25 watt valve pa bass cut question

[Bruce / Mission Amps]

Right off the bat I really need a better picture of that resistor that is blown/broken in half and it looks like there could be some omssions or errors on your schem.
It is possible to have B+ on the grid of the second triode but how is the cathode grounded for bias reference?
Is the second triode using another 220K anode resistor, which is written in but shown as a cap?
If there is just a tiny amount of NFB delivered to the cathode of triode #2 I guess a very high value is OK but I am not so sure that 470K is really a 470K on the cathode of the second 12AX7 triode.
I don't see any way to bias the triode unless it is fixed bias brought out by the plate supply, to the grid, forming a voltage divider on the grid... which again looks to be the voume control.... which would be painfully noisy and not very effect as it would change the bias point and the volume!! Not right.
It looks like a simple voltage divider for the negative feeback where the cap bypasses the NFB voltage at specific frequencies to ground, around the cap through the switch.
This is kind of what a presence control would do when bypassing NFB to ground around the 4k7 or 5K pot in that control.

[9 volts]

fixed schematic

Sorry the 220k resistor is fixed. Two separate resistors to plates of the first two triodes.
I've added a coupling cap before the volume in the schematic...whoops. (the amp actually sounds quite good so the errors will be with the schem). I'm currently rechecking again. Awa were based on phillips I think.

[Ptron]

I'm sure there a lot of folks who know better than me but I'll offer my two cents here. Resistors commonly come in decimal multiples of 6.2 and 6.8. If you get that portion of the circuit straightened out it might be possible to deduce what that resistor should be, but there are quite a few things with the schematic that don't look right to me.

First, the 2.2M input and 1.8k cathode resistor are certainly grounded. Looks like probably via the ring of the in jack. That junction of the to 8uF cap -'s and the two 1k cathode resistors are probably just all grounds. That would make the 1.5k and 2ohm the Cathode bias resistors for the power tubes and the 20uF the cathode bypass cap. Speaking of which, having two resistors in series like that seems highly unusual to me.

The junction of the two 220ks at the grids of the power tubes is probably a ground and perhaps the junction of the 2.2k and 2.2M between stage 2&3 is too? It's hard to say because Stages 3&4 look to be the phase inverter but it's not right the way it is. I don't get what that 56k and 10uf in series is (is the 10uf a cathode bypass in the P.I.?) and the feedback, which the bass cut looks to be a part of, often goes to the phase inverter. I don't think that junction between the .1uF and the 470k right at the end of your turquoise line should be there. I would look for where you might be missing points of the circuit that are grounded to the chasis.

[9 volts]

awa schematic

Thanks for the tips.
Ptron, you were pretty much right in your observations.I've corrected the mistakes in the schematic and have also included photos of the 1.5k to 2ohm resistors (second photo). I've also uploaded a detailed photo of the bass cut circuit. It's covered in two photos. The first shows where the 470k connects to the cathode (first photo) and the second (third photo) shows where actual bass switch etc. ). Anyway... thanks again.
Ps- The 10 uf cap is non polar.
I'm working on a 'version four' schematic which is a little easier to read. Will post when completed.

[Carlos]

im looking at mine and that resisitor is blue red brown gold, so i guess that makes it 6.2K? with 5% tolerance, im following your project, today i put a new power cord on some sylvania 6l6's and some nos 12ax7s, ran my cd player into the phono and i have sound yippee! i removed all teh radio garbage from teh circuit. keep going with this man its been great.

[Carlos]

extra info

This site is excellent, it has a little bit of info on these awa/phillips/stc PA amps
and lots of information in general on austrlian guitar amps
http://www.ozvalveamps.elands.com/awa.htm