Broke Amp Blues...

[GibsonGM]

You're more than welcome, Luke, I'm really glad it worked!  And YOU just fixed your first amp, man!  )
Vibration can do nasty things to amps, which is why ppl. pay big bucks for point-to-point, well-soldered boutique stuff.    Jacks to this every once in a while.
Enjoy the amp, AND the pedal!   

[Luke51411]

Now that its fixed, I'm thinking about fixing the taper of the volume on it. Its basically full volume at three and not much between that and nothing. From what I've read it sounds like the volume pot is a 50k reverse audio. If I were to swap pots, would I be able to just put in any old pot I have for stomp box use or would I need to get something that has a higher power rating?

[Luke51411]

Also, I never noticed before since I usually have it at lower volumes and/or have effects on but I noticed that at higher volumes it's not a very clean amp on the clean channel. I'm guessing this was designed in to try to emulate an overdriven tube sound? I looked at the schematic and d1 and d2 appear to be clipping diodes in the clean signal path is that correct? Would I be able to modify for a cleaner sound by removing/changing/putting these diodes on a switch? Would there be potential to harm the amp in doing so? I don't know that I want to do this but I would at least like to have a better understanding of what is going on in there.

[GibsonGM]

Normal channel volume, R13, looks like it's an audio taper pot, 50K (that just means log taper, but not reverse)...BUT  It's weird how it says "50k15A" - maybe the 15A is a part number?  You might try a linear taper in that spot, see what you get!  Yes, a standard pot should be fine.  1/2W.    I would measure the pot myself, in place, and try to determine what taper is actually is, as that label seems off to me.  Then just 'get the opposite', if that makes sense.

Yeah,  the amp is probably designed to break up a bit, as you noticed, to try to make it sound like a 'big guy'.  But, the clipping diodes you correctly spotted appear to be channel 2, your dirty channel.   There are also LEDs plus diodes in the feedback loop of the opamp stage next to D1 and D2.  That's what tells me it's the dirty channel - no WAY could that design be clean, ha ha!    The breakup you notice is probably a combination of driving a low-power amp hard and the speaker getting hit; a lot of these do that, sort of "Marshall-esque". 

U5B, below everything, is acting to power up the FETs (Q1 & Q2) you see associated with each channel, to select clean or dirty channel when you press the switch...it's a sneaky way to confuse us as to which channel is really what, ha ha....   

[Luke51411]

So basically I was mostly right. Maybe I understand more than I think!

[GibsonGM]

So basically I was mostly right. Maybe I understand more than I think!

You show a LOT more understanding than I *assumed* you'd have, being kind of new, yes!  Seems maybe you've been paying attention, ha ha.   The FET switching in things like this is very easy to throw you off, it still does that to me at times.   After you look at schematics for dozens of amps and stuff, you start to see patterns, building blocks that tell you right up front what they do.   That helps counteract the seemingly maze-like paths signals seem to be taking!   

We've probably created a monster, now, ha ha...dig in, Luke, just assure that you're being safe with ANY mains-powered equipment!   Good reads are sites about 'tube amp repair' and the like.  There's good stuff at GEOFEX (link at top of page) concerning safety, etc., too - check it out!

[Luke51411]

I probably won't deal with mains power again for a while though this episode has got me wanting to build a tiny giant amp now. I'm starting to pick out a few patterns just from stompbox work. I've come a long way from a few months ago when schematics were Greek to me. Many hours with a breadboard solved that one!

[Gus]

The 25R  looks like a nice design  to me. 

Look close at the output stage it uses current feedback.  The Speaker is part of the feedback loop.

Now you might not like a amp with voltage feedback for guitar because you have a 25R

One might be nicer with guitars and open-back speaker cab and one might be better for home audio.

http://sound.westhost.com/project56.htm

Speaker(s) and the cab are a very big part of the sound of a guitar amp

[duck_arse]

http://support.fender.com/schematics/guitar_amplifiers/Frontman_25R_schematic.pdf

this diagram shows "A", "B" and "C" pots. with B and C in the tonestack, we'd expect them to be lin and log, no? and the "normal volume" A taper is wired in an unusual manner. does fender have their own taper codes, or are they using the same system we had in australia 35 years ago? and what is with the grounded wiper?

[GibsonGM]

(?) pretty strange, eh Duck?

I have a Hot Rod Deluxe, and it too used a weird tapered pot. I replaced it to make the volume USEABLE instead of just "IMPRESSIVE" - you could not go higher than 2.5 or it would be too loud (rev log it seems).   Putting in a linear (I believe it was B, not A taper, was years ago) fixed it.

Don't really know what they're doing with the 25R here, too bad we can't lay hands on the pot and take some readings!

[Luke51411]

If I get around to swapping it out I'll take some readings, not sure if I have a 50kb pot around so I might have to wait for my next order. I don't think I'll try it but what effect would it potentially have to stick a 100kb pot in there?