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Old 26th May 2013, 10:59 PM   #1
billbo is offline billbo  Australia
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Default Simple low voltage tube preamp stage

Just though I would share this circuit which I quickly whipped up from some parts I had. It uses a 6418 tube driving into a source follower to the output. The circuit uses a little feedback for greater stability and can drive very a low impedance down to 1K ohm without any increase in distortion. Although the response is flat up to 35Khz (more like 70Khz) it seems to sound not as crisp and harsh on the highs and after listening for a while it is almost impossible to revert back to the op amp setup. The mids are highlighted and the highs are well presented but not over exagerated. I have only setup the gain at 2 but it has an open loop gain of only about 6. The feed back network can be omited if desired.
It is a very affordable solution if you want to try a tube output as a preamp
I invite all out there for your opinions about this.
Sorry for the poor schematic quality. I just quickly hand drew it.

Regards
Billy D...
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Old 27th May 2013, 12:40 AM   #2
DUG is offline DUG  Canada
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How is bias calculated with a directly heated cathode? Is your circuit considered biased at 0V or half of 1.2V?
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Last edited by DUG; 27th May 2013 at 12:44 AM.
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Old 29th May 2013, 12:34 PM   #3
billbo is offline billbo  Australia
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Default RE: biasing

A very good question indeed. So good I had to revisit my design. In escence the bias takes the average voltage that is across the filament (approx 0.6 volts). In real tests it showed to be about 0.1 to 0.3 of a volt. I considered this is too low and would pose a problem driving the tube positive. This is not a real problem if you can drive high current from the source but it is not recommended for audio. I did more research and redesigned the cct. to have more grid bias. (about 1 volt). The revised cct. has a 100 Ohm resitor which will give 1 volt bias and raised the supply voltage to 30 volts. This has greatly reduced distortion and has made the cct. more stable.
Once again sorry for the hand draw schematic.

Regards
Billy D...
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File Type: jpg 6418 buffer_2.jpg (379.1 KB, 146 views)

Last edited by billbo; 29th May 2013 at 12:56 PM.
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Old 30th May 2013, 03:38 AM   #4
DUG is offline DUG  Canada
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Glad I could help.



Schematic was readable...no reason to apologize.
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