Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: flora on July 07, 2010, 11:01:36 am
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Non - reverb Fenders sound better to me than Fenders with onboard reverb... I think there's too much treble in the reverb circuit , ( for my taste ). I've tried adding a reverb tone pot ,( Allen Amps version ) but it didn't seem to do much. What can I do to tame the high end of the reverb ?
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Try a 12AU7 reverb driver. I think it sounds a little smoother and less treble.
Dave
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First thing I'd try is putting a 250 to 500pF cap parallel with the 220K resistor to ground after the 100KL reverb pot. If that doesn't do it for you, #2. Play with the 10pF/3.3M mixer resistor/cap values. i.e. 20pF/3.3M.. 20pF/4M.. 47pF/5M.. #3. Try putting a shunt cap between the reverb pot's ground lug and ground. .001uF , .002uF.. up to .0047uF. Hopefully someone who knows more than I will chime in. Also, while you're in there.. Replace the 100KL reverb pot with a 100KA (audio) pot. That will give you more control 1-5 on the knob.
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Thanks.. the amp has a 100K audio pot already. I've tried the 12AU7.... but that's not exactly giving me the results I'm looking for. I'll try the 250pf across the 220K to ground next. Does anyone know of an effective reverb tone pot set-up ?
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I'm surprised you find it too trebly - the reverb has plenty, to more than enough, bass in it on all three BF Fenders I've made. Is this a new AB763 clone or a vintage amp?
If it's vintage, my first guess would be drifting capacitor values - starting with the cathode bypass caps.
Have you changed anything else in the reverb circuit? For example, if you reduce the grid return resistor (a.k.a. "grid leak") right in front of the 12AT7 reverb driver's grid, you would need to increase the value of the 500 pf cap leading to that resistor correspondingly to keep the frequency response the same. IOW cut the grid return to 500K and double the 500 pf to .001 uf. IIRC tubenit experimented with increasing the value of that cap to boost upper mids coming through the reverb.
Hope this helps,
Chip
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I'm used to playing through Bandmasters , Showmans , etc. When I play a Fender with onboard reverb , it simply sounds like a gimmicky-shrill-harsh compromise to get reverb/amp in one package.
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Hmm, I guess tone is in the ear of the hearer. The Fender reverb amps are pretty well considered some of the best ever built.
I personally don't like AB763 reverb as-is, but a ridiculous number of people do. I find that with a 12AU7 in the driver socket, it is pretty much as good as it gets.
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I'm used to playing through Bandmasters , Showmans , etc. When I play a Fender with onboard reverb , it simply sounds like a gimmicky-shrill-harsh compromise to get reverb/amp in one package.
Vintage amps, reissues, all of the above?
I can't help but ask what amp's reverb does please your ears?
Chip
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The AB763 reverb circuit was designed to sound bright. I've never heard any other stock built-in reverb circuit that sounds better. You can increase the 500pf input cap to allow lower frequencies to hit the springs, but if you go too big, you may find you quickly swamp the springs and it will have a mushy, boomy, boing-boing sound. I think Tubenit uses a .001 cap here to get a fuller, less bright sound. I trust his ears plus I've heard some of his amps. Sounded good.
You can also increase the .0033 plate coupling cap for the reverb recovery stage.
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In my V-Verb amps I use a 250k pot for reverb tone,just like the tone control on a Tweed Deluxe.It takes the high-end zing off the reverb and makes it warmer.
Just take a look at a layout for a Tweed Deluxe.
If it's a vintage amp,you will have to adjust a few values so it doesn't have the extra treble.