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50′s Flot-a-Tone Repair / Refurb

Recently, a friend asked me to fix a Flot-a-Tone tube amp that his uncle gave to him. His uncle, apparently, purchased it around 1958 making it particularly crusty, and dilapidated, and awesome. Here are a couple of photos from the beginning:

Flot-a-Tone Front

Flot-a-Tone Front

Flot-a-Tone Back No Chassis
Flot-a-Tone Back No Chassis
Flot-a-Tone Chassis

Flot-a-Tone Chassis

Flot-a-Tone Circuit

Flot-a-Tone Circuit

Reversing the Amp

I had a hard time finding any information on the amp. There was no model number on it anywhere and I didn’t find any schematics online that seemed to match the topology. This is a 2x6L6 cathode biased push-pull amp with a 6SC7 preamp tube and a 6SL7 phase inverter. Since I couldn’t find a schematic online I drew one before I did anything:

Flot-a-Tone Schematic

Flot-a-Tone Schematic

Some things that I noted:

  1. The tubes appeared to be original (ouch) and definately needed to be replaced.
  2. The electrolytics likely needed replacing.
  3. The phase inverter is odd.It looks like a cathodyne but instead of setting the plate and cathode R’s the same and taking each side into the power stage they set the gain to ~1 (cathode to ground) and one power tube takes its input from the second gain stage and one from the phase inverter. Saved a resistor I guess.

Trying To Not Die

The first thing I did was replace the unpolarized AC plug with a grounded and polarized plug.

Flot-a-Tone unsafe power cord

Flot-a-Tone unsafe power cord

The process for doing this is:

  1. Wire up the hot and neutral to the primary of the power transformer
  2. Wire the ground to a dedicated screw and nut connected to the chassis (I use a nylong locking nut)
  3. Cut out the capacitor that was previously connected between one side of the unpolarized plug and the chassis to create a reference. If you leave this capacitor in you will have a dead short to ground after connecting the safe power cord.

Hemp Speaker

When I took the amp apart I noticed that the paper cone from the original speaker was very ripped and so we decided to replace the speaker. The owner selected a Weber Blue Dog with a hemp cone. Here it is:

Weber Blue Dog

Weber Blue Dog

Blue Dog Hemp Cone

Blue Dog Hemp Cone


Blowing It Up

I replaced all the tubes and fixed the input jacks (put 1/4 inch on everything because who still owns a microphone with a screw on Amphenol connector? I know one guy but that is it). I fired it up through my current limiter (a 100W light bulb in series with a power outlet to limit current in the event of a dead short). At this point I hadn’t replaced the capacitors. Everything checked out so I fired up the amp and played for a while gradually increasing the volume to maximum. After a few minutes on max there was a pop and a fizz. The fuse blew and the amp smelled like burning. A diagnosis revealed that the output transformer high-voltage winding shorted to the core. That would also need replacing. Here is a photo of some bench testing in which you can see the current limiter on the floor connected to the light-bulb and amplifier:

Flot-a-Tone Bench Testing

Flot-a-Tone Bench Testing


Red Plating

I replaced the output transformer with a much beefier one from, replaced all the electrolytic capacitors (and replaced the power resistors since the old ones wouldn’t fit to the terminals of the new ones), and replaced one of the coupling capacitors that appeared to be leaking oil after the output transformer burn up. After powering it up again the amp worked and sounded pretty good but after a few minutes one of the power tubes red-plated.

Finishing the Job

I switched the tubes and the red-plating stayed on the socket (not on the tube) indicating that the problem was in the circuit driving that tube. Some analysis with a meter revealed that the power stage coupling capacitor that I didn’t replace was leaking a large amount of DC (about 15%). I replaced this coupling capacitor and also the stage 1 to 2 coupling capacitor (just because it was probably dying too). Finally, the amp was complete. Solid, stable, and rocking. I drove it hard with some P-90′s before taking it over to my buddy’s house for a long jam and break in and it performed great.


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Posted in Guitar Amps, Tube Amps
13 comments on “50′s Flot-a-Tone Repair / Refurb
  1. David Anderson says:

    I reviewed your schematic, and I recently restored essentially the same Flot-A-Tone amp for a client of mine (sn 1347). I also had to draw up the schematic. On the one I worked on, the filament winding was center-tapped. I did the restoration first since I knew the old electrolytics and wax-dipped coupling capacitors would be bad. I also found that, in stock form, the output tubes were running ridiculously hot, even for 6L6GCs, so I had to re-engineer the output section a bit by 1) changing the cathode resistor to 270 Ohms and putting a 50k shunt resistor to ground to lower the screen supply voltage. (I have the feeling that various different power transformers were used in these, so that might be the issue.) The volume pots in this one were from 1952, but it had a 1959 Consolidated speaker that’s still good. I’d suggest getting the original speaker reconed as it’s quite good.

    The one I worked on had the tremolo feature, but it’s very badly implemented and basically musically useless. It works by putting the preamp section into a low-frequency oscillation! Sounds awful.

    Are those red “Tiny Chief” capacitors in yours? They are notorious for failures.

    The client played it for a while, but starting having microphonic issues with V1. I had to iso-mount that tube socket to eliminate the problem, and we decided to replace the original volume pots since they kept going scratchy despite heroic attempts to clean them. It’s a fantastic amp.

    • jim says:

      On this amp that I worked on the filament winding wasn’t center tapped (I did create a fake center tap with two 100 ohm resistors). I ended up replacing all the caps including the Tiny Chief ones (all but one was leaking significant DC). I ended up tossing the speaker figuring it was junk. Oh well on that…

      • David Anderson says:

        The only other difference I noticed was that my client’s amp had, factory-stock, a 0.0022uF capacitor bridging the 6L6 plates, either for high-end roll-off or prevention of oscillation. Perhaps the tremolo circuit made this necessary. The tone control pot is 250k, not 500k, no big deal.

        I’d measure and calculate the 6L6 plate dissipation, just in case it might need a cathode resistor tweak. I think they just took the 200 Ohm value from an early 6L6 datasheet.

        The original speaker in it is pretty close to a Jensen P12N, nice Alnico magnet.

  2. Charles says:

    I’m repairing a 1946 Minshall Organ. It’s an electro-mechanic organ based on static electricity pickup of metal reeds oscillating through the input of compressed air (just like an old Wurlitzer). I was new to this at the beginning of the project but have been reading much and ordering very useful books.
    Question being, it seems a relatively simple task to replace a 5 pin amphenol socket (1 grounded pin/1 support pin/1 tone change switch pin/1 transformer-related pin/ forgot about the last pin) used as an audio output for the whole tube-amplifier just like traditional electric organs by a 1/4 ” jack, but are there any important details I should take in consideration before doing so???
    I might have more questions later but don’t want to bombard all at once.
    Thanks for the time,

    • jim says:

      If you just need to replace the amphenol jack it should be pretty straight forward as long as you find a replacement (some can be hard to find) and wire it the same way. I don’t have a lot of experience with some of the old odd connectors which is why I just replaced the two microphone connectors with normal 1/4″ jacks on this amplifier. If you have a more specific question relating to it perhaps I can help further.

  3. Charles says:

    Question being how can I replace an amphenol 5 pin female leslie socket by a 1/4” female socket?

    • jim says:

      I was able to replace mine because mine were just the two conductor (signal/ground) amphenol connectors and that translated nicely to a tip/sleeve of a 1/4″ jack. indicates that two connections were for line AC (probably for the leslie motor), black is ground and green was for the speaker. If you’re just connecting a speaker perhaps the black/green would work but you’ll need to take measurements to ensure you don’t hook line AC up to a speaker. I did that once when I was a kid thinking I would hear 60 Hz. The speaker blew up.

  4. Charles says:

    Should I change the 5U4G of my amp if it is at least over 60 years old, still glows a little, and still generates heat? Possible gas leakage with time?

  5. Rouel Sanchez says:


    I’m not really knowledgeable when it comes to tube amps so I hope you guys can help me out. I have a what appears to be a 50′s Flot-A-tone. It has 2x6l6, 1 x 6n7, 1x 5u4, and 1x 6sc7.

    Here’s the thing. I assumed that since this is american made, its voltage input (From the wall wart) is 100v. Is my assumption correct? I live in the philippines, and the voltage here is set at 220v, so I used a step down transformer between the wall wart and the amp’s plug/power input, so that I can achieve the 110v which “I assumed” is the input voltage of the amp ….but I noticed in your diagram that after the switch, you indicated 220v,… should that be the power input instead of 110v? The amp works…but it is not as loud as I “assumed” it would be.(There’s no 110v or 220v mains input or any schematics or any info inside the amp)

    Your help will be greatly appreciated

    • jim says:

      That 250V noted after the switch is the rating on the fuse. Wall AC here in the US is 120v today but when this amp was made it was a bit lower (110-117v range). I have no idea if Flot-a-Tone made amps for overseas but it is possible. If you have a variac or an AC power supply you can tell for sure with this test:

      • Unplug the amp from the wall
      • Discharge the filter caps so you don’t kill yourself
      • Disconnect the 6.3v heater secondary winding from the tube heaters
      • Set a variac or AC power supply for 6.3 VAC and hook it up to the 6.3V heater secondary
      • Measure the AC voltage at the primary. It will read around 110V or 220V probably. Which ever it is was the intended supply voltage

      Transformers work both ways so the idea is that instead of stepping down 110v or 220v to 6.3v we’re going to step 6.3v up to either 110 or 220. Hope this helps…

      • Hi!!!!

        Sorry for not being able to dropby this page earlier. Thanks for the info sir Jim, and I will try out your suggestion…however, I dont have a variac yet, but I can post the results as soon as I get one :)

        I do have a follow up question that I also hope you guys can shed some light on. You see, there is no data whatsoever indicated on the the Flot-A-tone amp. The voltage here is 220v, versus the US voltage of 110v. I am not entirely familiar with this amp’s history.The amp currently works. But what if the previous owner accidentally plugged it directly into 220v ac? Would that have an effect on the amp? on the Transformers? How will I know if that happened to this amp in the past? Is there a way to check?
        Again, your reply will be much appreciated.(“,)

        Thanks :)

  6. Billy Stephens says:

    I have what appears to be a 1948 Flotatone which is very similar to the one on this page but it has 6V6 power tubes, 2 6SC7′s and a 5U4. The power supply does not have 3 power taps but only 2, as I said a bit different but at a glance you would think they are the same. I believe the output transformer is toast. Which one would you suggest as a replacement?

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