Wright Mono 8 Schematic

[Paul Birkeland]

The 170 Ohm plate load on the second section of that 6DN7 is extremely unlikely.

[jdrouin]

Good catch. It's actually red-violet-brown-gold (2-7 X10) or 270 5%.

Also, any idea why the number "2" is stamped on that carbon film 3.09K cathode resistor for the 1st half?

[Paul Birkeland]

That's also not particularly plausible.  (Run it through spice or draw a 270 Ohm load line on the datasheet)

I would say that there's a white or silver band between red and violet that just isn't super visible. That would give an operating point of about 20V of bias and 20mA into a ~3K load assuming there's about 400V of B+ available.  It still seems a little off, but it will pass signal (the lower values of plate load will choke the available signal voltage at the grid of the 300B).

[2wo]

Don't know if you had a look at the old JE Labs 300B, a classic 6SN7 design. This was my firs SET project. here is a Google link...John

https://www.google.com/search?q=je+labs+300b&tbm=isch&imgil=9e0F-l2Ak9c_QM%253A%253B9kRUhpvyoqiamM%253Bhttp%25253A%25252F%25252Fwww.audiocostruzioni.com%25252Fa_d%25252Felettroniche%25252Ffinali-integrati%25252F300-b-daniele%25252F300-b.htm&source=iu&pf=m&fir=9e0F-l2Ak9c_QM%253A%252C9kRUhpvyoqiamM%252C_&usg=__yi_064IQDMIapNKlYlkLL3eBD6Y%3D&biw=1280&bih=671&ved=0ahUKEwj1pcyU9sDWAhXHj1QKHY39D_MQyjcIPQ&ei=GEXJWfW5Jsef0gKN-7-YDw#imgrc=qPQk-wPJF2GZpM: 

[jdrouin]

Thanks. Yes, I made a breadboard recently to try different designs, starting with the Fi Primer 300B and then the JE Labs. Really good sounding amps, and it's fun to hear the differences among the different circuits. Though the Fi Primer and JE are variants of each other.

They'd make great starter amps because they provide so much gain you don' need a line preamp. Put in a source selection switch and a volume pot, and you're good to go.

[jdrouin]

That's also not particularly plausible.  (Run it through spice or draw a 270 Ohm load line on the datasheet)

I would say that there's a white or silver band between red and violet that just isn't super visible. That would give an operating point of about 20V of bias and 20mA into a ~3K load assuming there's about 400V of B+ available.  It still seems a little off, but it will pass signal (the lower values of plate load will choke the available signal voltage at the grid of the 300B).

So, I read what you said and thought about it for awhile, and earlier this evening received pics from inside the other amp. The rake lighting in the attached image shows that there is no other stripe on that resistor -- it really is 270 ohm (unless the brown multiplier stripe is a discolored red or orange, but I don't think so).

I also learned that the power transformer was manufactured by DeYoung and is labeled with the number DMI 500-7694 0050, which Google shows me was also used on the WPA 3.5. I've written to DeYoung to ask for the specs, but it's possible that the Mono 8 ran at or near 2A3 voltages. Maybe he had one PT made for multiple models, and there was a higher voltage tap available for the Mono 8, but we'll see.

I tried the 250Vak and 300Vak 300B operating points in LTSpice just to see what the 6DN7 looked like.

250V (63mA)
B+ -- 306V
Plate/Cathode 1 -- 117V/4.4V (1.4mA)
Plate/Cathode 2 -- 257V/13.5V (13.5mA)

300Vak (77mA)
B+ -- 367V
6DN7 Plate/Cathode 1 -- 140V/5.3V (1.7mA)
6DN7 Plate/Cathode 2 -- 309V/16.3V (16.3mA)

In either case it can take about a 2V signal before clipping, so, feasible for use with a line preamp.

[Paul Birkeland]

6DN7 Plate/Cathode 2 -- 309V/16.3V (16.3mA)

16.3mA across a 270 Ohm resistor is 4.32V of drop.  That stage of the driver can swing down nice and far, but can only swing up a few volts...

(You'll also generate a lot of distortion with this loading)

[jdrouin]

Correction: The amp was not the later 6DN7 version but the earlier 6EM7 version. Silk screening on the chassis says 5Y3 Russian, 6EM7, and 300B, and the owner happens to be using a 6DN7.

I also corrected some schematic errors, making sure all of the back-bias components connect to their spots in the PS and that the negative side of the DC voltage source connects to the PS ground bus, where the PT center tap connects on the real thing. Also the ground bus connects to the ground tab on the RCA input jack, I suppose to carry the ground path through the interconnect to the line preamplifier. I'll have to redraw it but I think this is technically more accurate.

So, inserting a 6EM7 into the model and keeping the 300B at 300Vak, we get the following:

B+ 353.3V
300B -- 300Vak/59.2mA
6EM7-1 plate/cathode -- 171.6V/2.4V (776uA)
6EM7-2 plate/cathode -- 240V/33.9V (33.9mA)

That's pretty low current on that first stage of the 6EM7.

The model will take an almost 4V input signal before clipping.

[Paul Joppa]

The unbypassed cathode resistor of 1000 ohms provides considerable negative feedback. In conjunction with a 270 ohm plate load, the gain of that river stage is around 0.2 (i.e. -14dB. It also increases the effective plate resistance of the driver triode - that at least is not an issue since the output impedance is swamped by the 270 ohm resistor.

The loss of gain means also a loss of peak voltage. If the 6EM7 driver stage is running 33.9mA quiescent, then cutting that current to zero will produce the peak positive excursion of 9.15 volts - nowhere near the 45v needed to drive the output tube grid. PB already made that point, I'm just amplifying it (sic).

For the above reasons, I still don't believe it is a 270 ohm resistor, at least in the original design.

[Doc B.]

PB already made that point, I'm just amplifying it (sic).

[jdrouin]

"For the above reasons, I still don't believe it is a 270 ohm resistor, at least in the original design."

I think I understand your reasons, as I'm still learning basic tube operation. However, as an aside, it's funny that every time I post a George Wright schematic, people find some utterly baffling choices that can take up a lot of screen real estate. The 522K resistor from 6SN7 pin 6 to 2 in the WPA 3.5 is another case in point, where your own foray into the topic sheds some light on its role: https://www.audioasylum.com/messages/set/72516/re-wright-3-5s.

As a novice, it's amusing to see how George marched to his own drumbeat and continues to puzzle even experienced hands from the beyond. Wish I'd gotten into this earlier, as it sounds like he was a great guy and I would've loved to meet him.

Back to the amp, it appears to me that the 270ohm resistor is indeed intentional despite the loss of gain. The owner of the amps says he bought them directly from George in 2001 and they've never been modified. Maybe the oral history is inaccurate due to faulty memory or something, but every day he listens to and enjoys the very amps we're picking apart here. So it's not just a hypothetical.

Let's suppose that what I see as a brown multiplier stripe in several images is a discolored red or orange one. CB has already given his doubts about that, but here's how the model changes assuming a red (x100) or orange (x1000) stripe with the same op point of 300Vak/60mA on the 300B:

Red stripe -- Rp2 2.7K
B+: 354.4V
300B: 300Vak (60.2mA)
6EM71 plate/cathode: 186.4V/2.6V (834.4uA)
6EM72 plate/cathode: 196V/27.4V (27.4mA)
Can take up to 3.25V singal before clipping

Orange stripe -- Rp2 27K
B+: 357.8V
300B: 301.3Vak (63.3mA)
6EM71 plate/cathode: 227V/3.1V (994uA)
6EM72 plate/cathode: 70.9V/9.5V (9.46mA)
Can take up to 0.7V signal before clipping

And one final thing I want to bring up because it could be a clue to the op point of 6EM7 section 1: the 3.09K carbon film resistor at Rk has the number "2" printed on it in both amps. I've never seen that before. Does it have a bearing on the resistance value? Pic attached.

[Paul Birkeland]

Red stripe -- Rp2 2.7K
B+: 354.4V
300B: 300Vak (60.2mA)
6EM71 plate/cathode: 186.4V/2.6V (834.4uA)
6EM72 plate/cathode: 196V/27.4V (27.4mA)

That is making a lot more sense.  In this case, 73-ish volts are dropped across the 2.7K resistor, so there's adequate compliance between the plate and power supply to get some decent signal voltage out of that stage.

On the possibility of 27K, now the plate voltage is so low that you're running out of room in the opposite way compared to the 270.

[jdrouin]

That is making a lot more sense.  In this case, 73-ish volts are dropped across the 2.7K resistor, so there's adequate compliance between the plate and power supply to get some decent signal voltage out of that stage.

That's what I thought too. What about the less-than-1mA current on the first stage though? Is that too low? I've never modeled anything which had that little current before.

[Paul Joppa]

The 6EM7 small triode (first stage) has a high gain (60) and a high plate resistance (40kOhm), but a lowish transconductance (1600uMho). That means it will necessarily run at a low current.

 A fairly common "standard operating point" is to use a beam resistance (plate-cathode voltage divided by plate current) of 5 times the plate resistance. That is almost exactly what you have.

The other issue is whether the first stage can drive the second stage Miller capacitance, which is at most 50pF, probably closer to 30pF with the unbypassed cathode resistor. The calculation is a bit tedious, but you want the plate quiescent current to be 5 times the peak current needed to drive that capacitance at 20kHz. To do the calculation, you need to know the signal voltage peak at the second stage grid, which depends on the gain of the second stage. The unbypassed cathode resistor makes that voltage. That takes more than just the data on the spec sheet, so I'll leave it as an exercise for the reader  :^)

[jdrouin]

Thanks for walking me through that, Paul. I'll look in Morgan Jones for the calculations needed to figure out the peak and quiescent current needed to drive a given Miller capacitance at 20kHz.

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.

That means that with the rest of the circuit as it is in situ, using a 300Vak/63mA op point on the 300B, the model is as posted above, or:

Orange stripe -- Rp2 27K
B+: 357.8V
300B: 301.3Vak (63.3mA)
6EM71 plate/cathode: 227V/3.1V (994uA)
6EM72 plate/cathode: 70.9V/9.5V (9.46mA)
Can take up to 0.7V signal before clipping