Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: blueznet on July 18, 2011, 04:19:27 am
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Just built a AB763 40 watt amp. Amp is as quiet as can be but has way to much high end treble. If I turn the bright switch off it cuts to much treble. With bright switch on I am having to run my treble pot on about 4. Any higher and it is over the top.
Other than that the amp plays great with great low end, and mids. I have swapped tubes, chopped sticked everything, and tried multiple speakers.
What would be the easiest way to cut some of that high end down?
Thanks,
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What is your bright cap value?
If you're using the "stock" 120P, then try some lower values.....100P, 68P, or even 47P
G
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Yes it is 120pf. So going with a lower value, my bright switch can be on but will cut some high end, correct?
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What is the taper of your treble pot: linear or audio? With linear taper, treble comes on quicker. Full on or off will still be full on or off but 5 on one will be the same as ~7 1/2 on the other.
Not really a fix but if you're happy with the sound at 4 but wish your knob was pointing to 6 it may help you psychologically.
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Yes it is 120pf. So going with a lower value, my bright switch can be on but will cut some high end, correct?
That's not really what's going on, but in practice.....yes, you will hear less highs with a lower bright cap value.
All the bright cap does is allow more high frequencies to "bypass" the volume control at lower volume settings.....the higher the value (120P) the more highs get past the volume pot. The lower the value (47P) the less highs.
All of this is only effective at lower volumes. As the volume pot is turned up, the cap has less & less effect, until it has no effect at all when the pot is turned all the way up.
From your description of the symptoms & what you are trying to acheive, I believe the lower value bypass (bright) cap wil give you what you want, which is less perceived highs when the bright switch is on (but still MORE highs than when the bright switch is off).
G
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Yeah, try 82p or 75p. If that's still too much, you can put a resistor in series with the bright cap. Mesa used 82k in series with 120p on the Lonestar. You might try values between 10k and 100k.
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What is the taper of your treble pot: linear or audio? Full on or off will still be full on or off but 5 on one will be 7 on the other.
Not really a fix but if you like the sound at 4 but wish your knob was pointing to 6 it may help you psychologically.
Good point.....if the amp sounds good with the treble @ "4", then there's not really a problem, except possibly "That's not where I normally set the treble pot on my other amps".
So yes, check the taper of that pot....is it the same as you used on your other builds? Or maybe the pot is just "different" .....I've seen pots vary wildly in actual measured value as compared to what they are marked, so maybe try a different pot(?)
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What is the taper of your treble pot: linear or audio? Full on or off will still be full on or off but 5 on one will be 7 on the other.
Not really a fix but if you like the sound at 4 but wish your knob was pointing to 6 it may help you psychologically.
Good point.....if the amp sounds good with the treble @ "4", then there's not really a problem, except possibly "That's not where I normally set the treble pot on my other amps".
So yes, check the taper of that pot....is it the same as you used on your other builds? Or maybe the pot is just "different" .....I've seen pots vary wildly in actual measured value as compared to what they are marked, so maybe try a different pot(?)
Pot is a 250k Audio taper. 3/8" bushing, 1/4" shaft Alpha w/ solderable back. It doesn't require any soldering to the case anyway, only the lugs. That would be an easy thing to try first. I can just tac a new one in and make sure it's grounded to the chassis. I will let you guys know if that was the culprit. Thanks all yall for your input.
D-
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If it is an audio then that's not the problem I was thinking. If you somehow got a linear pot the treble would rise sooner.
No need to try a new one- just double check with ohmeter.
Put the pot on 5. If from center lug to each outer lug is about 125K then it's really a linear pot and try to replace it. if it reads ~25K from .02 cap's lug to center and ~225K from the 250pF to center than it probally really is a audio pot. If it measures backwards it's a reverse audio, even worse.
You could also try checking/changing the slope resistor(the 100K in the tone stack) first make sure it's a 100K and not a bad resistor. Even though I read the color bands I always check with a meter before I put a resistor in. Sometimes you get a resistor that measures high. Then try out a lower value 56K,47K,33K. I wouldn't use a pot to dial in the right resistance because that resistor has high voltage on it.
I've never liked the idea of a bright switch anyway. The amount of brightness it adds is dependant on the setting of the volume pot. So the way it works, playing with the volume real low gives a big boost to the brights while playing with the volume all the way up has no effect at all.
When you say the amp is too bright is it at the volume you normally play at or are you testing it out at low volume?
Isn't there a famous vox treble cut circuit out there? That'd cut some treble. I think it's basically a cap and a pot in series connected between the two power tubes grids.