Making amp less bright

[pops]

Hey all, i have a little amp i built that sound very nice with guitar, but i was thinking of playing mandolin thru it and i read here somewhere about losing or warming the highs by changing ??? plate resistor my old brain can't remember and can't find it. The amp is kind of a tweed princeton with an extra preamp tube and a 5E3 power section. I am sure there are a lot of tweeks i can do, but just thought i would put this out there and see if someone remembers or knows off hand. Thanks, really enjoy reading all the info on this site.

[xtian]

Roll the Treble control down (cheap and easy)

Remove the bright cap (if there is one)

A small cap ("snubber") across the plate resistor of a preamp stage will tame some highs

[sliberty]

A schematic would help.

[M Fowler]

So does it just have a Tone control or a tone stack treble and bass?

[pops]

It has a tone control only, and if you turn down the tone the mandolin will get wooffy, this happens on most amps as the mandolin is kind of a weird beast to amplify. It is naturally bright. The amp sounds great with a guitar and i have 12AY7's in the preamp so it is warmer. I built this a year or more ago so i don't remember which of the tweed princetons i started with, but added the 5E3 power section as that was the tube compliment of the chassis i had. Seems someone was talking about changing the plate resistor or something for another reason and the extra result was less highs. I just can't remember what it was and can't find it. Any other way would be good too. I am not after gain or volume so i will have to go in and see whether i even have cathode caps on the preamps, usually i like the flatter response that not using them gives. I could maybe change the brand of caps that would not be as bright as i know Mallory 150's are brighter than i like. Not sure what cap to go to, or what i put in there. Was playing it yesterday and decided i wanted it for mandolin and needed to warm it up. Thanks for any help.

[ToneMerc]

I would start by maybe decreasing preamp voltage and increasing coupler cap values verses swapping brands of the same values. I would approach this similar to tweakng for a harp.

TM

[Firestorm]

The mandolin has higher fundamentals than a guitar so cutting treble response may eventually compromise the amp's ability to reproduce the higher fundamental frequencies. What you may have been reading about plate resistors has to do with harmonic distortion. Lower value plate resistors have higher percentage 2nd order harmonic (and reduced gain). Higher value resistors, the opposite. But if you reduce the 2nd order harmonic, you are left with comparatively more 3rd order harmonics and if it's the odd order harmonics that were the problem, you will have made it worse. Mandolins generate a lot of sympathetic vibration in the portion of the string below the bridge. Maybe just try suppressors on those strings first.

[Jana]

If it was me, I would leave the amp alone and build a simple tone control in a small bud box. Wire it up with a pot and a cap like a tone control in a guitar. Experiment with the cap value. Adjust the pot to just barely roll off a bit of the high end.

[M Fowler]

If it was me, I would leave the amp alone and build a simple tone control in a small bud box. Wire it up with a pot and a cap like a tone control in a guitar. Experiment with the cap value. Adjust the pot to just barely roll off a bit of the high end.

Very good idea Jana.

[cbass]

A Really long cable? Perhaps

[sliberty]

If the tone control is basically the setup from a 5F2A, the .0005uF cap is effectively a bright cap. It sits across your volume pot the same way that the 120pF bright boost cap sits in a Twin Reverb. You could probably experiment with different values of this cap to provide more or less brightness. And, if you configure a switch to select between the standard .0005uF value (500pF) and whatever value you like better for the mando, then the amp could still be used "stock" for guitar. You could accomplish this with a push pull tone pot without drilling any new holes.

[Leo_Gnardo]

Might look into a "cut" control as on old Voxes. I've put 'em on a couple of sizzly-bright boo-teek amps. More or less a tone control run between output drive tube plates in push pull amps, or from output tube grid to ground if single ended.

In the end, it's a compromise between how much hi end to throw away to keep ears from cringing, and how much to keep for clarity. As you say, extra tough with mandolin, very peaky in the ouch range 1K to 6K. There are a couple circuits that can help put on some "dazzle" without making it too bright. I'm thinking BBE and Aphex "exciter" products.

[NickC]

If it was me, I would leave the amp alone and build a simple tone control in a small bud box. Wire it up with a pot and a cap like a tone control in a guitar. Experiment with the cap value. Adjust the pot to just barely roll off a bit of the high end.

Very good idea Jana.

As a variation on that theme, maybe try a JangleBox pedal. It's a compressor with EQ settings and gain control.

http://www.janglebox.com/janglebox.html

I use it for electric mandolin and electric banjo when playing live. Sounds good with guitars as well.

0.02

[M Fowler]

Or Merlin's B. book notes a good Tilt tone control and has graft showing responses with different cap/resistor choices.

[andrew]

JangleBox works great.