putting together a SE marshallish amp

[seljer]

Hi, this is my first post here I don't own a tube amp but I've been reading up on stuff and studying schematics for the past year or two.

Recently, somebody on another forum asked me to help him with modding his Marshall (master volume, taming down the bright channel, adding a bit more gain). With that done, he wants me to help him build an amp and I'm up to the challenge. Its just that I don't have a grasp over all the power amp stuff yet.
The preamp will be the basic non-master volume Marshall preamp with permanently bridge inputs, the power amp would be a single EL34 or 6L6, with the ability to easily change between different types of tubes without worrying about biasing (like on the THD Univalves).

For a start, I was thinking about just taking this http://ax84.com/static/corepoweramps..._Schematic.pdf and working off it. How critical is the bias here between different different types of tubes? When they're talking about self-biasing on the THD page they're basically talking about a cathode biased tube right, no other magic?

would it be possible to wire in both a socket for an EL84 and an EL34/6L6 like in this here slightly older AX84 schematic (though for up to 20W output like the power amp above)?
http://195.178.227.103/ax84/media/ax84_m215.gif
or at least implement the pentode/triode switch, in the same fashion as its set up there?

and finally, are there any major problems when adding the ability to switch between a tube and a silicon diode rectifier? if I understand things correctly the voltages change slightly but would it create any bias problems in a setup like this?

well anyway, I have a lot more reading to do, the internet really is an amazing source of information regarding this stuff

[f.leghorn]

PA Options

Either option will work OK. On the second one with the EL84, you have to ensure that it doesn't see more than about 350V for best tone, and if you put 450v to it look out!

I have actually done pretty much what you're proposing, and sold it to my dentist!

I put a conventional pre-PI MV (1M) in it, except also with a 1M grid leak on it and a 100K between the wiper and the PI input, to eliminate hi-freq. loss at lo-vol. Used a JJ E34L, this is the European equivalent of US JAN military spec. construction. I set it up so it's optimized for this tube.

I also put in a pentode-triode switch, but I don't personally care for the triode mode, it definitely loses something. Plus you have to pick your switch grade very carefully because switches of the type we normally use are not designed for 400 VDC!

Component tips: I used the new Tungsol 12AX7 and SOZO tone and cathode bypass caps, EXCELLENT combination!! Nails THE tone! I would also recommend the PT and OT from Dave Allen (www.allenamps.com) which are designed for SE operation. The OT is made by Heyboer and it rocks. Not expensive either.

Sorry, I see you are in Europe--Allen's trannies are 120VAC 60Hz only-- but if you're in Slovenia you should be able to get JJ tubes, made in Slovakia.

[rocket]

I think if you want a small marshall it would be better to stay with pp rather than SE.
Otherwise you'll miss the crossoverdistortion and the uneven harmonics created from pp.

[BlueToad]

I think if you want a small marshall it would be better to stay with pp rather than SE.
Otherwise you'll miss the crossoverdistortion and the uneven harmonics created from pp.

Seconded. Although I believe that you still aren't looking for crossover distortion, the push-pull stage cancels even harmonics, so you wind up with a much edgier sound. You can try using the SE output configuration, and it will probably sound good, but it's not going to be the traditional "Marshall" sound. You might try a push-pull EL-84 output stage, like from the marshall 18-watt amps. see http://www.18watt.com.
Carl

[f.leghorn]

SE vs PP

Well, all I can tell you is the one I built sounds very Marshally. I had it checked out by a friend who fronts a local metal band and runs two '70s Marshalls, and he said "it has the tone". My dentist loves it, he should, he paid me $1,200 for it. He runs it full out with both preamp channels dimed too. I have a '77 model 2204 myself.

If you push a SE operated EL34/E34L tube hard it will generate surprising amounts of even order distortion, even in triode mode, mostly 3rds but also a little 5ths. I confirmed this with John Broskie's SE AmpCAD software.

But I'll grant you this, it never sounds like it's right on the edge of oscillation, like some old Marshalls will do, it never gets that hairy.