peavey heritage vtx mods

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...)

  Whoopsie-daisy.  Ran into a slight problem here, literally.  Circuit board #2 conflicted with the power transformer, by just about 3/16".  I investigated relocating the transformer: would have been possible, but rather complicated.  Instead, I was able to get just enough clearance by drilling new holes in the circuit board and moving it 3/16" closer to the front panel.  Well, that's the kind of stuff one runs into when working quickly and not fully test-fitting at every stage.  Goes slightly against my trademarked policy, "move carefully and try not to break things" [tm], but there ya go.

  Board #2 is all wired to the front panel now; board #1 is in-process.  Note how I made additional screw holes to temporarily mount the boards a little further back from the front panel, for soldering access.  When the boards are in their final positions, the aluminum extrusion partially covers the boards. Once this part is done, it'll be time to connect the tube sockets to the boards.  And then the full assembly will be ready to connect to the VTX chassis. ...And all done!  Ready to connect to the VTX chassis and begin testing.

  Note:  I somehow ended up without 250p caps in my recent big parts order.  Not sure if I forgot to order them, or maybe they still haven't arrived, a few of the items had much longer lead times.  This is the cap on the treble pot of the AB763 tone stack (high voltage rated).  So, for now, I just put two 470p caps in series, for 235p.  Treble corner freq will be slightly higher, prob not enough to notice.

  Red/white twisted pair is heater power: white to pins 4 + 5, and red to pin 9, for 6.3VAC operation. Colour code for tube terminals: black = anode (plate), green = grid, yellow = cathode. This comports approximately with the "standard" colour code used in other vintage tube gear, such as Fender and Marshall -- except anode should be blue, not black.  Blue is not a colour available in 4-conductor phone wire, so...

Board #2. Important:  In addition to other component value changes noted in a previous post, there are two critical changes needed which I discovered as I soldered up this board: The two 220k resistors associated with the phase inverter need to be connected to ground, at the node where they join together. And the two 0.1uF capacitors which those resistors connect to, need to be rated for high voltage, i.e., 630V.  (In the wiring diagram I have them represented as just plain unrated low-voltage caps, i.e., 50V.)  The only other 0.1uF cap is in the tone stack, also rated for high voltage; so there are no low-voltage 0.1uF caps in this circuit.

Soldered up board #1... Q ...and installed a 12-position connector strip, to help organize the interconnecting wires between the old and new chasses.  This covers all the wires except the tube heater wires, those are soldered directly because of the high current.

  Here you can see my LED mounting technique.  The wires are soldered to the LED, insulated with heat-shrink tubing, and then the LED is epoxied into place in the panel.  This will make it a little complicated if I need to change an LED (I tested them first); but given the slick surface of the aluminum, I expect I could pry the epoxy off relatively effectively if I need to (likely destroying the LED in the process). You can also see, the last pot in the line, "presence", has to face sideways in order to fit underneath the wooden mounting bracket which goes nearby.

      Note that, by simply unplugging the connector, only one trace has to be cut, and no wires are cut. The wires which connect to the extension chassis are attached as per the wiring diagram. The 22uF electrolytic capacitors and the 330 Ohm power resistors are soldered to different pads in convenient locations on the given PCB traces, such that they end up electrically in parallel.  Capacitors are polarized as per the wiring diagram (+ towards the tube pins).  The resistors are raised a few mm above the board for heat dissipation.

Now that all the components have been shipped (just in time to get in ahead of the Big, Beautiful Tariffs), I've got the circuit boards cut to size, and a plywood bracket fashioned to mount them in the control panel extrusion.  Tonight I loaded the boards with the components, in accordance (more or less) with the wiring diagrams posted here before.  No soldering, yet.