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PostPosted: July 8th, 2020, 3:43 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 257
SoundMods wrote:
That's interesting in that he did not realize that with fixed bias he could have just dropped in the 6L6s, adjust the bias and it was a done deal -- as long as the B+ did not exceed the tube's ratings.

Another drop-in choice could have been KT-88s, 6550s, or even KT-66s without screwing up the amplifier.

Overcoming a mess is messy (pardon the pun). A buddy of mine came to me for a repair of a ST-70 that a Washington, D.C. hi-fi shop 'hot-rodded." I took one look at the mess (it was horrible!) so i told my friend it's not going to happen.

Then he proceeded to yank out of storage two more ST-70s -- one a kit-built piece and the other a factory job. I took the factory job and gutted it. I kept the steel and the transformers.

Using a full differential driver board from a now defunct company in New Jersey, nice mil-spec parts that serve the music, and as an added feature by installing a bias meter that fit a pre-cut now un-used hole hole with a 3-way switch to adjust left or right cathode current using a pot installed on top of the chassis -- when out of the circuit the tubes were run with grounded cathodes that much improved dynamics. I had all of the steel polished and chrome-plated and it has been in service for over 40 years now driving either Spendor speakers used as recording monitors -or- Quad Electrostatics for just plain listening pleasure. :thumbup:

A note about the 300Bs. My Carys have 300Bs with fixed bias used as drivers for the 845s that have cathode bias. Having diddled with bias levels from 45ma to 60ma I found a sweet spot at 48ma that opened up the sound. Worth a try.


Well, I can't find my CD-ROM of Sound Practices at the moment, but I think the whole point was that he liked the 300B, had a bunch of them, and came up with a nice, powerful and fairly simple PP circuit for 32wpc. I don't think it was a question of ignorance. I also don't recall *when* he built these but it may have been in the early 50's when a triode Williamson only got you 15wpc, and before ultra-linear was a common thing. The schematic indicates a Triad S-42, which was probably an S-42A, 4.5k p-p, 50W, a stock item with plate taps only.

I've thought about biasing them down in AB, just to see what happens. ;-)


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