Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Captain chunkulus on December 06, 2021, 12:13:41 pm

Title: crate turbo valve 50 weird bias issue.
Post by: Captain chunkulus on December 06, 2021, 12:13:41 pm

So this crate tv50 I have schematically is almost Identical to the blue voodoo 50. I was attempting to raise the bias to work with el34's. Looking at he jcm800 schematic I had it said about  -57 volts for the bias. I'm pretty much there almost spot on. This is a cathode biased amp just fyi. Here's the strange part and I know that I should know these things by now but I'm getting a reading on pin 1 of only 2.9mv when off of standby. Still getting -57 on the bias but only 3.09mv on pin 1. I'm posting the bv50 schematic. The bias resister is all I changed. its a 4.7k instead of the 15k that was there before. any help would be appreciated. The amp plays but doesn't sound right. has compressed sorta feel and the tubes{el34's} seem really bright. They don't seem to be red plating just really bright.

Title: Re: crate turbo valve 50 weird bias issue.
Post by: sluckey on December 06, 2021, 12:59:43 pm

A cathode biased amp does not need a negative bias supply. Maybe your amp is really a fixed bias amp?

-57v bias works well with 6L6 tubes but is too much for EL34s. You need to connect pin 1 to pin 8 for EL34s to work in a typical 6L6 socket. And you need a 1Ω resistor between pin 8 and ground to be able to monitor tube current.

Title: Re: crate turbo valve 50 weird bias issue.
Post by: PRR on December 06, 2021, 04:15:43 pm

That BV-50H is very strange. It does have pin 1 and 8 strapped. Then it has a fuse, then a 12Ω resistor. Moreover, the negative grid bias has a Zener in it??

Title: Re: crate turbo valve 50 weird bias issue.
Post by: cliffs_trip on December 14, 2021, 08:44:49 am

I've had a few of the BV amps and they have a weird Auto-Bias thing going on in that circuit.

This is straight from the manual :

• Auto-Biasing Circuitry: Want to change your sound? Change your tubes! Internal circuitry allows the use of either 6L6 or EL34 power tubes with no bias adjusting (page 7).

I did this with one of my BV50 or 60's and it seemed fine to just swap from EL34s to 6L6s and back. 

Basically the process is, turn off power, switch tubes, turn on power and let idle for 20 minutes, then it is ready.

Title: Re: crate turbo valve 50 weird bias issue.
Post by: PRR on December 14, 2021, 11:33:42 am

Sigh. So RTFM???

Title: Re: crate turbo valve 50 weird bias issue.
Post by: PRR on December 14, 2021, 11:35:43 am

> The bias resister is all I changed. its a 4.7k instead of the 15k that was there before.

And change that back!!!