Wright Mono 8 Schematic

[jdrouin]

I just heard back from an engineer at DeYoung, who gave me the specs and a drawing of the DMI 500-7694 they used to make for George. The HV winding is 620VCT 100mA. At 310V per side, it confirms my suspicion that the Mono 8 ran at closer to 2A3 voltages.

The drawing also clears up a mystery encountered in the 6EM7 version of the amp, where a light blue and a white/yellow wire from the PT are soldered together on a tab near the PS but are not connected to anything else. Turns out they are legs of the two 2.5V windings, which combines them into a 5V winding for the 300B filament.

It's hard to read, but the two yellow wires are a 5V 2A winding for the rectifier.

Just thought I'd share here.

[Paul Birkeland]

I'm sure Heyboer could clone those for you, and I've found their lead time to be very pleasing.

[Paul Birkeland]

The owner of the amps was kind enough to measure the 6EM7 second plate resistor, and it's 25.7K -- so that's an orange stripe, making it a 27K 5% resistor.
Orange stripe -- Rp2 27K
B+: 357.8V
6EM72 plate/cathode: 70.9V/9.5V (9.46mA)

Oh thank goodness, that makes a bit more sense.  You won't find a lot of sympathy on this forum for running a triode at 7% of its maximum dissipation.  With 375V available,  a 10K load line looks pretty nice.  25mA/20V of bias puts you on a reasonably linear cross section of the curves with gobs of headroom (but be sure your power transformer can deliver the extra current).

[jdrouin]

Thanks for that, CB.

I just came across a thread at AA where an owner of a Mono 8 w/6EM7 reports that George said a 6SN7 (among other tubes) could be swapped in place of the 6EM7, but for less gain.

https://www.audioasylum.com/cgi/t.mpl?f=set&m=1475

It makes me wonder if perhaps George did not substantially change the circuit for the Mono 8 for each "new" driver tube over the years, but kept it more-or-less the same and simply offered it with different input/driver tubes. Do any of you who knew George know anything about that?

Looking back over the images of a Mono 8 w/6SN7 that I posted at the beginning of the thread, with the new knowledge gained in the meantime, it appears the plate and cathode resistors are largely the same as in the 6EM7 model (with maybe a few ohms difference on triode 1 Rk) and that triode2 uses a bypass cap (the little white one that's hard to see) that's not in the 6EM7 model.

Back in reply #2, CB suggested a 470uF/10V bypass cap at 6SN7 Rk2. The little white one in the pic says "120" but I can't tell what that pertains to. It looks like a metalized film type rather than an aluminum electrolytic.

Anyway, putting a 6SN7 into the LTSpice model, with Rp1 100K & Rk1 3K | Rp2 27K & Rk2 of 1K, and 323V B+ available to that tube (there's 357V B+ for the 300B), we get:

B+ -- 323V
6SN71 plate/cathode -- 139V/5.5V (1.8mA)
6SN72 plate/cathode -- 177V/5.4V (5.4mA)
Can take up to 0.7V input signal before clipping

I tried my hand at load lines (attached), which look OK though I'm not sure I did it right.

6SN7 Triode 1:
B+ 323V
Rp 100K
323/100K = 3.23mA

So the line goes from 3.23 mA at 0V to 0mA at 323V. The line seems to match what I get in the model as 1.8mA at 139V. Does that seem correct?

6SN7 Triode 2:
B+ 323V
Rp 27K
323\27K = .0119

Let's round that to 12mA at 0 volts and 0mA at 323V. Likewise, this line seems to match the 177V/5.4mA point on the model.

What do we think about that? Should I get the couple of resistors and caps that I'd need, and do it up on the breadboard tomorrow?

[Doc B.]

George occasionally had people doing assembly for him. I'm afraid I don't know who. But perhaps someone who does know could point you to them, to see if they have any info for you.

[Paul Birkeland]

Yeah, those load lines look to be drawn properly.

The 6EM7 triode 1 is super wimpy, and putting a 6SN7 in its place will run the 6SN7 sub-optimally.

[jdrouin]

We also have that 1uF cap that's coupling some power supply noise to the output stage likely to cancel out some of the ripple in the high voltage supply. Your mystery cap could form an AC voltage divider with the 75K resistor to inject some more power supply noise onto the grid of the 300B.

For what it's worth, if you're building from scratch, I would leave out the 1uF cap, the 75K resistor, and the mystery cap with the 75K resistor.

I have all the parts for this build now except for that 1uF non-polarized metalized film in the PS that CB suggests removing. Maybe that's an RC snubber circuit meant to limit extreme voltage peaks from entering the power supply on startup, and protect components downstream?

While searching Mouser I came across an affordable Cornell-Dubillier snubber cap that would seem to fit the function there:

http://www.mouser.com/ProductDetail/Cornell-Dubilier-CDE/940C6W1K-F/?qs=sGAEpiMZZMv1cc3ydrPrF5lJsZdC82VlV9%252bBDCfCjzI%3d

The 4uF 10V silvery-blue cap that connects between the 270K and 75K resistors could also serve a similar function, I suppose, though that's in the ground network.

Attached is the (I hope) penultimate draft of the schematic and parts list for the Mono 8 w/6EM7. I missed the little coupling cap inside the 6EM7 and had to add it later as C0. Pending suggestions, I will do a nicer drawing and scan it better, and/or offer LTSpice files.

I only have one 6EM7 on hand and don't want to buy a matched pair, so I'm going to breadboard the 6SN7s to hear what we get and can tweak from there.

Need to get my hands on a pair of MQ TFA-204 right quick, though. The big 5K:8ohm Electra-Prints I'm using don't have a place in the lower voltage rows of the WE op points table.

[Paul Birkeland]

C6 and R13 don't serve any purpose that I can determine. 

[jdrouin]

The circuit is up and running om the breadboard, and sounding good on the test speakers. Right channel 6SN7 plates are a few volts higher than their left channel counterparts, but everything else is closer.

300Bs are now running at 310Vak/65mA, so a slightly larger dropping resistor after the rectifier tube, up from the 33ohm one currently in place, should bring it down to the target 300Vak/60mA.

Left
Rout -- 372
B+1 -- 370
B+2 -- 344
6SN7
P1 -- 148
K1 -- 6
P2 -- 186
K2 -- 5.8
300B
P -- 359
K -- 49
G -- -13 (DC)

Right
Rout -- 372
B+1 -- 370
B+2 -- 344
6SN7
P1 -- 153
K1 -- 5.8
P2 -- 193
K2 -- 5.7
300B
P -- 359
K -- 49
G -- -13 (DC)

[2wo]

Cool breadboard...John

[jdrouin]

Thanks! I had a chance to get back to this and update the schematic. Here is the 6EM7-300B version in beta 2, using call-outs to note the connections of various nodes to the back-bias circuit in the PS.