Food for thought; preamp-triodes in class AB

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6/21/1999 3:12 PM

Mike Burgundy

Food for thought; preamp-triodes in class AB This is something I´ve been toying with in my mind;     the H&K Creammachine uses a 12AU7 in single-ended class A through an output transformer to emulate real amp output. This will yield mostly even-order harmonics, while a balanced powerstage produces odd order harmonics in overdrive, so why not make a balanced "poweramp" with two triodes and a xformer (even if this tube is definitely not designed for this)?     I´ve done some reading, but since I´m not particularly theoretically gifted I thought you guys might like to join in.     The choice of a 12AU7 makes sense; highest maximum power dissipation, lowest Ri. I get that.     Secondary impedance 8-10 ohms, 3W should be way more than sufficient, but what about the primary? I´ve tried comparing penthodes to pentodes in triode configuration, but this wasn´t really a help.     I figure the primary impedance can be chosen within a reasonable margin, but this will have a significant influence on the sound.     Increasing Ra decreases distortion, increases power to a degree,but also limits headroom. Ra value probably also mucks with harmonic content. On a triode, Ra doesn´t have to be lower than Ri, okay....     Where the heck does this leave us with a 12AU7? There is obviously nothing in the datasheets....     I *think*, since Ri is still a lot higher than any penthode in triode configuration (12AU7; 7k7, 6L6 1k5) we also want a higher Raa - something like 30k-50k maybe? Or even 100k?     Anyone have ideas or experiences?

 

6/21/1999 3:23 PM
R.G.	About a year ago I built a 12AT7 output stage using a Circuit Specialists transformer at 20Kct to 8 ohms. Worked OK. I haven't had time to do much messing with it. Seems like it was shy bass response, but I got overloaded at work and had to drop it for a while.         I think something like 20K is a good starting point, and the CS transformer is also a great buy, about $3 as I remember.

 

6/21/1999 7:42 PM

Doc

Check the ampage Schematics section for the Moonlight Amp. This amp uses a p-p 6SN7, which has the same ac plate resistance as a 12AU7 tube (7.7k in a 250v circuit). Back when Stephen Delft (aka Kiwi) was developing this amp, I mentioned to him that the textbook load for audio should be from 2x to 4x the plate resistance, and was surprised that he recommended a much lower load. He had actually tried varying the load on the output stage over quite a wide range, noting the performance. It turned out that for pleasing guitar sound, half the textbook recommended load was the best. His schematic shows the OP trans ratio as 6 to 15k range for best operation. The 30k load just didn't sound as good. I think the Moonlight is a class A circuit. Maybe the best sounding load for the stage run in class AB will be different. You'd have to experiment. As R.G. mentioned, a typical replacement fender reverb driver transformer is a good starting point for a single ended amp (it has no CT lead). Any small OP trans from a low power p-p stereo amp may be suitable. Remember, you can double the reflected load by connecting an 8ohm load to the 4ohm secondary tap.         By the way, check out the Bench Test section of the latest (Aug '99) Guitar Player. On page 110 of the review of the Gerlitz G1 Revelator Dual Amp, they mention that each amp channel uses a p-p 5814 (industrial 12AU7) run in class AB, with a small output transformer and dummy load. Now all you have to do is find out what reflected plate load they settled on.

 

6/22/1999 1:15 AM

anonymous

>>On page 110 of the review of the Gerlitz G1 Revelator Dual Amp, they mention that each amp channel uses a p-p 5814 (industrial 12AU7) run in class AB, with a small output transformer and dummy load. Now all you have to do is find out what reflected plate load they settled on. >>             Ahh, yeah. Now how would I do that then? Call them, ask them, and they would go "oh, okay, it´s x.x kOhms, wanna know more of our secrets?" ;'))     Anyway, the lower primary is a good pointer, I´ll see what I can find at a second-hand tube-junk store in a near-by town... (YES! They DO exist! Praise the Lord!)     If I put up a "this-is-just-a-sketch-schem", would you all be interested? (Maybe we could turn this into a collective AX84-type project? I´d really enjoy that! The aim is to supply a balanced power amp output, preferably capable of driving a speaker, but also a regular guitaramp, combined with a speakersim. and line out. I´m thinking 1 12AU7 for "power", and two 12AX7 pre (one triode with poweramp)

 

6/22/1999 1:16 AM
Mike Burgundy	woops forgot to sign...

 

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