While direct-coupling a PP output has to be one of the worst things for global warming, it does produce a couple watts of interesting sound. The above is my shopworn circuit, now with a SE 27 directly-coupled to a "compact" or long-tailed-pair PP2A3, and then a nickel S245. The second 27 has a grounded grid and an adjustable current set resistor in its C4S and a separate 1.8K cathode bias resistor. The circuit has also worked well now with 6N1P, 6C45s and 6SN7.I've tried joining the cathodes of the first "stage", but so far have prefered the separate cathode bias and resulting SE circuit. This may change if I ever get a proper CCS in that mix.
The 27 and its 4.5 mA of plate current make for a kind of wimpy driver, but do take the amp one step closer to using a grid choke and no attenuator in the input. For the moment I'm using the primary of a B7 as a grid choke, which I think gives a hint of what the real item will offer.
A key to the low noise and great sound is the adjustable C4S. Maybe when DC offset is nulled, PSU noise rejection is at its greatest? With other inputs, the noise has dipped to a stable 0.4mV, which is as low as my DVM will go.
A direct-coupled phono stage set up this way might be interesting. Grid choke, SE pentode (maybe Gary Pimm's 6BN11 idea, here), 6SN7, and a 5Kct:500 linestage OPT. The input would have to be a pentode, so as to apply Gary's current-balance regulator in the output.
That's the latest... --keto
Follow Ups:
- 27-PP45 - keto 21:11:42 05/17/04
(1)
- In Reply to: 27-PP2A3-S245 (pic) posted by keto on May 15, 2004 at 08:10:11:
Interested in trying cooler than 60mA operating points, like the ones on the WE 300B info page (see link), I installed a 2.5K unbypassed resistor in the direct-coupled circuit, pictured in the previous post with SE27 driver, which now has an EXO-173 as simple grid choke (the 27, that is). A couple worn ST45s are now producing 1W of PP music... --keto
- update... - keto 19:11:46 05/29/04
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- In Reply to: 27-PP45 posted by keto on May 17, 2004 at 21:11:42:
In this circuit, my tired but trusty Valve Art 2A3 pair are better than the 45s or TJ300Bs. Using a pair of the 20uF/600V sprague/atom electrolytic caps (from Ned's triode electronics), I've raised B+ voltage and general output stage voltage potentials to cap-input levels. I was worried that just 20uF + 20uF wouldn't be enough capacitance. Maybe the BFFC casts an inductive shadow equal to its physical one? Works fine, anyway. Mistakenly thinking I'd burned up my Pimm CCS, I had installed the 2.5K (output) cathode bias resistor. Turns out one of my solder joints was bad, and the CCS is fine. In the meantime, I think its fair to say that in a direct-coupled circuit such as this, a 2.5K Rk is a pretty good performer.
I've been adjusting-away humm by adjusting the current of one of the input triodes (the non-signal one -- still prefer a single ended first stage, an echo of something Kurt Strain posted a long time ago), but I'm now thinking that the adjustment may just effect relative noise (via gain) of the two halves, and not really effect DC balance of the OPT primary.
The grid choke input has a lot to do with the sound; a stable load for the source and a stable source for the load.
Wagnerian singers and orchestra stomping around at the moment; crystal clear and miles from audible distortion. That's the latest. --keto
- update... - keto 19:11:46 05/29/04
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