Fender Frontman 15R mods

[Renegadrian]

I got this amp for cheap it had a little TLC (new trafo and 2050) now it works as it should.
not the best sounding F amp shall I say, it's not bad but I guess there are points of improvement both in the amp itself and in my knowledge - I googled a bit but the only advice from untrained people is throw it on a stairway or pretend to loose its power cord

First let me link a schematic so you can follow my thoughts
SCHEMATIC
Also keep in mind I want to get rid of the internal shit 8' speaker and cut the amp to head dimensions and get a speaker jack to connect it to a proper and better sounding 12' cab.


right...correct me if I'm wrong, all the Rs and Cs from C7 to pin 5 of U2b should be a low pass filter?! Was it meant to correct the frequencies to said inefficient speaker?! Should I just bypass that network and get straight to U2b?!
possible mods I was thinking about:
C7 1µ NP
C11 100p
C12 100n
R19 2.2k
R22 680k

Drive volume seems is quite unusable, it goes from 0 to...like 2 or 3 with no 1
normal volume is the same but worse.

2050 arrangement is different to the usual schem you see online, output is taken from C23 also cr3 and 4 are a news.

what would you sugest to make it a proper real amp?! Are my suggested changes worthful?! what can I change to have a better response on vollume pots?!  Let me know your thoughts on that!

[PRR]

Change the speaker.

Don't over-drive the input; turn your axe down.

[Rob Strand]

It's too much of an open problem.   The way to start would be do decide what solid state amp you actually like then make some mods to push the Frontman closer to that.   You have to treat fixing the clean and drive channels separately.  They are different things and require separate tweaks.

right...correct me if I'm wrong, all the Rs and Cs from C7 to pin 5 of U2b should be a low pass filter?! Was it meant to correct the frequencies to said inefficient speaker?

All that is is some pre-EQ for the drive channel.

Drive volume seems is quite unusable, it goes from 0 to...like 2 or 3 with no 1
normal volume is the same but worse.

The levels can be tweaked relatively easily.

2050 arrangement is different to the usual schem you see online, output is taken from C23 also cr3 and 4 are a news.

The R60, R41, R42, C23 etc parts try to emulate the higher output impedance of the tube amp.   It gives you a bit of bass and treble boost because the higher output impedance interacts with the speaker's impedance..   IMHO this type of circuit is good to keep on a ss amp.   It usually works best for 12" drivers.   For an 8" speaker it may or may not work in an optimal manner as the inductance is lower and the resonant frequency of the speaker is higher.

There are a few of odd-ball things on those fender amps:
-  The first stage is a noticeable high-pass filter.   Not many other amps use this.  I think the motivation for this stage was to add roll-off like an o/p transformer, even through it's at the first stage. 
- The clean channel cuts the lows as you boost more.  Maybe that's so you can get some crunch instead of just clean.
- The tone controls are different to what you would get on a tube amp.

One place to start is to look at some other SS amps.

[Renegadrian]

What I really just don't get is the clean volume pot in the feedbback loop and not at the output of the IC...

[Rob Strand]

What I really just don't get is the clean volume pot in the feedbback loop and not at the output of the IC...

It's a trick which achieves three tasks with one knob.  It was invented by Peavey.
- When the gain control is advanced the opamp part has a lot of gain.
   That lets you get a bit of distortion/crunch.
   The low end is also cut so the sound doesn't get too muddy.
  [EDIT  Actually in this amp the large value of R11 doesn't let you get crunch from first stage.]
- When you back off the gain control the opamp gain is reduce and this stage no longer clips.
   In this case C5 add a bit of brightness; a bit like putting a cap across a volume control in a tube amp.
- When the gain control is on zero the output is zero.

If you look at the MXR distortion+ for example you cannot adjust the drive control to get zero output when the drive control is on minimum.

If you have a post opamp volume control to ground, like you see on a tube amp, you can't set the opamp gain too high because the opamp might clip.  So the opamp gain must be restricted and that  means you can only get clean.

The way to look at the circuit is one side of the pot is operating like an MXR distortion+ drive control.  The other side of the pot is kind of acting like a normal volume control:  The 2.2k resistor R12 forms the "R1" resistor of a voltage divider and the pot forms the "R2" resistor of a voltage divider.   

Look at the cases of gain full, gain zero and then gain in the middle as three separate cases and you should see what I mean.

[Renegadrian]

If you have a post opamp volume control to ground, like you see on a tube amp, you can't set the opamp gain too high because the opamp might clip.  So the opamp gain must be restricted and that  means you can only get clean.

Yes but that is supposed to be clean!!! 

[Rob Strand]

Yes but that is supposed to be clean!!!

Some amps let you get a bit of crunch from the clean channel so you can switch between crunch and distorted or, clean and distorted, or, clean and crunch (using distortion channel on low).   It's just a way to get a bit more out of the amp.

[Ben N]

It's been a while since I had one, but the speaker is definitely the weakest link--upgrade there, and it will instantly sound a lot better. IIRC, there might be enough room to squeeze a ceramic 10" in the existing cab, but you'd probably have to cut out the existing baffle flush with the cabinet, then mount a new one on cleats.

[printer2]

I have a 25R, I want to reduce the gain of the distortion channel and put more of a flat frequency response through it. I understand why it was rolled off but I am not interested in tons of clipping. One day.

[Renegadrian]

Ben, as I already wrote the cab is getting cut to become a head. Printer2 I'd start lowering the resistor in the FB loop

[vigilante397]

I had a 25R a while ago, and I did some simple mods:

- removed the preamp and replaced it with a 12AX7 preamp
- removed the power amp and replaced it with an EL84 power amp

Sounds great now

[Rob Strand]

- removed the preamp and replaced it with a 12AX7 preamp
- removed the power amp and replaced it with an EL84 power amp

Sounds great now

And then you sent it off to a custom shop to upgrade the case so it looked like a Marshall amp (Sorry, I was just being silly and used Marshall) 
The Frontman is gone man!

[Mark Hammer]

I have a Champion 110, which has the same front end as the Frontman 15R and 25R, and probably a bunch of other budget solid-state Fender amps.  I found the overdrive channel annoyingly shrill.  I also find the volume control for the clean channel is VERY twitchy at low volumes, and often end up using the overdrive channel simply because I can turn the gain and volume down.

So, a cap in parallel with R22 - 100-150pf is a reasonable choice - can tame some of the shrillness of the overdrive.  The clean Volume pot can have its behaviour improved a bit by either replacing R13 with an anti-log pot, or by installing an 82k-100k fixed resistor in parallel with the right side of the pot; i.e., from the junction of C6/R14 to ground.  That ought to make it possible to dial in lower-volume sounds.

Other than that, a larger speaker, and larger cab oughta do you.  As I often note, there is much competition in the range of "economy" amps, and manufacturers have to find corners to cut to be able to make a little profit on them.  One of those corners is cab size.  Adding a mere inch on each dimension would increase the cabinet size from .695 cubic feet to .92 cubic feet; about 32%.  This would increase the internal cabinet volume, and along with that, the bass and perceived volume.  Closing the back would help too.

But all of that would increase the shipping weight. materials cost, and shelf-space for inventory, so they keep it small.

[Rob Strand]

I also find the volume control for the clean channel is VERY twitchy at low volumes, and often end up using the overdrive channel simply because I can turn the gain and volume down.

I think the Peavey's used a 10k linear pot and a smaller R10.    Forming a divider from R12=2k2 and a 50k pot like the Fender is going to be touchy.  Peavey used 2k7 and the 10k pot.   Some Peavey's were a bit touchy around setting 3 or 4 *but* they were dialing-up heap more gain when the gain was maxed out.   Maybe look at a few Peavey schematics.

I should have said.  Instead of changing the pot you can just make R12 bigger say 12k or 15k.

[printer2]

I had a 25R a while ago, and I did some simple mods:

- removed the preamp and replaced it with a 12AX7 preamp
- removed the power amp and replaced it with an EL84 power amp

Sounds great now

I have enough pine cabs sitting around that I don't have to use pressed sawdust for a new amp. Bought the amp for $25 not working, was something minor, curious if I could make it sound better (ignoring the recommend speaker upgrade, why bother, use it in the pine cab instead).

[vigilante397]

I have enough pine cabs sitting around that I don't have to use pressed sawdust for a new amp.

I don't I'm terrible at woodwork, so if I want to build a small combo I usually try to pick up a cheap/broken amp, like I did the 25R.

[Renegadrian]

I also find the volume control for the clean channel is VERY twitchy at low volumes, and often end up using the overdrive channel simply because I can turn the gain and volume down.

I think the Peavey's used a 10k linear pot and a smaller R10.    Forming a divider from R12=2k2 and a 50k pot like the Fender is going to be touchy.  Peavey used 2k7 and the 10k pot.   Some Peavey's were a bit touchy around setting 3 or 4 *but* they were dialing-up heap more gain when the gain was maxed out.   Maybe look at a few Peavey schematics.

I should have said.  Instead of changing the pot you can just make R10 bigger say 12k or 15k.

You meant R12!? Gotta try it. Any suggestions for the drive volume!? It jumps from 0 to 2. Sometimes you need 1 but its not there.

[Rob Strand]

You meant R12!? Gotta try it. Any suggestions for the drive volume!? It jumps from 0 to 2. Sometimes you need 1 bit its not there.

Yes, R12!

Not sure why the Drive would jump.   Sometimes the pots get dirty/old.  Do you think it could be that?   You could try cleaning it.

I have a 1x12 Laney solid state combo and it has sweet spot on drive control.  The pot is worn in that position.   (By previous owners not by me.)

[Mark Hammer]

I have a 1x12 Laney solid state combo and it has sweet spot on drive control.  The pot is worn in that position.   (By previous owners not by me.)

That's a job for Stabilant 22.

[Rob Strand]

That's a job for Stabilant 22.

I've never tried products like that.  Any success stories with pots?

To be honest I don't use that amp much.  It was a roadside rescue and it has been in that state for a while.  My original plan was to try to clean the pot and if that didn't work maybe replace it.    I think it's a 65W model with two channels, reverb and a Celestion 12" speaker.  It's not a bad amp at the end of the day.

Maybe this one,
https://www.ultimate-guitar.com/reviews/guitar_amplifiers/laney/hcm65r/

Schematic is here,
https://elektrotanya.com/laney_hcm65r_sch.pdf/download.html

- Fairly conventional  tone controls on both channels (slightly tweaked values).
- Clean channel is very minimalistic,  not much added EQ other than the treble cap on the volume pot.
- Drive channel is a 4069 distortion.    IIRC the max button is a switchable post distortion mid scoop.