Fender 140 mods/tweeks?

[bobshbob1]

Hey all, I recently picked up a Fender 140 head for cheap and am considering doing some tweaking on it. It's 140 very loud watts with an ultralinear OT, point-to-point wired, and some cool features you don't normally see on Fender amps. But for all the power and volume it packs, the amp is pretty dull-sounding. Too much bass, not nearly enough bite, and the boost knobs only seem to affect different midrange frequencies.

Basically I'm wondering if there's some change to the tone stack or something that I can make to liven things up a bit or if this is just an inherently dull-sounding amp. I've tried removing the negative feedback loop, which amounted to lifting one resistor, and that did make some difference in the dynamics/responsiveness, but that too-mellow tone is still there.

Haven't been able to find much information on it aside from the schematic which I'll post and a single blog entry (which is where I read about removing the NFB loop); if anyone is familiar with these amps or has any input on ways to get a little more bite out of them it would be appreciated. I know how to safely work on amps and am pretty handy with soldering but I still don't entirely understand what goes on inside these beasts.

http://www.stratopastor.org.uk/strat...-140-schem.pdf

Thanks!!

[Enzo]

Amps are amps, they all work the same way, so making an amp brighter should respond to similar tactics used on other amps. How fresh are your power tubes? For that matter, have you swapped in diffferent preamp tubes? You have an insert jacks for "effects" which you can use like an FX loop. Send the preamp signal to some other amp for a listen, and plug a signal into the return to see how the powr amp alone sounds. That helps localize the deficit to either the preamp or the power amp.

[olddawg]

Interesting, those things and the 120watt Twins usually sound like a cross between a Lear jet and an ice pick. I would do as Enzo says (who wouldn't?) and then swap some tubes. If it has an effects loop or Pre/main break in jacks you can always put an equalizer or a compressor or both in there. Btw, that amp will sound pretty lifeless until you turn it up past at least 4. That might make your ears bleed and drive everyone from the room.

[pdf64]

Every tube stage has a snubber, rolling off top end; that all adds up. Consider removing some.
There's a 220pF / 2n2F capacitive divider; try removing them.
The fx loop is very poorly implemented; the signal level can be enormous, so take that into consideration if using it.
Bear in mind it's a 30 year old amp, resistors may have drifted significantly, electrolytic caps developed high ESR, screwing up the tone.
And yes could just be tired tube/s.
Pete

[bobshbob1]

Thanks for the replies. I hadn't really considered the tubes being an issue because they all look brand new, but I know that doesn't mean all that much. I won't get a chance to work with it til the weekend but I'll definitely try tube swapping and using different pre/power amps.

Also pdf64: When you say "snubbers" are you talking about the 500 pf caps on most of the preamp tubes?

[pdf64]

Yes, I meant those 500pF plate-to-cathode caps, mounted on the preamp tube sockets; sorry I should have been more clear with that.
Check the power tube dissipation. My experience is that the bias supply voltage is insufficient for some current production tube makes, eg Winged C. There are some tweaks to bring it up a bit (reduce the limiter resistor, fit a full wave bridge rectifier etc).
It's been reported that Sovtek WXT+ 6L6 are well suited to these high VB+ UL amps, I think these are the same as Electro Harmonix branded 6L6.
Pete.

[bobshbob1]

Finally got a chance to tinker around with the amp this past weekend. The effects loop is a single stereo jack on the back of the amp, so I guess I'd have to use an adapter cable that I don't have readily available to compare pre/power amp sound.

I did try removing just one of the 500 pF caps (the one on V4, didn't want to try changing too many things at one time) and that brought some sparkle back to the sound, especially when it was cranked up good and loud. Next time I get a chance to work on it (it lives in my band's practice space and that's the only place I can really crank it anyway) I'll probably try removing another. I don't know the math behind what frequencies these caps roll off but it definitely seems to be in the audible range, which I hadn't expected. Also putting a 12AX7 in the PI spot made it overdrive earlier, but also made the sound a little crisper.

Thanks again for the replies. It seems like every little change has gotten me closer to where I want to be tone-wise, so I'll keep tinkering and maybe have an awesome amp by the end of it.