Monkeymatic Penta (Matchless Clubman inspired)

[martin manning]

I don't understand why you would want to waste another minute tweaking a circuit that is not functioning correctly.

Um, because it sounds good? But I guess you're suggesting there's a wiring fault (or defective tube) causing the issues, therefore, don't bother with tweaks until I solve this issue.

Exactly. Did you check g1 (pin 9) to see if that is the source of the plate and screen current?

[xtian]

Did you check g1 (pin 9) to see if that is the source of the plate and screen current?

No. How? Look for a voltage drop over the 100K resistors in the tone stack?


BTW, I ordered a pair of Russian EF86 to help troubleshoot.

[martin manning]

Did you check g1 (pin 9) to see if that is the source of the plate and screen current?

No. How? Look for a voltage drop over the 100K resistors in the tone stack?

Yes, see if there is any DC voltage at the input grid.

[xtian]

see if there is any DC voltage at the input grid.

I read -0.15vdc at pin 9 of the EF86. Which resistance do you use to calculate current? Plate load?

[martin manning]

It'd be the resistance that the current you want to calculate is flowing through, i.e. The tone stack. But I don't understand the negative voltage (to ground, right?), it should measure 0V.

[xtian]

I received two Russian 6Ж32П tubes today. Like the one on the right side of this photo: http://en.wikipedia.org/wiki/File:Four_ ... es_(1).jpg

Plugged them into my Clubman build, and read the voltages:

Code: Select all

       pin 1(scn)  3(cat) 6(plate)
Russian A    154   3.44   149
Russian B    152   3.80   116
JJ EF86       55   0.00    61

On the scope, the Russian tubes don't distort at all with 1KHz 1v input.
The JJ distorts but does not clip.

The screen values should be lower than the plates, right?!?

Anyway, can't crank it up now. Will report tomorrow on the sound.

Wish I had more EFs to try.

[martin manning]

Are those Russian tubes supposed to be equivalent to EF86? What values of Ra, Rg2, and Rk do you have installed now? As I recall you were changing those around trying to get the JJ tube to work. I suggest you put them back to the schematic values. Then, if these tubes are EF86 equivalents, look for voltages to be like the ones shown in the voltage table.

[xtian]

Are those Russian tubes supposed to be equivalent to EF86? What values of Ra, Rg2, and Rk do you have installed now? As I recall you were changing those around trying to get the JJ tube to work. I suggest you put them back to the schematic values. Then, if these tubes are EF86 equivalents, look for voltages to be like the ones shown in the voltage table.

Martin! Always with the practical questions!

Yes, the Russian tube are equivalent to EF86. Now I have reverted to values per the schematic: 330K Ra, 2M2 Rg2, 2K7 Rk.

And, what do you know, the voltages are nearly identical to the voltage chart on the schematic: 126 on plate, 94 on screen, 2.45 on cathode.

So this whole episode was prompted by the bizarre behavior of the one JJ EF86 tube I ordered. Which actually sounds pretty good. Anomalous tube, or typical for brand?

Anyway, now to a real question of import. The whole point of this amp was to be able to drive the EF86 into distortion, but this isn't happening. The distortion is coming entirely from the PI.

Merlin says adjusting the screen voltage is the most effective way to adjust gain is to adjust the screen voltage drop:

The screen voltage has much more control over how the pentode amplifies than the anode voltage. Lowering the screen voltage reduces headroom and compresses the grid curves. Do not exceed the rated maximum screen voltage or average power dissipation!

http://www.valvewizard.co.uk/pentode.html

Should I continue to play with the 2M2 screen resistor?

[martin manning]

Ya, go for it! Maybe put a 1M pot in temporarily between the 2M2 and the screen grid to find something you like.

I think that JJ tube is JJunk, DOA.

[JazzGuitarGimp]

I will refrain from brand-bashing. But I will say this (again), I will never buy another JJ - anything.

Qustions:
Have you measured the gain of the first stage?
Have you measured the loss through the tonevstack?

Merlin says the best way to coax "pentode sound" from a pentode is by hitting it hard with a preceeding stage. I haven't done a load line, but based on your supply voltage to the 12AX7 and its plate load resistor, I'm betting you don't have much gain there. You might consider splitting the AX7 and putting one triode before the tone stack, and the other one after the tonecstack, but before the EF86.

[jelle]

I guess the real question is: why is the JJ misbehaving. With that knowledge you might be able to make the others misbehave too. On the assumption that misbehavior is fun behavior...

What happens if you connect the cathode to ground directly on those Russian 6Ж32П. Be careful! You might blow it up...

Or just use JJ EF86 tubes if they have 'the tone'.

[Darkbluemurder]

If you don't get any voltage on the EF86 cathode, the cathode bypass cap could be shorted. I would check that first.

Cheers Stephan

[xtian]

If you don't get any voltage on the EF86 cathode, the cathode bypass cap could be shorted. I would check that first.

Cheers Stephan

Thanks. I had already removed the bypass altogether for testing; it was fine and worked the same without it.

[xtian]

Finsihed and looking plush.

[M Fowler]

Yeah that amp does look great and I like the logo

Oh and I love JJ tubes they work wonderfully on variety of amps.

Mark