peavey heritage vtx mods

OK, done (I think) with this reverb damper and the associated changes to the amp cabinet.   I made a video, demonstrating the operation of the damper mechanism, including the first audio test: I hooked the reverb output up to a stereo amp, and manually bumped the springs with my finger to create a reverb sound.  With damper off and damper on, there's a clear difference in tone and decay time: pretty much right along the lines of what I was hoping for -- at least as far as I can tell from this primitive test. https://youtu.be/n60kjWGzs7A

I was going to have the reverb damper control panel located two plywood layers deep, but this would have necessitated cutting a nicely-shaped oval hole in the original baffle to line up with the oval in the new baffle.  This would have been difficult since the old baffle is not removeable, so given the protruding edges of the cabinet sides, it wouldn't be possible to get my power jigsaw in there to do the cut.  I would've had to use some kind of small hand scroll saw, which I don't have so first I'd have to go tool shopping...  And anyway, I didn't really like the knob being that far back inside a "tunnel" of plywood layers.  So I realized, I could just cut out a larger square-side notch in the old baffle, as seen, and then mount the panel at the level of the back of the new baffle, so going through an oval hole in only one layer of plywood.  Looks better, and easier to implement. As you can see here, the reverb base mounts with the control panel protrudin...

In order to facilitate installing this new reverb damper mechanism, I will mount the reverb tank on a plywood base.  (This will cause the tank to protrude higher into the space behind the speaker; I hope it doesn't interfere with the speaker, I haven't carefully tested this!  If it doesn't all fit properly, I'll just have to start making changes...)   ...As you can see, I've used a cannibalized 6mm pot body for the control shaft, so that I can attach the same type of knob that the rest of the control panel will have.  I'll cut a hole in the speaker baffle to expose this reverb damper panel.  With my usual laser-printed label and epoxy finish, it should look pretty decent! The knob travels through about 180 degrees; ccw is damper-off,  cw is damper-on.  The bent wire rises up and contacts the springs, when the reverb tank is in place.  The wire will be covered with felt -- at least, that's what I'll try first.

When I tested the spring reverb, back in San Diego, it was roughly functional, but performance was terrible.  When turned up at all, it would start to go into acoustic feedback.  It was necessary to keep it pretty low, to get any usable reverb sound at all.  This is not what I want; if I'm using reverb, I want an overwhelming wash of wet sound!  If I'm going for a subtle edge of reverb, then in live situations, I'll just as well turn it all the way off for a tighter sound.  In recordings, of course I'll be using digital reverb in the mix.  I'm no special fan of spring reverb, I hate the "twangy" or "drippy" sound, I'm no surf-rocker or rockabilly player.  I use spring reverb if it's all that is available, and the more it sounds like plate reverb, the more I like it.  But if I'm using it at all, then I want a lot of it. Since it's obvious that less-than-careful people have been inside this amp before me, and the reverb tank had cle...

  The parts listed under "cathode bias" are to be soldered to the VTX power tube board.  Everything else mounts within the extension chassis.  Most components mount on two circuit boards, as seen in my drawings; a few components mount directly to the back of pots. The stars next to some component values are my non-specific indication that these components should be "up-rated". For capacitors, the voltage rating should be higher than the +HV rail of the circuit in question.  In this amp, with its relatively high +525V rail, the starred capacitors should have voltage ratings of 630V or higher.  Unmarked capacitors should generally be rated at 50V. For resistors, the star means that the wattage rating should be higher.  Unless marked otherwise, in this amp, I am using 2W for starred resistors.  Unmarked resistors are 1/4W.  The up-rated resistors should be mounted a few mm above the board, and similarly separated from any other components, for proper...

  (Updated drawings, minor corrections.) (More corrections:  Electrolytics are 22uF @ 50V, not 25uF.  Decoupling caps are 22nF @ 630V, not 20nF.  In cathode circuit of phase inverter, it's a 5.6k resistor, not 5.8k.  Also, note that in my actual layout of the two circuit boards, I have placed all of the HV power supply components (2 x 10uF electrolytics and 2 x 10k @ 2W resistors) in one group at the right end of board #1.  Finally, there is an additional 100k LED resistor next to the two shown, on the left end of board #1, and there is a 10M LED resistor next to the HV supply components on the right end of board #1.  These are for the RGB pilot LED (red = +15V, blue = HV).) (Still more corrections:  The two 220k resistors associated with the phase inverter, need to be grounded at the node where they connect together.  And the two 0.1uF caps connected to these resistors need to be rated for high voltage (630V being my standard throughout the ...

 

I've decided to concentrate directly on producing a representational wiring diagram, without making a real schematic diagram first.  Maybe I'll make that later.  Everything I'm planning to do is on a collection of different schematics, but not all in one place.  As you can see, I adapted my panel graphics PostScript file to display images of each front panel item (pots, switches, etc.), plus the two tube sockets.  Then, I'm just hand-drawing the components and wiring in between, which I'm in the middle of doing as I write this...

So the Fender amp circuit which is closest to my new proposed design (in terms of power amp), seems to be the 5E5.  This is a Tweed with push-pull 6L6 pair, cathode-biased, and cathodyne phase inverter.  However, this circuit had no negative feedback. Cathode resistor was shared between both 6L6 tubes, 250 Ohms @ 10W.  (And 25uF cap.)  This was with B+, at the output transformer center tap, of +395v.  We will have +525v.  So I'm thinking, maybe 2 x 330 Ohms @ 10W, just by mindlessly scaling up by a little over 20% for the higher voltage. The 5E5 used a 12AX7 for the PI, with a standard gain stage in front of the cathodyne stage.  The obvious approach would be to send NFB to the cathode of this gain stage, with master volume ahead of both.  However, I'm wanting to reserve the extra 12AX7 stage for switchable boost in the preamp, thus MV between the two stages (and VTX preamp output also feeding in at this point, ahead of the MV).  So instead, ...

  Here's the front panel extrusion before drilling.  The laser-printed paper graphics have been sealed onto the panel with clear epoxy. Here I have drilled the holes for the controls, and the two extrusions are mounted (temporarily) on the wooden blocks which hold the whole assembly to the original VTX chassis. This shows the cutout I had to make in one of the mounting blocks, to allow clearance for the presence knob.

  Here are the interconnecting wires which will connect to the new circuitry.  As you can see, the wires pass through three holes to the 2nd chassis which will attach below.  This will all be covered up when the preamp board is re-installed, so I document it here. Through the left hole, pass the +15VDC and gnd (thick red & white wires), the footswitch wires (four wires inside jacket -- see below), and the VTX pre-out signal (thin red). Through the middle hole, pass the negative feedback wire (black), and phase inverter signals, phase(+) (yellow) and phase(-) (green). Through the right hole, pass the +HV and gnd (untwisted thick red & white), and the 6.3VAC and gnd filament power (twisted thick red & white). This documents the footswitch wires.  The colour assignments are (unfortunately but unavoidably) different from the colours of the wires in my "rewiring the footswitch" work. black = FS.2 = rvb red = FS.3 = phs green = FS.5 = lead yellow = FS.6 = norm ...

  This new panel design embodies my recent changes: boost switch, making use of the available extra triode stage now that I'm switching to a cathodyne phase inverter; and output jack, to allow patching the tube preamp into the VTX preamp (or anywhere else), independent of the preamp select switch. As you can see, most of the wires I'll need to access, to attach the new circuitry, are available on this 6L6 power tube board.  And this will be the only board that I need to modify, as shown. BTW, notice that, the way they have things connected, you definitely cannot run 7027 tubes in this amp.  These are functionally the same as 6L6s, but they have two of the octal pins which the 6L6s don't use (pins 1 and 6), connected redundantly to other pins.  The way Peavey has the sockets wired would short the cathode to two of the grids in a 7027.  Not sure why they did this, other than for mechanical stability and without thinking of the 7027. I won't have to attach anything...

 

  I've drilled holes in the VTX chassis (pretty tough steel!) for the mounting blocks for the aluminum chassis extension, and also a couple larger holes for wire feed-through. I think I will have the two 12AX7s facing backwards towards the 6L6s, i.e., horizontal orientation, rather than pointing downwards; this will work out better in terms of space inside my extrusions -- unless there is not enough clearance behind the 6L6s, in which case I'll have to re-figure things a bit.

 Before even starting to build the electronics portion of this amp, already I'm contemplating some changes. First, and simplest, I'm going to add an output jack to the AB763+ preamp.  This will be a simple, non-switching jack, always active regardless of the position of the preamp select switch.  This will allow the preamp to be used independently as a general processing element -- including, it will thereby be possible to patch the AB763+ preamp into the input of the VTX preamp, with a cable, thus duplicating my typical amp configuration of old, where I'd put a tube preamp in front of a Peavey solid state preamp for more and better gain. In a more significant change, I'm now thinking I will configure the power amp as cathode-biased, with the simpler and more primitive one-triode phase inverter (what's it called?) rather than the long-tailed pair.  This will free up a triode section to become a switchable boost stage, which I've wanted.  These changes will both ...

  The new second level panel will be formed from this 1+1/2" aluminum extrusion.  The front panel will extend across the full 24+1/2" width; there will be a 6" section in back which carries the two 12AX7 tubes. As you can see, the best place to position this back section turns out to be offset from center towards the right, more or less underneath the "norm" channel EQ and the phaser knobs.  In order to keep critical audio wire lengths a bit shorter, I'm going to re-arrange the ordering of the control subgroups within this panel.  The VTX preamp controls (which are just switching logic, no audio) will be to the left, then the AB763+ preamp in the middle, then the power amp controls to the right.  This will put the input jack for the AB763+ preamp right near the center, rather than at the left end, which is a bit strange, but I think it's the best way.  The McGrath logo will go to the far left, then the three subgroups will be closely spaced in the given...