Bare Knuckle Pickups Forum

Forum Ringside => Tech => Topic started by: Keven on October 21, 2012, 08:54:42 PM

Title: Tube types for a randall LB50 head
Post by: Keven on October 21, 2012, 08:54:42 PM

Hey guys,

according to the manual, the Randall LB50 is supposed to be self biased and it says i can use a variety of tubes to my liking.

so far i tried mesa branded 6V6, redplate... jj 6v6, redplate. WingedC EL34, redplate...

it seems odd that so many tubes will redplate.

it comes stock with JJ E34L. i'm going to buy another pair and see if they redplate. the current ones don't redplate at all!

I'm not with the amp at the moment but i do plan on using my bias-rite to check the plate voltage and see what happens.

here's the manual:

http://www.randallamplifiers.com/images/stories/manuals/LB50C-LB50H-LB112MANUAL.pdf

also, what preamp tubes would be decent these days? haven't had much experience with these so far. budget isn't really a limit.

thanks guys!

Title: Re: Tube types for a randall LB50 head
Post by: jpfamps on October 22, 2012, 05:30:44 PM

By self-bias, I assume the amp is cathode bias.

This would be optimistic to get 50W from a pair of EL34s in this configuration.

The E34L biases similar to a 6L6, so I would expect the cathode resistor used would work for 6L6s as well.

Genuine EL34s would probably require a larger cathode resistor to bias correctly, and hence the red plating.

6V6s have lower dissipation rating than El34s, and as I expect the plate voltage is way over spec for a 6V6, this may account for red plating with these valves.

Title: Re: Tube types for a randall LB50 head
Post by: Keven on October 22, 2012, 07:47:32 PM

So according to you i could safely replace the E34L's with 6L6GC. that's good to know!

Title: Re: Tube types for a randall LB50 head
Post by: jpfamps on October 23, 2012, 10:29:37 PM

Quote from: Keven on October 22, 2012, 07:47:32 PM

So according to you i could safely replace the E34L's with 6L6GC. that's good to know!

Yes, that substitution should work.