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Re: [Amps] 4-400A and QB4-1100 amp - TSPA

To: "John T. M. Lyles" <jtml@lanl.gov>
Subject: Re: [Amps] 4-400A and QB4-1100 amp - TSPA
From: R.Measures <r@somis.org>
Date: Wed, 1 Sep 2004 09:52:56 -0700
List-post: <mailto:amps@contesting.com>

On Sep 1, 2004, at 8:17 AM, John T. M. Lyles wrote:


The 1960 vintage Heathkit Chippewa amplifier model KL1 was a dual 4-400A amplifier which worked well in class AB1. It was described in Electric Radio magazine a few years back, with some modifications to improve it. I believe that there is an online index to ER which you can look up, and order a back issue. I believe that earlier W6SAI Radio Handbooks (like 21st edition) elaborated on the craft of building tetrode amplifiers. About that time the 3-500Z was really becoming a favorite, as a grounded grid zero bias (low bias) class B amplifier. Setting up tetrodes for best IMD and linear operation requires a bit more care, as you have both a screen and control grid bias voltage to deal with. Some have found that grounding the grid and screen for RF (and applying the DC bias to both) and driving the cathode is a good way to handle a 4-400A.

J. T. -- For a g-g 4-400 or 4-250, DC bias is needed for obtaining enough ZSAC (75mA min) at lower anode supply voltages, however, at c. 3500V-DC, no bias is needed.


The Johnson Thunderbolt amplifier had 4-400As but it ran them in AB2 at too low an anode voltage, leaving it short of output and prone to being overdriven (causing IMD).

I have a Chippewa that I am going to restore someday, as it got "modified" by someone. I saw another one that had been converted to grounded grid, but wasn't interested as it had been too modified from original, and for that an SB220 or Drake L4 would be adequate. Problem with Chippewas is that they had a separate huge power supply chassis with 866As and it is usually missing or burnt up.

73
john
K5PRO



Message: 1
Date: Tue, 31 Aug 2004 18:23:08 +0200
From: "ON5KM" <on5km@planetinternet.be>
Subject: [Amps] search 7527A design
To: <Amps@contesting.com>
Message-ID: <005601c48f76$d160a020$a284efd4@computer>
Content-Type: text/plain;       charset="iso-8859-1"

hi ,
who has a link or info to a good stable design using 1 or 2
7527a's (qb4-1100 or 4-400) ??
all info welcome .
tnx on5km stef
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org

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