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#26 pre amp
#26 pre amp
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Old 1st July 2011, 11:35 AM   #1061
andyjevans is offline andyjevans  United Kingdom
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Just a quick word about batteries for grid bias. I got some A23 batteries to try out. These are 12v and quite small - 28mm long and 10mm diameter. Users of battery bias may find these more useful than the usual PP3 at 9v, though clearly those are widely available and have a longer life. Anyone tried A23 batteries or can suggest if they are powerful enough for the job?

Energizer A23 Pack of 2 Batteries: Amazon.co.uk: Garden & Outdoors

They fit N size battery holders
N Size Battery Holders : Battery Boxes : Maplin

Andy
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Old 1st July 2011, 11:53 AM   #1062
andyjevans is offline andyjevans  United Kingdom
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Actually, I found a 15v battery, 45mAh:

VARTA|V74PX.|BATTERY, 15V | Farnell United Kingdom

Haven't looked for the battery holder yet.

This raises the possibility of running the 26 at 180v and 15v bias, which would be about 5mA. I'm starting to wonder if that could be used as a driver tube for an output like a 2a3 or even a 300b!!

Andy
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Old 1st July 2011, 02:49 PM   #1063
coolzero is offline coolzero  Sri Lanka
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Thanks Andy. You confirmed my observation with CX-326 plate curves. I was little in doubt whether I did it right.

Rod, thanks for your information. This is something new for me to learn. BTW, I tried different tubes with similar plate/bias voltage figures but scenario was almost same. Anyway I will test the few of these tubes for gas as you described and report.
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Old 1st July 2011, 02:57 PM   #1064
coolzero is offline coolzero  Sri Lanka
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9V Energizer worked just fine for me. Hope this will give good result as well. Since I have switched my bias supply to SMPS, it is much easier now to control bias voltage across wider range making use of a pot.
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Old 1st July 2011, 03:02 PM   #1065
Rod Coleman is offline Rod Coleman  United Kingdom
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#26 pre amp
Kanishka, are you sure that the SMPS bias is not generating the noise?

Many SMPS operating from the mains line can show leakage current from L to safety Earth. Even if the ripple on the output is low, you may get noise from this current leakage.

Comparing with a battery will prove quickly if this is worth looking into.
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Old 1st July 2011, 07:33 PM   #1066
coolzero is offline coolzero  Sri Lanka
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Quote:
Originally Posted by Rod Coleman View Post
Kanishka, are you sure that the SMPS bias is not generating the noise?

Many SMPS operating from the mains line can show leakage current from L to safety Earth. Even if the ripple on the output is low, you may get noise from this current leakage.

Comparing with a battery will prove quickly if this is worth looking into.
Thanks Rod. This make sense. I will change the bias supply and see. I remember when I used battery before, it was less noise. BTW, is there an easy way of getting rid of this leakage from SMPS? If there is, it will reduce my time to build a different PSU.

Last edited by coolzero; 1st July 2011 at 07:47 PM.
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Old 1st July 2011, 09:04 PM   #1067
Rod Coleman is offline Rod Coleman  United Kingdom
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#26 pre amp
For bias, you need only a tiny current, so unregulated RC-filtered supply works fine.
Maybe a shunt regulator if you want to ride first class.

Some SMPS leak more than others, & it's hard to predict a good one. I don't know of any reliable fix for the leakage.
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Old 2nd July 2011, 06:25 AM   #1068
coolzero is offline coolzero  Sri Lanka
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Thanks Rod. I'll try "SALAS" LV probebly.
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Old 2nd July 2011, 12:00 PM   #1069
merlin el mago is offline merlin el mago
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Good choice coolzero.
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Old 2nd July 2011, 01:52 PM   #1070
andyjevans is offline andyjevans  United Kingdom
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Just a quick observation - I tried the globe 26 against the ST version, and found I preferred the globes. Slightly more definition in the mid and treble. Anyone else have views?

andy
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