Counterpoint SA-220 Tube section advice needed

[KBK]

I've got two Counterpoint SA-220 FET output-Tube hybrid power amplifiers to fool around with.

I'd like to mod the PS (first job!) for the tube section. It is a Tube rectified PS, done with a 6CA4 (aka EZ81) Rectifier tube.

According to one engineer I was speaking to, this particualr tube in this particular amplifier is quite overloaded as far a current draw goes. I was about to upgrade the PS caps that are in there now, but he says the unit has too much capacitance already. The heavy draw makes the tube nosier, not quieter. The 'slap' of the rectifier turning on and off is far worse than it should be, due to the heavy current draw in this design. After he explained to me what he meant, it made sense.

He also said, that the unit had a 'motorboat' issue, early on, as the windings for the tube PS are on the same trasformer as the PS for the FET output section. Go figure, eh?

The heater supply is under driven. I was toying with the idea of removing the mini 9-pin socket and retro-fitting a octal tube socket, and seeing what a bigger rectifier might do, but I need abit of advise first. 1 amp for the 6CA4, the 5AR4 uses 1.9 amps. I think it can do it, as the (heater rectification) rectifier is not hot under it's current stressing. Perhaps I can add a simple heater trasformer into the mix on the unit. I also have mounting space for Inductors, for a bit of filtering. There are no inductors right now, in the stock design. Shame.

Should I just go solid state here, and forget about it?

I'm not looking to design a car, here. Or send men to the moon. I'm looking for what is ultimately a simple, elegant, and photogenic fix ot the unit. No heavy mods and **** hanging all over the place. KISS. I will have to re-sell the units some day, maybe sooner than I think. So keeping the mods clean and simple is the way to go. Increase the value of the unit, not decrase it by scaring the potential customers away. Which, in my opinion, something as simple as a larger rectifier, the good ole' 5AR4, GZ34, or the ..uh..memory here..5U4GB?maybe that one too.

Any quick fixes? like an octal, in your opinion?

[SY]

Resale? Don't even think about mods.

[KBK]

well, I'm gonna anyway. Not that concerned about how much they sell for, if I do end up selling them, that is...: things are going farily well, yes. Actually, i may never sell them.

I'm going pro on the loudspeaker design thing, and will have to have two racks (or more) of amplifiers sitting around that I can swap out to check the passive crossover designs. Or, just tons of different sounding ampliers, period. I'm up to about 7 amps or so right now, and I'm a bit short on the tube units. I'm a bit tired of the electrocutions.....(see short comment above for the joke part)

"Electrocutions? Good.. Down the hall, first B+ rail on the left...."

[fdegrove]

Hi,

What exactly is the 6CA4 feeding?
A pair of 6DJ8s?

No idea how much capacitance they stuck right of the rectifier but if it can take for a few days it can usually take it for ages too....

Oh, what do you mean by underdriven? Too low voltage on the heaters of the tubes?
If so, how much do you measure?

As for the rest, I'm with SY here.

Cheers,

[KBK]

The 6CA4 is feeding 5 6DJ8's. The unit, in stock form, has about 800uf of capacitance. B+ rail is running at about 365 VDC or so. I haven't investigated much furthur than that, as of yet. I'm waiting to get a schematic.

The 800 uf is distributed as 8x100uf 450 volt snap mount UCC caps.

what I mean, is the SS 25 amp rectifier for the DC heaters is definitely underdriven. Don't know about the winding in the tranny, though! I wouldn't want to overtax that. That is what I meant about possibly running a separate heater transformer for the heavier heater on the larger rectifier. The 6CA4 is 1 amp,and the 5AR4 is 1.9 amps on the heater. that's substantial increase overall (5x 6DJ8 + 1 Rectifier tube, at 1 amp or 1.9amp) in heater current draw.

[audiousername]

800mF is an awful lot of capacitance for any valve amp, especially when it's only the input section, and valve rectified.

The 5AR4/GZ34 or 5U4 have 5V heaters/filaments, so they will need a different heater/filament voltage from the 6.3V 6DJ8s. Also, the 5U4 is directly heated, while the 5AR4/GZ34 is indirectly heated, but the cathode is internally connected to the heater. So you basically need a separate 5V winding, which will be at the potential of your B+.

[SY]

If you're going to do it, do it right. A Maida-style regulator will give you a very, very solid supply rail.

[KBK]

Originally posted by SY
If you're going to do it, do it right. A Maida-style regulator will give you a very, very solid supply rail.

And to quote the Director of the hospital, In the film "The meaning of Life", when he is told that the woman is about to give 'birth', ...."What sort of thing is that?"

Sounds like some sort of tube hybrid regualtor? maybe a SS regualtion of tube? Something simple, I hope?


Thanks for your effort!

PS: I have both the RC-30 RCA book, and I believe what is one of the last renditions of the Sylvania tube reference manual. I'm thinking of find in a way to get them on the company server, since I paid for the bloody thing and it's huge bandwith use. Got about 160 gig doing nothing...

[SY]

It's a machine that goes, "piiiiiing." Most expensive one we've got.

The Maida regulator can be found in National Semiconductor's regulator application notes. It's basically an LM317 with some guard circuitry around it. Very simple, very effective, 98% as good as a super-complex ultra-high-performance discrete regulator (overkill for your application).

EDIT: Michael Maida, National Semiconductor LB-47, from "Linear Applications Handbook." It's also reviewed in Morgan Jones "Valve Amplifiers."

[KBK]

Thanks Sy.

BTW, a project for the folks here, might be to crash into Paul McGowans party (a-la 'Blazing saddles'...ie, the fight crashes through the wall into the cafeteria), and design a 'tracking' AC regulator. Something with a phase shift to the 60hz signal (cap-ind loading anyone?), to give the unit some'headroom' to work with. What I am speaking of is the PS Audio 're-generators' that Paul is selling. I figure even a Tranny that takes the AC, cranks it up by, let's say 25%, then, reg it back down to the desired 120AC. USe something that tracks with the 120AC, and regs what is above that 120.

Should be very doable, and efficient. Huge headroom in terms of thermal capacity and current throughput. Just spec a big-*** tranny....

Is that a stupid idea? It's been floating on the edge of thought for quite some time and PS Audio's $3500 toy may be nice, but is it really nessessary, in the end, for maximum fidelity? Can we do similar for considerably lower cost? Ie, for about $5-600 in parts, make one with three-four times the current capacity?

I realize that draw-phase angle may shoot this idea in the foot, but it may be doable. I haven't looked at it too closely.