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Old 5th May 2009, 02:28 PM   #1
00940 is offline 00940  Belgium
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Default Scavenging a broken oscilloscope to turn it into a preamp ?

I've a dead Heathkit oscilloscope on hand. Geek quickly convinced me it wasn't really worth fixing.

Still, inside the dead beast, we have the following tubes that could be useful:
3x 12AU7
1X EZ81
1X 6AN8

There's also a good transformer, with strong 330-0-330 and 6.3-0-6.3 secondaries (among others).

Here's the idea I came up with. It's a simplistic preamp based on ideas from Tubecad, the Janus shunt regulator and the CCDA preamp ( http://www.tubecad.com/2007/08/blog0117.htm and http://www.tubecad.com/2009/03/blog0161.htm among others).

As the preamp has nearly constant current draw, the shunt regulator doesn't have to work too hard. It's set up to draw around 7ma. The rest of the power supply will be : ez81-330r-20uf-330r-20uf (I've got lot of voltage to burn...). This gives, according to PSUD, around 306VDC and some 3V of 100hz ripple for the regulator to deal with. All the 20uF caps will be 400V MKP.

I've got some questions though...
- Should I change the way the first tubes are biased, from resistors to Led ?
- Is it really necessary to pass to DC heaters or can I simply use AC ?
- Is it worth my time to build such a thing or would it sound so-so ? (it is for a friend who asked me to build him a preamp).
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Old 5th May 2009, 03:43 PM   #2
pointy is offline pointy  United Kingdom
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may be keep the transformer for something else and use in its place back to back 12v transformers with a small transformer for the heaters. but the ez81 and the ecc82s should go well in a preamp and it should look good to.
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Old 5th May 2009, 04:07 PM   #3
pointy is offline pointy  United Kingdom
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or.......you could put a big light bulb in the place where the ray tube was and write 'POWER ON' over the end of the bulb.
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Old 5th May 2009, 04:33 PM   #4
00940 is offline 00940  Belgium
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Actually, I don't think I'm going to re-use the enclosure of the oscilloscope, it's huge (see : http://www.heathkit.nu/IO-12_1968.jpg ).

Another question... Couldn't the 200k R9 be replaced by a CCS ? (with something like three 2n5401 cascoded)

Thanks in advance for any help
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Old 6th May 2009, 01:41 AM   #5
pointy is offline pointy  United Kingdom
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it should be ok with the resistor any how.............

ie . get it working good and then play later on
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Old 7th May 2009, 02:52 PM   #6
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Ben,

The 12AU7/ECC82 has (IMO) a deserved unsavory reputation as a voltage amplifier. Do you need gain in the line stage? The 'U7 is OK as a cathode follower and the 12BH7 usually drops in for voltage amplifying, as long as sufficient heater current is available.

Consider a choke I/P filter. IMO, it is a much better way to dispose of unwanted Volts. LESS waste heat is generated!
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