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May 26, 2022, 05:31:42 PM

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Fender eighty five - how to mod it - less glassy

Started by nooneknows, April 13, 2022, 05:53:54 PM

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caused by a pure nostalgic wave, I purchased an old fender eighty five, in very good condition.
It was my first amp, back in 89/90 , can't remember exactky. At the time it seemed it was very musical. With my current ears...mmmh, not so much.

Clean channel is beyond brittleness, it's almost a kinfe, dirt channel with gain at 0 is far better but still it sounds 'strange', at least compared to my princeton RI.
85 it's glassy, it's thin. I've noticed that if I connect directly the guitar to the amp in the sound is far better, it's only low (and driy, with no vol control or reverb, ok).

I checked the only schematics I've found, the studio85, I assume it's very similar if not identical.
The clean channell seems not having any real filtering, just like a common fender first tube stage, then the signal is routed directly to the tone control or to an additional  'dirt' stage, much more filtered, it depends on the channel switching.

To me the tone stack , in common with the two channel, seems a lot like a standard fender tone control, I simulated in the response and it's comparable, except a bit attenuated.

Ok, Are there any known mod to bring some warm and or get rid of all this glassyness ?

thank you in advance for your kind help.


I've found the schematic for example here:


Welcome Marcello, :)
My guess try this;
Remove C21 (lift one end to test)
That will remove some hi Freq

Also you can raise the value of C25 (Add 5 or 10uF there)
That Should (hopefully) add a lot more bass.
those 2 mods will help balance the system tone shape.


There is Deluxe 85 and Studio 85, but there is also a separate model that is simply "85".  I have it on paper in my stored files - not really accessible.  I have been meaning to dig out some of the old paper files , but no idea just when.  But I do have an inventory of them and it is in there.


Mine is the "Eighty Five", not the Studio 85, I thought it was the same. I haven't opened it yet to check if the schematics I have for the Studio 85 is usable indeed.

BTW, maybe the speaker contributes to the glassy vibe: I have a sidekick 25 reverb too, it was almost unusable until I replaced with a Jensen C10R the original Fender cone, the sound is still a bit boxy but it is way way more nicer and rounder now.


Place a graphic EQ (or multi-band parametric) in the FX a loop and set for Clean channel.

Does adjusting this EQ help the brittleness problem? Is there any setting that makes things tolerable/good?

If not, then EQ or filtering mods probably won't do what you want. I'd look at adding a FET based "warm up" stage to the input preamp. My favorite FET is the J113 and I've made many of the Danyuk-based FET triode sim circuits with them - with the Danyuk equations, the J113's only get about +12dB of gain, and you and resistor-divide that back down to unity if needed. And, only requires a few parts, and can be built easily on a small piece of a proto-board.

If EQing fixes the issue we'll enough, then I'd put the preamp and post-limiter circuits in a SPICE simulator, and see what I could tweak/add to mimic the EQ.

My gut feeling is that the lack of warmth is a non-linear issue, and not EQ. But this way you can test that idea. You could also place very low distortion tube-screamer style boxes in the FX loop, or at the input to the amp (normal stomp box style) to further suss out the issue of EQ vs. nonlinearity.

Hope that helps,



Dis-qualifier: I'm a newb....  but I spent a lot of time playing with a similar Fender tonestack in a Frontman Amp. In these analog SS amps, Fender tried hard to make them mid-scooped like their tube amps. Unfortunately, it comes off horribly. That sounds like what you are running into. Way too bright... like treble-beam on steroids. And I had three to test... all were the same. It's by design.

Also, I hunted for Studio 85 schematics but I too could only find the Deluxe and the Studio versions. But, they have the same tone stack. My WAG is that the standard 85 is the same.

So I put the tone stack for the Deluxe/Studio into Duncan's Tone Stack calculator. Here's what it looks like with bass/mid/treb all at "5":

And here's the only way to get it close to flat: Set the bass a little below 1, mids to 10, and treble to 1:

With the Frontman, I was able to get a good sound by allowing a slight mid scoop with B/M/T at 3, 8, 2. Not quite flat. But it left me with nowhere to go in terms of tone on the amps. EQ'ing for different guitars wasn't really possible.

As an experiment, try the settings I suggested for "flat"... and also the ones that worked for me. See how it sounds. If it's better, then you have the answer. You can either EQ that way, or spend time modding the tone-stack, or sell it and buy something that sounds better.


thanks for the replies.

Looking at the schem again, I will do the C21 lift for sure, it's clearly an high pass at low volume.

I don't think the C25 mod works instead, it's an high pass but I calculated a cut freq of 22 Hz, so enlarging the cap doesn't seem so effective.

verifying the first stage, it seems similar to a VCVS opamp high pass, but I don't understand why the 510KR is not connected to the opamp output but to a resistor network, I'm not able to calculate it. Am I wrong? Any idea?

thank you


The input amp is configured as a kind of multiple feedback 2nd order HPF. R6, R7 and R8 set the filter gain.

The C1, C2, R4 R5 network is the HPF. Changing the 510k resistor will alter the LF edge of the HPF (changing any of those 4 components is going to adjust that frequency).

As is, the HPF fc is 60Hz, well below the ~88Hz low E-string on a standard 6-string guitar. This stage has no HF compensation in the feedback path, so the HF breakpoint is super high, well into the MHz range (basically, the op-amp internal compensation is setting the 1st order HF edge frequency, so the op-amp itself is going to set that part of the FR).

This is interesting as I would expect to see the standard 22pF - 100pF cap in the feedback loop to stabilize. If you want to soften the HF response, add a cap in parallel with the FB resistor.

Have you determined if the brittle-ness is an EQ thing or a lack of Nonlinearity in the input stage?



Quote from: willpirkle on April 20, 2022, 08:49:01 AMHave you determined if the brittle-ness is an EQ thing or a lack of Nonlinearity in the input stage?

Not yet since the amp is in my other home, I think I'll check it in a couple of weeks, for sure, I'll try with my old GE7 as you suggested.
thank you!


From my experience a lot of Hi-Freq hash comes After the pre stages. I've found a lot of hi freq crap can be removed before power stage which in most SS amps runs clean.

I would try adding a higher value cap at C26 at U5B (Preamp output). It's 22pF on the Deluxe 112 Plus Schematic I'm working from which will likely be similar as most of this era SS fenders use very similar circuitry so just find the Preamp output section. 
You can just leave the 22pf in place and bridge with higher values until you find a sweet point. You may even need 1nF.

The input section on most of this era of SS fenders are close to identical, it's just a hi pass filter which is helpful but it and the rest of the circuit does not have enough suppression of the high frequency,, so the result is a very harsh treble so when in distortion modes they sound crappy and harsh.
I resolved this crap hifreq issue in a Fender Performer 1000 by doing the above and swapping to a darker speaker.
I doubt these amps will ever win awards but you can improve them somewhat.


I finally opened my Eighty Five. Some values are a bit different than the schematics of the Studio 85 I posted, for example C21 is 2n2 and C23 is 560p, the rest seems similar.

Anyway I desoldered C21 and I have to say I solved my problem, the tone of the clean channel is way way nicer and now usable.

I then put a 150pF over C26 and the harsh spikes are gone.

I think I'll stay with this for a while, to get used with the new tone.

thank you all for all the hints, hope this will be useful for other users.


Good to hear it's worked for you and having the feedback may help others who struggle with these Amps. 8)
A lot of SS Fenders from this era tend to suffer from excess treble response.


BTW, any hint about the speaker too? Any experience with different cones?

I am partial to Jensen C12R and C12K (I had them in my old Peavey Classic 30 and they were both delicious compared to the original mounted, with a preference for the C12K) but I don't know how they sound compared to the one installed in the Eighty Five, which I suppose it could be an Eminence, rebranded.


Speakers are a rabbit hole but a general rule for balance is; If the Amp is dark ,,use a bright speaker.
If the Amp is bright use a dark speaker.

Compare the cost;
You just altered the Amp and it cost you 50cents for a cap.
So even if you have to try several caps to find the right tone it's still only costing pennys.

While speaker swaps may cost you hundreds before you find the right one.
Then there is the Cabinet the speaker is mounted in,, another rabbit hole. :-X

Be warned some of the fancy painted magnets will cost. :o
For me personally I tend to like Sealed Cabinets for single speaker combos.



Quote from: phatt on May 04, 2022, 08:26:15 PMFor me personally I tend to like Sealed Cabinets for single speaker combos.


I was just talking about this with a co-worker.  I believe combos should be open-back, the way God intended.  Just kidding, though I am an open-back believer.
Life is what you make it.
Still rockin' the Dean Markley K-20X