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Old 5th January 2004, 12:19 AM   #1
AKSA is offline AKSA  Australia
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Default 6ha5

Folks,

Has anyone ever used a seven pin, sub-miniature 6HA5? I have four I'd love to try out; 1963 tube, GE, branded 'Sears'.

This tube is a 14.5mA/V, mu 72 12mA single triode for use in VHF as a grounded grid amplifier. Plate dissipation is 2.6W.

Anyone know what the sonics are?

Cheers,

Hugh
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Old 6th January 2004, 12:40 AM   #2
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Why don't you give 'em a try? Data sheet: http://frank.pocnet.net/sheets/135/6/6HA5.pdf

I have a couple of dozen myself, 6HA5, 6HM5, 6HQ5 all seem to be close equivalents - some are marked with more than one number.
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Old 6th January 2004, 12:46 AM   #3
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Hi,

Quote:
Anyone know what the sonics are?
Similar to 6GK5.

Cheers,
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Old 6th January 2004, 12:47 AM   #4
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When selected, darn nice tube for a phono input stage. Works pretty well with medium-output MCs. The poor man's 417a.
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Old 6th January 2004, 01:24 AM   #5
AKSA is offline AKSA  Australia
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Thanks Guys,

Just set it up on the workbench.

With B+ of 280, 136V on the plate and 11.75K plate load (12.2mA), cathode resistor/cap is 150R//470uF. In this configuration it will easily pull 160Vpp output with 4.5Vpp input. 3dB points are 4Hz and 140KHz, square wave battlement to about 15KHz.

For a stated mu of 72, this is an Av of just 35.5, disappointingly low, I'm quite surprised.

Really need more voltage gain. Might have to move to the 6GK5.

Cheers,

Hugh
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Old 6th January 2004, 03:41 AM   #6
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I'm not suprised, you're encroaching mighty close to Rp. It's what, 5k? (Rp = mu/Gm) I'd guess less current.

Tim
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Old 6th January 2004, 06:09 AM   #7
AKSA is offline AKSA  Australia
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Tim,

This is not a particularly linear tube, designed for grounded grid CRT VHF duties. It was running -1.77V of bias at 12mA with a cathode resistor of 150R.

I find a convenient way to figure voltage gain for audio tubes with bypassed (common) cathode at Class A plate currents is quite simple.

Divide the plate load by rp. In this case, 11.75/5 gives 2.35.

Now, multiply the published mu by 2.35/1 + 2.35. This factor is around 0.7, so Av should be around 50. It isn't, it's quite a bit less. This, I feel, is a comment on the linearity of the tube, and perhaps the fact that it's really not designed for common cathode.

It's a real pity, because they are dirt cheap, and run up to 22mA in a tiny package 3/4" in diameter and 1 1/4" long.

Ah, well, I'll try the 6GK5 next........

Cheers,

Hugh
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Old 6th January 2004, 12:11 PM   #8
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The 6GK5 may give you similar results. Why not try them in grounded grid?
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Old 6th January 2004, 12:38 PM   #9
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Default UHF triodes

Hope you have more luck with these things than I did . A few points to note with most UHF triodes :

Device inconsistency between samples
Rarely meet book spec for gm
Microphony !

Others to try are PC900 and EC97 . Why not go the whole hog and look for some 416B ? These have stupendous gm and mu of about 250

316a
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Old 7th January 2004, 12:06 AM   #10
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Hi,

Quote:
Why not go the whole hog and look for some 416B ? These have stupendous gm and mu of about 250
Sorry to put a cold blanket on this but they're even more problematic IME.

Cheers,
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