Super Champ XD to Brownface Princeton w/verb..

[Deric]

I must have too much time on my hands.....

A while back I ended up with a dead Super Champ XD. DSP board was shot but everything else was fine. Gutted it then stared at it for 8 months or so.... Finally decided to build a Brownface Princeton (6G2) and add solid stated spring verb.

The verb is lifted straight out of a Blues Jr. Power supply for the verb and bias are from the stock Super Champ XD. Added a boost/tone stack lift via switched tone pot and a "Soul" contol (variable negative feedback).

No "Mojo" parts...just what was laying around.

After struggling with the bi-polar power supply for the verb for a while finally decided to finish the rest up. Fired up first time with a few issues. Amp was dead quite as far as hiss/hum but the trem was ticking pretty bad. Rearranged some grounds and got most of it. Replaced the wire form the .01 cap to pin 7 on V2 (pin 2 on V2 on the stock Fender layout) and that killed the rest of the noise. Verb worked great for a while then amp went dead.... Didn't have a cap between the PI and the verb....duh! Added a cap and all sounds good!

B+ is about 50v high so plan on adding a mosfet B+ reducer. Also need to design and layout a new bi-polar PS/bias/verb PCB.

Thanks to all those here who have been answering my dumb-ass questions!

[Deric]

Couple more pics....

[Deric]

...and...

[Deric]

Finally been playing with this some more...

Dropped the B+ by 70vdc or so and never could get it to sound right... After Google-ing a bit it seems that most (if not all) of the brown Princetons had considerably higher voltages than the Fender schematic shows. Pulled the mosfet B+ reducer and it was better but still sucked. Removed the reverb completely and it started to sound much better. Upped the preamp dropping resistor from 10k to 27k and got 180-190vdc on the first two stages. Started sounding really good - stock Super Champ speaker sucks though...

Thought about scrapping the verb altogether...but...

The reverb send just sucked way too much signal and took all the balls away. Ended up taking the send right off of the first stage coupling cap via a 220k resistor. Has zero effect on the tone but....it's before the volume control so even with the volume turned down you still hear the verb (if the verb control is turned up). The verb sounded REALLY good though and I kinda like the way that as the gain (volume knob) increases the verb decreases (well...really the verb just doesn't increase with the volume knob). The return took some tweaking too... Ended up adding a 330k resistor at the grid of V1b. Used a 470k resistor from the reverb return to the grid to mix the verb. Very little tonal effect and sounds good.

Swapped out the negative feedback resistor to 8k2 and added a 50ka pot in series. Maxxed out it's roughly equal to the stock 56k but you can turn it down to clean up/tighten things a bit if desired.

The trem was really good but too fast... Swapped the first .01 cap for a .02 and all is good. Still gets faster than needed.

When all was done had a few noise/ticking issues that were easily solved with a chopstick and a little wire nudging.

Sounds good for now. The stock speaker sucks... Plugged into a Creamback it sounds wonderful! Throwing back in the cab to beat up for a while. It just might be worth buying a speaker for.... (I got a shit ton of 12's laying around but no 10's...ugh).

Still need to deal with the reverb power supply.. Gets hotter than I'd like...will deal with that another day.

Working on a schematic and will post if I finish it. (If.....)

[JazzGuitarGimp]

You might consider a voltage follower opamp stage to drive the reverb circuit. It will present a very high imput impedance, so no load on the preamp stage where you want to drive the reverb from. Just use a TL072, connected to your bipolar supply with 100nF ceramic bypass caps from each rail to GND. Use the non-inverting input of the opamp (pin 3 or pin 5) as the input. Connect the output to the inverting input (pin 1 to pin 2 or pin 7 to pin 6) and use this as the ouput to drive the reverb. If you don't have a use for the second opamp in the TL072, ground the non-inverting input, and tie the ouput to the inverting input.

[Deric]

Thanks for chiming in Lou!

I thought about that approach but as it is I'm quite happy with the reverb. The only drawback at all is the verb is still there when the volume is down. Luckily, I rarely play with the volume all the way down.

I was taking the reverb send after the 2nd stage coupling cap and using resistors to mix it back in at the same point. Besides the loading I noticed another issue... The send was after the point where the negative feedback is connected. At extreme settings (though not likely to be used) I'd get feedback in the reverb circuit. Wasn't an issue with the stock 56k feedback resistor but anything lower than that (which is great for cleans with lots of verb) caused the reverb to feedback.

I also really like the way the verb seems to "turn down" as the volume increases. I usually use less verb as the gain increases so....works OK! Of course you can always turn up the reverb knob if you want more... I've also noticed that with the "fixed" send level the reverb always sounds good.

Going to play it as is for a while and see how it works. If I like it I'll draw up a new PCB with a better verb power supply and call it good. If not....

Do need to find a new speaker fer sure!! The whole point of this build was to learn something and reuse parts so not looking to spend too much. Looks like the Jensen C10R is a popular (and cheap $$$) choice for brown and blackface Princetons so may try one of those...

[Deric]

Been playing with this more and more... Sounds great. Ordered a Jensen C10R speaker to try.

Edit: Put a C10R in and like it. Louder and brighter (in a good way). Was still a little harsh when pushed. Added a grid stopper to the phase inverter - MUCH better. Can't stop playing it.

Updated schematic as is attached.

[Deric]

Not a lot of interest here but.... Faceplate and Logo from Joe at Amplates.com

Been playing this thing a ton. Really like it. Still haven't rebuilt the power supply for the verb but it's been holding up so far and sounds great. Did up the PI grid stopper to 820k. Also added a switchable cathode cap on the first stage. Selects between the stock 25u and 1u. Really tightens things up when pushed with humbuckers. Schematic updated...

[sonicmojo]

Awesome result and a much better amp in the end. Reverb, trem,...what more can you ask for in a Fender amp. I'm eyeing an early 80s Fender Concert locally for just over $200 and may do something cool to it like you did with this.

[martin manning]

Good stuff Deric! Following your design evolution is interesting and instructive. Speaker reports are always useful too.

[Structo]

That looks great!

Glad you were able to tune it to your liking, that is the art.

[10thTx]

What a fantastic job you did with this!

with respect, 10thtx

[Deric]

Thanks guys! This was a fun build. Turned out to be a great sounding amp. Still plan on updating the reverb power supply but I think I'm done tweaking the rest.

[Leo_Gnardo]

Not a lot of interest here but.... Faceplate and Logo from Joe at Amplates.com

Super Chump ! HA! That's terrific, & brown Princeton one of my favorite amps. Great mod you done there sir! FWIW one of the first mods I did @ 1986, back when Champs were cheap and nobody thought of drilling one up, boogie-ized a VibroChamp & got 19W at clip, a screamin' little anklebiter. Simply blacked out the top of the A in the logo, then it was the Chump. Good to see you've carried on the name - made my day.