to disable tremolo. I was fighting motorboating issues that actually got worse after replacing caps. Sure enough, the tremolo circuit is the source of the motorboat sounds. Since it's not real useful anyway, do you just leave your tremolo turned off or did you redesign? I may just add a jack in there, according to your schematic, to interrupt the trem circuit because it is quite annoying.

One other question, what kind of wire goes from pin6 of the 6SC7's to the tremolo control on your amp? Mine had some obviously non-stock twinax, but changing it did not alter the flup, flup, flups coming out of my speaker.

[gtmusic23]

Hi all..... I know this thread is 10 years old but I have been working on one of these flot-a-tone amps and the tremelo circuit had been ripped out. I would certainly like to see the schematic that Rhodesplyr drew out. I see the link to it but it simply returns invalid file. So it must have been or something. Does anybody have a copy of this circuit?

[dmeek]

I believe it is this one:
Flot-A-Tone amp 1952.pdf

[gtmusic23]

Thank you dmeek, I believe this is pretty accurate based upon the layout I started to draw.

[shiftertoe@gmail.com]

Finally had something to share, just got this same "1952 Flot-A-Tone" amp in my shop and was researching and found this thread. The can capacitors are 30MFD/450WV, my circuit matches the sketch posted above but with 50MFD/50WV Cornell Dubilier cathode caps, that are in small rectangular metal housings with two chassis bolts. Speaker measures 5.5 ohms. Hope this helps.