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Regulator Tubes ? - Hoax or just a cruel joke?

Posted by Bambi B (A) on July 7, 2009 at 07:58:59

PHOTO: TOP: The back of the Audio Research D115. The regulator in this photo is the KT88 shaped tube near the centre. BELOW: The power supply chassis of the Audio Research SP10/II preamp showing the 2- 5881 regulators and 12AT7. Both from ARC Database

Mates,

I'm reviving my main Audio Research tube system- stored during a period of what seemed to be constant moving. While taking stock of some long-intended tube changes, it occurred to me that the regulator tubes in the SP10- 2-Sylvania 5881's, and in the D115- 1-Philips ECG 6550 are quite elderly- the SP10 5881's are from 1987 and the D115's Philips ECG 6550, is there since 1998.

I had always had the thought that the regulator tubes did not have a significant affect on sound, but in a recent thread on another subject- I can't find it at the moment- said that the sonic impact of the regulator was greater than the output tubes. I also believed that rectifier tubes did not have a lot of say, but as enough people seem anxious to pay $150 for whatever metal-base Mullard GZ34 for a Dynaco ST70, there must be something noticeable!

1. What are regulator tubes doing?

> And how are they doing it? It can't be that all the current going to all 8-output tubes of the D115 is passing through the regulator and it's acting as, well, a valve.
> And, if not all the component's current is passing through, why should the SP10 have two-5881's- to make the power supply a dual mono design?
> What is the 12AT7 in the SP10 likely to be doing for a living?

2. How much sonic impact do they really make?

3. Are regulator tubes operating under significant stress?

4. What 5881 and 6550 would you recommend for regulator tubes?

> As I need only 2 for the SP10 and 1 for the D115, if there is a significant sonic impact, I'd be prepared to get some semi-fancy NOS.
> Doing a bit of looking into it, I see that Tung Sol 5881's have a great sonic reputation, but don't handle as much current and that the later Sylvanias are more rugged. The SP10 has Sylvanias, which may have been the only real 5881'a still available in large quantities in 1987.

5. Would the KT88 operating parameters make those better or worse as regulators in place of 6550's?

6. Are the 12BH7 and 12AT7 in the Audio Research SP8 power supply components and if so, is the 12BH7 used as a regulator?

7. What is the 12AT7 in the SP8 doing?

It seemed a good time to address several power supply tubes issues at once.

Thanks as always!

Cheers,

Bambi B

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