Old Teisco running... almost. Hmmmmm.

Well, I got the Weber power transformer transplanted in the old Teisco and I get decent tones, but a loud 60Hz hum which increases with volume. Since there's only one tube (a 6C6) before the volume pot, it's got to be 'round there someplace. Of course, that's the part of the amp I touched the least.

I've replaced the PT, filter caps, cord, power switch, fuse holder, and removed some mojo, I suspect. I did remember to ground the center taps of the PT.

The 6C6 tube is one of those with the grid connector on top with a shielded wire. I s'pose I could check it, but I suspect shielding only would make a difference with RF interference, right? The input jacks are flimsy and a mess, so I can check grounding there. The heater wires don't seem to be near the grid, so I don't think the hum comes from there.

Hints about tracking this down?

 

I'm pretty sure it's 60Hz, and it increases directly with volume, so I suspect something with the first preamp tube. The heater windings have a center tap which is referenced to ground.

The filter caps had been replaced once, but to be sure, I replaced them again. The original (disconnected) cap can was 10/10, but probably had a direct cap off the rectifier that was 20 or so. The replacements were 22 - 22 - 8, and I replaced them with 22 - 22 - 10 (500V F&Ts).

Voltages look good (B+ around 360).

From the thread about diagnosing hum you (Celeste) sez: The two major sources of 60hz are heater to cathode coupling and magnetic coupling between the PT and OT

Since the OT is on the speaker frame and the hum is volume dependent, I'm not too worried about it.

Would heater/cathode coupling be a tube-internal thing? If so, I've got a couple of replacement 6C6's on order. I don't see any physical "nearness" of the heaters and the cathode leads. The 10uF bypass cap is old and I haven't replaced it yet, but I would expect the only thing there would be low-end response, right?

The input jacks are pretty dodgey, so I'll play with them and maybe try the 6V battery on the heaters trick.

 

First of all, Celeste, you Rock! Never cease to amaze... Thanks for providing that kind of background.

Since this layout (although with cheezy hookup wire) looks obsessively neat, any potential "Bozoing" would have been done by me, which is highly likely.

I've got a couple of 6C6s coming which will probably rule out the heater cathode coupling, but hey, worth a shot. I've got a few #80 rectifiers, so I can swap them and rule out issues there.

Thanks again; I'll keep ya posted.

 

PBC (Potential Bozo Confession): On my way to work, I was thinking about this 6C6 tube and remembered that someone (I guess that'd be me) powered up the amp with the 6C6 rotated by one pin. All I was getting was Buzz (no signal) and I tracked it down to that. In my defense, there's a huge tube shield around V1 and although the heater pins are a little larger in diameter, it is pretty easy to put the tube in wrong and I did. I had powered it up "softly" (light-bulb current limiter) and these old tubes are probably pretty tough, but I suppose putting plate voltage on one heater or heater voltage on a cathode could have damaged the 6C6.

I'll test this theory when AES sends me my 6C6 tubes.

 

Teisco-San Lives!

Well, I found a loose heater wire, not at V1, but at one of the power tubes, so I had an open heater circuit. Bad trip to buzz town.

So, a new PT, new e-caps, new power cord and switch, a bunch of spare tubes, and much, much labor from me and help from you guys and the dinosaur lives. I learned a lot, which is what it's all about for me.

It sounds kinda like I expected it would. It's probably sort of close in volume to a what I think a TV-front deluxe would be (push-pull, paraphase inverter, 2 6V6-like power tubes and a 5Y3-like rectifier). It's pretty squishy as you might imagine.

Thanks again for your help guys and gals.

Jim

 

Thanks for the congrats...

I think most of the signal gain must come from the 6c6 V1. The first 76 tube must do a little post-tone stack gain make-up and the other two do the phase splitting.

I still need to clean the amp up a bit for my friend, and I'll take some glamour shots and post them.