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Well, having fixed my stupid mistake of leaving the OPT secondaries floating...

I have to say that this is a lovely amp. Probably the old timers here have experience with the Williamson, but I've never actually built one. To recap, I used 1625s (the 12v version of the 807 with a different base) and 12SN7s because they tend to be much cheaper. The OPT is a Pacific Transformer copy of a Peerless 16309 that I got for a Heathkit W5M project that I never got around to. I have a pair of the originals (with nickel lams) but sadly one has an open primary winding.

Anyway, this is a delightful amp. I can't say I miss the PP 300Bs at all. It's very sensitive to components, of course. I threw some cheap electrolytics on the breadboard but subbing the one good poly-oil cap I have at the input of the PS tells me that's definitely the way to go. It went from rather wispy to developing muscles, like feeding spinach to Popeye. But it has all the transparency and immediacy you could ask for. And so much easier than struggling with the drive requirements of the 300B!

It definitely needs a preamp. I was a bit disappointed at first, using it with a DACT stepped attenuator at the input--it was rather veiled and distant-sounding. I removed the attenuator, used a fix input resistor and borrowed a Cary preamp from work. All the difference in the world--it sprang to life.

If you can get away with 12 watts and want a sweet budget PP amp, I'd recommend it. Raphaelite sells 10K PP transformers on eBay for around $100 a piece, or I think Edcor makes a stock 10K 20W OPT. Vintage 807s and 1625s are quite cheap, like a matched quad for well under $100. You could also use the 6BG6 which is an 807 with an octal socket. You could also use a KT66, of course, but I think the NOS 807's sound better than any of the current KT66's.

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Walt

Well, not always, especially if you're winging it. I replaced the remaining electrolytics with some better ones this afternoon, and it took on more body. The Cary preamp isn't bad, fairly neutral. I'm really enjoying this little amp. It's clean and transparent with exceptional bandwidth and control--which was the whole point of the Williamson, after all. It's been very interesting to explore the genesis of this circuit. I would dare to say that as more companies wrought variations to produce more power, it lost something, though I've read high praise of the 6146 version.

Seriously, folks with efficient speakers should give this a shot. You can google "musicain's amplifier" and find the original article.

I am surprised that you don't have enough gain, with two 6SN7's cascaded and then a diff amp after that. IIRC, this is similar to the topology that you ended up with after you removed the offending split-load phase inverter from your Williamson.

There is now that I removed the DACT attenuators. For reasons I don't understand they really killed the sound. I suppose a carbon log pot would be fine, since sensitivity is supposed to be 1.5 V. That said, I'm rather enjoying the flexibility of the preamp.

I switched to KT66s. They're more mellow, quite nice, really. The 1625s are a bit crisper. But I noticed one was not behaving nicely. I have some more on order.

Another thing I noticed was that many of the early Williamson designs used a 5V4 rectifier with 425-0-425 on the plates. That's supposedly beyond the 375 V rating for these, but they seem to work fine, and they're cheap enough. They're one of my favorite rectifiers. They give you a bit of delay on the B+, a bit more voltage on the plates, and they sound lovely.

I have a question
Is not the B+ voltage based on the power xformers secondary. Maybe limited to what the glass rectifier specs, but the tube dose not set the voltage

Well, different rectifiers have different voltage drops, depending on the internal impedance. If I change the rectifier I get a different B+. In his follow-up to the original article, "Gilding the Lily," Sprinkle suggests substituting the 5V4 for the 5U4, thereby increasing the B+ from 400 to around 450vdc and slowing the voltage ramp-up. He also suggests removing the triode connection and employing the 50% screen taps to create an "ultra-linear" operation and reduce distortion. (David Hafler responded with a letter to the editor firmly stating that this was NOT "ultra-linear" mode!) I have not tried this.

Turns out I mucked something up when I swapped in the KT-66's. That has been corrected and they are now MUCH better than the the 1625s.

I just remembered that a few years back a friend here in Oregon was cleaning out has late grandfather's attic and came across the old boy's handmade Williamson amplifier It was in pretty rough shape so he gave it to me, knowing I like to tinker with that stuff. I stuck it in the storage shed and forgot about it untill tonight. Pulled it out and sure enough, it has an original Partridge WWFB OPT. I'll have to measure the taps and see if it's still good. Sigh. Wonder if I can find another one. Guess I'll institute a search on eBay and see if one comes up. Might have more luck on eBay UK.

Don't worry about a pair. Time to set up a 1st rate mono system. People tell me mono is making a comeback. I still have my father's first corner cabinet with a University coax inside.

Maybe the club should do a mono demo at CAF sometime?

I’ve been thinking about that since last CAF...

Roscoe

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