Monkeymatic Penta (Matchless Clubman inspired)

[xtian]

I'm about to start building the board. Layout posted below, if anyone wants to play "spot the goof."

[xtian]

I love this stage in the build. Nothing's gone wrong yet!

I also hate this stage in the build; I've forgotten to order a can clamp and a couple caps. *facepalm*

[mark enger]

xtian what ot are you using spek wise, i love cathiode biased el34
amps , and i have a matchless 1 12 combo sitting here, now a layout mark

[xtian]

Mark, I'm using a salvaged PT/OT set from a 35 watt, EL34 PA amp. I can only guess at the specs, but am just hoping it sounds good!

[Darkbluemurder]

Never built a Clubman but a Chieftain w/o reverb. That was my 3rd complete build and it was the most straight forward. I played it on several gigs with a country rock band and was very happy with the tone I was getting. I used a turret board and even put in two spare turrets so that I could change it later to Clubman specs!

I used a TAD JTM45 PT and a Marstran JMP50 OT.

Good luck!
Stephan

[xtian]

All finished and powered up. Getting sound out, but family is asleep, so can't turn it up above a whisper. Voltages on V2 and V3 are wrong. Time to put myself to bed, too, and get a fresh look in the morning.

[xtian]

I followed the (suspect) schematic dated 2009. My voltages are way off. Power tubes look OK (340 plates, 23 cathode), but V1 (12ax7) has 315 on plates. V3 (12ax7, the PI) has ~140. And my V2 (EF86) has only 102 volts on its plate, and I'm reading 0 across the cathode resistor, which can't be right, because I'm getting audio out of this amp.

I can't find anything miswired. So, start playing with the dropping resistors in the power supply? Do I start at V3 because it's closest to the source, and work toward V1?

[Tillydog]

Which schematic did you follow? (I haven't checked your layout.)

In one of the schematics you posted (the gif file) the B+ nodes are lettered, and they're wrongly identified (dropping 200V through a 6k8 resistor feeding a dual triode....???)

The pdf version (where the B+ nodes are given as colours) looks more believable.

From your voltages, it sounds like you followed the wrong one.

100V on the EF86 could well be OK.

[xtian]

You're right, Tillydog, I followed the GIF. Thanks! I'll swap the dropping resistors (220K vs 5K6) and see what that does.

[xtian]

MUCH better. Thanks again, Tillydog!

See voltages attached. Overall, about 20% lower than the schematic, which doesn't bother me (though I may replace recto tube with diode bridge).

Anyway, EF86 voltages are still too low. Only 36v on plate. I see that I have contributed to this low voltage by subbing a 10K resistor for the 8K2 dropping resistor, and a 2K7 for the spec'd 2K2 cathode resistor for the EF86.

Guess my next step is to drop than 10K power resistor to raise the voltage at the plate.

[Tillydog]

Glad to hear you're making progress

It sounds like you have a wiring fault / short on the EF86 cathode - you should be seeing some voltage there. I would concentrate on that area before adjusting the B+

[xtian]

Hmm. I tacked in another 10K in parallel to the existing 10K, making the dropping resistor 5K, and this raised my plate voltage to 36.8. Yes, basically no change.

Do I need to encourage my EF86 by lowering its cathode resistor, thereby making it draw more current, thus raising the plate voltage?

OK, I'll peer closely at my wiring again.

[xtian]

I had grounded pins 2 and 7 (the shields) on the EF86. Any trouble with this? I ran my 2K7 and bypass cap from pins 1 and 8 to pin 2 as a convenient ground location.

That's the only possible issue I see. Closeups attached.

[jelle]

I can't really see it well, but did you reverse your electrolytic cap?

[cbass]

I can't really see it well, but did you reverse your electrolytic cap?

Yes in the last photo it looks backwards