Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on July 06, 2011, 11:22:10 am
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On my amps that draw from the D-style amps ............ I have been using Mat's PAB (preamp boost) & discovered it is wired differently thatn the Dumble #124 & #183 amps.
My conclusion is that Mat's PAB will give a much slighter boost in volume and overdrive than the 124/183 design.
Am I thinking thru this correctly?
With respect, Tubenit
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Is there a superfluous 22meg resistor in the drawing(s) on the right? Looks like it isn't in the circuit regardless of the switch setting but maybe it's an "anti-pop" resistor.
Chip
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The superfulous 22M resistor may be there because the original D-style design had a rock/jazz switch connected with the PAB. So maybe it served a purpose in that.
For my purposes, I think I have found a simpler approach for what I want to do. In fact, I am thinking that I could use a different value than 22M to "dial in" the amount of PAB boost that I would want.
Since the PAB disconnects the treble and bass pots, then maybe I could control the amount of disconnect by trying different values to lessen the amount of boost and volume jump between the PAB on and off?
With respect, Tubenit
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My conclusion is that Mat's PAB will give a much slighter boost in volume and overdrive than the 124/183 design.
Yes, I believe that is correct.
Matt's PAB effectively disables the Bass pot except for its interaction with the Treble pot. IOW with PAB on, the Treble pot is still grounded through the Bass pot wiper. The position of the Bass pot will have an effect on the frequency band controlled by the Treble pot and also will control the signal level coming out of the tone stack. Turn the Bass up, and the PAB should have a greater effect.
OTOH the "Dumble" PAB shown on the right appears to completely disable the Treble control (22meg to ground) and effectively turns the Bass control all the way up (22meg between wiper and "top" of the pot). Almost like an old stereo "Loudness" control on steroids. :grin:
In three out of the four shown in the last post, the PAB switch doesn't affect the Bass control at all. They do eliminate the interaction between the Bass control and the Treble control which I think happens in the very first example, first post.
Knowing that you (tubenit) like mids, I wonder if this PAB that seems to focus only on the Treble control will get where you want to be. What if you used "tubenit's PAB" upper right above as one side of a DP/DT switch and used the other side (pole) to switch the mids? Maybe a resistor between the top of the Mid pot & the wiper? Maybe a switchable version of LC's "girth" control?
I hope my analysis is useful even if parts of it are wrong.
Respectfully,
Chip
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Chip,
I always appreciate your replys. Thanks! You consistently make good observations and give food for thought.
Ironically, this is the first amp I've built where I find myself actually dialing down the mids somewhat. What I mean by that is I am using the midboost switch maybe only 25-40% of the time & instead of dialing the 100k mid pot up, I actually and dialing it down to about "3".
However, I do like your idea of using the other half of the DPDT to add some type of color or tone to the PAB tone. I'll explore that and report back at some point.
Maybe I could use a DPDT center off and have on, off and on with some color added?
Thanks, Tubenit
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Given the way the Mid pot is wired in this circuit, it sounds like a 50K pot would make more sense for you. More usable range.
Your on/off/on switch idea is cool. One position could be full treble boost, another treble boost with mid boost, and the third spot "normal".
Is this tone stack driven by a cathode follower? If so, switching the slope resistor might be an interesting option.
Glad my posts are helpful. I love reading about your thought process in amp design and tweaking.
Cheers,
Chip
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One thing you have to realize is,is that a 22meg resistor has nothing to do with shaping the tone.It basically blocks the signal completely as electricity follows the path of least resistance.So playing with the value won't have any effect at all all the way down to a very low level.
It simply blocks the path.Why Dumble used 22meg is a good question,maybe he had a few laying around and used them to confuse people.:)
It does not form a voltage divider because the value is too large.It's like putting a big wall in front of the electrical path.
Take another look at #124.The switch seems to be wired differently that your diagram. I could be wrong.
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One thing you have to realize is,is that a 22meg resistor has nothing to do with shaping the tone.It basically blocks the signal completely as electricity follows the path of least resistance.So playing with the value won't have any effect at all all the way down to a very low level.
It simply blocks the path.Why Dumble used 22meg is a good question,maybe he had a few laying around and used them to confuse people.:)
It does not form a voltage divider because the value is too large.It's like putting a big wall in front of the electrical path.
Take another look at #124.The switch seems to be wired differently that your diagram. I could be wrong.
My guess is that the 22meg is an "anti-pop" resistor and the value is more or less random.
Chip