Bias on a Champ Amp

[LooseChange]

I am confused. I  notice on a Champ amp the bias is always around 42ma. This alway seems high but they are all like this. Running very hot. Am I missing something about Cathode Bias? Should I change the Cathode Bias resistor?

[HotBluePlates]

Flesh out your question some. Should you change the cathode resistor which way? And why does 42mA seem high to you?

It seems to me that if you have a 14w tube, 42mA on the plate implies ~330v plate to cathode, and with a 470-500 ohm resistor you have another 19-21v from cathode to ground (plus a minimal screen current drop). So it seems like the whole shebang was designed for a 350v B+ (not counting any voltage drop across the OT).

But the thing is hottest while sitting idle. At full-roar, there is additional power being pulled from the supply, but it's very, very little extra in class A. And everything going to the speaker reduces the amount of power being dissipated by the plate.

As time went on, the B+ climbed, cathode R stayed the same. Did the tube actually draw more current, or is the cathode voltage marked on the schematic a hold-over from previous schematics? I don't know. But I haven't seen a red-plating 6V6 in a Fender-built Champ yet.

You can always raise the cathode resistor value if it makes you feel better, but it takes a fairly big change to get any reduction of current. Like going to 650+ ohms just to reduce idle current a few mA's. I have not tried the adjustable cathode bias (shown by O'Connor maybe?) that has a pot running from the cathode resistor to G1. I don't know if that would have more action to it.

[LooseChange]

I guess I don't know my stuff when it comes to Cathode Bias.  I have a 1ohm bias sense resistor in there and I took the measurement.  My bias chart says bias at 375v is 22ma for a 6V6 at 70% dissipation.

[phsyconoodler]

With a Class A single ended amp you can run at close to 100% dissipation because as HBP said,the current stays basically the same at idle to full throttle.
  Does the amp sound good?Yes? Leave it alone.No?Then start looking at why.

[Fresh_Start]

LC - you know cathode bias fine but this is a single-ended amp as physco said.  Runs Class A by definition.  100% of maximum plate dissipation at idle is fine because it actually drops when you apply a signal to the amp (OT draws more current for the speaker and screen grid draws a little bit more too).

If you're talking Blackface Champ, 350 VAC on the plate is fine.  My experiments with a 5F2-A circuit lead me to want about 300 VAC like the 5E1 Champ.  Just a matter of taste.

Cheers,

Chip

[rafe]

I posted a couple of really informative champ sites when I got my 74 Vibrochamp. I was concerned because after I repaired the amp, the 6v6 was still running hot,  in one article the author states (I'm paraphrasing) the 6v6 tends to red plate a bit at idle and will do such for years... you can change out the resistor and extend your tube life or enjoy why we enjoy champs and change out power tube every few years ....I have been running mine a lot, as stock, slight redplate at idle......for months now with no harm. It's a small exotic racer meant to be run in the redline IMHO  

 http://archive.ampage.org/threads/1/gagd/032174/Vibro_Champ_single_ended_6v6_questions-1.html

that is the article I mentioned

[HotBluePlates]

One slight correction I would add to the linked post:
The value of idle current is determined by the cahode resistor and tube transconductance. Different tubes will settle into a different level of idle current with a given cathode resistor, so that some might tend toward overheating and others will be fine.