Fender Bandmaster 350K Treble Pot 70k tap

[gary sanders]

...whats up with that?

[sliberty]

It's also listed on the schematics of the 6G6A and 6G6B Bassman amps.

[martin manning]

...and the 6G16 Vibroverb.

The often repeated story is that Fender used these because he got a load of them cheap. I don't know if I believe that, but I think this was something from the Hi-Fi world. Anybody have an explanation or a reference? I'm curious as to what the supposed benefit is too.

This subject came up a few weeks ago, and I noticed that this feature had been replicated in the reissue Vibroverb, so it was seen as important for some reason.

More discussion here: https://tubeamparchive.com/viewtopic.ph ... highlight=

[David Root]

That story is the same one I heard. It's quite plausible, Leo was a smart businessman. You can get 16mm versions of them from Weber or Hoffman Amps. I used them when I did my brown Vibrolux w/reverb build.

I recently cut the tap connection part off two of them so I could fit them side by side in a CE Dumble chassis where I drilled out the manual OD & PAB holes to use as send and return pots in a buffered loop. Too close together for regular size pots.

[martin manning]

After doodling with simulations (while watching the Tour de France being won by an Australian for the first time ever; congrats to those down-under!), my conclusion is that the tapped pot stack gets a significant treble reduction that the "replacement" configurations don't achieve.

The tapped pot version, at 0% rotation, has a knee at around 100Hz and is (predictably) 20dB down at 1kHz. The successor AB763 tonestack eliminated the tapped pot, and produces an essentially flat response from ~700Hz on-out with the treble control at 0%.

One would have to wonder if so much treble reduction is really useful, and judging by the configuration of the later tone stacks, maybe not. It'd be interesting to know where the treble control is typically set on the amps with this feature. Anybody have some first-hand experience?

BTW, I believe the 70k tap is at noon on the tapped pot (70k out of 350k, 20% log taper). The response is flat when it's set at that point, and it looks very much like the AB763. So, if the treble pot isn't really ever set below noon, then the later tone stack doesn't give up anything and provides more adjustability over the useful range. Versions that try to reproduce the response of the tapped pot tone stack (by adding a resistor and a cap to ground at the bottom of the treble pot) don't come any closer to it than the AB763 stack.