Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shooter on March 09, 2014, 10:55:49 am
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The partial schematic below shows the changes I made from the original, in order n in red.
I have about 400mV AC p-p in at pin 4. I have (original) 4.92 V AC p-p out (junction of 1meg n .022uF). with all the “mods/changes” the lowest I can get is 4.5V AC p-p.
I’m trying to lower the gain of the overall preamp section to something close to the G1 peak AC, about 14 VAC (28V AC p-p?) for the PA tube (6550) I currently have 48V AC p-p clean and about 56 V AC clipped. Short of just eliminating one triode stage, is there a way to “set the gain of the tubes” without more clipping?
I can do it by just turning the gain and drive down but I’m trying to get the “full” range of the pots. The gain pot is about right, giving me “back” what the tone stack ate up.
Anyway, thanks again for any help
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Short of just eliminating one triode stage, is there a way to “set the gain of the tubes” without more clipping?
Disconnect the cathode bypass cap. If that doesn't get you where you want to be replace the plate load resistor with a 250K PEC pot wired as a simple three terminal volume pot (voltage divider), not wired as a two terminal variable resistor.
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Short of just eliminating one triode stage, is there a way to “set the gain of the tubes” without more clipping?
Hope these help shooter.....
http://vimeo.com/27777346 (http://vimeo.com/27777346)
http://vimeo.com/33244955 (http://vimeo.com/33244955)
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Thanks guys, I kept putzing and wound up at the next gain stage where I have a drive pot and paralleled resistors til I got close. I get 28vac p-p at 500hz, 42vac p-p at 1khz, and 29vac p-p at 8khz. (all tone knobs set for max signal on the scope)
Was wondering if I should be using rms instead of p-p? also since i'm not to musical, what amount of preamp clipping/compression is reasonable (understanding that's like asking me how much red should used for a painting !!), the 42volts is clipped one side about 5-10%. I'm shooting for clean to about 6or7 on G1 of the PA tube.
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insert a voltage divider either before or after the stage. before: insert a resistor on top of the 1M pot. after: in series with signal, between .022 and 330K.
If the circuit distorts when the 1M pot is at 50%, install a 1M-R on top of the pot, or swap pot for a 500K and insert a 500k-R on top. If this stage doesn't distort, but it is the next stage that distorts, insert a resistor to make the 300K the R2 of a R1/R2 voltage divider.
either one of these will lower gain through the entire stage and allow for full range of pot swing.
what amount of preamp clipping/compression is reasonable
I don't know, but I know it when I hear it. ;-)
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that's my problem, I know how much red when I see it! i'm in the process of printing out the tube doctors "common gain stage" 35pgs, by the time I get through that, with understanding, I don't think I will like red any more:)!!!!