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Re: Gordon Rankin: "2a3's..what a waste of time"...So what you're really saying is...

Posted by Paul Joppa (P) on March 3, 2007 at 08:10:19

In Reply to: Re: Gordon Rankin: "2a3's..what a waste of time"...So what you're really saying is... posted by lovetube on March 2, 2007 at 18:31:31:

The original version was published in the old VALVE magazine/newsletter under the title "SEX and the single 300B" (because it was a rebuild of the original S.E.X. amp, long discontinued). It's in the November 1997 issue, with several mods documented in later issues. All this was long before I started doing designs for Bottlehead.

The original circuit used a 6SN7 in mu follower, set up so that a 6SL7 could be substituted directly. For my ears, I quickly decided I preferred to take the drive from the lower plate rather than the upper cathode, and also that I preferred the 6SN7; it was later modified to use a current source, and the 6J5 substituted.

The power transformer, also discontinued many years ago, had two windings at 2.5v, 1.25A, which could be wired in series for 300B or parallel for 2A3.

The output tube ran at 325v and 55mA with a 1200 ohm cathode resistor, with a Magnequest TFA-204 output transformer of 3K primary impedance. Total power supply voltage was about 400v. This is a plate dissipation of 18 watts, exceeding the 2A3 rating of 15 watts. The Chinese 2A3s of 10 years ago had a reputation for being rugged but having relatively short-lived cathodes, and they were quite inexpensive, so I did not mind the thought of replacing them frequently - "smoke 'em if you got 'em"! I measured 6 watts at 10% THD at the onset of visible clipping on the scope.

This amp served me well for many years, but I don't think it's practical today because of the unavailable power transformer and the excess plate dissipation. It is a direct ancestor of the current Bottlehead Paramour. I'm happy to answer specific questions about that design here, but any detailed discussion probably belongs on the Bottlehead forum.

A friend, who posts here as "richardl", has built a variant of this design using the Magnequest PGP8.1 power transformer which produces even more voltage; he uses the Sovtek 2A3 which is pretty rugged. The current version uses a choke-modified mu follower, but we are working on a modification to a choke loaded driver with separate voltage regulation.

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