Sears 40XL

[Just Mike]

I wrote about this amp a while ago. Didn't get much interest, and I forgot about it. But I'd like to get this thing working now and use it asa project amp. If I can't find a schemo, maybe someone could just give me some voltages to check? It's got 2 EL84's, 3(I think, I'm at work, so I'm going on memory) 12ax7's and a rectifier tube. The OT's been replaced, and it DID work with the replacement OT, but the output was real low.
I know I should probably replace all of the caps and resistors due to the age of this thing...anything else?

[RWood]

Mike,

I just took one of these in, I will post here what voltages I get when I open it up. Right now I am scratching my head trying to figure out how to remove the chassis from the cabinet!

RWood

[RWood]

Voltages

The hardest part about opening it up was the pilot lamp retaining spring, which sandwiched the faceplate to the chassis. Oy.

Here are the voltages I got after changing the power tubes' cathode resistor from 120 to 150 ohms.

V6 6X4
Plate to plate 321 vac
Cathode 350VDC

V4 & V5 7189s
Plates 348 vdc
Screens 325
Cathodes 10.62

V3 12AX7
Plates 173 / 139

V2 12AX7
Plates 106 / 110

V1 12AX7
Plates *** / 295


*** On Pin1 of the input tube, I could not get the voltage to settle. Even after replacing the two coupling caps and plate load resistor that lead from that pin, and trying a different tube, the voltages here bounced around continuously in the 160 to 220 range.

On the B+ side of the (330K) load resistor I get a steady 315 vdc. On the far side of the two new (Mallory 150) coupling caps I am reading from -12 vdc to + 12 vdc, also bouncing around.

Can someone help me understand what is going on here with the voltages and what I need to do to fix it? As it stands the amp works, has a midly unpleasant fuzz tone from 1 on up. Mind you it is biased pretty hot even with the 150 ohm Rk.

Thanks,

RWood

Btw, this amp came stock with Sprague 220 orange drop couplers and tone caps and Planet filter caps

[Enzo]

Use a clip wire and ground the grid of that triode, pin 2. Any difference?

What is the cathode doing for that triode, pin 3?

Is the cathode resistor intact and close to value? DOn;t measure the resistor itself, measure from pin 3 of the tube to ground. That way the conections are included.


Uh wait.... could this by any chance be the tremolo tube? Does turning the speed control affect the rate of voltage bouncing?

[RWood]

could this by any chance be the tremolo tube? Does turning the speed control affect the rate of voltage bouncing?

Yes indeed it is!

Oddly, the voltage settles as I increase the speed and or intensity. Oh well.....you nailed it!

Thanks Enzo.

[Enzo]

SO it is working normally then. Your meter doesn;t respond instantly, it gives average readings. The faster the signal oscillates, the more your meter falls behind. A scope would show the real story.