Bottlehead Forum
General Category => Technical topics => Topic started by: jdrouin on September 11, 2017, 12:33:50 PM
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I thought I'd ask this question here because some people used to know George Wright, and some of his ideas contributed to early BH products. I've made a breadboard to try different amplifier circuits, starting with old standards like the Fi Primer and JE Labs 300Bs. I love Wright equipment, and have successfully breadboarded an amp that is the front end of his WPA 3.5 (2A3) but with a 300B output stage.
I'm curious to try the Mono 8, and while I haven't found a schematic anywhere (I know, he never wrote them down), I did attempt to reverse-engineer it from hi-res pics in a recent internet sale.
There are 3 components in the 6SN7 stage that I can't quite figure out the values of. Does anyone recognize them or happen to know what their values are likely to be?
I've attached pics from a recent internet sale, a drawing which duplicates and simplifies the layout, and my attempt at rendering a schematic.
What I don't know are the following three things in the 6SN7 stage (squared-in with red pencil in the drawings):
* Small red coupling cap from plate 1 to grid 2 (pins 5 —> 1)
* Second cathode (pin 3) resistor and little white bypass cap (peeking out from under the big red coupling cap)
(common values here are usually 23K-30K and 47uF 160V, but who knows?)
* Silvery-blue “F1" cap (I might see numbers 047 -- possibly 0.47uF?)
(it connects between the 270K and 75K resistors that go to the ground leg of the first [rightmost] 22uF/450V PS cap)
Many thanks for any insight you might provide, and sorry for posting so much info.
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George used a combination of new parts and parts from a huge stash of surplus parts he got from an old surplus electronics place called Radar Electronics. The parts that are not marked could well be from the hoard. Like many smaller manufacturers he seldom built things the same way twice. Best bet is to use your estimate of the most commonly used values, as George's stuff was pretty traditional.
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The first coupling cap would be fine as a 0.047uF.
I drew a loadline for a 6SN7 assuming about 300V of B+ available and a 27K load. 4V of bias and 5mA of plate current looks reasonable. That's a cathode resistor of 800 Ohms (820 is fine) and a 470uF/10V bypass cap should do the job.
If you build the amp as-drawn in your schematic, you'll smoke 2A3's and potentially your output transformer.
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Thanks, Caucasian Blackplate. I added those values into a SPICE model and got the bias and plate current you suggested. Wave form looks good too.
Do you think 0.47uF for the other unkown cap -- the one that connects between the 270K and 75K resistors -- is reasonable?
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Hmm, I didn't notice that the choke is in the ground leg (I had initially thought there was positive voltage being applied to the 2A3 grid, which isn't such a great idea).
Normally we would see the 270K resistor go from 300B grid to ground. 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.
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Thanks. I hear what you're saying. Wright designs seem to have an unorthodoxy that goes against modern wisdom. I'll try it as-is on the breadboard once I understand what it's doing (which you've helped me to do -- so thank you), and can then make changes like you suggest to hear what happens.
The thing that's bugging me now, though, is that 750R cathode resistor on the 300B. The earlier Mono 8s had a 6SN7 voltage amp and driver and the MQ FS-030 OPT, which is rated at 70mA quiescent current. So unless I'm missing something, in order to use a 750R resistor and not exceed that 70mA quiescent current, you'd have to run the 300B at 275Vak (according to my LTSpice experiments, anyway), which would result in like 5W output, not the 7W (12W peak) that George claimed on the earlier models.
I must be missing something.
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George changed the designs on these amps frequently and was known to not really keep notes about what he was up to.
The 70mA rating on the FS-030 is a recommendation. I wouldn't expect a performance meltdown running it at slightly higher currents.
Assuming that we can just make the B+ whatever we want but are constrained by a plate current limit of 70mA, and output transformer impedance of 3K, and a cathode resistor of 750 Ohms (this is not normally how we design a tube amp, lol), then we have a bias voltage that's about 50V. That would indeed lead one to conclude that you aren't going to get a lot of power out of the output stage!
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"this is not normally how we design a tube amp, lol"
I know, right?
Attached are two LTSpice screenshots that show the circuit with your originally-suggested res/cap values.
One shows the 300B running at 275Vak/70mA, and the voltages work out as follows:
6SN7 1: 153Vp, 6.4Vk
6SN7 2: 161.6Vp, 4.6Vk
300B: 328.5Vp, 53Vk
The other shows the 300B running at roughly 350Vak/91mA, with voltages as follows:
6SN7 1: 195Vp, 8.2Vk
6SN7 2: 204.4Vp, 5.9Vk
300B: 420.1Vp, 68.9Vk
I don't think either of these can be right.
Would an FS-030 be capable of handling 91mA?
Also, in LTSpice each of these can take a maximum signal voltage of just 0.25V before clipping. The Mono 8s were designed to work with his line preamps, just as the WPA 3.5 was. The WPA 3.5 requires an input signal of 3-4V for full output.
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At the old MQ website, the product info for the FS-030 said it could be gapped for up to 80mA. My guess is that gapping for more than 80mA would reduce the primary inductance enough that Mikey didn't want to do it.
Your simulations show that the first stage has more bias voltage than the second stage. Though you are able to clip the amp, I don't consider this ideal. You could go for 8V of bias and 4mA (2K cathode resistor) and see if things improve.
Your comment about the gain is spot on. I remember reading a Stereophile review of a 300B amp some time ago where the review thought that the amp didn't have enough gain but totally ignored that the input sensitivity of the amp was 1.5V for full output (it was a power amp, intended to be driven by a preamp).
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The original spec for the FS-030 was 60mA maximum. That is of course Mike's conservative rating. I believe this was chosen to match the 300B, that being the reference operating point in the WE data sheet - 300v p-k, -61v bias (cathode resistor ~1000 ohms, OPT 3000 ohms).
The RCA spec for the 2A3 is 250v p-k, 60mA, -45v bias (750 ohm cathode resistor), 2500 ohms load. George usually followed these specs. At one point, he used the DS-025 output transformer, matching the 2A3 spec of 2500 ohms.
All his SE 2A3 circuit variations I have seen had a cathode follower for the second stage, with the first stage plate direct coupled to the second stage grid. This is basically the same topology as the original Foreplay, which was designed by George. The coupling cap from the second stage cathode to the 2A3 grid was fairly large, 0.5 to 1.0uF as I recall. This allows the 2A3 to be driven into grid current (class A2), making possible a claim of 8 watts, as long as it sees short, infrequent transients, and you don't ask uncomfortable questions about distortion. It's still a 3.5 watt amp measured in the normal way. [I have myself used 275v/55mA/3000 ohms, which works pretty well.]
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Since you are starting from scratch, and have FS-030's, I'd not worry too much about what was actually in the Wright amp, but rather where you might want to be with the output stage. Western Electric was thorough to a fault, and I'd certainly start by reviewing their operating points (this may be a little quicker than tweaking the Spice model).
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Caucasian: Oh yes, I know that WE op point sheet well. ;) Also, your suggestion of 2K for the second 6SN7 cathode resistor is close. I read your comment on my phone, and then looked at one of the pics I'd attached and could clearly see the stripes are brown-black-red, or 1K. Unless it's grey-back-red, which would be 8K, but probably not. Funny, the colors didn't show up as clearly on my large computer monitor.
Paul: Yes, the WPA 3.5 as well as WLA 12 amps I've looked at are exactly as you described. Interesting thing about the WPA 3.5 is that it takes the signal from the 6SN7 2nd cathode, rather than the second plate as in the Mono 8, and capacitor-couples it to the 2A3 grid. That's what I did with the bastardized WPA 3.5/300B that's playing *very nicely* on the breadboard now. Btw it runs the WE-recommended 300B op point of 350Vak/50ma into a 5K load (I've got big 5K Electra-Print OPTs from an earlier project, not MQs).
Back to the Mono 8...
I suspect that one or both amps were tampered with, since they not only have some different component brands but different values as well. For instance, in one amp the 6SN7 cathode 1 resistor is apparently a 292 ohm carbon comp (red-white-red-black-black) -- unless that means 2-9-2-0-0 or 29.2K. In the other amp is a much larger wattage 3.9K 5% (orange-white-red-gold) carbon film.
But I hadn't thought about that second black stripe, so let's run with this happy accident...
Suspending our disbelief in my stripe-reading abilities for just a moment, let us speculate that 29.2K is a viable option in 6SN7 1 cathode. In the model, that drops the input sensitivity to about 2V before clipping (2.25V with a 39K resistor) -- much more realistic for use with a line preamp. If the same resistor in the other amp is not 3.9K but 39K (I don't think it is, but still), then there's still a difference between the two that calls into question the authenticity of the specimens before us. But let's try it anyway.
The below trials were done by running a 300B with 750ohm cathode resistor at 300Vak, which results in 77mA current. That's a little north of the FS-030 manufacturer specs but possibly within its realistic limits. That might just reach the 7W that George claimed for that particular version of the Mono 8.
With a 29.2K 6SN71 cathode resistor, we get these rounded results:
6SN71 -- 296Vp, 15Vk
6SN72 -- 190Vp, 5.8Vk
With a 39K:
6SN71 -- 308Vp, 16Vk
6SN72 -- 190Vp, 5.8Vk
And for the hell of it, with 8K on the second cathode resistor:
6SN71 -- 318Vp, 16.7Vk
6SN72 -- 312Vp, 14.5Vk
Edit: That last one can take up to 3V and a little more before clipping, which is closer to the WPA 3.5 input sensitivity.
All are within spec -- if a bit hot -- for the 6SN7 but it's odd that the first triode has higher voltage than the second, as I almost always see it the other way around.
Not sure how that would sound, but that's what the breadboard is for. Should I try it?
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Funny, the colors didn't show up as clearly on my large computer monitor.
Welcome to the 21st century world of over-hyped, supersaturated smart phone displays. It's like Apple and Samsung want everyone to feel like they're on acid all the time.
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If you have a bread board, try a single 6AB4 with a constant current source running 2-3mA of plate current and biased at 2-3V.
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Thanks for the alternative tube suggestion. Something to consider when I start designing my own circuits down the road.
An owner of the Mono 8 with 6DN7 input/driver (ca. 2001) sent some good pics of the interior, and I was able to generate a schematic (attached). I'm more interested in the 6SN7 version, and more examples will be necessary to get a handle on that one.
Still stumped on that 750R cathode resistor for the 300B. Would require a low voltage op point (ca. 250Vak and only 4W output power) to stay around 60mA plate current, unless George had his FS-030s custom airgapped for the 70mA range.
Is there some possible alternative that I'm not thinking of here? I've been studying schematics for 2 years -- a lot less than you guys -- but have never come across a 300B SET that's designed like this.
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The 170 Ohm plate load on the second section of that 6DN7 is extremely unlikely.
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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?
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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).
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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: (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:)
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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.
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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.
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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)
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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.
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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.
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PB already made that point, I'm just amplifying it (sic).
https://www.youtube.com/watch?v=LukyMYp2noo
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"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.
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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.
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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.
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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 :^)
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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
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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.
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I'm sure Heyboer could clone those for you, and I've found their lead time to be very pleasing.
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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).
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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?
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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.
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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.
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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.
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C6 and R13 don't serve any purpose that I can determine.
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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)
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Cool breadboard...John
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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.