Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Paramount => Topic started by: jlschweiger on November 05, 2011, 05:59:22 PM
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Still wet behind the ears here, should I be checking the driver tube bias every so often as a "check up" or does this only need to be done if the driver tubes are replaced? Really enjoying the Paramounts...
Thanks for any replies!
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No checkup necessary. The only indicator of needing a new tube is if the one in the amp sounds like it is worn out, i.e. the overall sound is getting flat. Even if a tube is getting long in the tooth sonically it won't hurt anything.
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I'm considering trying some tube rolling with some WE396A's as the driver tubes.
Would I need to check/adjust the bias if I substitute these for the original driver tubes?
Thanks!
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Yes, the bias should be adjusted when a new tube is substituted. I would check it again after about 100 hours - it probably won't have drifted, but if it does, adjust and check in another 100 hours. After it's stable, most tubes will go for years with very little drift. But each tube is different - even the same brand, date, and type - so re-check when you change tubes.
I think you want to be within 5% if you are set up for 2A3s; the 300B with cap coupling can be off by 10% with little audible effect.
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Mr. Joppa,
Thank you for your reply, Sir!
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Another question on the same topic.
If you change the output tube, do you need to readjust the driver tube bias?
Deb
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Since this came up with Paully recently, the soft start manual says to turn on the amp and wait for the tubes to glow but doesn't say allow it to warm for 15 minutes before setting the bias. I take it that the B+ is stable as soon as the tubes conduct.
Is that right?
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@Deb, the output tube - 2A3 or 300B - has no effect on the driver operating point because the grid draws no current. So changing the output will not have any effect on the driver and no adjustment is needed.
@Grainger, once the tube is conducting the shunt regulator kicks in and the power voltage for the driver is stable. But more relevant is that the driver is loaded with a current source, which (once the soft start has kicked in, about 30 seconds) sets the current, no matter what the supply voltage might be. In our experience with 5670 tubes, they stabilize quickly and further heating (as the envelope and internal parts temperature stabilizes, maybe 30 minutes?) has no detectable effect on the bias needed to obtain a stable plate voltage. But it's a reasonable precaution to re-check the bias after 30 to 60 minutes in case your particular tube in not as stable as the half-dozen we tried. Especially if you have changes out the tube for another type...
Again in our experience the 5670 does not drift, so the 100-hour check showed no change. But since many tubes these days are showing up with incompletely formed cathodes, I recommend the check anyhow.
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Thanks, that is exactly what I needed to hear.