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Re: [Amps] adding second tube to AL1200

To: Amps <amps@contesting.com>
Subject: Re: [Amps] adding second tube to AL1200
From: Tony King - W4ZT <amps080605@w4zt.com>
Date: Thu, 18 May 2006 00:17:45 -0400
List-post: <mailto:amps@contesting.com>
Thanks Rich!

That point is valid for adding a second tube to almost any amp whether 
it is home brew or commercial.  Adding a second tube is never as simple 
as it sounds.  As pointed out by others, most of the time the power 
supply can't handle it and it goes down hill from there.  Adding the 
second tube and failing to consider the impact on the plate tank can be 
a recipe for lots of future problems. Many have done it, found resonance 
SOMEWHERE with the existing plate tune cap and figured they got lucky. 
All the while the tank cooks and things get dicey.

Anyone considering adding a second tube to increase their power should 
carefully consider the impact on the plate load Z which will change the 
needed values for c1 and L to maintain the design Q.

For anyone contemplating the addition of a second tube... Remember, the 
BEST you can gain is 3 dB by adding a second tube. that's ONLY half an S 
unit!  If it's that important, you should consider a different amp with 
a bigger tube in it.

On the other hand...  Replacing two tubes with ONE tube can be a very 
effective way to lower operating costs due to tube replacement prices, 
improve cooling, reduce instability (as in the case of replacing a pair 
of 572B's with a single GI-7B) and be successful in your conversion 
project with much less chance of failure.  It is also feasible to 
replace one tube with another, like replacing the the 3cx1200A7 with a 
GS-35B (filament supply needed). In a case such as this, you can save 
money on tube costs, gain a little headroom, improve cooling and use all 
the existing components in the tank and power supply.  No great increase 
in power but effectively modernizing the amp with an old tube that's 
inexpensive and readily available.

73, Tony W4ZT



R L Measures wrote:
> Kevin -- The stock tank inductances >40m in the Al-1200 are not  
> adequate for one 3cx1200A7, much less two.
> Doubling the diameter of all  tank L conductors, and halving their uH  
> would be a good place to start.  Also, with 2x the circulating  
> current in the tank, the dissipation in  the bandswitch contacts  
> increases 4x, so a bandswitch cooling fan might be a good idea.
>     I did not reply only via Verizon because this item might be of  
> interest to amplifier builders.
> cheerz
> 
> On May 17, 2006, at 12:52 PM, Kevin Adam wrote:
> 
>> Wanted information on adding a second tube to AL 1200 has anyone  
>> done this if so can you give the details as to what all is envoled  
>> what parts you had to change if any please help
>> Reply to N9iww At Verizon dot net
>>
>>
>> N9IWW
>> Kevin Adam
>> 1239 West Till Road
>> Fort Wayne, Indiana 46825-2145
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