OT: Fender Frontman 25R (original issue)

[Ge_Whiz]

Hi guys

Just picked up a secondhand Frontman 25R for a good price. Just like the reviews say, rather noisy when cranked up but a lovely clean tone and the reverb springs crash musically when you kick it (gently!).

As usual, no foot pedal. To save me taking it apart, can anyone please give me a clue as to what the foot pedal wiring is like? Does it end in a mono jack or stereo, i.e. SPST or SPDT? Does the pedal have a LED indicator?

I have several old Marshall footswitches that I can bogle for this, so I'll not be forking out for a new one - sorry Fender.

Thanks for any help you can proffer.

[Mark Hammer]

The schematic is available here: http://www.mrgearhead.com/faq/schematics/fender/Frontman%2025R%20Schematic%20653.pdf

The footswitch would appear to be restricted to selecting clean vs dirty, without having any impact on the reverb.  I suspect the easiest way to make the reverb foot-switchable without inviting pops and clicks is to simply shunt  C25.  That would ground the output of the reverb tank and the input of the gain-recovery stage, which would yield about as little reverb signal as one might imagine.

C25 is located near the back of the board, approximately halfway between the mid and bass controls.  There's obviously a lot you *can't* tell form a drawing like this, but my sense is that this would make a decent location for installing a footswitch jack in the back.  Since both the suggested reverb-cancel and the channel swap involve grounding a line, you could stick a stereo jack in the back, run the channel-switch control-line to it as well, and have a single stereo-plug going in the back for reverb/gain changes.  

Just bear in mind that while the channel-swap control line has no audio in it, the reverb-cancel is tied to the signal path so a shielded stereo cable is probably a good idea.  Capacitor C27 limits high frequency response in the reverb section, but in tandem with the 220k feedback resistor this only inserts a rolloff starting around 15.4khz.  Since you wouldn't expect 15khz bandwidth from a spring, you can probably safely increase the value of that cap from 47pf to 100pf to reduce bandwidth to 7.2khz and increase noise immunity.

[Ge_Whiz]

Blimey, thanks Mark. I only wanted to know how many rings on the plug and got the full analysis! That's great, because it would be good to be able to switch the reverb too. It doesn't have a front-panel on-off control, only a 'mix' control, so your analysis is much appreciated.

Heh, heh, heh - come meet my soldering iron, little amplifier... :twisted:

[Ge_Whiz]

Postscript: the 25R was noisy in the shop, where it was running alongside a switched-on Line 6 Spider; but get it home, surrounded by other working amps and no special power line conditioning, and it's actually very quiet. Lovely tone, and cranked up it really shakes my trousers - even at only 25 solid-state Watts.

[D Wagner]

Ge_Whiz,

Is there any chance that you could scan the copper side of the PCB for that amplifier?  It looks like it could be a very do-able build with that bit of "information"  

For curiosity's sake of course!

Derek

[Ge_Whiz]

Sorry to disappoint, but there are too many screws and fasteners to get in there easily, and I'm not planning to wreck it yet.