peavey heritage vtx mods

  Again, caveats from the previous post apply here as well.  Most significantly, I have not yet built the boost circuit, nor even verified whether the VTX power transformer can handle the additional heater current load.  And some of the wiring of the phase inverter, vis a vis nfb, has not yet been implemented or tested.  So these circuits are to some extent, aspirational.

In order to debug the issues I've been having, I've finally undertaken to draw up a proper schematic of my circuit, and my mods to the VTX power amp. Not shown are the VTX preamp, which I've made basically no changes to (except for swapping the yellow and green LEDs), and my boost circuit, which I haven't actually built yet anyway.  (The original boost circuit has been subsumed into the phase inverter section.) Certain aspects of the schematic reflect how I plan to change the design, not exactly as it stands now.  In particular, the NFB signal returning to the cathode of the third triode, and the cathode resistors of the fourth triode correspondingly changing to adapt to NFB no longer connecting there.  Also, I show the additional HV power supply components for the boost circuit (B+3), which are not there yet.

Well, I started implementing the changes I've identified as needed.  And unfortunately, I broke something.  I removed the boost switch from the signal path, putting the third triode stage into the path following the mixer resistors for both preamps -- or so I thought.  But instead of getting louder, actually everything got quieter.  The VTX preamp was very quiet even at max volume, and interestingly, the AB763 was even quieter, barely audible at all.  So at least I managed to more than equalize the levels of the two.  But clearly, something is wired wrong. Taking a break from it as I regroup.  Tomorrow, I'll try to debug this.  I'm glad I've heard it get loud before these changes, or I might worry that my cathode-biased power amp design was faulty.

OK, yep.  I hooked it up to the real 12" speaker, and through the AB763 preamp, with the boost stage engaged, it gets *pretty* loud.  Almost loud enough, I suppose -- certainly louder than I'd want it, most of the time; however, I don't think it's actually driving the 6L6s to anything like the full 100W.  Maybe it's 25W or so.  I'd have doubts about being able to keep up with a loud drummer.  In fairness, given the single 35W Celestion, I'm not eager to push 100W anyway.  But I want the circuit to be capable of it! With the boost off, it's simply not very loud through the AB763.  And through the VTX, it's never remotely loud enough.  Must be a good 12dB below a reasonable loudness. So I could "fix" the VTX situation with some opamp work I guess, but all in all, the power amp driver is just not sensitive enough.  Claiming the third 12AX7 section for boost in the AB763 was just wrong, it is needed to gain-up before the lossy cathodyne stage....

So I hooked the amp up to a tiny 8 Ohm speaker (computer type, must be about a 2" driver).  And good news is, there's audio, and the basic systems seem to work as intended.  There are a bunch of caveats, though.  Which is to be expected, I'm just happy there was sound at all, given that this is a total redesign of the circuit, never before tested in any form. Both preamps work, but the AB763 is noticeably louder than the VTX.  It's awfully hard to tell, with this tiny speaker, how loud the real amp will be.  Maybe not nearly loud enough.  Might be, at full volume, that it's just putting out about 1W or so.  Or maybe it's just due to speaker inefficiency, etc..  I think I'll have to rig up a way to connect the real 12" speaker, before putting everything back in the cabinet, because I might have a serious problem with signal levels in this new circuit. The new AB763 chassis needs to be grounded.  I put a screw hole for a ground strap, but I hav...

Connecting the new circuitry to the VTX chassis.  Most connections are through the 12-position "Euro" terminal strip, except heater power (to the left) which is hard-wired. First power-up, with no tubes inserted.  LEDs ahoy!  This just verifies that the +15V power supply is working, and I haven't made any grievious errors in the wiring. I verified that the channel-select 3-way switch works, turning on and off the yellow and green LEDs on the VTX, and the preamp-select 3-way works, turning on and off the white LEDs (yes, those are white, not blue as they appear in the camera). And I verified that the HV supply works, by taking it out of standby and observing that the blue LED of the RGB pilot light illuminates.  (It's a bit dim with the 10M resistor though, I guess I need to decrease the resistance; gotta take care with the power dissipation...)