[ARC5] Smart People: Triode Connecting Pentodes

[Geoff]

The TZ 40 at a mu of 62 is about the lowest Im aware of.

I wonder if a 304TH in AB1 would qualify at reduced HV? I believe 
audiophools run them in Class A.

Carl


----- Original Message ----- 
From: "Fuqua, Bill L" <wlfuqu00 at uky.edu>
Cc: "ARC-5 List" <arc5 at mailman.qth.net>
Sent: Thursday, November 14, 2013 12:25 PM
Subject: Re: [ARC5] Smart People: Triode Connecting Pentodes


>  You know, I can't think of any actual (real) low-mu zero-bias triodes. 
> Not a one.
> But the article indicates this configuration in grounded grid produces 
> much less distortion.
> But the question is, does the also apply when in grid driven 
> configurations?
> Grounded grid reduces distortion due to the negative feedback from the 
> input impedance
> in the cathode circuit.
> 73
> Bill wa4lav
>
> ________________________________________
> From: arc5-bounces at mailman.qth.net [arc5-bounces at mailman.qth.net] on 
> behalf of Fuqua, Bill L [wlfuqu00 at uky.edu]
> Sent: Thursday, November 14, 2013 12:16 PM
> To: kgordon2006 at frontier.com
> Cc: ARC-5 List
> Subject: Re: [ARC5] Smart People: Triode Connecting Pentodes
>
>   An very interesting connection used with the "forbidden" tubes for the 
> hi-mu connection is
> to connect the control grid to the cathode and apply signal to the screen 
> grid. This produces
> a very odd result. A low-mu zero-bias triode.
>  You can make a nice grounded grid amplifier with 4CX250B tubes this way 
> but it requires
> more drive since the power gain is only around 5 or so. But just about all 
> the input power
> goes straight to the output and the screen dissipation is practically 
> zero.
>  This is mentioned in one of Bill Orr's articles on super-cathode driven 
> amplifiers.
>  http://mikea.ath.cx/eimac/eimac08.pdf
>  This triode configuration with the right tube will allow you to operate 
> class A with positive
> grid bias and very little grid current. But the low screen current is due 
> to the geometry of these
> 4X and 4CX tube where the screen grid elements are in the shadow of the 
> control grid elements.
> 73
> Bill wa4lav
>
> ________________________________________
> From: Kenneth G. Gordon [kgordon2006 at frontier.com]
> Sent: Thursday, November 14, 2013 12:06 PM
> To: Fuqua, Bill L
> Cc: ARC-5 List
> Subject: Re: [ARC5] Smart People: Triode Connecting Pentodes
>
> On 14 Nov 2013 at 16:39, Fuqua, Bill L wrote:
>
>>    I believe by connecting the grids together you produce a high mu
>>    zero bias tube.
>
> Yes. According to Bill Orr in a document he wrote for Eimac, connecting 
> the
> grids together results in a hi-mu triode, while connecting the screen to 
> the
> plate results in a low-mu triode.
>
>> 813,803, 4-400, and many others are done this way for grounded grid
>> applications.
>
> Yes, but as Bill points out in that article I mentioned, SOME tubes don't 
> work
> well in that condition, while others do. He lists several of each.
>
>> Bill Orr wrote a number of articles on this.
>
> As I said, published in an Eimac document which is available as a PDF on
> line somewhere.
>
>>   The down side is that this technique does not work well for class A
>> or near class A operation since the grids have to have a positive bias
>> on them to get the needed quiescent current.
>
> Makes sense
>
>> And the grid supply has to
>> provide considerable current.
>
> Again according to Orr, in tubes such as the 4X150 and 4CX250 family, even
> operating in Class AB2, grid current can be as high as 3 or 4 AMPERES !!!!
>
>>   I once used four 6AG7's in grounded
>> grid as an amplifier following my CE 10A.
>
> I remember that circuit. The CE-20A uses two 6AG7s as linear amps. They
> work well.
>
> Ken W7EKB
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