WARNING O.T. tubeampstuff...

[Johan]

I posted this over at ampage a few hours ago, but dont really expect a good answer there..its been a few years since I posted at ampage so this is probably not fair..but my experience over there is something like "whoever posts more often is right.." rather than actual knowledge so just in case, I try here too..


"I've been running my -72 Marshall 50watt cathodebiased ( to reduse anode-cathode voltage ) with 6v6 for a while and it works but it just dont sound the same as when the cathode is straight to ground....so i was looking around and had a look at some old Musicman schematics and found that they run EL34's with almost 700 volts on the anodes..this would normaly fry EL34's but the trick was that they had a voltage devider, taking the screen grid voltage down to 350 volts. and IIRC they where pretty reliable...does anyone here know if I could do the same trick with my Marshall..it would mean 470V on the 6v6 anodes and around 235V on the screen grids..CAN THE 6V6s TAKE IT?
in the Musicman's the Cathode is raised about thirty volts but they still run with way higher voltages than those tubes are supposed to handle..."

thanks for any help..

Johan

[puretube]

NO.

EL34 is specd. for Ua = 800V; Ug2 = 500V (max);
6V6  is specd. for Ua = 315V; Ug2 = 285V (max);

I`ve seen commercial amps with EL34 @ 800V recently.

[Johan]

thanks puretube

the only specs I looked at for el34 is the ones given in the tube amp book ( forth edition ) saying maximum plate supply 500volts..so I just thought if that one can take a few hundred volts to much, and I know I can run 6v6 at 420volt in a deluxe reverb ( 100 volts over specs ) maby this would work too...

Johan

[The Tone God]

The specs that Puretube posted are the offical specs. Fender took advantage of the safety underrating margin running them at higher voltages. You can take a look at the old 6V6 specs in the book you have to compare the spec'd voltage to what voltage the amp put on them.

Running 6V6s at 420v is already pushing the tubes beyond their design specifications. Trying to run them at 470v is asking too much. There is not that much leeway in the specs to keep inching them towards that goal.

My advice would be to NOT do this.

Useless tidbit: The Svetlana (SED) EL34 is offically spec'd at 600v but unoffically can take 800v.

Andrew

[dosmun]

You may try posting or E-mailing at www.metroamp.com.  A friend of mine owns the site and is very knowledgable with Marshalls.

[puretube]

ok: one thing to take into account, is the fact that the specified max.-voltages stand for the absolute voltage between plate and cathode, and screengrid and cathode, (for a warm tube at its working point),
which is not neccessarily equal to the "B+" voltage,
which can be higher than the absolute Ua:Uk, depending on the plate-load (transformer) and the biasing...


but anyway: this sounds too high for me with those tubes (how about the caps?)

Take into account also the Qa max. (power dissipation) !

yes, some people do use a B+ of 1000V with EL34...

[Lonestarjohnny]

first off, the 800 volt rating came frome the fact that Mullard and amperex EL 34 tube's came from the Factory wth a 800 V rating, no other tube could handle this kind of voltage for any length of time, your 6V6 tube's are rated at 420 v. you run them on
high voltage and your gonna cook them, and maybe your amp too, on the musicman amps I have worked on, they are Biliniery, and yes they do have a voltage devider network built out of diode's and capacitor's, but on your Marshall I don't think it would work, No Biliniery output tranny,
JD