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

[Carl]

>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 at gmail.com>
>> To: <amps at 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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