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Heathkit AA-121 feedback cap question



 
 
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  #1  
Old January 20th 05, 12:01 AM
Kevin Killebrew
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Default Heathkit AA-121 feedback cap question

Hi,

I'm restoring my Heathkit AA-121, and notice that the cathode cap/resistor
network coming off the first 6AN8 contains a 50uf 15v electrolytic coupled
with a 680 ohm resistor, and then connects to the feedback loop going to the
OT secondary via another resistor/cap network. I hate the idea of any of
the signal path going through an electrolytic but am not sure if it's worth
replacing with a large (and expensive) film or poly type. I've seen some
Solen's and Dayton 50uf poly's commonly used in speaker crossovers that
might work/fit since there is a lot of room in the amp chassis. My question
is: Is it worth doing? The 50uf poly's are around $13 each. The money's
not a big issue, but I don't want to do it if it really doesn't matter. I
really do like this amp.

Kevin Killebrew
Austin, Texas


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  #2  
Old January 20th 05, 03:07 AM
John Stewart
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Kevin Killebrew wrote:

> Hi,
>
> I'm restoring my Heathkit AA-121, and notice that the cathode cap/resistor
> network coming off the first 6AN8 contains a 50uf 15v electrolytic coupled
> with a 680 ohm resistor, and then connects to the feedback loop going to the
> OT secondary via another resistor/cap network. I hate the idea of any of
> the signal path going through an electrolytic but am not sure if it's worth
> replacing with a large (and expensive) film or poly type. I've seen some
> Solen's and Dayton 50uf poly's commonly used in speaker crossovers that
> might work/fit since there is a lot of room in the amp chassis. My question
> is: Is it worth doing? The 50uf poly's are around $13 each. The money's
> not a big issue, but I don't want to do it if it really doesn't matter. I
> really do like this amp.
>
> Kevin Killebrew
> Austin, Texas


You have been listening to much to the 'boutique' audio end of the buz. It would
take a kilobuck's worth of equipment to measure the improvement resulting from
that change, if in fact there is one. For example, any of the tubes in the amp
have more non-linearity than that cap will introduce into the audio path.
Fortunately the applied NFB fixes many of the problems. If it were a serious
problem at the time of design it would have been fixed then.

Some people really think they can hear a difference. Biggest problem by far in
any of these systems is the loudspeaker(s) in it's box & the listening room.

But do it if it gives you comfort.

My 2 cents, anyway. John Stewart


  #3  
Old January 20th 05, 02:28 PM
TubeGarden
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Hi RATs!

Unlike global negative feedback, upgrading the caps may improve your pleasure
without hidden costs

But, be sure you have the speakers you really like, if not deserve, before you
spend time and money on other portions of the system.

Happy Ears!
Al



Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead
 




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