Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tubefiend on February 29, 2016, 08:50:25 am
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On my recent build of an SE parallel preamp / parallel output EL84's, I came upon an enlightening conclusion. You can have too much of a good thing. I have a parallel AX7 as my first stage, into the first half of another AX7, into a Marshall style tone stack, and then into the second half of the AX7, then into parallel EL84's. The original idea, was a high gain SE, and I must say, I succeeded. A little too well.....
I had to go back and add a switch to ground the grid of the second half of the first preamp stage when desired, it was just too much to clean up with the gain pot setting. Next time, I'll forgo one stage of the preamp. It works quite well with the switch to effectively shut off one half of the first tube, but what's the point? When would you ever need that much distortion? Maybe next time, a gain pot with a SPDT switch to make it easy (easier) to switch between the two. I'm starting to believe that anything over two triode preamp stages is a waste of time.....I'm going to go one step further and swap out the first 12AX7A for a 5751 that I'm waiting to arrive. Looks like a much better fit for this amp. Any other ideas or suggestions?
Pete
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I am currently working on a preamp that has 2 12Ax7, an 5879 and EF86. Certainly all in a row will create a load of distortion if bypassed, but I am really liking the tone of just a little added gain at each stage. Seems a lot smoother.
What my idea is to try to get the tone of power tube distortion from a preamp. Ain't gonna work and pentodes make some wierd sine waves, but it sounds pretty good.
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Leo was not as dumb as he looked.
You want ~~20mV at jack to put ~~20V on power tube grid.
Gain of 1,000.
A "boost" tone-stack may have loss of 10. Then you need gain of 10,000.
Reverb mixer usually has loss; as do most low-level tremolos. So a full-feature amp may need gain near 50,000.
One 12AX7 is gain of 50. Two of them is 50*50 or 2,500. Three is 125,000.
Simple amps (up to AA-Champ) should have TWO small stages before the power bottle. (Push-pull power stage needs one more small stage to get push-pull drive.)
Three stages will do very lossy (tone, rev, trem) schemes.
More-than-three only makes sense if YOU "add loss" between stages. The 7 and 9 stage corrupters in the big head-banger amps tend to go gain-of-50, loss-of-20, etc. The overall gain is not insane. Each stage overloads slightly before each earlier stage. To make it "interesting", losses are staggered in amount and tone-shape so the sound changes as more and more stages hit their stops. IMHO, this is advanced tonal design. Takes MUCH time. Get someone to pay you to do it.
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And Marshall (and others) added voltage dividers in between preamp stages when there were 3+ stages. It seems foolish, build signal up then knock it down then build it up then knock it down. But the point is to gain harmonic enhancement from each stage, and all the stages together provide way more gain than needed to drive the output section, as PRR points out.
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gain harmonic enhancement
does the enhancement come primarily from the tube,
or the passive parts in and out of the tube?