Biasing

[Wyatt Turk]

Ok time travelers time for another brain teaser.

On the subject of auto biasing a valves output let me through this one at you.


to keep the circuit symetrical in functionality lets Bias the HT rail for the output valves.. What do you people think?.

I want to hear the grey matter crank on this one...

Wyatt

[SY]

I'm not quite following your question. Do you mean loading the tubes with a current source?

[Sch3mat1c]

I think he means using plate resistance to tweak plate current? Ack...

Tim

[SY]

Maybe. In an exchange of emails, it seemed to me that he was asking about using voltage regulation. But the question here seems to conflate regulation with control of idle current, so now I want to figure out what he really means.

[Wyatt Turk]

Calm down people!!

I was asking to regulate the high tension voltage rail say keeping it as for my example at 360v.

At this point the current has not much baring on the overall current as the valve runs at it's peak no matter what the current has been set to.

I have seen a few circuit that employ this theory but I was asking other peoples opions so that the overall toneage eg. does not suffer under constant load.

We in Australia are only starting to get back into valve equipment and for one they are rare, two not much has been done since the late 80's.

I started my electronics career in 1972 for a company called Kriesler Electronics that got eaten up by the Phillip's Corp. and people hated trasistor equipment "Boy would I like to take a bet then".....

I am building a proto type guitar amplifier around the 100w stage it's to be a cross between a Marshall and a Mesa Boogie more like a Marshall on steriods. Since valves eg are very expensive in Australia EL34 ave price is $35 each. I was interested in say placing a $20 circuit in my proto type to save on valves.


Cheers

Wyatt

[SY]

Well, regulating the HT rails won't do anything to control the idle current- without a proper biasing scheme, you'll end up with a puddle of melted glass where you used to have an output tube. Now, regulation can be a good thing as a separate issue. But if you're talking about control of idle current, you're down to deciding between a cathode resistor bias arrangement (so-called automatic bias) or a negative supply for the grid (so-called fixed bias).

[Sch3mat1c]

Also, guitar amps depend on bad regulation for distortion and sustain - you want stiff power supply rails for hi-fi.

Tim

[Enzo]

I agree, a regulated B+ is the opposite of what you need. The "sag" under load gives the amp sustain - helps it "sing." The tubes don't wear out from lack of regulated B+. I only buy matched sets of output tubes. If you want to use unmatched tubes you can bias each side separately.

In fact, many prefer the sound of the amp with a tube rectifier rather than solid state diodes. The voltage loss across the tube recto and its variance under load give a pleasing sag. Blues players like it. The screaming heavy death metal cats don't go for it as much.

Older Fender designs used a bias balance control, though the overall level was fixed. You adjusted the control to even up the sides. A lot of guys modify them back to a bias level control and just use matched tubes.

I am assuming you are running class AB. Class A doesn't change the current load the same way. It is already running ful out.

A good resource for making your own amps is www.ampage.org

(Sorry, I am not up on HTML and all that or I would make this a link)

And there are many resources and links once there.

[Enzo]

Cool, it made it a link for me, my other forums don't.

[Geek]

Wyatt,

Your question has actually given me ideas for great sound generators

Thanks!