BF/SF Vibro Champ and Champ voltages and bias come up a lot. Our amp-wise friend muchxs used to warn us how high these ran from the factory, with B+ in the range of 410-420V or more, and on the positive side, our tone-wise friend @Wally has noted these amps can sound great biased hot, possibly up to 120% MPD or more.
Sure enough, my BF '67 VC came to me with B+ 423 on 120V at the wall, so I fitted a 470Ω sag resistor between rectifier and cap can to drop the B+. Later when I built a 7%/12% bucking transformer I found I could remove the sag resistor on a 115V supply.
However... for the last few months it turns out *I had the amp plugged into my non-bucked power strip.* Recent VC voltage threads make me look back at my voltage tables, and I realized that wasn't the plan. But -- the amp sounded great.
So today I popped it open and measured on and off the bucking transformer:
TBH, I'm not sure it sounds *quite* as magic on the 115V supply. But it's awfully close. Since I like to use heater voltages as a guide for "how low to go", I put the amp back on the bucked powerstrip. Then too, although the nominal US supply voltage was apparently bumped up from 115 to 120VAC in 1967, these amps were designed before that, so it may also be smart to run them around 115V from a 'vintage voltage' point of view. If you don't have a bucker, let me say I did like my 470Ω 5W 'sag' resistor...
But VC people: Even on 115VAC, I'm still at 123% MPD if I use the conservative 12W norm for 6V6s. So if you've built or bought a Vibro Champ, maybe don't freak out if you get bias figures in the 100-120% MPD range, and maybe don't hesitate to use a 14W spec, especially if you run a JJ or good NOS 6V6. Modern issue 6V6s may not be as robust. At a minimum, look for red plates in a nice dark room...
PS: The AA764 schematic voltages seem *really* low; maybe they're pre- or early production, and even the later AB764 plate and screen voltages are probably wrong (since they're the same as the AA764 despite the big jump in B+).
Sure enough, my BF '67 VC came to me with B+ 423 on 120V at the wall, so I fitted a 470Ω sag resistor between rectifier and cap can to drop the B+. Later when I built a 7%/12% bucking transformer I found I could remove the sag resistor on a 115V supply.
However... for the last few months it turns out *I had the amp plugged into my non-bucked power strip.* Recent VC voltage threads make me look back at my voltage tables, and I realized that wasn't the plan. But -- the amp sounded great.
So today I popped it open and measured on and off the bucking transformer:
TBH, I'm not sure it sounds *quite* as magic on the 115V supply. But it's awfully close. Since I like to use heater voltages as a guide for "how low to go", I put the amp back on the bucked powerstrip. Then too, although the nominal US supply voltage was apparently bumped up from 115 to 120VAC in 1967, these amps were designed before that, so it may also be smart to run them around 115V from a 'vintage voltage' point of view. If you don't have a bucker, let me say I did like my 470Ω 5W 'sag' resistor...
But VC people: Even on 115VAC, I'm still at 123% MPD if I use the conservative 12W norm for 6V6s. So if you've built or bought a Vibro Champ, maybe don't freak out if you get bias figures in the 100-120% MPD range, and maybe don't hesitate to use a 14W spec, especially if you run a JJ or good NOS 6V6. Modern issue 6V6s may not be as robust. At a minimum, look for red plates in a nice dark room...
PS: The AA764 schematic voltages seem *really* low; maybe they're pre- or early production, and even the later AB764 plate and screen voltages are probably wrong (since they're the same as the AA764 despite the big jump in B+).
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I compared again yesterday -- the 17.2W did sound sweeter and sexier than the 14.7. It's subtle, but Jason's trick of recording to compare showed it. My enjoyment of the amp the last few months, while it was 'mistakenly' running on 122V, has got me thinking about putting some dropping resistance in the heater circuit -- I'm not happy with that 7.0V on the filaments. Hmm, lessee, how'd I figure that for these single-strand heaters? (Insert squirrel-wheel noise from brain...)
