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Old 25th February 2008, 09:10 PM   #21
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Look at the "toroidal amps" thread and you'll see two different implementations of the bias I described. Unfortunately it most definately places some woping big caps in the signal path. However I think your fear is a little overstated - but thats just my opinion. I suppose its a good thing to aim for though.

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Old 26th February 2008, 12:27 AM   #22
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How much are NOS 6B4G's?
I've got about 20 or so originally boxed, un opened US Army NOS 6B4G's in my collection, so I'm ordering a pair of Hammond 1615's (5kCT primary) with my 1650W's (for another project) to make a ~7W push pull class A.
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Old 26th February 2008, 02:20 AM   #23
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Quote:
I've got about 20 or so originally boxed, un opened US Army NOS 6B4G's in my collection
You might want to open one box and look carefully at the tube itself. There were some "6B4G's" made by Sylvania or Phillips that were actually re pinned 6AV5GA's. A true 6B4 is DHT. You should see a filament and one grid. Most are of the twin plate variety similar to a 2A3. Sylvania made some "6B4's for the US Army that contained 6AV5GA guts. They have straight sided glass, and contain a beam pentode that is triode wired. I have "tested" one of these and they don't sound half bad, and seem to function like a 6B4 should. I ran mine in SE mode and had no problem getting 7 watts out of one tube, but I might have ignored the ratings a bit.
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Old 26th February 2008, 02:32 AM   #24
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Originally posted by G.Kleinschmidt
How much are NOS 6B4G's?
I sold a used pair for $80. So, not cheap Depends on brand though.

Cheers!
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Old 26th February 2008, 03:52 AM   #25
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Quote:
Originally posted by tubelab.com


You might want to open one box and look carefully at the tube itself. There were some "6B4G's" made by Sylvania or Phillips that were actually re pinned 6AV5GA's. A true 6B4 is DHT. You should see a filament and one grid. Most are of the twin plate variety similar to a 2A3. Sylvania made some "6B4's for the US Army that contained 6AV5GA guts. They have straight sided glass, and contain a beam pentode that is triode wired. I have "tested" one of these and they don't sound half bad, and seem to function like a 6B4 should. I ran mine in SE mode and had no problem getting 7 watts out of one tube, but I might have ignored the ratings a bit.

Iím reasonably sure theyíre of the curved bulb variety. Not sure about the brand. Iíll have a look tonight if I get the time.


Quote:
Originally posted by tubelab.com
I ran mine in SE mode and had no problem getting 7 watts out of one tube, but I might have ignored the ratings a bit.

LOL. Just a little.


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Originally posted by Geek
I sold a used pair for $80. So, not cheap Depends on brand though.

Cheers!

Not an economical alternative to new 2A3's then.

Cheers,
Glen
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Old 26th February 2008, 07:56 AM   #26
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OK, the 6B4G's are a mix of Westinghouse, Ken-Rad and Sylvania. They all look internally identical to the NOS pair of RCA 6A3's I've got.
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