[Amps] Biasing a Class C modulated final

[Carl]

The original 6L6, which is metal, is not highly prized by the audiophools. 
The 2E26 is a reduced power choice and goes for pennies. Another interesting 
and cute choice is the 815.

One way around the clamp tube is to select a tube that runs at a reduced 
voltage so that when there is no bias the plate dissipation is low. A 
problem with the clamp tube is as it ages the threshold needs to be 
readjusted, a real pain in the DX-100 and other rigs of the era where the 
6AQ5 is being run pretty hard.

One tube that is often overlooked and can be found NIB for around $20 or 
less is the 4D32 which was used in the Collins 32V series, Johnson Viking I, 
and Hallicrafters HT-20. The 3E29, 829B, 5894 family are close equivalents 
when sections are run in parallel.

Carl
KM1H


----- Original Message ----- 
From: "Roger (K8RI)" <k8ri at rogerhalstead.com>
To: <amps at contesting.com>
Sent: Tuesday, February 17, 2015 9:13 PM
Subject: Re: [Amps] Biasing a Class C modulated final


> Not sure the 6L6 would be a good choice due to price, unless you have 
> some, or a good source.  The price on them is astronomical.  My guitar amp 
> uses 6 of them.  I could purchase a nice, new amp for less than replacing 
> those tubes.
>
>  73
>
> Roger (K8RI)
>
> On 2/17/2015 5:14 PM, KA4INM wrote:
>>   On 02/17/2015 03:11 PM, Paul KW7Y Baldock wrote:
>>
>>> I am building a 2x813 class C final modulated by 2x813.
>>
>>> The typical way to bias the class C final is with a combination of fixed
>>> bias applied to the control grid and bias derived from the rf drive
>>> through a grid resistor.
>>
>>> Rather than fixed bias applied to the grid it seems that I could achieve
>>> the same effect by using a pseudo zener in series with the cathode,
>>> similar to what is used in cathode driven linears.
>>
>>> Any thoughts on this approach?
>>
>>   I am guessing common cathode RF amplifier, a more standard way to do it 
>> would be an adjustable bias supply and a clamp tube on the screen grid. 
>> (6L6 or similar) look into any pre-1960 ARRL hand book.
>
>
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