Will this work as a front end for a three stage SET GM70 amp

[djnagle]

Hi All, I posted this in the GSG amp forum but maybe here is a better place. I am new to this forum....so "HI". Here is what I posted in the GSG forum;

I am building a GM70 SET amp and was thinking about using the JE Lap 6SL7/2A3 for the front end but then have been following Ashoks progress on his Get*Set*Go amp and was wondering if this would work as a front end. I would just not use the OTs and go cap coupled to the GM70 tubes. What do you think.

[kheper]

I have seen a few schematics for driving the gm70 with a single volt amp, but most designs lean toward:

Volt Amp -> Volt Amp -> Cathode Follower -> GM70

Volt Amp -> Power tube -> GM70

Volt Amp -> Power Tube -> Interstage -> GM70


Look in the "files" section of the gm70 yahoo group for some schematics.

http://groups.yahoo.com/group/GM70/


Here is a GM70 driven by a single 12GN7A.

http://diyparadise.com/nikcgm70.html

I do not know how much swing the JE Labs design can provide. GM70s are relatively easy to drive, but if you are using a low B+, you may be disappointed with the output.

[djnagle]

Thanks Kheper, I have a 1300v PTX but will bring that down to 900v. I want to stay away from 1000v or better.

[kheper]

Here is a schematic for a GM70 driven by a 12HL7 pentode. B+ = 930V.



Pentodes like the 12HL7 are supposed to swing more than the 6SL7.

The power supply for the above could be simplified with a 0-400V voltage doubler. You could tap off of the first electrolytic cap to get ~400V to drive the 12HL7. With this, you would not need a separate transformer winding.

[soundbrigade]

I've seen similar with a Russian 6E5P as driver. Scan you a pic tomorrow.

[azazello]

GM70 start to sound good with B+ more than 1200 v, Ra = 12kohm and You
will take 14-15 watts. 800-900v is not serious mode with 7-8w. You better use
2 parallel 2A3 /or 1 300B/ with B+a-k=300-350 v, Ia = 2x 60 ma /70/, and Ra = 1 - 2 kohm.
I'll prefer 6S33S that will give me 13-14 w in mode B+=200 v, Ra = 1,5 kohm and Ia = 280 ma.
Very good driver is russian 6E5P ! B+a-k=180-190 v, Ia= 40 ma
http://klausmobile.narod.ru/td/data/_6ee5p.GIF
http://frank.pocnet.net/sheets/113/6/6e5P.pdf

[soundbrigade]

The pic:

[djnagle]

GM70 start to sound good with B+ more than 1200 v, Ra = 12kohm and You
will take 14-15 watts. 800-900v is not serious mode with 7-8w. You better use
2 parallel 2A3 /or 1 300B/ with B+a-k=300-350 v, Ia = 2x 60 ma /70/, and Ra = 1 - 2 kohm.
I'll prefer 6S33S that will give me 13-14 w in mode B+=200 v, Ra = 1,5 kohm and Ia = 280 ma.
Very good driver is russian 6E5P ! B+a-k=180-190 v, Ia= 40 ma
http://klausmobile.narod.ru/td/data/_6ee5p.GIF
http://frank.pocnet.net/sheets/113/6/6e5P.pdf

I've heard that about needing 1200v B+ but then I started listening to GM70 amps that ran at under 1000v and thought they sounded great. I don't want to go over 900v because of corona effect.

[djnagle]

The pic:

Yep, that is a clean schem. Don't know why, but I have it stuck in my head that a three stage would give a bit better dynamics. I am not concerned about power as this will only drive my midhorns that are 115db effiecent.

[djnagle]

Here are pix of my chassis. It is all glued up and waiting for the top plate. So, I guess my origanal question still stands. How do you thing the GSG amp would work at the front end for the GM70?