When I used the 160 selector, the Plate Voltage fell to about 395.
Well, then that's too much of a good thing.
The same thing happened when I used my 6 Volt "wall dropper"...the heater Voltage fell to 4.8-5.0 Volts...depending on time of day.
My wall Voltage is typically 122-127 Volts.
Believe it or not, some old equipment purposely ran tubes at slightly reduced heater voltage for longer life and less noise. So running the heaters at 5.something volts is fine. You may have reduced performance from some tubes, especially as you get down to 4.xx volts.
But the math doesn't add up if the ~40v reduction by using the 160v tap causes the same effect as a 6v reduction. I'd still advise you to se a bucking transformer to knock wall voltage down 12v before feeding to the amp, and it will solve all problems.
If I have ever heard anything about Vox amps in general, and the AC100 in particular...it is that they had a reputation for "Burning Up".
Mostly, that was the AC15 and AC30, because all tubes were inside a box formed by the L-shaped chassis and the cabinet, and only tiny vents were provided. So that's all about normal tube heat being confined without sufficient airflow, which then heated the chassis and cooked the internal components.
The AC50 and AC100 had MUCH more substantial vents. Even if they didn't, you wouldn't fix the ventilation problem without redesigning the chassis and cabinet. So
In My Opinion, all this worry about the voltages is much ado about nothing.
The AC100 in question appears to have the opposite philosophy. It almost looks as though it was designed to fail at full bore ...
For all the worry over screen resistors, I have never seen a tube with a melted screen in the last 20+ years of doing this stuff. I say that while also having a 6V6 that redplated to such an extreme there is a hole melted in the plate. FWIW, the tube still functions and sounds pretty good! The hole in the plate happened before I got it, and I just keep the tube as a novelty.
The AC100 wasn't "designed to fail," it was designed to assure the full 100w output at full power. If you read the old tube books,
tube manufacturers recommended no series screen resistance for class AB amps, because it reduces the maximum power output for the reasons I explained before. Hence, no screen resistors in the AC100. Or in most tube hi-fi's, like the McIntosh amps I have which were intended to run 24/7 for years.
The AC100 in question appears to have the opposite philosophy. It almost looks as though it was designed to fail at full bore, which is probably why you don't see them in pictures of your favorite rock bands.
I think you don't see them because of market forces, Vox's business model and poor business deals.
Vox didn't build its own amps, they were subcontracted out to other shops to produce while Marshall made its own amps. An English band could probably get 100w (or bigger) amps quicker from Marshall. American bands pretty much couldn't get them at all, because of Vox's distribution deal with Thomas Organ, who was selling solid-state amps using Vox's name in the U.S. and pretty much cut off Vox's biggest potential market. And Vox spread itself thin trying to make too many different products (guitars, pedals, amps, PA's, most of which flopped eventually).
Add to that that the circuit design is clean-only where a Marshall 100w could rock, and you've got the death-knell for big Vox amps.
So I'm still at the position that this amp would be fine stock if the wall voltage is reduced. Screen resistors weren't added until the modern Vox amps, and most of that design is "me-too" rather than engineering-driven (you find this is true if you do a search for "Vox AC100 history" as there's a site with all the 60's AC100 variants and their schematics & internal photos).
Topic: B+ Drop Question (Read 9731 times)