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

[John T. M. Lyles]

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. 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 at planetinternet.be>
>Subject: [Amps] search 7527A design
>To: <Amps at contesting.com>
>Message-ID: <005601c48f76$d160a020$a284efd4 at 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