[ETO_Alpha] Re: Alpha 76

[George & Marijke Guerin]

Dave, Robert, et. al.,

The original Alpha 76 with two tubes, and an input swamping resistor had an 
input impedance close to 50 ohms.  With the 3 tube 76C(A) or 76P(A) and the 
low drive modification, the swamping resistor was a smaller wattage higher 
ohm value, to keep the input impedance close to 50 ohms.

If one uses a pair of 3CX800A7 tubes, the input impedance is around 25 
ohms... as each tube is about 50 ohms input impedance with grounded grid 
configuration.

If the amp is modified to 2 x 3CX800A7's make sure there is a 2:1 or 2.2:1 
UnUn from the 50 ohm input to the cathode circuit similar to the 77SX.  I 
would also leave the swamping resistor in the circuit!

73        George    K8GG

----- Original Message ----- 
From: "Dave Haupt" <emailw8nf at yahoo.com>
To: <eto_alpha at mailman.qth.net>
Sent: Sunday, June 19, 2005 1:31 PM
Subject: [ETO_Alpha] Re: Alpha 76


| Robert,
|
| The 76 feeds the cathodes through a broadband network
| with no tuning elements.  If other amplifiers use a pi
| network input, then the 76 might be lower gain (e.g.
| "hard to drive").
|
| Note also that there is a "low drive" modification to
| the 76, documented on the AlphaRadioProducts web page.
| Make sure that you've performed that modification.
| The original 76 was designed to deliver 1kW INPUT
| (roughly 650 watts output) from a 100W exciter
| (typical exciters those days were 180 watts INPUT, and
| delivered 100 to 120 watts output).  When the FCC
| increased the amateur power limit to 1.5kW output,
| many existing amp designs needed to do something to
| increase the gain.
|
| What Dahl xfmr did you use?  I have looked at the
| possibility of running my 76 on higher anode voltage -
| at stock anode voltage, the 3CX800A7s are being pushed
| a bit for cathode current.  One solution would be to
| add a third tube, but not sure there's room.  I was
| hesitant to increase the anode voltage because that
| might put the tune cap and bypass caps at risk.  I
| gather that you haven't had any problems with them?
|
| 73,
|
|
| Dave W8NF
|
|
| >Date: Sat, 18 Jun 2005 00:30:17 -0500
| >From: Robert Wood <rwood90 at direcway.com>
| >Subject: [ETO_Alpha] Alpha 76>
| >
| >Have Alpha 76 converted to 3CX800's and the Dahl
| >higher >voltage
| >transformer.
| >Others with experience on said conversion
| >and feeding the inputs to these tubes.   Seem harder
| to
| >drive then other 3cx800 amps I've dealt with.
|
| >73 w5aj
|
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