webcor 4905

[DummyLoad]

table-top amp i picked up in late 2015. has been kicked out the way in the shop long enough. so, time to do something with it or salvage iron and scrap. unit did fire up but needed PS filter attention. has a 6-1/2" woofer and 2" weeter with a 2.2uF x-over network. one 12AX7, 2 x 12AB5 and a 5Y3 rectifier. simple layout: voltage amp -> level control -> concertina -> output. not enough gain for geetar though.   

odd PS connection scheme and cathode coupling of the output stage. yes, the PT connections are correct - "A" node to G2 and "B" node to plates. schematic was transposed from paper copy glued to inside of cabinet. i confirmed connections when i opened it up. photo of original schema and transposed with CAD copy attached. have photos but not worth the BW to post them...


amp used ceramics for coupling caps, paper caps for phase inverter to power stage coupling caps, and paper phenolic sockets. low cost parts. low cost cabinet materials with a burlap grill cloth.


attached schema of proposed mods.  nothing too elaborate and i should be able to squeeze in a 7pin bottle. another 150mA load on the filament string secondary shouldn't be an issue.


--pete

[sluckey]

"A" node to G2 and "B" node to plates

Don't think I've ever seen that before.

[92Volts]

The separate OPT winding used for cathode feedback is also something I've never seen before. Obviously the idea is similar to ultralinear mode, but I haven't seen it implemented this way.

Also if I'm interpreting the phase correctly, I think this is providing positive feedback?

[sluckey]

Also if I'm interpreting the phase correctly, I think this is providing positive feedback?

How can you tell? I don't see any phase marks.

[terminalgs]

webcor used lots of funky OT's on tape players and amps like this.      I bought a webcor amp like this but it had an extra 12au7 out front for a mic.  I couldn't get it do anything interesting for guitar as an amplifier,  it was maybe 2W or 3W.  I think they were sold to go along with a record player or a tape player to call bingo at the Elks lodge, or as P.A. in the elementary school cafeteria.


I like how the schematic has signal voltage measurements for 1Khz all along the way to the speakers.


the 12.6V filaments limit your options for using other tubes.  Maybe a 12V6GT or 12AQ5 and an SE OT? but 280V B+ will also limit possibilities (is there a 12V EL84?).


you might opt to use it with just a couple 12AX7's and a preamp unit, or a matchless hotbox overdrive device.


Below is what I did with my webcor cabinet.  this was my first chassis build for an amp that I bent up myself.   These days I visit the fastener aisle at the hardware store and don't need to weld nuts and bracket like I did ten years ago... :-D.   The circuit is a 5E3 with the 12AX7 in the upper chassis.  Two chassis and a weber AlNiCo 10" was a super tight fit.  The dog is 13" at the withers (for reference).











Also a note about the "6V6GT"s you see above:  these are the russian "equivalents" 6n6c's I think.  absolute garbage, not capable of living at 400VDC+ at the plates for very long.

[DummyLoad]

meh! if i do decide to, i'd use it as tubed with simple mods. maybe would make for a decent 5W P-P practice amp? if you look at the mods schema, i added a 12AT6 for concertina and converted the 1/2 12AX7 concertina to another gain stage. i guess it'd be either a P-P champ or partially castrated 5E3...call it ace!


--pete

[mresistor]

I like his neckerchief..   

[terminalgs]

meh! if i do decide to, i'd use it as tubed with simple mods. maybe would make for a decent 5W P-P practice amp? if you look at the mods schema, i added a 12AT6 for concertina and converted the 1/2 12AX7 concertina to another gain stage. i guess it'd be either a P-P champ or partially castrated 5E3...call it ace!

I could never get anything but fuzz out of it (like what you get out of an AC/DC amp..),  but maybe b/c I was using the mic input which had an an extra 12AX7 triode.




With those signal voltages,  maybe its a 12K:8ohm OT?  The cathode winding, which I would think would really be on the secondary side as a tertiary winding.  I would think you could re-wire the cathodes in a more familiar way,  but you'd need some load on that cathode winding., maybe a 1K resistor? 

I like his neckerchief.

In CAT sprint testing Dolly has clocked 100yds in 8.5sec (24mph),  in Dock Diving her record is +17ft.  She is a beast.

[mresistor]

I like his neckerchief.

In CAT sprint testing Dolly has clocked 100yds in 8.5sec (24mph),  in Dock Diving her record is +17ft.  She is a beast.

Dayum !   that is one helluva dog !   

[PRR]

> "B" node to plates.

Interesting the parchment shows 280V to the OT and then 285V arrived at the plate.

The A-B swap makes so little sense that I suspect it was Happy Hour at the bar and the gaff never got caught as it went to production and printing. So buzz is not as low as hoped, not bad enough to care.

The NFB OT was a standard item at Webcor. Good tiny hi-fi. May be a bit clean for geetah.

[92Volts]

How can you tell? I don't see any phase marks.

I'm assuming the end of a winding toward the top of the schematic is in phase with the end of other windings oriented that way. This is true for the 2 halves of the plate winding (or their phase would be wrong for PP), but it's only an assumption that the cathode winding is phased like that.

[sluckey]

Each end of the cathode winding will be 180° out of phase with each other. Same as each end of the plate winding. But with reference dots (or something) there's no way to tell if the phase relationship between the two windings is such for correct feedback. It's a 50/50 assumption. If not right, just switch ends for one of the windings, makes no difference which one.