July 8th, 2020, 3:43 pm
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.
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.