64 bandmaster modded for EL34s

[randomhitz]

I had this mod done about 18 years ago. Have run it that way ever since. It sounds great but recently I wanted to try a set of JJ 6L6s in it. I have this admittedly rusty memory that the guy who did the mod said I now could run either EL34s or 6L6s. The mod was something like a simple jumper on the octal socket that was necessary to run the EL34s but didn't affect the operation of the 6L6s. Does any of this make sense to anyone? Anyone done this mod?

Thanks
Randy

[Alnico Sunburst]

You'd want it rebiased for the 6L6's but your friend is correct. 6L6 is a beam tetrode, while an EL34 is a power pentode. In the EL34, the third grid is connected to pin 1, while the cathode is connected to pin 8. In the 6L6, the cathode is connected internally to the beam forming plates (a clever dodge to avoid infringing on Phillip's patent on the pentode) and come out to pin 8, while pin one is left unconnected (it orginally connected to the outer envelope in metal encased 6L6 tubes, but it's not needed now). So, with the jumper in place, you can use either tube.

[randomhitz]

Thanks. Do you have a preferred biasing method?

[Alnico Sunburst]

Do you have a schematic for your amp? It will be available for free on a variety of sites. The schematic will tell you the voltages you would read at various parts of the circuit. The grids of the 6L6s are tied to a common supply coming from a separate winding on the power transformer, through 220k resistors. You will, on various Fender schematics see various numbers given for the voltage feeding the grids, depending on where the plate voltage is set to.

Read this first: http://tone-lizard.com/Biasing.htm

and then I'd set it to what it says on the schematic.

As Mr. Jamz says (I met him at a ham radio swap meet years ago in Winnipeg. Good guy, knows his stuff.):

If the sine wave looks good on an oscilloscope (the top and bottom half clipping at the same time, with a slight crossover notch when the wave is 'just' clipping), and you have the expected output power without the plates glowing red, who cares how much plate current the tube is drawing? Not I. The number will vary.

He explodes a lot of myths, but it can be a humbling experience to find out a lot of the things you thought you knew were utter b*******. I'd recommend downloading and saving as much stuff as you can from his site now, as he has stated his intention to possibly let his domain lapse in the new year.

[randomhitz]

Alnico Sunburst--Thanks for the link to Tone-Lizard. Great stuff!

[SamBooka]

Originally Posted by Alnico Sunburst

Do you have a schematic for your amp? It will be available for free on a variety of sites. The schematic will tell you the voltages you would read at various parts of the circuit. The grids of the 6L6s are tied to a common supply coming from a separate winding on the power transformer, through 220k resistors. You will, on various Fender schematics see various numbers given for the voltage feeding the grids, depending on where the plate voltage is set to.

Read this first: http://tone-lizard.com/Biasing.htm

and then I'd set it to what it says on the schematic.

As Mr. Jamz says (I met him at a ham radio swap meet years ago in Winnipeg. Good guy, knows his stuff.):



He explodes a lot of myths, but it can be a humbling experience to find out a lot of the things you thought you knew were utter b*******. I'd recommend downloading and saving as much stuff as you can from his site now, as he has stated his intention to possibly let his domain lapse in the new year.

COOL LINK :)