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Old 17th May 2008, 04:29 PM   #1
wicked1 is offline wicked1  United States
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Default 300b-98c mesh bias

Hello,
Im trying to find the proper bias point for the valve art 300b-98c mesh plate valves.

I've found articles saying to bias the mesh plates low, but that was specifically talking about the TJ's I think...
I found an article saying these VA 98c's can take the full 40 watts because of their titanium plate.
I found another article saying to bias them at "1.2 volts" and Im not really sure what they are talking about, because everything I've learned so far talks about milliamps when biasing.
Then if the person writing that has their amp set up like the tubelab, 1.2 volts would be 120ma, which is WAY high!

so, does anyone have any advice for this? I have them at about 65ma right now, and they are missing a little of the power I had w/ the solid plate tubes. (which were biased at about 80ma)

thanks!

(maybe I should mention b+ is 375v before it goes through the transformer and a resistor, so is probably more like 365v at the tube)
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Old 17th May 2008, 05:32 PM   #2
kmtang is offline kmtang  Canada
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I believe the amplifier that you have is with fixed biasing for the 300B tube, right?? Is it Single Ended or Push Pull type?

I have a pair of this Shuguang 300B-98C myself and running them in single-ended mode.

It sound rather nice with B+ of 405V with auto bias with 880-ohm cathode resistor bias.


Johnny
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Old 17th May 2008, 05:55 PM   #3
wicked1 is offline wicked1  United States
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single ended, and it's the bias I set w/ a pot. (Tubelab SE design)
I'm not sure if that's what is called fixed bias or not. Doesn't seem fixed to me, because I can change it to whatever I want.

So, with the way yours is set up, have you ever calculated how much power you're pulling through the tube? How many watts?

thanks
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Old 17th May 2008, 09:15 PM   #4
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by wicked1
I'm not sure if that's what is called fixed bias or not. Doesn't seem fixed to me, because I can change it to whatever I want.
Yeah, the terminology is a bit confusing. Fixed bias, means fixed relative to common. For AC inputs, the grid to cathode change exactly follows the signal. It doesn't depend on tube characteristics, or plate voltage. Yes, you can set up the amp so that it can be easily changed, but once set it remains fixed, even if you change tubes.

Cathode, or auto bias, is set by the current through the tube, and can change with tubes and B+ changes. If the cathode resistor is not bypassed, the grid to cathode voltage is attenuated some (cathode degeneration - a sort of negative feedback).

Sheldon
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Old 18th May 2008, 06:32 AM   #5
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I figure it's about 7W that he's putting out.

Fixed bias means that the negative bias voltage is set on the grid of the tube, while cathode bias is set on the cathode.

DanL
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Old 19th May 2008, 01:43 PM   #6
wicked1 is offline wicked1  United States
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I'm wondering how much power the tube itself is burning. Not the output audio power. It's got to be something between 20 and 40 watts.
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Old 19th May 2008, 02:00 PM   #7
kmtang is offline kmtang  Canada
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With 375V B+ and 65mA idle current, the DC dissipation power is 24.4W. The output power is just the same when you set the idle current to 80mA. The DC dissipation power is increased to 30.0W which is still within the max power limit of 36.0W from the sepecification of WE 300B.

By changing the bias current, the sound character will change because of the differences in transfer characteristic. Therefore, it is a bit of fun to try out what's the idle current will give you the sound you most like.

Johnny
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Old 19th May 2008, 02:37 PM   #8
kevinkr is offline kevinkr  United States
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Actually my data sheets for the WE 300A/B indicate that the max plate dissipation is 40W..

Also I have found that the maximum power for a given distortion % increases significantly with increasing bias current. In my amplifier 60mA idle current at 400V is good for about 7Wrms at 5%, increase this to 80mA and for the same distortion % produces about 9Wrms. Note that this is somewhat dependent on the actual tube, and the actual numbers may vary somewhat from sample to sample. Note that in a sense I am splitting hairs as the minimum usable difference is 3dB and this is nowhere close.

The 5% number actually comes directly from WE and seems to be about the % thd where the distortion apparently (?) becomes audibly objectionable in 0 fdbk SE amplifiers. Certainly sounds objectionable when I run my amp that hard..

Interestingly there was a commercial Japanese design that ran a single 300B right to the limit and claimed 17Wrms output. IIRC there is a schematic for this in Jean Hiraga's "Tube Amplifiers" book. I bet tube life was less than impressive.

I have been thinking of trying the TJ mesh plate 300B in my amplifier at some point, replacement JJ are getting expensive enough that I am inclined to try something else at some point. I liked the WE 300B that I tried in this amplifier, but the small improvement in performance over the JJ did not justify the really steep cost difference and I did not buy them. Operating conditions in this amplifier are 400V, with fixed bias over a range of 50mA - 100mA possible. Comments?
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