Blast from the past... Flo-a-tone

[Sowhat]

Just got a Flo-a-tone amp in for repair, no speaker or cabinet, just the chassis and control panel... D.O.A. I'm guessing it once worked back when Elvis recorded Hound Dog.

I'm imagine this is from the early 50's and looks like a pretty standard dual 6V6 with steel case 6SC7's for phase inverter and preamp. These look like the original tubes. I'm wondering if anyone remembers these and knows the speaker impedance, I know back then you could have some pretty high impedance speakers on things or not. The output transformer is small, choke size so we're not dealing with a lot of output power here... maybe 15 to 18 watts tops, I have not checked the B+ yet so I don't really know where it falls. If anyone has a schematic that would be cool to have.

[Sowhat]

Just got a Flo-a-tone amp in for repair

Whoops!!! that's Flot-a-tone not Flo-a-tone.

[Sowhat]

Hi all,

Just though I would put this little saga to an end and show you what I found. I decided to make a schematic of the amp because the only one I could find was a pencil sketch one of what appeared to be a similar model. Working on the output section I noticed a slight difference between what I had and what I found on the web. It appears that my job has a 0.0068uf 600 volt cap across the output transformer primary and the one in the found sketch don't. I believe this is an oscillation suppression cap, at any rate, it was shorter than Billy Barty. I took it out of circuit and the amp came right to life (well... there's your problem!).

If you can't tell, my drawing is on the left and the sketch I found is on the right.



Oh, yeah... thanks for all your help on this. :-)

[nashvillebill]

Sorry but I've never heard of a Flot-a-tone, is it a Magnatone derivative?

[Chuck H]

You're welcome for all the help!!!

Suppressor filters on OT primaries can take a real beating! The voltage rating for that cap needs to be about three times the B+ to survive. A straight shorted cap seems like a bad idea since it means for any frequencies above the caps knee the tubes will be playing into a dead short. The more typical "conjunctive filter" would use a resistor of 1.4x the primary impedance in series with that cap. The resistor also needs a high voltage rating. IMHO just because the circuit used the cap only as the stock circuit doesn't make it a good idea. If the amp sounds good without it then you could always just leave it off.

You can determine the speaker impedance by measuring the OT ratio and speculating based on the power tube data. That's what I do with unknowns like this.

[Sowhat]

You're welcome for all the help!!!

Suppressor filters on OT primaries can take a real beating! The voltage rating for that cap needs to be about three times the B+ to survive. A straight shorted cap seems like a bad idea since it means for any frequencies above the caps knee the tubes will be playing into a dead short. The more typical "conjunctive filter" would use a resistor of 1.4x the primary impedance in series with that cap. The resistor also needs a high voltage rating. IMHO just because the circuit used the cap only as the stock circuit doesn't make it a good idea. If the amp sounds good without it then you could always just leave it off.

You can determine the speaker impedance by measuring the OT ratio and speculating based on the power tube data. That's what I do with unknowns like this.

Yeah, that's what I did, it's a 40:1 so an 8 ohm speaker seems most appropriate with 10K plate impedance for 2 6V6's. That suppressor could very well have been an add on by someone over the past 50 years. It don't look stock if there is such a thing in a Flotatone. I have been running it without that cap because I don't have a replacement in the shop right now and I'm not seeing any HF oscillation or funny business on the scope. I since have got a good look at some other Flotatones and they don't have this cap, I just may leave it out. Now all I have to do is measure the max wattage so the customer can purchase an appropriate speaker for this relic restoration.

[Sowhat]

Sorry but I've never heard of a Flot-a-tone, is it a Magnatone derivative?

Just do a images.google.com using Flotatone as the searchword and you will see a bunch of them. It's an original being made from 1946 to 1960 and was later bought out by Supro or so I think. This one is a 1948 model... one of the early ones but they did not change much over their manufacturing lifetime except to go from 6V6's to 6L6's as far as I can see.

[Sowhat]

Oh yeah, just a little more about this thing. Someone previously jumpered around the power supply capacitors with some new caps which was probably a good thing seeing that the existing can capacitors were shot. The supply is supposed to have a B+, screen, and low voltage taps to the amp. The way it was rewired the screen tap was soldered to the low voltage tap leaving the screen tap location feeding nothing. I got some gain back into the amp resoldering the screen tap and filter cap back in it's correct place in the voltage divider chain... I knew something looked wrong about it the second it came in the door.

Who ever did this recap did a horrible job but I left that to my customer to fix or not fix as he see's fit, my job was only to get it running, he's the restorer.

Here's how I got it...



and here's the fix... it's not pretty but making it so is not my job here, I just make it operate.

[JoeM]

True point to point!

[Sowhat]

Now all I have to do is measure the max wattage so the customer can purchase an appropriate speaker for this relic restoration.

Just to go full circle on this, the amp measures out to be 13.586 romping stomping watts... what a monster! That's reached at about 1/2 rotation on the volume with a 440hz 250mv sine signal. Above 1/2 rotation you get some pronounced distortion, at times almost a triangular wave or maybe trapezoidal wave but certainly not a sine... that should sound pretty interesting. Now, this is all with original 1948 vintage 6V6's and original 6SC7's. New tubes may perform better but that distortion may be the vintage bomb someone is looking for.

[Chuck H]

Nice job. Thirteen and a half watts for a pair of cathode bias 6V6's seems alright to me. Incidentally... I don't see a 0V grid load on one of the power tubes in the schem. How it's working without one is a mystery, but it might explain the weird clipping. There should definitely be a 100k to 220k to ground from each power tube grid. There's no possible way the amp was designed with only one grid having a 0V reference. Restoration guy or not, I'd put one in there. In fact I would probably add a 1.5k to 10k grid stopper to each grid as well just ahead of the grid loads.

[Sowhat]

Nice job. Thirteen and a half watts for a pair of cathode bias 6V6's seems alright to me. Incidentally... I don't see a 0V grid load on one of the power tubes in the schem. How it's working without one is a mystery, but it might explain the weird clipping. There should definitely be a 100k to 220k to ground from each power tube grid. There's no possible way the amp was designed with only one grid having a 0V reference. Restoration guy or not, I'd put one in there. In fact I would probably add a 1.5k to 10k grid stopper to each grid as well just ahead of the grid loads.

Oh, it's got one, it's just that my schematic fragment don't show it, here's the schematic I found on the web which shows the entire amp...



That's an interesting phase inverter, don't you think?

[Sowhat]

That's an interesting phase inverter, don't you think?

Yes it does although the lytics have been replaced with close values but not identical ones.

[Chuck H]

That's a very interesting inverter. Hardly an inverter at all, with the exception that it inverts Sort of a poor example of a paraphrase type.

If the amp you have is wired as per the schematic there is no grid load on the lower 6L6. And there should be. You can see that there is no 0VDC reference resistor between the DC blocking coupling cap and the grid of that tube. It should be right where the red "X" is. Without it the cathode's positive voltage references to, what? The grid can't be negative WRT the cathode if the grid is left floating like that. I'm surprised it hasn't gone into thermal runaway. Who say's the guy that drew that schem knew enough about the circuits? You REALLY want to put a 100k resistor from where I placed that red X to ground. Research it if you must.

[g1]

Yes it does although the lytics have been replaced with close values but not identical ones.

Sorry, I deleted that post as I was seeing the drawing properly .
(I had asked whether the drawing matched the actual circuit).

Chuck has a good point, maybe the resistor is in circuit and the author of the schematic missed it?