Vintage-amp-ifying the Fender Prosonic (yay the 90s! boo the 90s!)

[Snfoilhat]

I figure it's not too soon to treat the 80s and 90s amps in a way similar to the heavily studied, copied, hyped, and reinterpreted 50s and 60s amps, by working out some more or less faithful clones using the common parts and construction techniques most hobbyists use. You can still find an original Super Champ or Prosonic for about the cost in parts of a good copy, which argues against my thesis here. All the same I'm going to take a swing at detuning some of the 1990s-ness of the Prosonic and see what it sounds like built more in an earlier style.

1st, a Prosonic has been on my list for years, and only this week did it finally occur to me that if I just pick the non-reverb (head only) version, it's both more likely to run correctly than the reverb (combo) version and it will fit in Princeton repro chassis. Every similar repro chassis wider than the PR is drilled for 2 channels and I've never wanted to deal with that.

2nd, I found a front panel slide switch I'm hoping will simplify (and 60s-ify) the user interface and the channel switching function, since I'm not going to build anything that requires a footswitch and want to eliminate the relays.

Looking at my collection of Prosonic gut shots I've been diagramming the interesting use of dual gang pots in the production amps. It looks like only two of them really needed to be dual though all 7 or 8 were.

I ordered a fresh PR chassis, and my hope is that there's just enough space between input 2 and the volume pot to squeeze in this 3PDT slide switch, which AES doesn't carry (I didn't know it existed) and I ordered from Mouser. It's a little wider than the typical bright switch.

Here's a copy of the Prosonic (non-reverb) schematic

And then I've been working on cleaning it up -- removing the relays, LED, footswitch, simplifying the rectifier+and biasing switching scheme, reworking the power supply according to the power transformer I want to use, and some other minor changes.

https://robrobinette.com/Generic_Tube_Amp_Mods.htm#High_Voltage_Tap_Adjustable_Bias

The grayed-out portions need to be reworked based on the new PT and the power tubes chosen. I don't think I'll pick 6L6GC but something smaller.

I don't expect doing a new turret/eyelet layout of an amp designed for PCB is going to be a cakewalk. You can see in gutshots that virtually every signal lead is shielded. But I think the part count looks manageable for the space and some of the lead dress can be copied from the original.
That's it for now

 

[Wally]

You can still find an original Super Champ or Prosonic for about the cost in parts of a good copy,

Who builds a copy of either one of these amps?

 

[Wally]

Ime, the head version of the Pro Sonic does not sound like the combo. The one head I have heard was harsh and thin compared to the three combos I have owned. The three combos all sounded exactly alike…I owned them all at the same time.

 

[Snfoilhat]

Who builds a copy of either one of these amps?

The cost in parts (like to the would-be home builder/end-user of a Super Champ or Prosonic), assuming retail prices for components and cabinet is, for the sake of conversation, about $1000. Approximately what an average, real Super Champ or Prosonic costs used today. Likely part of the reason that none of the usual players makes a kit or clone Super Champ or Prosonic. Another is surely that neither is nearly as popular as the old amps.

Ime, the head version of the Pro Sonic does not sound like the combo. The one head I have heard was harsh and thin compared to the three combos I have owned. The three combos all sounded exactly alike…I owned them all at the same time.

That's an interesting observation. The Prosonic's reverb recovery and mixing stage is different from every other Fender ever made, as far as I know, and maybe losing that in the non-reverb version has some unpleasant global effect on the EQ. On the other hand it could have been an idiosyncracy that one head unit.

Thanks!

 

[Wally]

On the other hand it could have been an idiosyncracy that one head unit.

and so I observed to my friend who owned the head. I wanted to open it up and see what was going on As far as any differences in the components, what the voltages and biasing numbers were, had it been modded??? He just wanted to sell it, and so he did….via Reverb.com, iirc. I would not have bought it the way it was unless I knew why it was sounding as it did. I was surprised at the difference in sonics.

 

[Snfoilhat]

Chassis and front panel controls for the Prosonic non-reverb

7 full-size potentiometers, indicator light, at least one input (two on the original), and I like to move the power on switch to the front like in the Rivera-era amps.

Will these doublewide switches fit between input 2 and the first pot? Yes!

 

[Snfoilhat]

Power transformer choice and mounting -- I want to get all of the chassis modification out of the way before any other work begins, though I always seem to end up drilling a few more extra holes along the way as a new layout develops

This is my favorite power transformer for smaller amps -- close in size to the Princeton Reverb / Champ PT, but more current handling on the filaments and lower voltage on the HT, plus being stand-up frees a little space inside the chassis

Choosing this PT w/ 250-0-250 VAC @100 mA helps narrow down on the power tubes. One pair of 6V6GT or 6BQ5/EL84 are the most likely choices

Other bottom panel modifications include a hole for dedicated piece of hardware for the chassis ground nearby the power cord inlet, and the output transformer will likely be bigger than the princeton OT this chassis is drilled for, and so will need wider spaced mounting

 

[schmee]

There are fans of the Pro Sonic for sure. I just have never been able to bond with one. I can bond enough with most Fender old school tube amps, but the Pro Sonic's I have tried just sound terrible to me. I have no idea why, I just couldn't get a normal Fender sound out of them. A buddy had one of the foam green tolex ones I tried to use. There was one on a back line I used also.

 

[Wally]

@schmee, what is a ‘normal Fender sound’???

 

[schmee]

@schmee, what is a ‘normal Fender sound’???

.....anything BF or SF from Deluxe to Showman!

 

[Wally]

.....anything BF or SF from Deluxe to Showman!

What about the almost two decades of high quality Fender amps that preceded the BF/SF era???? One of those tweeds was the basic blueprint for a huge number of amps that have been used worldwide since the introduction of the first Marshall????
I hold that there are a fair number of varied ‘Fender‘ sounds….and all of those different sounds are ‘normal Fender’ sounds. And…fwiw….I can play surf music on a Pro Sonic…quality sonics.
I was playing on one of my Pro Sonics some years back when a fellow stopped by. He happened to have been one of the only people I know of who bought a new Pro Sonic Around here. He commented that he had owned on, and I acknowledged that I should have bought it for $475 when I saw it in a pawn shop…..sea foam green LO first year Pro Sonic. I went back the next day, and it was gone. Anyway….I sat there and put the amp through its various paces which yield a very wide variety of sonics. When I had gone through them, he said: “I never knew it would do all of those sounds!” He was amazed. The trick is in knowing how to run the machine, ime. A Pro Sonic can do almost anything a person wants it to do if they understand the amp. I kind of regret selling my last of three that I owned…all at the same time. I may be able to buy one of them back, though, if the young fellow doesn’t find a way to use it. It doesn’t travel as well as does his Super Champ.

back to the program…sorry for the diversion, @Snfoilhat.

 

[Snfoilhat]

Normal / Drive switch

Drilled all the other holes marked out in the painter's tape. Next is working on control panel graphics

 

[pbenn]

Earnest Lee in Toronto at Grossman's used a Pro Sonic about 15 years ago, I remember.
Loud blues band stuff.
Although perhaps Grossman's is more of a Deluxe Reverb room.

 

[Snfoilhat]

Slowly filling in details of the layout, which because it's an adaptation means there are unknowns to face sooner or later. Checking to see how much space will be left in the upper left corner to fit adjustable fixed bias components

Cathode / fixed bias switch (not the 3P3T bias and rectification switching of the original, and not the DPST shown either , but a DPDT on order). This will be always tube or always solid state diode rectified. Haven't decided. Impedance selector switch -- cutting that 21 mm diameter hole and trying to keep it circular isn't the most fun

Working on front panel graphics. Easiest is to go with knobs with the numbers on them, like this familiar style

Alternatively you can choose a knob with an indicator line and put the numbers on the front panel, but then you need to plan out the diameter of the knob so the numbers are visible. I imported these number graphics from an amp I did with Marshall-style knobs (d=0.77 in), but that may not work with a larger knob like chicken heads or the oven knobs I got to really like in my Ampeg-looking amp

Yellow front panel, black lettering, and gold-capped Marshall knobs would be the easiest choice, from a computer wrangling point of view -- but then it wouldn't be as obvious that this amp is a tribute to (ripoff of) Zinky/Fender! So I'll at least check the chicken head datasheet and see if a fit is possible

TBD

 

[Snfoilhat]

Repeat the process for the rear control panel

The unused holes in the chassis can be filled with something like JB Weld or not, and the back graphic will at least cover them

 

[Wally]

Fwiw, I find the option of tube or SS rectification in the Pro
sonic to be very useful.

 

[Snfoilhat]

Fwiw, I find the option of tube or SS rectification in the Pro
sonic to be very useful.

I don't doubt it! I'm still new at searching mouser and maybe that's why I couldn't track down an appropriate switch (3P3T I think) to give the same simultaneous rectifier and biasing choices. Two independent switches could do it of course, giving 4 choices

My rationale for leaving it is that a stout power supply running a pair of 6L6CGs with the option to run tube vs diode is going to have a pretty noticeable and usable swing between modes. This smaller power supply running 6V6GTs is probably always going to be somewhere in the middle, with the PT on the spongy side (vs. stout) but the tubes just not capable of drawing all that much

***

I'm a big fan of the builders on tdpri who are big fans of Sandy and other engravers, but here's my control panel method for the nth time. $6 for the whole sheet of gloss cardstock, plus a couple bucks worth of matte clear sprayed on the back (to prevent the adhesive bleeding through) and the front (for longevity, to knock the gloss down, and build up the surface a bit to make the plates look a little more silkscreened and less like paper). So minus my time, that's about $5 per amp

Bonus: I made a couple copies of entries in the sibley guide for a few common backyard birds for my friends' kid and got them laminated at the printer. Something to keep by the back window that looks on the bird feeder

 

[Snfoilhat]

Gorilla glue (clear, non-foaming) has a decent amount of open time, bonds to metals, and doesn't bulk up and try to lift the face plate away from the chassis. I clamp the chassis so that the side I'm working with is face up horizontal and spread the glue with a scrap of cardstock so it's even and totally covered. Then dab a little water onto the back side of the (now sealed) graphic with damp paper towel. It will slide freely on the layer of glue without wrinkling. I shine a light in from inside the chassis to ensure all the holes in the chassis line up with the white circles, then let it cure. Then I spray on the clear. Last is to cut out the circles with a razor knife

I ordered 7 black chickenhead knobs as a nod to the original prosonic

 

[Snfoilhat]

Making a color key for the leads to the 3PDT switch helped me see a numbering mistake I made on the schematic. In the original a simple SPST switch on the front panel talked to the relays which ran the real channel switching. I had to do some rearranging to ensure that all the Normal connections were on the same throw direction, and that Normal was up from the player's perspective. The schematic shows the switches in Drive mode. This Switchcraft switch isn't threaded and needs nuts on the hardware, the bottom of which was a pain to get started so I wanted to get this all out of the way before installing any other nearby parts. Then I got the power supply going

Little details of chassis housekeeping so far include running both leads along the body of the fuse holder so that a piece of heatshrink will fit over and seal it, cutting a rectangle of fiberboard to insulate the indicator light from the chassis floor, making a tube socket cover out of a scrap of copper sheet for V7. I think I'll build the bias supply on terminal strips. This PT needs a virtual center-tap, and instead of running the pair of resistors from the light terminals to ground, I think I'll dedicate a little space on the circuit board for heater voltage elevation and the resistors will live there

Drafting a layout of the circuit board is the next big task

 

[trancedental]

Those faceplates look really good Snfoilhat, what type of printer did you use for them?

I've only got an inkjet printer here to use, maybe I should take my faceplate designs to the print shop?

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