Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on March 31, 2017, 09:49:17 am
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I built this amp about 7 or 8 years ago, it's a little different from the attached layout (no boost switches) but basically the same. The amp is just too bright for me now. In a band situation it cut well and the bright was Ok (it's very loud for an 18 watt). Now as we age the old band seems never to get together again so I'm kind of playing to jam tracks. I've tried a few thing to get the brightness out but just still too bright.
I have the preamp cathode bypass cap, R on the TMB channel the same but on the normal I used a 4.7uf, I haven't tried to raise this to so 50uf as I don't like a real bassy tone either. I'm using a well broken-in Weber Blue alnico 12 30 watt, use a strat with singles.
Any suggestions?
Sincerely,al
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Do you have a schematic to go with this layout? I'd be curious to try changing the value of the slope resistor to a few things, see if bringing up some of the mid range helps with the brightness.
-Brett
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Have you used the Duncan Tone Stack calculator?
http://www.duncanamps.com/tsc/ (http://www.duncanamps.com/tsc/)
Plug in your current values and then work out your changes there, should make it easier to see what your proposed changes will do.
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I built this amp about 7 or 8 years ago, it's a little different from the attached layout (no boost switches) but basically the same. The amp is just too bright for me now. In a band situation it cut well and the bright was Ok (it's very loud for an 18 watt). Now as we age the old band seems never to get together again so I'm kind of playing to jam tracks. I've tried a few thing to get the brightness out but just still too bright.
I have the preamp cathode bypass cap, R on the TMB channel the same but on the normal I used a 4.7uf, I haven't tried to raise this to so 50uf as I don't like a real bassy tone either. I'm using a well broken-in Weber Blue alnico 12 30 watt, use a strat with singles.
do you have a schematic?
I would bump the 4.7 to 10 or 22uf. with a 4.7uf by-pass cap the gain on that 12AX7 drops for anything under 1Khz. you might not like too much bass, but with a 10uf cap or a 22uf cap you can let your bass tone knob alleviate that.
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I add the schematic to my first post.
as far as the tone calculator, maybe the normal channel V1 bypass could be raised higher than 4.7uf but I like to keep the flabbiness out, the schematic calls for 220 uf w/820 (off if you used the calculator)....?
Slope is 56K, middle of the road is around 47K. I believe lower it would raise the gain, I lean toward a cleaner amp.
Hoping to get some advice on an exact change that is know to cut the overall brightness.
Though about taking off the 500pf on normal volume, first and second leg only, keeping the other 500pf.?
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I built this amp about 7 or 8 years ago, it's a little different from the attached layout (no boost switches) but basically the same. The amp is just too bright for me now. In a band situation it cut well and the bright was Ok (it's very loud for an 18 watt). Now as we age the old band seems never to get together again so I'm kind of playing to jam tracks. I've tried a few thing to get the brightness out but just still too bright.
I have the preamp cathode bypass cap, R on the TMB channel the same but on the normal I used a 4.7uf, I haven't tried to raise this to so 50uf as I don't like a real bassy tone either. I'm using a well broken-in Weber Blue alnico 12 30 watt, use a strat with singles.
Would changing that bypass cap to say 22uf, take away some "overall" brightness? Yeah I can lower the bass pot. That change would only do the first channel, so I guess get the .68uf out on TMB channel raise that to so 10uf, might not even hear that difference?
I attached the schematic to my first post.
do you have a schematic?
I would bump the 4.7 to 10 or 22uf. with a 4.7uf by-pass cap the gain on that 12AX7 drops for anything under 1Khz. you might not like too much bass, but with a 10uf cap or a 22uf cap you can let your bass tone knob alleviate that.
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Easiest try, just bypass your tonestacks completely.... I think just lift the ground to them, and then clip in a .022 coupling cap to from the preceding plate to the following stage's grid. Also, tacking in a couple K bypass caps where there are none in the preamp?
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Put a 100pF cap between pins 1 and 6 of V3.
Install a cut control after the PI.
Try a darker sounding speaker.
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The 100pf cap helped but like to cut more, ideally the cut control would be best but I'd have to take out the 2nd input for the pot, kind of liked that for lower volume settings.
I did this to the normal channel, helped a lot. See attachment. Could I replace the coupling cap in the TMB channel to volume from .022 to .0047...?
Would increasing that bright cap help, to say 250pf
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Is the excessive brightness from the Plexi channel? The R5 value of 820 combined with bypass cap of only .68 is like a treble boost.
--Craig
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The amp is what it is, I learned here that bandaids aren't the cure. If an amp's circuit is on the bright side change the circuit which is what I did. I found a revised 18 watt Plexi circuit on the Trinity site that is like a Marshall lead but (Brown). I changed the circuit and the amp sounds great, not too bright for an 18 watt now. Nice to be reminded that the first stage by-pass caps and R has a great effect on the tone, sometimes the simple stuff evades, at least me, ha.
Anyway, this new circuit has a Fat Switch bypassing V2's cathode cap, side going to the gain pot. I don't have a place for the switch and don't want to drill the faceplate.
I was wondering if I could use a unused spk jack and run a foot switch instead? Can this be done without a relay or will the amp be subject to buzz and hum when the fat switch is on? I did this once with a cathode bypass cap on the first stage but couldn't get rid of the hum when on.
Thanks for all the advice.
Sincerely, al