Transformer shunt bias adjustment

[dwhutchens]

I'm biasing a pair of EH 6v6 tubes. I'm using a Fluke 8012a bench meter. I connected it between the center tap and each side of the OT. On one side I get 24ma and the other reads 26ma. At that bias setting I have 414vdc plate voltage and -33vdc bias voltage. I'm calculating about 9.9 watts on one tube and 10.8 on the other. Is it that simple or am I missing something. What is the maximum wattage I should set these tubes to?
Thanks,
David

[pdf64]

Yes it is that simple, just check that your wall supply voltage also, to ensure that it isn't unusually high / low, which would give you misleading results.
For max wattage, use 70% of max rating ie 8,4 watts, see Aiken for rationale. Unless you prefer the sound hotter or cooler, it's your call.
Peter
http://www.aikenamps.com/Why70percent.html

[Tom Phillips]

Yes. It's that simple and your bias setting is within an acceptable range.
The final proof is the sound. If you like it at the current setting I would leave it as is.
Maximum rated plate dissipation for a 6V6 is 14 Watts. (But some tubes were rated at 12)
You are at an average 74% of max which is not uncommon.

Warning: Asking questions about bias settings is sur to get you lots off different opinions.

Regards,
Tom

[tubeswell]

Never had any experience with EH 6V6s, but the New Sensor "Tung-Sol" RI 6V6GTs seem to be able to handle about 78% okay (touch wood).

[dwhutchens]

I sounds pretty good at that setting. I can't find a good spot in the chassis for a bias pot so I had a pot clipped in with alligator leads. It was set at about 25k when I took it out so I had to put a couple of resistors together parallel to get close. I wound up with -34vdc so its a shade cooler now. I ran out of time this afternoon (kids football game) so I didn't get to listen to it.
David

[tubeswell]

I ran out of time this afternoon (kids football game) so I didn't get to listen to it.
David

Ah yes - I know that feeling. Well if you come back to it later you'll probably here something different again. My ears hear things differently at different times of the day - Like I quite often don't hear my wife telling to to take the kids out if I am busy testing out my tweeks ;-)

[Enzo]

DOn't get into a numbers game. Bias is not nearly as critical and so many people make it out to be. 70%, 78%, 65%, whatever. Nothing in these amps is critical or high tech. If your balance is off a couple mas or your % is off a few points, don;t sweat it.

Think of tire pressure in your car. You'd likely notice if one tire was at 10lb and the other three were at 34lb. Or you'd note a difference if ALL the tires were at half pressure, or at 70 pounds. But if the spec said 34 pounds and yours were at 33, do you think you'd notice anything at all? Or if one or two tires area couple pounds different from the others? Assuming you are not a professional race driver.

There is a broad range of perfectly reasonable and proper bias settings. And not everyone is concerned with power tube breakup, or for that matter screaming overdrive. A lot of the differences in bias setting show up the most when the amp is pushing hard. If you play mostly on 4, then it matters less. In my opinion.

[dwhutchens]

I like to play on 12 quite often. I just want to make sure I don't melt my tubes

[Amp Kat]

I like using those little small trim pots for bias supplies because you can put a dab of silicon and practically stick them anywhere. I use the solder to hold them in tight and then the silicon underneath for insulation. You don't need anything that is big or requires power because there hardly is any power and the current is extremely low.

[dwhutchens]

Thanks, I may give that a try if Radio Shack has a value I can use.