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:
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:
Some things that I noted:
- The tubes appeared to be original (ouch) and definately needed to be replaced.
- The electrolytics likely needed replacing.
- 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.
The process for doing this is:
- Wire up the hot and neutral to the primary of the power transformer
- Wire the ground to a dedicated screw and nut connected to the chassis (I use a nylong locking nut)
- 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.
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:
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:
I replaced the output transformer with a much beefier one from http://www.musicalpowersupplies.com/, 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.