Thanks all for the reactions & additional mod-suggestions !
*** tone-stack ***
The mid-lift I tried already and volumewise it's a good jump.
At the tonelizard-site I read the treble-pot would more or
less keep it's functionality but not really so here. The tone
gets quite a bit darker.
I'll keep this switch though and yes, while at it why not add the mid-control.
I'll add it in the form of a footswitch & 25k pot.
The 1 M pot is a nice combination though.
And I forgot about that 1M across the switch, good to have that, thanks.
*** feedback ***
Yesterday I changed the feedback-circuit (2700 & 47 Ohms):
keeping the 2700 and replacing the 47 Ohms by 100 Ohms
and a 100 Ohms lin pot across it. The wiper of the pot goes
to the 2700 Ohms resistor.
The effect is zero... turning the pot gives no audible difference.
After some q&d calculations (see also RDH-4, pages 334/335)
I think I know why.
The open loop gain of the second 12AX7 stage & the 6V6 is
simply too low.
So no or less feedback won't raise the outputlevel much,
say only by 3 dB.
So the NFB-pot has no real use.
I'll be trying to lower the 2700 feedback-resistor, so the
pot does something. Range is then from no feedback
(wiper to ground) to more feedback (fully CW),
about 7 dB change in output level.
*** triode-switch ***
Nothing yet, will add. Actually strange one doesn't see this
more often - probably it's not really a worthwhile addition ?
I'll try though and add that screen-grid resistor.
*** other ***
have a switch with a middle position off. and have the option of changing the first cathode cap to .68uf-no cap-25uf...
That sounds like a nice one. That 0.68 is a Marshall-thing, right ?
BTW, if you're wondering if I'm totally messing up & drilling
this Vibro Champ into pices, no.
No additional holes in the chassis; I've made a metal backplate
which is clamped between the two wooden side panels. The existing screws keep it in place.
On this panel I've added things like the NFB-pot and I
will add the mid-pot here as well. There's also a 3R9 power resistor
on the inside of this panel and some speaker-jacks and a jack for the
Everything can be restored again - FWIW for a SF...