Saturday, September 24, 2016
Biasing The Vox AC50
Every now and again you tumble across something that stands head and shoulders above the rest, and I had that experience the other day with a JMI produced Vox AC50, circa 1965.
It came in to have the bias tweaked as the owner had had a power tube quit and replaced them. Then, the sound disappeared.
Being no slouch, he also knew enough to have the installation finaled out with a bias setting and a quick visual inspection to make sure nothing had been damaged. The problem consisted of a blown B+ (or HT if you prefer) fuse. Good work by the design engineers to fuse this important circuit, and many a Fender could have been saved from major repairs if they had incorporated this idea early on.
In any event, this amp has a hybrid bias feature that was universally scorned when Fender tried it for a brief moment back in 1969-1970, and that is a fixed bias supply and a whacking great cathode resistor for each power tube.
I've yet to figure out just why this feature was used and discarded because it does work pretty well and it also offers some protection if the fixed bias supply quits on the job. You won't have a melt down.
In any event biasing the AC50 is a relatively simple task as the factory recommended setting is measuring 2.2 volts across the big cathode resistors and adjusting the individual bias level controls to suit.
It's noted right on the schematic there.
This is without a doubt one of the best clean sounding big amps I've ever heard. It's got all the sparkle of a classic Fender, and the midrange is more thoroughly developed as well. I missed the chance to try my Gretsch through it but I bet I woulda found what defined George Harrison's distinctive tone.
Fact is, I may just build one this winter. Stay tuned.