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In Reply to: RE: OPTs? posted by Frihed89 on November 29, 2015 at 04:15:00
The output transformers aren't pictured, but rather the plate chokes.
I would suggest Google searching "Western Electric Parallel Feed".
Follow Ups:
It took me an hour to find the trail to Lynn Olson and then another hour or so and two more discussions, elsewhere, about the differences in OPT operation...and in the prices.
The more I read about it, the more it seems that parallel feed amps, as exemplified by Western Electric, are an über-exotic design hiding in plain sight. I wonder why more designers to explore it?
" Seventy years later , the technical reasons for parallel feed are as compelling as ever. Parallel feed allows a substantial reduction in core size in a SE transformer, and this in turn reduces stray capacitance, improving HF response as much as one or two octaves. Distortion goes down as well, since the core no longer has to accept a heavy DC current, which uses up most of the "permeability allowance" of the core. With no DC flowing through a primary, the transformer designer can take advantage of low-distortion cores that would be unusable for conventional SE or even PP applications, such as high-nickel, mu-metal, amorphous, or even more exotic materials.Connecting the return of the primary to the driver-tube cathode gives a further improvement by shortening the AC loop around the audio circuit; in addition, careful ratioing of the cathode-bypass and transformer-return capacitor can essentially cancel most power-supply noise. (A vacuum tube amplifies the difference between the cathode and the grid; if noise that appears at the plate can be reduced in the right ratio at the cathode, it will cancel PS noise appearing the plate.)
The Western Electric cathode-coupled parallel-feed circuit has been around for a long time; it predates the invention of indirect-heated triodes, power rectifiers, pentodes, and negative feedback. It's right there in the WE 11A, an amplifier from the dawn of radio in the early Twenties. Instead of coming up with clever "audiophile" names, I'd like to nominate Western Electric coupling as the title for this circuit. They did it first, they might as well get the credit."
Lynn Olson, nutshellhifi.com
Big J
"... only a very few individuals understand as yet that personal salvation is a contradiction in terms."
Edits: 11/29/15
"The more I read about it, the more it seems that parallel feed amps, as exemplified by Western Electric, are an uber-exotic design hiding in plain sight."
While I agree with what Lynn says about the advantages of a parafeed OPT, the plate choke still has some of the same limitations of a series feed gapped OPT.
It seems to me that a active CCS instead of a plate choke would allow the full advantages of the parafeed OPT to be realized.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
You are correct - the higher the impedance of the plate load, the better the sound. I've done the experiment in a preamp, with four different values of plate choke inductance plus a current source.
As noted below, electronic current sources for power amplifiers have some practical issues (of which I feel the safety risks of high voltages are the worst). So I elected to increase the plate choke inductance instead. It is nearly doubled from the previous design. That pushes the parafeed cap/plate choke resonance down to 7.5Hz.
If a 300B amp had 450V of B+ available before the OPT/plate choke, how much B+ and how much CCS dissipation would be required if one wanted to use a CCS? (let's assume 70mA for plate current)
Now that you have those numbers, what solid state components are available that can handle those demands?
(Things get pretty crazy in a hurry!)
Also, since there is no energy storage with the CCS option, you'd need higher B+...with inductive loading, turn down conduction and plate voltage exceeds B+. You'd need to split the dissipation between the final's plate and the CCS. In order to take 'full' advantage of the parafeed OPT, you would need 2x the idle voltage on the final's plate, plus CCS headroom.
That is substantial dissipation for a SS device. With another 500V on top of the 2x one could make a fine, cascade triode/pentode CCS and avoid heatsinks.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Yeah, that's what I was going after. 1000V of B+ for a cap coupled 300B makes its own set of problems.
"...you would need 2x the idle voltage on the final's plate, plus CCS headroom."
For practical reasons I was thinking a CCS loaded 45.
On the other hand Gary Pimm's "Active 300B Rev3" looks good.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
.
Have Fun and Enjoy the Music
"Still Working the Problem"
No reason you couldn't use a pentode as the CCS. Yes, you'd have to have a seperate heater for it, but hey, who doesn't love having one more tube glowing for all to see?
.
Have Fun and Enjoy the Music
"Still Working the Problem"
.
Have Fun and Enjoy the Music
"Still Working the Problem"
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