[ARC5] AM linear amplification

[AKLDGUY .]

Hi Robert

That variation on the driven grids is interesting as I had never heard of
it. All the factors you mention are favourable. I'll keep that
configuration in mind.

73 de Neil ZL1ANM

On Tue, Dec 20, 2016 at 5:04 PM, Robert Nickels <ranickel at comcast.net>
wrote:

> On 12/19/2016 9:27 PM, AKLDGUY . wrote:
>
>>
>> Class B is interesting. It's not widely known that the 807 / 1625 is
>> capable of Class B zero bias operation, with screen grid connected to
>> control grid*. This configuration eliminates both the negative grid supply
>> and the screen supply.
>>
>
> Tying the grids together treats the tube like a triode.   There's another
> class B configuration in which the control grids are grounded and the drive
> is applied to the screen grids instead. This circuit also eliminates the
> bias and screen supply components, and was used by Gonset in the modulator
> for the G-77 mobile transmitter, with a pair of 6DQ6 sweep tubes as the
> modulator.   I got curious about it and did some research and found hat the
> circuit was patented in 1944 and assigned to Guided Radio Corp.   US patent
> number 2,358,148,  which was allowed 4 claims.
>
> One claim was based on the higher input impedance when driving the screen
> rather than control grids:
>
> "The increased impedance results in greater independence between the plate
> current and the plate voltage when excitation is applied to the control
> grid  and it is possible for the plate impedance to vary considerably
> without introducing serious distortion or loss of power. Also, a much
> higher plate voltage can be applied to these tubes without exceeding the
> limits of normal plate dissipation than is possible when the tubes are
> operated as beam power tubes."
>
> Another claim was based on longer tube life due to reduced heating and
> improved efficiency.
>
> The inventors also claimed the distortion levels in this circuit (using a
> beam power tetrode) are "substantially less than obtained with zero bias
> class B operation or with beam operation" and say the circuit is
> "substantially linear even at low input levels"... "similar to those
> obtained in the operation of class A triodes". I'd read that distortion at
> low signal levels was a limitation of the triode-connected tetrode
> amplifier.
>
> Around the same time I had a Multi-Elmac AF-68 on the bench and since I
> was replacing driver transformer anyway, I first wired it up as per the
> Guided Radio patent, with audio drive to the screen grids,  no bias,  and
> the control grids grounded.   It worked and sounded fine with equally good
> fidelity, but I wasn't able to get above about 85% modulation with the
> existing driver stage.   I put it back to the stock configuration but this
> proved that the grounded-grid tetrode class B amplifier is well worth
> exploring. The G-77 schematic is available on BAMA for anyone who'd care to
> have a look.   Others may have also used this circuit but it was a new one
> to me.
>
> By the way, Guided Radio was in the business of providing what were called
> "divers radios" which weren't radios at all but rather what we'd call an
> intercom for two-way voice communication between a tender and submerged
> divers.   So it made sense that the company would be interested in
> efficient audio power amplifiers.
>
> 73, Bob W9RAN
>
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20161220/04a75d24/attachment-0001.html>