Ben N 4/23/2018 7:04 AM
Exportizing my Vibro Champ - Part II: alternative switching.
So we got her up and running fine. But I'd like to have line voltage switchable, without cutting or drilling the chassis. I've got some recessed slide switches, like the ones in desktop PCs, on the way for this. So I'm thinking that a round DPDT or DPST power switch in the pilot light hole would let me have my cake and eat it, too, especially the neon illuminated kind (or LED + resistor). But I haven't been able to find a switch on Mouser or Digikey that would fit the opening. Any suggestions? Anybody do anything like this before?

I'd also like to use an IEC 320 for mains, again without doing any harm to the chassis. I think the least destructive way to do this is to mount the IEC in a small aluminum enclosure and mount that to the inside of the cabinet below the upper panel. Any thoughts on this?

bob p 4/23/2018 9:39 AM
external line transformer.
Steve A. 4/23/2018 8:57 PM
I assume that you already replaced the PT with one that handles multiple voltages. I have no idea what vintage of Vibro Champ you have but is there a hole for a ground switch, etc., on the rear panel that could be used for a voltage selector switch?

I personally have no objection to drilling a hole in the *bottom* of a BF/SF chassis for adding switches, especially a voltage selector switch that you certainly do not want somebody to mess with accidentally. But if it is a very valuable vintage age I'd probably pass on that suggestion....

Steve A.

P.S. I bought a 120vac LED panel light which turned out to be much smaller and dimmer than I expected. Also many of the BF/SF amps mount the 100 ohm ground reference resistors in the filament supply to the lamp socket. (Look at the layout drawing for your amp.)

EDIT The schematic posted in your earlier thread does not use the 100 ohm ground reference resistors on the filament supply but instead has one of the 6.3VAC legs connected directly to ground which can be noisier than a filament supply with a center tap- or the two 100 ohm resistors which create a "virtual" center tap.

Also unless you plan to go back and forth between countries that use different voltages I would skip the voltage selector switch and just rewire the PT as necessary.
Ben N 4/24/2018 3:31 AM
Thanks for the detailed reply, Steve. There was no need to change the transformer, since the tranny in this particular early 70s VC comes with both taps -- per the schematic posted in the other thread, the two halves of the primary coil are either wired in series for 117v or in parallel for 234v mains. So that's an easy enough operation, one I have done on other amps. Unlike my other SF Fenders, there is no ground switch, or anything, on the back panel--just solid steel, 1.7mm thick by my Q&D measurement. (The courtesy outlet on those amps also makes a nice spot for a voltage selector or an output impedance selector.) I'm not going to hack that up. While your suggestion on putting the switch in the bottom of the chassis is interesting, I don't think it is worth the effort and the damage, so I may just skip the selector altogether, as you suggest, or maybe mount the switch inside the chassis somehow.

You are right that the filament winding has no center tap, but I plan to make an artificial center tap (2x100R) and run a twisted pair to the filaments rather than the current arrangement of each filament being locally grounded. To me that's a no-brainer mod to do as a matter of course with any amp that grounds the filaments directly.
Enzo 4/24/2018 10:26 AM
I also was wondering what the point of the voltage selector might be, unless you are going back and forth between 120 and 240v countries. I would just wire it for what I need.

If I had to have a switch, I would put it inside. WHen someone changes countries, it is a small price to pay to open it up to flip a switch. PLus any switch accessible from the outside is BOUND to wind up in the wrong position at some point causing confusion. An interior slide switch would take up little room and be of low profile.
Ben N 7/11/2018 2:32 PM
Belated update: I finished. The amp is running happily on 230v like, well, like a champ--with the added bonus of a 5 meter power cord. The original power switch seems to be fine rewired as a DPDT per the schematic. I took the advice not to bother with a voltage selector, and and am happy I did. She works great, plate voltages are actually a tiny bit low all around, especially in the trem, and she plays immaculately clean almost to dimed--whereas before she was fuzzy at 3. Replacing the tubes along with the rewired PT primary solved all the problems I had been having with redplating (the 6V6) and intermittent "tremolo". With the twisted pair heater wiring and DC elevated virtual center tap, she is really much quieter as well--so quiet that my original intention of replacing V1 grid and plate resistors with metal film seems pointless. I'm going to go back in at some point to measure all the passives for drift and maybe try to get the plate voltages up to where they should be, but for now she sounds gorgeous (especially with a Strat) and there are higher priorities (like tuning a booster to effectively goose a 5W amp with an 8" spkr). Thanks again, everyone.