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Re: [Amps] The GU84B and GU78B tubes / some brief history

To: "Vic K2VCO" <k2vco.vic@gmail.com>
Subject: Re: [Amps] The GU84B and GU78B tubes / some brief history
From: "Carl" <km1h@jeremy.mv.com>
Date: Mon, 29 Oct 2012 12:38:05 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>


I presume you mean the modified B&W 852!


** Its an 850A that Ive rebuilt and slightly modified the L values. The 4X1 is a Hi Z tube


That's what I'm planning to use for my 4CX1000A amplifier. I simply lifted the strap from the cold end of the coil so I can insert a 160m coil. The one thing that I couldn't figure out how to do was to add a contact in the 160m position to switch in an additional capacitor across the loading capacitor. The rotor wiper for the B&W switch interferes with one of the contacts no matter where you position it.

I know B&W made a version of the tank that supported 160m. I bet they used a narrower rotor wiper.

** Its been ages since Ive seen one but I dont remember any special place to hang a cap. They just added the contact and a modified coil. Im using a toroid for 160.



I am probably going to gang a small rotary switch with 60 degree indexing to the B&W tank and use it to operate relays for the grid coils and the loading padder.

** In the past Ive used a WW2 BC-375 switch for padding when I didnt have a big enough vacuum variable. This time around Im using a 2500pf version that Ive been hoarding for 20+ years.

Carl
KM1H




On 10/29/2012 8:23 AM, Carl wrote:
Ive even run swamped grid old glass triodes without neutralization. Currently it is a pair of Class C 6C21's in parallel that runs up to 20M and Ive no doubt it would work higher. That amp ran 250TH's earlier.

During the 70-80's I built and sold plenty AB1/AB2 single and dual 4X1 amps and never needed neutralization, even on 10M at 5KV.

A swamped grid Class C 4X1 covering 160-10 and driven by a 32V2 is the next project for my own use. Guess what tank circuit Im using (-;

Carl
KM1H




----- Original Message ----- From: "Vic K2VCO" <k2vco.vic@gmail.com>
To: <amps@contesting.com>
Sent: Monday, October 29, 2012 10:59 AM
Subject: Re: [Amps] The GU84B and GU78B tubes / some brief history


Why do you say that a swamped grid-driven tetrode needs neutralization? I haven't seen this in published designs. Isn't a 50-200 ohm swamping resistor enough to stabilize the amplifier?

Agreed on the other points!

On 10/28/2012 11:57 PM, Jim Thomson wrote:
## actually, they are not apples and oranges. One has an extra grid in it, requires a regulated screen supply, neutralization, and some method to cancel out the Xc between the cathode and control grid, so u just cant slap a 50/200 ohm.
passive grid resistor in there.

--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/

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--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/



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