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

[Snfoilhat]

Thanks @trancedental , i always expect the guy at the print shop to set up the wide plotter/poster printer because that’s what he did once when i got a 23”-ish wide panel printed, but he went to a machine that looks like a copier and loaded this 19” photo(?) paper.

 

[Snfoilhat]

DIY layout blues

Here's a photo I picked up in 2018, don't remember the source and can't give credit

First little detail that threw me off was I always have the tube sockets oriented with pin 9 toward the front of the amp -- I don't know if there's a standard and I missed the message. Anyway these have 678 to the right from the tech's POV and 123 to the left. Next, this layout has grouped the components, moving from right to left, V1b, V2b, V1a, V2a, [V3b empty], V3a, V4. So I already had to backtrack from the generic 1960s Fender type layout I started drawing naively.

Has anyone gotten DIYLC running well with Ubuntu / deb based kind of system? I'm linux dumb and would appreciate any help you can give. To save myself from all these pencil eraser crumbs I'm making at least, and maybe the world gets a layout they can use in return at the end of this project

 

[Snfoilhat]

The princeton reverb chassis looks so spacious when you start out...

Here's the first draft -- even if it improves substantially, i don't see being able to fit the power supply parts. So I'll work on a smaller board to sit somewhere in the left end of the amp, maybe above the unused V7 cutout, maybe above the power transforme. See the original (weirdly distorted photo), same source as the previous one. Panorama mode?

 

[Snfoilhat]

If you haven't tested your power transformer and primary-side wiring (AC cord, fuse, on-off switch) yet, around now in a build isn't a bad idea. Just make sure you have the secondaries managed in some way (e.g. floating electrically and secured physically)

This will all get covered up presently and would be a pain to have to excavate:

I added a short twisted pair off the lamp terminals parallel to the rest of the heaters to go to the virtual center tap, now relocated (re-relocated) back to right where it happens to be in the original Prosonic. Even if my main circuit board doesn't tighten up much in revisions, I got a lot done on this smaller board and I'm more confident now everything will fit in the end.

That lowest terminal on the speaker impedance switch will be a pain to get the soldering iron to later, past flying heater wires and everything else, so I got that going first, with enough slack so that the whole speaker jack assembly can hang outside the chassis when I install the remaining tube sockets and wire them.

I'm waiting on a few parts (like the 1kV caps that protect the main rectifier diodes), but first thing once I get going again will be to complete the stuff shown here and get some voltages for B+, heater elevation, and bias. They're interdependent, so some of the resistor value choices are guesses that can only be confirmed after turning it on and measuring.

I think it will be nice to work on the other side of the amp knowing all this stuff is settled.

 

[fxrskrsa]

Here is a shot of my recently acquired Prosonic which has had the board rebuilt by an amp specialist in the UK… fantastic amp

 

[fxrskrsa]

 

[fxrskrsa]

 

[Snfoilhat]

That’s really cool @fxrskrsa thanks for sharing that. I see that this builder also had to separate the ss/tube and fixed/cathode choices into two individual switches. I don’t think that rotary switch Fender used to switch both at once is a part easily found

 

[fxrskrsa]

That’s really cool @fxrskrsa thanks for sharing that. I see that this builder also had to separate the ss/tube and fixed/cathode choices into two individual switches. I don’t think that rotary switch Fender used to switch both at once is a part easily found

I hoped the photos might be helpful, please let me know if there is anything else I can pass on, I am not technical but will do my best

 

[Snfoilhat]

Testing the inputs can save time and trouble later. In this case I caught a small error -- I wired up the 1M grid leak by rote in the 60s Fender style. If you check the schematic you can see that the Prosonic designers arranged the grid stoppers and grid leaks to form a small voltage divider. It would work fine either way in this case, but serves as an example of why checking can be helpful. Save you that stress at startup time having to make a thread that says tubes light up but no sound help appreciated.

First is to know what to expect, then to measure, then to compare.

Instrument input 1 (tip) to ground (sleeve) on this cable I'm using for convenience. Two 47k in parallel plus 1M = 1.02M

Instrument 2 input (tip) to ground (sleeve). 47k and 1M in parallel plus 47k = 91.9k

Instrument input 1 (tip) to the grid of the first gain stage (in this case, the end of the lead that will be soldered to V1 pin 7). Two 47k in parallel = 23.5k

Instrument input 2 (tip) to grid. 47k.

Maybe we don't trust Zinky et al. and want to verify how much sweet instrument signal is going to ground by not using the '60s style inputs. Using input 1, calculate the voltage divider.

Use 100 for an input voltage (unrealistic but convenient) and you can read output voltage as a percentage. the 1990s lost us 2.3%!! I'm not sure if this part of the design goes under the category "Yay the '90s!" or "Boo the '90s". For now I'm sticking with the original.

 

[Wally]

For now I'm sticking with the original.

Original??? original BF arrangement or original Pro Sonic?

 

[Wally]

Okay, @Snfoilhat, you ‘liked’ my post; but you left me hanging with no answer???.

 

[Snfoilhat]

I really appreciate your interest in this amplifier @Wally !

Original to the Prosonic. I'm also going to put that tiny (5 pF) cap from the grid to the plate of the first gain stage in. Who can say what was going on when they were working on these. Some tiny extra attenuation of high frequencies, like to the level traditional 68k grid stoppers would have done? Some radio interference? I'm gonna take their word on it.

***

Parts! Remember when the scientists in Jurassic Park wanted to clone T. rex, but they had to use a bunch of DNA from less awesome (nonextinct) animals? In this analogy, the real Prosonic is the tyrannosaur and the Blues Jr. and Princeton Reverb parts I'm using are the frogs

I don't know if this is going to happen, but I'm mulling it over. I can't avoid building a head shell for this forever.

 

[Wally]

Snfoilhat, Bruce Zinky revisited the Pro Sonic. I have the ZINKY mods copied in my files, but one thing Zinky advises is to cut all of those small caps on the tube sockets out. If you want those mod suggestions from B.Z., PM me. I’ll see if I can locate them.

 

[Wally]

And….I like that vertical cab.

 

[JDB2]

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.

The two Prosonic heads I owned did not sound harsh or thin. For comparison, I also owned a combo at the same time. As I recall, a coupling cap is a different value in the head and as a result the lower bass is filtered out. I thought about swapping out the cap to give it the same value as the combo but never did that before selling them.

 

[Wally]

The two Prosonic heads I owned did not sound harsh or thin. For comparison, I also owned a combo at the same time. As I recall, a coupling cap is a different value in the head and as a result the lower bass is filtered out. I thought about swapping out the cap to give it the same value as the combo but never did that before selling them.

Yes, as I noted I wanted to investigate that pRo Sonic head, but my buddy simply moved it on to be somebody else’s problem. There was no mistaking the difference between that head and the three combos I had.

 

[Snfoilhat]

@Wally PM sent, thanks!

***

Taking my own advice about checking the power supply mid-build:

I double checked everything visually (some of those leads you see floating around will be energized, so shouldn't be touching each other, the chassis, or anything else), ran it through the current limiter to check for faults (also checking voltages to see anything way out of line), then swapped to a normal outlet to test the voltages w/o the limiter.

B+ unloaded was around 400 VDC, and I'm thinking this will sag to somewhere between 350 and 400 when loaded. Heaters VAC was good and heater elevation was around 70 VDC. Bias had a range of -22 to -26 VDC.

I looked at a few comparable 2@ 6V6GT amps' schematics to see the nominal bias voltages. Brown Princeton 6G2, -35 V bias, ??? on the plates (it says 315 but I'm not sure that's accurate). Princeton non-reverb AA-964, -34 V bias, 420 V plates. Super Champ, -33 V bias, 385 V plates.

So I guessed that my first draft values probably weren't going to get me a cool enough range. It's hard to say, since everything is going to change under load, but I took a guess and replaced a 27k resistor in the bias supply with a 39k, and got a range of -27 to -32 VDC. And we'll just have to see where it all ends up and adjust from there.

 

[Snfoilhat]

Epoxying the extra holes in the back panel

Eyelet board

 

[Snfoilhat]

Edit: All that hedging and trepidation because the SLO was so noisy and unplayable at startup. This thing already runs pretty good! Normal channel works, drive channel works, master volume works and just has some noise near it's extreme end. Drive channel sounds pretty rad. Gain 2 control seems to thin the sound, and Zinky describes it as "like a compressor" so there may be an issue there with my wiring of the control.

Bias switch works, and I can see a drop in B+ when switching to cathode bias mode (flowing more current than in fixed mode). Gotta dial in the fixed bias. I took resistances of the output transformer primary leads during the build so that I could use the voltage drop method of estimating plate current.

I'll bias it, see if I can tidy the lead dress w/ respect to that noise at high master volume settings, and then button it up in a different amp's head cab to show everyone reading along what it looks like. And a sound clip. And an as-built schematic.



There are all the parts and a bunch of wires. It remains to be seen if it's a musical instrument. What I learned from the (paused) Super Lead Overdrive -based amp build is that debugging something like this can be just as much work as it took to get to this stage, so I think I'll work on the cabinet for a minute and recharge.

Alternatively there's a Princeton Reverb style combo cab I built years ago that needs a little work and a Jensen C10N 4 ohm speaker I never seem to have anything to pair with, so maybe it's time to fix that up. Still I think this makes more sense as a head.

Thanks!