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In Reply to: RE: Yep - that is the one that... posted by RC Daniel on May 09, 2013 at 20:20:44
I drafted a derivative version for Mike at Magnequest, to use some of his iron in parafeed. I never built it, nor do I know anyone who has, but I'll share the circuit and notes if anyone is interested.
Follow Ups:
Here's the details. OK to build for personal use, keep Joe Robert's name and mine on it, all the usual.
Notes on the M91 derivative
by Paul Joppa, September 2012
In the first issue of Sound Practices magazine, v.1n.1 dated "Summer 1992," Joe Roberts presented a version of the Western Electric Model 91 amplifier, modernized and simplified for use as a hifi amplifier. This circuit is a slight modification of Joe's version, to use Magnequest transformers in parallel feed. A few other changes, based on Bottlehead's experience with 300B amplifiers, have also been incorporated, notably the use of DC for the 300B filament.
This design has not yet been constructed or tested, but is only a small deviation from similar designs that have considerable experience behind them. No guarantees are offered!
Component notes
R1 - this is the power supply bleeder, 50K ohms at 25 watts, wirewound.
R2, R3 - these were a voltage divider for the driver heater; the present arrangement uses the 300B bias to provide this function instead.
R4 - Joe used the original WE 880 ohm resistor, which is not an easy value to find. I show a 1K resistor, which is what we use in the Paramount and works very well with a 3K output transformer. This reduces the 300B current about 10%, keeping the PGP8.1 slightly cooler, too. The 50W rating is overkill but that's what Joe used and it does make it highly reliable.
Rstop - I use these 220 ohm grid stoppers on all tubes, though it's especially important on the 300B which has a high transconductance. Quality carbon composition is recommended.
R5 is the input resistor. You can substitute a 100K level control if you like; I'd recommend the PEC mil-spec carbon composition pots as being most in keeping with the intent of this design.
R6 is the driver bias resistor, 1200 ohms nominally. You may need to experiment with this to obtain the desired 170vDC at the plate - you want to be within +/-20v. Joe used carbon composition but I would avoid that here; metal film or wirewound would be good. The dissipation is tiny so anything ¼ watt or larger is fine.
R7, R8 are the screen power voltage divider. Joe chose 5-watt wirewound; the 30K R7 can be 3 watts but the 75K R8 must be 5-watt rated.
R9 is the driver plate resistor. Joe used carbon composition but in this position I would strongly recommend metal film or non-inductive wirewound. Minimum power rating is 2 watts.
R10 is the 300B grid resistor; Joe used carbon composition. I would use a smaller value of 250K, to allow for more grid leakage than original WE 300Bs.
R11 is the driver power filter, 27K 1 watt minimum. Again I would avoid carbon composition in favor of metal film or wirewound (does not need to be non-inductive).
C1, C2, C8 are power supply filter capacitors, all 15uF, 600v minimum. Joe used 1000v PIO caps that he had on hand, but suitable rated motor run caps should be equally good. Film caps will work well but might change the sound a bit. In my opinion, 15uF is the minimum capacitance but up to 27uF should work well without changing the character of the sound.
C4, C5 are cathode bypass caps; Joe used 16uF 75v or 100v oil or film. I would use a larger cap on the 300B, preferably 30-50uF, and a 150v rating, for better bass and greater reliability. The driver cap needs only a few volts rating. In my opinion, oil caps are likely to give the more traditional sound.
C6 - 8uF, 200v (minimum) again, PIO will likely give the more traditional sound. This screen bypass is sonically significant.
C7 - Joe specified a 0.1uF 600v Vitamin Q PIO, which is an excellent choice. A teflon V-cap might be another good choice.
Cpf - the parafeed capacitor should be between 4.5 and 18uF, I recommend the middle value of 8.2uF for the best compromise between deep bass and bass power handling. A 600v rating is the minimum, 800v or 1000v are preferred. My experience is almost entirely with polypropylene film caps, but PIO and motor run caps are worth exploring.
D1, D2, D3, D4 - I have specifieded these 8-amp Schottky rectifiers, type 80SQ045N, for their very low voltage drop, which makes is possible to obtain enough voltage from a 6.3v winding and a choke-input filter.
L1 - 10 henries, filter choke, 80mA or greater. The Triad C7-X is widely available and inexpensive. If you can find an older one that is layer wound, that would be more in keeping, but the modern bobbin-wound (made, I believe by Hammond now) will work just fine.
L2 - I chose the Magnequest EXO-003 plate choke for its high inductance. It is only rated 60mA but we have used it at 70mA with great success; the rating is quite conservative. For complete overkill, you can ask Mike about a BAC-80.
L3, L4 - 6mH 2A 0.3 ohm filament chokes, Hammond 155B. These provide choke-only filtering of the 300B filament power. An alternative is the Bottlehead FC-1 which has dual windings, so only one choke is needed per channel.
T1 - Magnequest PGP8.1 power transformer (PGP8.2 is the 240v version). This was a workhorse for Bottlehead for many years. The twin 2.5v windings were derived from a 5v rectifier winding on the classic original design, and I have returned them to that function here.
T2 - Magnequest TFA-2004 parafeed output transformer, 3000:VC. Extremely good-sounding transformer, this is a parafeed version of the TFA-204 which was sold by Peerless as a replacement for the WE91 original. Parafeed allows significantly deeper bass and freedom from saturation. It is available in a variety of core materials, though plain M6 or M4 will likely give the more traditional and expected sonic. Consider nickel if you are using the V-cap for coupling - a different sound, but cleaner.
Other notes
I chose a 6SJ7GT, which is quite similar to the original WE310A without the grid cap or 10-volt heater. It's not identical, so some adjustment of the bias may be needed to obtain the correct plate voltage (see note on R6). I retained Joe's 5AR4/GZ34 rectifier. Maximum rectifier filament current is 2 amps; do NOT use a 5U4!
Joe used a center-tapped AC winding for filament power. With the modern 300Bs I have tested, hum is much worse than with a hum pot, and even a hum pot still gives around 5mV hum which is too much for many speakers. So I suggest this choke-filtered DC filament supply with hum pot. The PGP8.1 can be wired to give a center-tapped 5v winding for the tube rectifier.
By using the same winding for filament power and the driver heater, the driver is biased up to about +70vDC. This is rather close to the maximum specification of 90vDC, and thus much more than the original +30v, but the simplification seems worthwhile to me.
I have retained a copy of this page on my HD. There are a couple of neat tips/ advice I have noted also.
Thanks again for your efforts and contributions - they are highly valued.
Cheers.
“As long as we have any intention to be right… we should be wary. So long as words have the slightest ego attachment, they are dishonest.” Charlotte Joko Beck
Thanks! Or you're welcome! Take your pick :^)
The essence of a creative community is "each one teach one". I hang out at these forums because I learn stuff, so I try to share what I've learned on the forums as well.
Hi Paul... my build of your design is well on it's way.... all Magnequest iron, PGP 8.1 PS, TFA-2004 OPT (cobalt), EXO-003 plate choke and BCP-16 Ni grid chokes.... with Paul's permission, I would be happy to supply a parts list if anyone is interested....
awsjr,
I am working on the same build, -- PJ's all sound practices WE91 with magnequest iron. I like your layout, can you share with me the CAD file? Have you completed it? How does it sound? hows the hum, quiet?
David
hi David..
yes , finished the build in Dec 2014...http://www.audioasylum.com/cgi/vt.mpl?f=magnequest&m=11464
how something sounds is always subjective... to me they are excellent... minimal hum...
as far as cad files, they are in NX... attached jpg of the top plate... if you would like some others message me... thanks -Al
Edits: 06/29/15
I'm not sure if it would be a good idea to use grid choke on the 300B with a high plate resistance driver like a 6SJ7.
Edits: 05/12/13
That was my thought as well, but recall having been corrected on that assumption in past. My guess is that Paul knows what he is doing... but I'd want to try it both ways (grid L and R) before I recommended one approach over the other.
Cheers.
“As long as we have any intention to be right… we should be wary. So long as words have the slightest ego attachment, they are dishonest.” Charlotte Joko Beck
As long as the grid choke inductance is high enough and the capacitance is low enough, it should be OK. At AC, the driver's high impedance is shunted by its plate resistor (91K), so it's maybe 10 times the resistance of a triode. Mike is not specific but says "thousands" of henries (!) for his grid chokes... I haven't used them myself due to magnetic hum pickup issues, but as long as you're patient and willing to move things around if necessary, give it a try.
Consider me interested.
Cheers.
“As long as we have any intention to be right… we should be wary. So long as words have the slightest ego attachment, they are dishonest.” Charlotte Joko Beck
...I'd be really interested in taking a look and consider building it as my second project... if Mikey still has the transformers available.
Share it to where you think best... just let us know the location!
Thanks Paul.
“As long as we have any intention to be right… we should be wary. So long as words have the slightest ego attachment, they are dishonest.” Charlotte Joko Beck
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