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Old 20th January 2005, 02:41 AM   #1
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Default pre-amp with 6as7??

decided to use the 6as7 as a pre-amp tube.

should I

A) transformer load it... either SE or Parafeed SE
B) resistor load it.

btw, i need gain of 8-10 so either 2 needed... or?

thanks for putting up with my indecisiveness, as my current pre just doesn't have the "guts"... its a EL84 (triode mode) with plate chokes...

Thanks for ANY help.
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Old 20th January 2005, 03:09 AM   #2
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The 6AS7 has m=2, so the stage gain of a single 6AS7 cannot exceed 2. You could try finding a 6528, which is similar (designed as a series-pass device), and it has m=9, but terrible input capacitance.

Here is a preamp by Pete Millett, using several low-mu valves (including 6AS7) with an active load using EL34s.
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Old 20th January 2005, 10:58 PM   #3
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Default Re: pre-amp with 6as7??

Quote:
Originally posted by AudioGeek
A) transformer load it... either SE or Parafeed SE
Sorry, I missed this bit yesterday.

Of course you can transformer load it (assuming your pocket can stand it ), though you will need around a 1:4 turns ratio to achieve a gain of 8. This means that your output impedance has will rise from around 280W to 4.5kW.

Also your inductively loaded EL84 triode preamp will have an input capacitance of the range around 10pF, while an inductively loaded (choke/transformer), or CCS loaded 6AS7 will have an input capacitance more than twice that, around 22pF.
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Old 23rd January 2005, 09:55 PM   #4
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Maybe try 01 / 01A / 01B / 01C?

it is an old ST or globe UX4 DHT, =8 ; if i recall.

Not sure if they are so cheap anymore, kind of ancient.
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