tube wah/amp

[birt]

i've been very busy trying to build an amp that has a built in tube wah. i wanted it to have a standard expression pedal to control either the wah frequency or the blend between clean and wah with a set frequency (the other parameter being controlled by a pot on the amp).
it's kinda influenced by the MRB on old solid state vox amps.

so i started drawing the amp and came up with something between a vox and a fender with a big muff style tone control (thanks AMZ). the first tube has 2 parallel triode gainstages. one of these has a twin T filter as local negative feedback. both are then blended as found in the "panning for fun" article on Geofex. the way the expression pedal works is based on the "Rock 'n Control" by The Tone God.

the amp itself works with very low hum and noise, in a tiny chassis.
the extra circuitry (filter, expression pedal control, blend) doesn't really work.

the schem:


-the filter doesn't work as shown. it just makes the signal dull and the frequency doesn't 'move'. i don't think negative feedback would be different when comparing a transistor gainstage with a tube or opamp gainstage. so there must be something else going on there. i guess it has something to do with the impedance of the triode and the impedance of the filter.

-the switching works. when the mode switch is closed and the 4PDT relay is powered optocoupler 3 has to control the wah frequency. the pot on the amp has to control the blend. when the mode switch is open optocoupler 1 and 2 have to control the blend and the pot on the amp controls the wah frequency. when the expression pedal is unplugged the relay is not powered (no ground connection with closed mode switch because it's grounded with a stereo jack) and the tip of the jack input is grounded (so opto 1 and 2 control blend and are set to 'dry signal only'). i hope this makes sense?

-the blend with opto 1 and 2 has a big volume dip in the middle of pot travel. can i fix this by lowering the value of the trimpot?

-the blend circuitry with the 100K resistors also lowers the signal level a lot. in the panning article there is a 100K pot with 15K resistors but i thought only 10K from signal to ground would also lower the level a lot. so i chose a bigger pot, had to use bigger series resistors... it seems like a vicious circle?

-the tonestack doesn't do much, but it works.

-voltages are:

V1
1: 136Vdc
3: 1,17Vdc
6: 134Vdc
8: 1,14Vdc

V2
1: 201Vdc
3: 1,6Vdc
6: 218Vdc
7: 47Vdc
8: 90Vdc

there is 30,5Vdc over the cathode resistor of the power tubes.