BF vibro champ bias

tsutt · Post by **tsutt** » Mon Jan 19, 2015 3:41 am

Ok what should it be biased to. what is the real world expectation. i have never heard one in person only on youtube so not sure what original sound like this is a triode kit originally. just components. and what about filtering what effect does the power node ecaps do to sound?

Stevem · Post by **Stevem** » Mon Jan 19, 2015 12:28 pm

We need to know first about how much B+ voltage the power supply has after the rectifier, what I can tell you is My Champ has 415 volts on the output tube plate and I have it pulling .045 amp, a tad hot but the output tube is a JJ 6v6 and has a 2 watt larger plate rating than a standard 6v6.
You can go look up the tube spec chart for a 6v6 and get a really good feel for what it should be set at current wise from the spec listed for class A single tube.

tsutt · Post by **tsutt** » Mon Jan 19, 2015 2:55 pm

my plate voltage is 344. I know how to adjust it. just wondering what it really should be. where did u get info that jj can go higher? it was at 15 watts pd but I thought 6v6s are supposed be at 12 max. so I lowered it to 11.77

billyz · Post by **billyz** » Mon Jan 19, 2015 4:05 pm

I'm pretty sure class a can have the idle current near or even at 100% plate dissipation. I usually figure 14 w for a 6v6gt. Some old ones were 12w like plain 6v6, no suffices. I think 80-95% sounds great in a champ. They actually cool off when playing them.
Be sure to allow an air gap between the cathode resistor and the bypass cap.
25uf sounds good as does 100uf, might increase the voltage rating to 50v, but I've seen so many old ones at 25v and they worked a long time. 470 ohms is fine. I have gone as high as 1k, still sounded good.

When doing your calcs remember the screen current.

martin manning · Post by **martin manning** » Mon Jan 19, 2015 4:23 pm

billyz wrote:When doing your calcs remember the screen current.

And that plate voltage is Va-Vk. Pa is then (Va-Vk)*(Ik-Is). Ik is Vk/Rk, and Is can be found from the voltage across the screen resistor and its resistance value. Or, you can just ignore the screen current, and know that your Pa calc is 5-10% on the conservative side.

Stevem · Post by **Stevem** » Tue Jan 20, 2015 12:18 pm

The RCA tube spec states that for class A with a 6V6GTA you want 315 volts on the plate, a OT with a 8k impedance and the max idle current should be .034 amp!

tsutt · Post by **tsutt** » Tue Jan 20, 2015 2:28 pm

Stevem wrote:The RCA tube spec states that for class A with a 6V6GTA you want 315 volts on the plate, a OT with a 8k impedance and the max idle current should be .034 amp!

And your at 415v and .045a isnt that kind of hot. This is why i asked what what real world is.

And i did not account for the screen. i will try that. Thanks

billyz · Post by **billyz** » Tue Jan 20, 2015 3:09 pm

This guy does a good job of explaining it . Of course Martin does too. But why reinvent the wheel .

http://www.ampbooks.com/home/classic-ci ... power-amp/

tsutt · Post by **tsutt** » Tue Jan 20, 2015 5:47 pm

Billyz, Im not sure what that is saying. me and books dont get along to well. i have to SEE and DO to learn.

Isnt screen voltage supposed to be less than plate voltage?

billyz · Post by **billyz** » Tue Jan 20, 2015 6:45 pm

tsutt wrote:Billyz, Im not sure what that is saying. me and books dont get along to well. i have to SEE and DO to learn.

Isnt screen voltage supposed to be less than plate voltage?

yes, The Fender 5F1 schematic shows a 10k resistor dropping to the screen. So it would be lower.
that page merely showed and explained how the champ arrived at its design. But , yes the screen supply is drawn wrong. Look at the Gibson single ended too.

A quick and dirty way is the measure the voltage on top of the cathode at the pin. maybe 19vdc ? Then measure the voltage at the plate, 344 vdc ?
subtract the the cathode voltage and you have 325vdc. 14w/ 325 equals 100% dissipation for the tube. or 43ma. forgetting the screen current of maybe 5-10%.
I like to measure at both places , because I can attached the black lead to the chassis and just probe around with the positive lead.

So if you get 43ma with out accounting for the screen then you are probably golden. 43ma -3ma for the screen is 40ma, or 93%.

I could be wrong about all this , but it works for me in practice. Of course this assumes a proper load. Champ maybe 8K ? and the speaker load is reflected to the power tube by the output transformer.
I would assume since this was a kit and a clone of the champ, the parts were chosen to copy the original fender design. .

tsutt · Post by **tsutt** » Tue Jan 20, 2015 7:21 pm

I looked up the print on the ot its 5k or 8k so I will have to check how its wired.

billyz · Post by **billyz** » Tue Jan 20, 2015 7:46 pm

tsutt wrote:I looked up the print on the ot its 5k or 8k so I will have to check how its wired.

Probably to be able to use an 8ohm speaker. Originals were 4ohm .

tsutt · Post by **tsutt** » Tue Jan 20, 2015 8:38 pm

yep its 8k and its got 4 or 8 ohm tap. I built it 5 years ago but never really liked it didnt sound good so i have started going through it and found a couple errors i made and some tweaking its sounding better i just want to do things right and obviously i dont understand some of this mojo you guys know.

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