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@rogerhalstead.com>
To: <amps@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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