Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Fresh_Start on July 13, 2010, 03:33:09 pm
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With all of these neat designs coming from tubenit, geezer, DBM & Co. using pentodes in preamps, I've been wondering if anyone has tried using a 6BM8 or another pentode/triode tube in a preamp?
The EF86 and 5879 are the two pentodes I seem to hear about the most. Looking at the data sheets, the 6BM8 pentode section has 1/2 the mu (9.5 vs. 21) of the 5879 but much higher potential plate dissipation (7 watts vs. only 1.25 watts for the 5879). Are relatively higher mu and lower current output part of what makes a good preamp pentode?
Are there any commercial designs which used dual triode/pentode tubes in the preamp?
Just daydreaming...
Chip
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dynaco and IIRC a geetar amp or two (ampeg & gibson) used the 7199.
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Dyna and the old Sunns used a 6AN8.
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Dyna and the old Sunns used a 6AN8.
The pentode section of the 6AN8 looks like a good driver for the triode as a concertina splitter:
http://www.schematicheaven.com/newamps/sunn_100s.pdf
What I had in mind was a 2-tube amp:
> input => triode => gain control => pentode
=> TMB tone stack => volume control
=> triode with NFB => pentode driving OT
Oh well, back to the amp I actually should be building right now...
Chip
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Are relatively higher mu and lower current output part of what makes a good preamp pentode?
Great question and I am hoping that someone takes a stab at it, at least with the pentode end of it, in a general sense. The difference in tone between a 12AX7s biased to idle at--say 1.25 ma, 1 ma and .75 ma--is noticable to many. Is the tonal or tactile response of a tube a function of the gain it is set up for or the current it is designed to run at. RPM or torque? Eeek, I am running off into the barrow pit! Regards
dennis
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Dennis - you might want to look over the Pentode Press Pentode Preamp Design tutorial (http://www.pentodepress.com/home/tutorials/lesson-001/) (see lower left frame). I've only skimmed through part of it, but it looks really helpful.
Cheers,
Chip
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Thanks Chip--I read through it a couple of times and, with a little more Merlin chapter 3 review, went back tonight to see what I could do with a 6U8A. Did a load of calculations stepping through it and dialed it in pretty close right off the bat but I no matter what I adjusted, I just cannot get a decent OD sound with it. I am again surrendering to the adage that I am not yet able make a audio silk purse out of a TV local oscillator/mixer combo; I could only get as far as a folding wallet this time. The Don Quixote in me I stills think that I can do better. I'll give tubenit's circuit a go--I really like the sound of what he did with his COS.
If anyone else has had any success with using a triode into pentode (6U8, 6EA8, etc.) for a OD stage, I would be more than interested. Again thanks, that was a another great read on preamp pentode design. Regards
dennis
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I built a pair of SE amps using just a 6U8A for the preamp. I used the triode then TS then pentode into a 6V6. Sounds OK, it took a awhile to dial it in but the sound is OK nothing special and not as good as a plain 12AX7. A little more gain but very light on the lowend which is what I was looking for in that design.
I did find that about 70% of the 6U8As I bought were too microphonic to use.