Monkeymatic Penta (Matchless Clubman inspired)

[xtian]

Thanks, guys, but no, the e-cap is pointed negative end towards pin 7, which is shield and is grounded. But maybe that's a good place to troubleshoot; I can remove that cap to see if it's bad.


I reverse wired the two tone pots. Duh. Also, they're useful only in the last 10% of rotation, even though they're audio taper. Still have to add the Cut control today, so I'll reserve my opinion on whether the tone stack sounds OK.

[Tillydog]

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

I'd looked at that in your previous pic, and the layout seemed OK to me, just leaving stray solder blobs, etc. to check for. Are you sure that the sleeved wire joining 3&8 isn't shorting to the bare wire between 2 & 7?

Might be worth taking some resistance readings with/without the EF86 installed to check.

You should be seeing a few volts at the cathode, whatever.

[xtian]

Yes, I removed the tube and checked to make sure I'm seeing 2K7 between pins 3,8 and ground.

Soon I'll try removing the 25uF cap to see if that changes anything.

I'm also installing the Cut control (was waiting on my Mouser package, but it had been sitting on my porch since Friday!).

[cbass]

I see now .It looked like the positive end was attached to the busbar.Sorry

[Tillydog]

Yes, I removed the tube and checked to make sure I'm seeing 2K7 between pins 3,8 and ground.

It might be worth checking that pins 2 or 7 aren't shorted to the cathode of the EF86.

You got another EF86 to try ??

Are you getting dead on zero volts on pin 9? I can't understand how there's enough current to drop the anode/plate voltage to 30V without anything appearing across the cathode resistor.....????

Unless.... Try re-flowing the solder joints on that 330k resistor from the EF86 anode/plate to B+

[xtian]

I added the cut control, officially finishing the build. That is, all the components spec'd are in place now.

I'm stumped by the low voltages on the EF86. But first, some observations:

This is a great sounding amp! With the MV wide open, it's really quiet at idle, but super loud when played. Must be the cathode biased power section.

Although the tone stack, as I have it wired (following the schematic), is NOT user friendly, there is a sweet spot at the extremes of the bass and treble controls that sounds wonderful.

So, back to the low voltages--maybe this is how the JJ EF806S wants to run? Anyone used one before? I don't have any other EF86 tubes to try.

Anyway, back to play testing.

[milkyBK370]

I have been following this thread since its inception. Now that the amp is up and running and sounding they way you want, can you post the finalised layout? I'd love to build this one.

[xtian]

I have been following this thread since its inception. Now that the amp is up and running and sounding they way you want, can you post the finalised layout? I'd love to build this one.

I will, after making some more important changes this week. I reversed the Treble, MV and Cut controls.

Also, I'm going to remove the bright cap on the Volume control, and increase the 220K grid leak resistor that follows the Volume control to 470K, per the PDF schematic posted above. Hopefully this will help push the tone and volume controls into more usable territory.

I'm taking this amp tonight to my AC/DC tribute band rehearsal, so I'll have a chance to play it loud.

[Tillydog]

So, back to the low voltages--maybe this is how the JJ EF806S wants to run?

Glad to hear you're having fun!

Did you look at the 330K plate resistor on the EF86? If there's a dry joint there, it could be dropping much more voltage than it should.

If you measure the voltage either side of the 8K2 resistor feeding the EF86 B+ node, you can work out how much current is flowing to the EF86 - does that tally with the current implied by the voltage drop across the 330K resistor (plus a little current for the screen)?

[rp]

Looking at your gutshot, I like your board, a nice clean reduction of a matchless ptp, better than their ptp. And this is coming from someone pretty dedicated to building ptp. Wonder why Matchless were so committed to the cluttered, hard to assemble, hard to service PTP style??? I do it for fun, I wouldn't do it if I sold amps, especially series of amps. I guess once you have a successful shtik you must stick to the shtik. Let us know when you get that thing sounding like Robert Cray's.

[xtian]

Did you look at the 330K plate resistor on the EF86? If there's a dry joint there, it could be dropping much more voltage than it should.

I'm certain it's good. Now, I have replaced the 330K with 220K plate resistor, as in Vox designs. Higher plate voltage now: 158v, but still reading zero at pins 1,8. So weird.

If you measure the voltage either side of the 8K2 resistor feeding the EF86 B+ node, you can work out how much current is flowing to the EF86 - does that tally with the current implied by the voltage drop across the 330K resistor (plus a little current for the screen)?

I'm using 10K in place of the 8K2 dropping resistor. So, 9.2v dropped across this resistor gives me 0.92mA. Can that be right?

Also, 169v drop across the 220K plate resistor gives me 0.77mA.

That's stupid. It should be eating a few milliamps, right? Maybe I should take the next step toward the Vox design and change the cathode resistor to 1K5. What do you think?

Looking at your gutshot, I like your board, a nice clean reduction of a matchless ptp, better than their ptp

Thanks, RP! Of course, while I'm testing and refining, it's getting uglier and uglier…

[xtian]

Oh, maybe the quiescent current consumed by the preamp tubes is very small, but ramps up when in use! I see that my V1 is also resting at 0.67mA.

[xtian]

Wow! Latest round of changes put this amp in the zone! Opened the master and the gain all the way and jammed with my SG. Suh-weet! Loud enough to make me a little queasy, but clearly not gonna cut it at my AC/DC rehearsal, where I normally play my JCM 800 2203.

Now that I have removed the bright cap and fixed the Treble control (swapped the orientation, and moved the 0.01uF to the other side of the pot, per the PDF schematic) the tone controls are all more useful. The Bass control gives a tremendous boost in gain in addition to bass when twisted clockwise.

While I was playing, I left my DMM clipped on pin 8 (cathode) of the EF86, other lead clipped to ground. STILL not seeing any voltage develop over the 2K7 cathode resistor. WTF?

[xtian]

Photos attached.

[xtian]

Incomplete, but hopefully accurate layout attached. Send me any corrections you find, please, if you use this. Illustrator file available by request.

Disclaimer: This is no longer a Matchless design, only based on. Many values are changed, two source schematics combined, other refinements made.