JM Signature Build

[martin manning]

The layout doesn't have balance resistors in parallel with the caps on the stacked reservoir (see the schematic), and you really should add them as Steve is suggesting. Trying to keep what's there as much as possible, I'd do something like the modification shown below. You could do all this without lifting the filter board by running a separate wire from the screen cap negative direct to the star ground, and running the diagonal jumper between the main filter caps on top of the board, but you'll have to get the existing jumpers (between the lower ends of the main filters and the one from the screen negative) out somehow. The balance resistors can be 220k-270k, and at least 1/2W.

[67plexi]

Martin, Thank you for clearing that up. I tried to post photos but my
anti-virus software is blocking me posting images.


Steve.

[M Fowler]

I put my 220k or 270k balancing resistors under the board because I had less room then you do.

[dreric]

Finally had some time to go through this one and clean up a few things, the amp works now! For those interested the mistakes I found were:

1. 2 UN-soldered joints, one cold
2. In an attempt to color coordinate the preamp tubes I thought one thing and did another, reversed pins 1 and 3, 6 and 8 on three tubes.
3. Forgot the the PT had a center tap on the heaters and installed the 2-100k to ground.

If I wasn't self taught I'd be embarrassed over these amateur mistakes, but fuck it this is how I learn.

I also followed Steve and Martin's advise and rewired the power supply using the 220uf caps.

So how does it sound? OK more distorted that it should be and the power is about a third of what I would expect. I think the problem is in the preamp, here ate the new voltages: anything jump out at anyone?

V1
pin 1: 191
pin 2: 0.03
pin 3: 1.76 vdc
pin 6: 194
pin 7: 0.02
pin 8: 1.66 vdc

V2
pin 1: 274
pin 2: 0
pin 3: 2.0 vdc
pin 6: 264
pin 7: 0
pin 8: 2.14 vdc

V3
pin 1: 406 vdc
pin 2: 0
pin 3: 5 vdc
pin 6: 404 vdc
pin 7: 0
pin 8: 5.05vdc

V4
pin 1: 217 vdc
pin 2: 0
pin 3: 1.7
pin 6: 209
pin 7: 0
pin 8: 1.63 vdc

V5
pin 1: 409
pin 2: 0
pin 3: 84
pin 6: 409
pin 7: 0
pin 8: 84 vdc

V6-9 power 6v6
pin 1: 46 vdc
pin 3: 439vdc
pin 4: 440vdc
pin 5: -35vdc

[martin manning]

Progress...

V5 (the PI) should not have zero volts on the grids, so check the measurement and wiring there. BTW, you should not measure the grid voltage at pins 2 and 7. Instead, measure at the junction of the four resistors (2x1M, 820, and 24k) so your meter's input impedance doesn't change the bias point.

I don't understand the 46V at pin 1 on the 6V6's.

[dreric]

This is getting frustrating. I'm 99% sure that I've gotten all the bugs out of the wiring. Fired that amp up, still weak on volume then it made a slight "whump thud like sound" and wen silent.

I checked all my voltages the are as follows

V1
pin 1: 195
pin 2: 0
pin 3: 1.76 vdc
pin 6: 204
pin 7: 0
pin 8: 1.7 vdc

V2
pin 1: 281
pin 2: 0
pin 3: 2.0 vdc
pin 6: 274
pin 7: 0
pin 8: 2.1 vdc

V3
pin 1: 410 vdc
pin 2: 0
pin 3: 5 vdc
pin 6: 410 vdc
pin 7: 0
pin 8: 5 vdc

V4
pin 1: 220 vdc
pin 2: 0
pin 3: 1.7
pin 6: 209
pin 7: 0
pin 8: 1.63 vdc

V5
pin 1: 414
pin 2: 54
pin 3: 73.3
pin 6: 416
pin 7: 52.6
pin 8: 73.3 vdc

V6-9 power 6v6
pin 3: 448vdc
pin 4: 447vdc
pin 5: -39vdc
bias @ 33ma steady

So................I'm wondering if the whup sound was a tube failing? does that make a sound?

There seems to be a slight, I mean hard to hear high pitched sound, you couldn't hear it over talking, could that be parasitic oscillation, how would I know.

[martin manning]

Voltages look reasonable, except maybe the PI.

In your schematic you show both the PI and reverb driver being powered from the same B+ node, while in the layout 3A goes to the reverb driver and 3 goes to the PI. Which node is connected to which and what are the voltages?

Try removing the reverb driver tube and see if that has any effect.

[M Fowler]

The PI goes to 3 and the reverb goes to 3A

[stephenl]

Looking at the manual for thr JM Sig, there's acontour control on the reverb return. From the description, it sounds like a tilt control. I wonder if it's implemented like this: