Kofi Annan in: "Push and Pull with Me"

[cerrem]

As far as simplest EL84 P-P amps of good sound...
I always prefered the Fisher SA-16 or the SA-100 ....
Have a look at those schematics.....
You can simplify the power supply...

Chris

[Sheldon]

Originally posted by Kofi Annan
So, can I use silver micas for some resistor bypass caps or will this cause a problem?

Yours in fickleness,
Kofi

In the Bevois Valley, they'd be ideal to bypass the feedback resistors. You won't find any big enough for any of the other bypass applications in that amp, but I guess they'd be dandy if you could.


Sheldon

[SY]

They're probably OK for the lead compensation cap, but a Wima FKP would be better. Same with the "slugging" caps across the grid resistors.

[Sheldon]

Why is that Sy?

Sheldon

[SY]

Subjectively audiophilia nervosa. Objectively, poor soakage characteristics.

[Kofi Annan]

OKOKOK!!!

Back after a short hiatus to make some Buschhorns and toy with the idea of a 7' version of the Fostex 225Ks. Can you guess which project won the approval of Mrs. Annan?

Anyhoo-- I intend to make this Bevois Valley project a reality but, as always, I'm a bit bufuddled by something. In this case, I am befuddled by the regulated HT supply.

Any suggestions on the best way to make a regulated HT supply for the Bevois Valley?

Oh yeah, and a recommendaiton for the next scotch I should try would help too. Finishing up some Macallan 12 and I may bag some Glenmorangie, but I could be persuaded in another direction by an audio geek with the proper booze ethos.

Kofi

[navin]

Originally posted by Kofi Annan

Oh yeah, and a recommendaiton for the next scotch I should try would help too. Finishing up some Macallan 12 and I may bag some Glenmorangie, but I could be persuaded in another direction by an audio geek with the proper booze ethos.

Kofi

Cardhu?

[Miles Prower]

Any suggestions on the best way to make a regulated HT supply for the Bevois Valley?

I did this voltage regulator for the screen supply of my current project (E-Mail for zip archive of design details). This is a smaller regulator, but the principle is the same. Starting with the requirements: Vo= 270Vdc; Io= 5.0mA -- 16.0mA. Since the main DC rail is at 365Vdc (nominal) that gives a Vpk= 95Vdc for the series pass device: a 6AQ5A operating as a triode. So you enter that on the plate characteristic graph. The extremes of the current lines intersect the main load line at a little less than -5.0Vdc and somewhere around -10.0Vdc, so use -10.0V. Knowing these bias voltages gives you the voltage across the DC voltage amp's Rp. Since this regulator uses an 0C2 for a reference, that gives a Vpp for the 6CB6A of: 270 - 105= 165Vdc. That gives the zero plate current point on the 6CB6A plate characteristic. You can then just play with possible load lines until you get one that looks viable. In this case, at the Vpk= 157.5Vdc (estimated bias for the 6AQ5A) and a plate current of 1.2mA looked good. This gave a value for Rp= 6.72K (select 6.8K). I picked off a couple of points for extremes of bias and current.

As for the voltage feedback divider, simply try to come up with a pair of resistors that'll divide 270Vdc down to 105Vdc (zero biasing the 6CB6A). These "next closest" values actually give +4.3Vdc, however, this is not a problem. Before starting this, simply set the 100K pot (used 10-turn cermet pot for this, since it's basically a "set it and forget it" control) to halfway. In practice, this actually gave too much voltage when tuned into a 50K load. NBD to adjust to 270Vdc. When operating with the 807's, the final measured screen voltage was 268.8Vdc. This doesn't vary under different operating conditions, nor when the line voltage shifts a bit. This circuit works, and gives dead quiet DC.

Of course, for more current, you won't be able to use a 6AQ5A. In those cases, use one of the triodes designed for series pass regulation: 6AS7A's, 2A3's, depending on what the plate dissipation will be. If paralleling series pass VTs is necessary, be sure to include a current balance resistor (~100 Ohms) in each cathode path. Grid stoppers, though not really required, aren't a bad idea and don't cost much anyway. So you might as well include it. Even more important is the Cdom to enhance Miller capacitance of the voltage amp tube (one of those rare cases where Miller capacitance is a good thing) since this is supposed to be a DC, not AC, amp. Also, the less AC passband you have, the less noise you have.

The 6CB6A is really good for this in that it has a heater-to-cathode voltage rating of 200Vdc. It's unusually high for a small signal VT.

Hope this helps.

[Joel]

IMHO, you should replace Mr Jones's "285V regulator" with a choke and a cap. Current draw of the first two stages is 16.6mA, so a Hammond 154E 20H@20mA, followed by a 100uF-350V cap will fit the bill nicely. You'll thank me later.

And, I prefer to poison myself with Lagavulin.

Joel

[dhaen]

Originally posted by gingertube
Some time ago I was in correspondence with Hammond seeking data to do some modelling. They sent me a spreadsheet with a lot of data on the entire 1600 Series Transformers. I actually intended to post it BUT how do you post an Excell Spreadsheet?

...snip..

Cheers,
Ian

Please just zip it and post as an attachment..