Single tube linestage schematic wanted: 8CG7 - diyAudio
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Old 14th April 2012, 12:23 AM   #1
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Default Single tube linestage schematic wanted: 8CG7

Recently done a few builds of various linestages using pre-fabricated pcbs and thought it was high time to attempt a first point-to-point wiring job.

Does anyone have a schematic which uses a single tube (starting simple!) and would be suitable for the 8CG7 (I have a few of them cheap from ebay)? Datasheet here:

http://www.pentalaboratories.com/pdfs/6CG7.pdf

The power amplifier it will be feeding is a 100k input impedance push pull 6v6 tube amp with 3v sensitivity.
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Old 14th April 2012, 01:28 AM   #2
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You could do something very nice with CCS loaded common cathode gain stages cap. coupled to parafeed step down trafos. The excess gain from μ = 20 gets converted into good drive. The Magnequest model B7-15K is well suited to this scheme.

BTW, regardless of the scheme you settle on, construct a current regulated 450 mA. DC heater supply. You'll be hum free and not have to struggle with proper heater operation.
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Old 14th April 2012, 04:24 AM   #3
mach1 is offline mach1  Australia
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Any design which uses a 6SN7 (and there are heaps out there) will work. The 6CG7 is its nine 9 pin counterpart.
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Old 14th April 2012, 06:30 AM   #4
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Appear to be plenty with two tubes - Franks schematic, Aikido, etc, but can't seem to locate a circuit that uses a single tube.
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Old 14th April 2012, 06:52 AM   #5
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by lordearl View Post
.... but can't seem to locate a circuit that uses a single tube.

for a hifi line stage I suppose there is not other option than cathode coupled
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Old 14th April 2012, 06:56 AM   #6
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Quote:
Originally Posted by lordearl View Post
can't seem to locate a circuit that uses a single tube.

For the simple reason that this tube with plate resistance of 7.7k is really not appropriate to be used "as is". It needs either a follower, srpp or some other means to get reasonably low (<1k) output impedance.
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Old 14th April 2012, 07:44 AM   #7
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by lordearl View Post

The power amplifier it will be feeding is a 100k input impedance push pull 6v6 tube amp with 3v sensitivity.
ah, I missed that part, and maybe others have too

hey, talking about 6V6, theres also a 6V6 linestage
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Old 14th April 2012, 11:49 AM   #8
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What would the output impedance of a cathode coupled 8cg7 linestage be?
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Old 14th April 2012, 05:41 PM   #9
DF96 is offline DF96  England
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Roughly double that of a grounded cathode, unless you added NFB to the second grid.
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Old 16th April 2012, 02:00 AM   #10
mach1 is offline mach1  Australia
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Read this article.

GeeK ZonE - The Underappreciated Hybrid Mu Follower

There are many different types of ccs you can use to accompish the same thing, several of which are documented on this forum. If you have to use one tube this the only way you are going to get an acceptably low output impedance (unless you go down the expensive output transformer route).
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