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

[Snfoilhat]

As built schematic with voltages -- the full set (blue) is in cathode bias mode. This pair of power tubes is pulling a lot of current with plate dissipation around 105%. In fixed bias mode set fully cool (-32 V) the voltages throughout the amp are higher. Still working on the final bias points for each mode.

***
I did the initial cuts for a head shell and an extension cab and the wood doesn't look too bad. It's been a wet winter and the wood (reused pine) wasn't in perfect shape when I got it. I'll continue to tweak the Prosonic and post in this thread, and will put the cabinet stuff in another since the DIY channel people asked for more speaker cabinet builds.

***

Got a few minutes of playing on it now — this thing is fun!!

*** Edit after working on the bias. I made a simple error calculating Vpk, and now I see that the plate dissipation in cathode bias mode is sitting right at 100% of 14W. I also got a set of voltages in fixed bias mode and -32 V bias comes out to 65% of max dissipation. I'm going to leave it like this for now.

 

[Snfoilhat]

Borrowing the cabinets from a previous amp in order to dress the Prosonic up for these photos:

Prosonic left, bass amp right

Control panel front

Control panel rear

This is a pair of 12" greenbacks I'm listening to the amp through now, and modeling the new 2 x 10" cab after (though I don't have a pair of tens to put in it -- that'll be a question for down the road).

The light in this room is weird this time of day and I'm no photographer -- apologies for the poor view


@Wally @fxrskrsa and anyone else with Prosonic experience, how would you describe what the Gain 2 knob does as you turn it up in the Drive channel?

 

[Wally]

Borrowing the cabinets from a previous amp in order to dress the Prosonic up for these photos:

View attachment 1101023
Prosonic left, bass amp right

View attachment 1101024
Control panel front

View attachment 1101026
Control panel rear

View attachment 1101027
This is a pair of 12" greenbacks I'm listening to the amp through now, and modeling the new 2 x 10" cab after (though I don't have a pair of tens to put in it -- that'll be a question for down the road).

The light in this room is weird this time of day and I'm no photographer -- apologies for the poor view


@Wally @fxrskrsa and anyone else with Prosonic experience, how would you describe what the Gain 2 knob does as you turn it up in the Drive channel?

looks good.
As I hear it, Gain 2 is a Midrange boost. The manual says that it functions as a compression circuit. I say it does that simply by pushing the mids until the circuit compresses. It is a fairly radical control. Ime, one would want to explore each small incremental adjustment in order to get to know it.

 

[Snfoilhat]

Oh yeah! I can hear the Gain 2 control now. I rechecked my potentiometer wiring against the Prosonic photos and confirmed it. Looking at the schematic I'm thinking that the Gain 2 and Master Volume controls are interactive, and until now I've had the Master so low that it was making the sweep of Gain 2 sound strangely. I tried input 2 (≈1/2 instrument signal) and once the Master is above 2 then I can hear the mids/extra something that the Gain 2 increases. Sounds rad

 

[Snfoilhat]

Recorded 90 seconds of audio. No guitar phrases to knock anybody's socks off. Just a few notes in the upper and middle registers where I thought it was cool to hear so much distortion content in the notes that wasn't just fizz. Then a couple chords to show how it retains some clarity even with bigger, dissonant chords. This was less than TV volume (preamp overdrive) and you can hear the pick noise. Low gain input and controls set as shown below.

I hear a liquidy bell/chime sound in the treble that mostly doesn't stray across the line into too poky, and sustain. But talking about sounds is weird and everyone hears differently and uses the language differently, so if you hear fizz, icepicks, and then some mud, kindly let me know!

 

[Pete Farrington]

In fixed bias mode set fully cool (-32 V)

For the bias supply, to increase the magnitude of bias voltage, it may be a good idea to reduce the value of the 220k dropper from the HT winding. eg 180k, 150k.
And increase the voltage rating, of the reservoir cap at least.

 

[Snfoilhat]

Small update to the amp: I added a pair of 200 ohm sag emulating resistors to the power supply to begin to get the operating zone out of the max dissipation area. New voltages are in purple. I realized after buttoning it back up that I didn't take the whole set of voltages, but I think you can look at the change in B+1 and estimate the rest. I haven't noticed any change in sound, but that's not surprising to me given how quietly I'm playing at home. All the distortion in this new clip is in the preamp.

Demo sounds: I think I have always played noodling/nonsense to try to get the listener's attention off my playing and onto the amp, but I'm not sure that really holds up. Playing familiar music sure does make every shortcoming really jump out.

So here's about a minute of the Dave Mustaine guitar part of Tornado of Souls (Megadeth, 1990, 195 bpm!). I can't downpick that fast yet and am still learning the song, so this is played at 90% tempo using the interactive play-along tab and backing track tools on ultimate guitar.

I estimate the new bias settings as about 25 mA, 60%, in fixed-bias mode and about 37 mA, 80%, in cathode-bias mode.

 

[JDB2]

See the original (weirdly distorted photo), same source as the previous one. Panorama mode?

Ha ha, yes, those are my photos, and it was in panorama mode to maximize the resolution. Nice work, by the way! Glad the photos could be of some help.