6V6's in a Simul 395

[fusionbear]

Does any one know if I can put 6V6's in an old Mesa 395 power amp if it is set to half drive or does half drive just mean that it runs two tubes instead of 4 per channel.

[fusionbear]

El Bumpo! Anyone care to comment?

[vibroverbus]

I don't know what the circuit looks like but assuming it's not too crazy, you should be able to, but I would absolutely check and re-bias with new tubes in there, and you probably have to build in adjustable bias as I doubt this amp has it.

Most likely 1/2 power does just cut out 1/2 the tubes. What would be tricky would be the "Class A / Simul Class" feature - I have no idea what that circuit looks like but you'd have to build the bias adjustment around that - presumably it's a simple resistance switching setup that takes the op point from "Frigidly Cold Mesa bias" levels to just "Damn Chilly Mesa bias".

Output impedance will change - 6V6's want about 2x the output impedance of 6L6's/EL34's, so whatever your output is rated for now, double it (or if it has an impedance selector, go 'down a notch' - ie. set to 4ohms for 8ohm cab).

[Blue Strat]

6V6s are not typically a good choice to replace 6L6s or EL34s...too much plate voltage.

The Mesas I've seen turn off 2 of 4 power tubes for half power mode. You can check this by removing the inner or outer 2 of your power tubes and see what happens.

[fusionbear]

You are right. HalfDrive=Half of tubes 2 instead of 4. The main reason I was thinking about it is that new 6L6's are taller than the old tubes and do not allow the top cover to go on.... what to do...

[John Phillips]

Look for some old-stock GE 6L6s. They typically have shorter bottles than most other types. The side benefit is that they sound fantastic too :).

[triodal]

what tubes r you using?
mine is EH 6L6GC and it sits just perfect...
Im guessing the tube sockets too high?Is this thing adjustable?
Im curios to know if i can put in el34 on 395..

i have the 395 schematic..
But It only show the bias supply circuit and few other notes..

[fusionbear]

I'm using the STR440's which i believe are chinese. The STR430's have a flat top and fit perfect. When I got it it did have the STR420's which I think were sylvania's, but they were blown. I don't want to modify this amp as it is a rare one, plus I am going to use it only for my "wet" effects rig. MKIV 1x12 wide body combo Dry with the DI>G-Major>Simul 395> 2-1X12 Theile cabs

[fusionbear]

what tubes r you using?
mine is EH 6L6GC and it sits just perfect...
Im guessing the tube sockets too high?Is this thing adjustable?
Im curios to know if i can put in el34 on 395..

i have the 395 schematic..
But It only show the bias supply circuit and few other notes..

It wont take stock Mesa EL34's, but I think GT and JJ can get you sets that will work for the fixed Mesa setting...

[braz board]

My Strategy has 520 volts on plates, that would fry any 6v6. Biasing is a real issue since Mesa makes these amps non-adjustable and I have yet to find what resistor to change. Mine runs a cold 25ma. built for long tube life not great tone.

[John Phillips]

25mA is 60% of maximum dissipation at a plate voltage of 520V, which is plenty hot enough. (Well, OK, 59% :)). That should give good tone as well as long tube life. You don't need to change anything.

Biasing up to 70% is not a good idea at high plate voltages, either for reliability or tone. It's not actually the idle dissipation that matters, it's where in the operating range the idle point is, because it's what happens to the tube under high signal levels that really counts. (Read justonwo's big thread about plate glow in a Marshall Super Lead - there's an explanation from Randall Aiken as to exactly why.)

It's funny how Mesa gets a bad rap for making their amps non-adjustable, but when you actually check them, you find that they've got it right... ;)

(Unlike the currently fashionable idea of biasing "to 70%" regardless of any other circuit parameters.)