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Old 8th September 2009, 03:17 AM   #11
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Originally Posted by SY View Post
Nice one, Pete, despite the lack of screen drive. Looks like you've got some new software to replace AudioTester- what is it?
Just posted it... here:

http://www.pmillett.com/hp_8903_software.htm

Home-grown!
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Old 8th September 2009, 02:00 PM   #12
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I figure I'd whoop up one of these cause I've got almost all the parts already, and then plenty extra for tinkering with Tubelab's screen drive design too. I've already got 6GV5's, and some iffy 6CB6's wanting for action.

Are there any complications or limitations in using higher uF caps in positions C3, C6, C25, ,C27?

Last edited by whitelabrat; 8th September 2009 at 02:14 PM.
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Old 8th September 2009, 08:33 PM   #13
vladn is offline vladn  United States
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Hi Pete,

Using stabilized G2 voltage with unstabilized bias voltage may cause significant bias current variation if mains voltage varies. Wouldn't it be safer to use stabilized bias as well ? (incremental complexity and power dissipation in bias regulator can be very small)
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Old 8th September 2009, 09:53 PM   #14
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Are there any complications or limitations in using higher uF caps in positions C3, C6, C25, ,C27?
I don't thinks so, should be fine.

Pete
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Old 8th September 2009, 09:57 PM   #15
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Hi Pete,

Using stabilized G2 voltage with unstabilized bias voltage may cause significant bias current variation if mains voltage varies. Wouldn't it be safer to use stabilized bias as well ? (incremental complexity and power dissipation in bias regulator can be very small)
Maybe so. I did think about it but decided to leave it unregulated.

The benefit of keeping it unregulated is that if the line voltage goes up, bias gets more negative so plate current drops... so you don't risk exceeding the plate dissipation rating. That allows you to more safely bias it close to the limits.

Pete
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Old 8th September 2009, 09:57 PM   #16
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Lovely use of those "useless TV tubes". This scores a 10/10

Cheers!
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Old 8th September 2009, 09:58 PM   #17
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Hi Pete,

No compactron this side of the ocean !
Should I bring a big box full to ETF?

Seriously, I could... they are cheap!

Pete
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Old 12th September 2009, 02:33 AM   #18
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I've dug through the great box of parts and I'm doing pretty well. I've got a burning question though. Why 1k carbon composition? I've got a mess of 1k's but are all metal film, but only 2 CC 1k's from "the shack".
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Old 12th September 2009, 03:37 AM   #19
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Why 1k carbon composition?
The CC's are grid stoppers. Their purpose is to be lossy (resistive) all the way from audio up to high (RF) frequencies - most metal film resistors are inductive, and can actually form a tuned circuit and promote oscillation at HF.
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Old 12th September 2009, 04:14 AM   #20
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The CC's are grid stoppers. Their purpose is to be lossy (resistive) all the way from audio up to high (RF) frequencies - most metal film resistors are inductive, and can actually form a tuned circuit and promote oscillation at HF.
Carbon films are less noisy and I've measured 1.2:1 SWR at 470MHz with a 2-watter. More than enough HF range for audio

Metal oxides (the Matsushita power types) are pretty good too.

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