A final update on how the Bonnet Box rebuild went. Thank you again for the great feedback given on this forum.
The attached schematic shows most of the mods.
I kept the power section, although I changed the PT secondary resistors to 75ohm 5watters, and the GZ34 to a 5U4 - this was to reduce plate voltage at the EL84s and the PI. It's worked nicely....so far.
The PI was redesigned in accordance with Sluckey's drawing of the Vox AC-30.4, which also uses a 12ax7 PI - that schematic is easy to find if interested, and it matched the quad EL84s nicely. Of course, this necessitated inserting coupling caps between stages, instead of the original design. Once those caps were in place, I was free to add the 18 Watt Tremolo tone stack (super simple for a newbie like me).
The EF86 input was reconfigured it for Hi and Lo imp (not shown on schematic). I reduced the cath res and added a 25uF bypass cap. Also, the schematic doesn't show it, but I removed the 33k res that goes to gnd off Node 3 - it was bleeding too much voltage away from the EF86, and once it was removed, the PV of the EF86 went from around 46v to 110v.
I removed the NFB, and I floated the filament AC with two 100ohm res to gnd, thus removing the hum balance pot, which I didn't like much.
There was a bunch of extraneous stuff removed, but I reconfigured the octo plug to serve the control panel. Control panels are a PITA, largely because they entail many more logistics when removing and re-testing the amp over time. However, the PT has a dedicated 6v winding for the idiot light, so I used that, running it up to the control panel.
The inputs and control pot connections were run, via shielded RCA cable, up to the control panel - more screwing around that took forever. It also introduces all sorts of hum, and the amp still has a little more than I'd like, so I keep checking for ground loops etc.
The original amp had a woofer, two mids, and a tweeter. I kept the woofer and a hi-mid (both 8ohm) to give me the 4ohm load the amp output was designed for. I experimented with the crossover filter, and it still works well. I know guitar amps don't usually have this type spkr config, but it's fun to experiment, and it means that when an MP3 player is plugged into the Lo-Imp input, the audio quality is a bit fuller than a single spkr would offer.
Finally, the thing sounded exceptionally loud and exceptionally good, breaking up beautifully right about the same time my ears started to bleed. So I added a Type 3 Master Vol, inserting a 1M audio taper pot right after the 0.1 coupling caps coming off the PI.
The MVol works, but there is a problem: it turns the vol all the way down okay, but it behaves sort of like a linear pot - the total volume kicks in with barely a quarter turn of the pot. This is a problem with some Fender amp volume pots - usually, changing those pots, which were linear, to audio taper fixed the problem. But this is already an audio taper, so I'm not sure what's up, unless the pot is mislabeled. Any suggestions very welcome.
That's about it. You'll see I rebuilt the cabinet from the old bonnet box in an attempt to retain some of the old styling, but it quickly becomes a logistical nightmare to do this sort of thing - headaches loom at every turn. This is why the control panel if full of patched holes - it took about ten tries to figure out how the thing would go together, as opposed to using a purpose-built cabinet. The results ain't pretty. Thus, Ugly Betty was born. And let's face it, the Bonnet Box was about as ugly as a stereo could get in any case. But I do admire the workmanship I encounter when modifying such a cabinet.
Cheers all.
Topic: Bonnet Box conversion 4x EL84s (Read 11211 times)