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Biasing DFDR

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9/2/2005 6:05 PM
Brad	Biasing DFDR How much current is usual for SF Deluxe Reverbs? I realize it varies, but I've heard advice ranging from; 20-25 ma. and no more! to 30 ma. as long as plate voltage is under 425V. What range do folks out there end up with? Thanks, Brad

9/2/2005 7:08 PM

Forget about a plain current measurement. You need to take into account both the plate voltage and bias current to come up with an idle dissipation in watts. Any idle dissipation (that sounds good to you) that is in range of the tube's max operating parameters is OK. For a hardy 6V6 (like the JJ) I would not go more than 14W idle (my Champ runs fine like this) but in a DR, 9-12W would be OK. Higher idle = less tube life.

9/2/2005 10:18 PM

Thanks coledavis,

Is the max plate dissipation a function of the tube type/design? Same for any 6V6? Or is it highly dependent on the manufacturer/materials?
I've read that 60-70% of max plate dissipation is the target range but where can I find out what the max is? I assume it will vary by manufacturer?

In my first post above I wasn't looking for a specific current number to set it to, just the range that folks commonly see in those amps for a rough idea of whether I'm in the ballpark.

The person who sold me some NOS tubes for this amp is who told me not to go above 20-25 ma. But to my ears it sounds a little cold. But knowing these amps put a lot of voltage on the plates I want to be careful with those NOS tubes.

Brad

9/3/2005 1:42 AM

It all depends on how you want the tubes to die. Do you want them to arc internally due to the high voltage or do you want to blow them up with too much current???

Just kidding.

But I mean to illustrate a point. Some techs believe that the higher voltage is a greater threat than the current. Since a hotter bias will reduce plate voltage a little these guys will keep reducing bias voltage until the plate volts read below 425. But voltage seems to be less of an issue with modern 6V6 tubes, though I can't speak from experience, I've read it hear alot. You could just bias at 70% dis. and call it good. 70% of 14 watts is 9.8 watts. 425 volts/9.8 watts is about 23 to 24 ma. But that never works for me. What I do is just bias the amp so it sounds good. If the tubes red plate on me I try to find tougher tubes and repeat. I have one amp that really cooks the EL84s it uses, but they last just long enough to keep me from cooling off the bias.

If your amp is using a GZ34 rectifier tube you could try a 5U4 to bring the voltage down a little. Then you could increase current a little for a warmer tone.

Chuck

9/3/2005 7:54 AM

What Chuck said, find a bias point that sounds good to you that is in a safe range and stick with it. If the tubes sound cold at 25mA, what's the point, you get more tube life but the amp sounds lousy. I'd rather enjoy myself with nice tone and live with a little less tube life

I know the JJ 6V6S is rated to 500V plate - it's right there on their datasheet, and I know of tube dealers deliberately redplating the tube and running it for hours with no ill effects. Which is not to say you should do this, but I would not worry that much about killing a JJ 6V6S that easily. Like I mentioned before, I run one in a Champ at 14W idle (around 430V plate) and it has run like that, cranked up to 10 with the tone controls bypassed for months, no problems.

9/3/2005 8:05 AM
coledavis	Sorry that should have been 330V plate, 14W idle.

9/3/2005 1:44 PM
Brad	Thanks guys. I'm going to try some things with it this weekend includung setting it just by ear, then taking measurements to see where they end up. I'm going to search for data sheets on the tubes I have as well..

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