Reduce treble in Fender Frontman 212r clean channel with circuit mod

[darkfenriz]

Just wondering how many of you recognized the input stage is actually an obsured version of input impedance bootstrapping?

[Justin Thomas]

I wouldn't worry about what the caps are made of. And I would bet nobody else here thinks you should concern yourself with that, either. Use what you got. That's kinda how we are here.

Justin

[dazz]

I wouldn't worry about what the caps are made of. And I would bet nobody else here thinks you should concern yourself with that, either. Use what you got. That's kinda how we are here.

Justin

Thanks Justin. Turns out I have an SPDT switch I can use to select between the 220nF cap or the 68nF one. Since the original 220nF in series with the suggested 100nF results in 68.75nF that should work well enough I guess

[J M Fahey]

Good catch
Not only that, itīs an active filter high pass cutting everything below some 66 Hz, what the Doctor ordered to cut thump and subsonic without affecting Guitar sound.

Frequency is setup by C3 and R4 , place those values in: RC pad corner frequency upper and lower cutoff frequency calculation filter calculate time constant tau RC voltage power calculator capacitance resistance - sengpielaudio Sengpiel Berlin which is a simple and useful RC frequency calculator.

Above 66 Hz Guitar "sees" C3-R4 ; C2 not so much and R5 not at all, because R5 is bootstrapped so in theory itīs infinite ohms.
Net input impedance is R4 = 510k which is fine.

Below 66 Hz U1a stops receiving sound (because C3-R4 attenuate it) , it stops bootstrapping and C2-R5 (same value as C3-R4 so same frequency=66 Hz) get into the act.
Net result is flat frequency response and 510k impedance above 66Hz and 250k plus 12 dB/oct cutoff below 66 Hz.

U1a also provides "plain" gain, [(R7+R6)/R8]+1 , about 4X which is shown on the schematic.

So U1a is "dual function": highpass and gain . Nice and smart

This input stage is an SS Fender staple, youīll see it all over the place, and comes from some late model Music Man ..... which makes it "Family" of course.

[darkfenriz]

Good catch
Not only that, itīs an active filter high pass cutting everything below some 66 Hz, what the Doctor ordered to cut thump and subsonic without affecting Guitar sound.

Frequency is setup by C3 and R4 , place those values in: RC pad corner frequency upper and lower cutoff frequency calculation filter calculate time constant tau RC voltage power calculator capacitance resistance - sengpielaudio Sengpiel Berlin which is a simple and useful RC frequency calculator.

Above 66 Hz Guitar "sees" C3-R4 ; C2 not so much and R5 not at all, because R5 is bootstrapped so in theory itīs infinite ohms.
Net input impedance is R4 = 510k which is fine.

Below 66 Hz U1a stops receiving sound (because C3-R4 attenuate it) , it stops bootstrapping and C2-R5 (same value as C3-R4 so same frequency=66 Hz) get into the act.
Net result is flat frequency response and 510k impedance above 66Hz and 250k plus 12 dB/oct cutoff below 66 Hz.

U1a also provides "plain" gain, [(R7+R6)/R8]+1 , about 4X which is shown on the schematic.

So U1a is "dual function": highpass and gain . Nice and smart

This input stage is an SS Fender staple, youīll see it all over the place, and comes from some late model Music Man ..... which makes it "Family" of course.

Now spice it. You may be surprised.

[J M Fahey]

Oh, you do it please and post results

[Dave H]

...please and post results

OK

[J M Fahey]

OK

[ATTACH=CONFIG]45586[/ATTACH]

Thanks.
Letīs see:

U1a also provides "plain" gain, [(R7+R6)/R8]+1 , about 4X which is shown on the schematic.

12dB=4X gain. Check.

Net result is flat frequency response and 510k impedance above 66Hz and 250k plus 12 dB/oct cutoff below 66 Hz.

Simulation shows cutoff frequency (9dB which is -3dB below average 12dB gain) as some 63Hz so within 5% .... Check .
Oh, and 12 dB/oct slope below that. Check.

Nice to have old Math and methods confirmed by fancy modern tools

[darkfenriz]

Input impedance rising in the inductive manner in the 100Hz-1kHz area plateuing around 2Meg - checkmate

[Dan Thomas]

Hey gang, I found this thread while searching for a way to tame the treble in our Fender Frontman 212r amplifier. I'm digging up this thread to add some photos of the mod suggested by JM Fahey in post #10. If someone else wants to do the mod, these photos may help. I did the mod and think it is worthwhile. The amp runs much warmer and is not so shrill. BTW just removing the bright cap does not change much. A very subtle difference. The warmth cap is what made the big difference . Since the mod is fairly straightforward, I won't add anything to his instructions from #10. The only thing I did different is to put the capacitors onto the switches instead of the circuit board. Cheers, David
[ATTACH=CONFIG]42495[/ATTACH]
[ATTACH=CONFIG]42496[/ATTACH]
[ATTACH=CONFIG]42497[/ATTACH]
[ATTACH=CONFIG]42498[/ATTACH]
[ATTACH=CONFIG]42499[/ATTACH]
[ATTACH=CONFIG]42500[/ATTACH]

Picked up my FM212R for $25, replaced speakers and saw these comments about the FAHEY Mod. Mounted my switches on the front each side of the FM212 logo. actually I'm quite happy with the way it came out, like that I can clip those highs. Also built up a clone footswitch for the Channel and Drive select.. Don't mind patting myself on the back, for less than $100 I got a screamer for my makeshift back line.