Re: 71A amp



Raymond Koonce wrote:

I posted a web page with additional photos and commentary. Take a look
at http://www.timebanditaudio.com/71A/71a.html.

Best regards,

Raymond

Very nice work Raymond.

I'd probably go broke if I limited myself to making 2W amps, but then if
I only made wooden boats then I'd also go broke.

In 2009, much technology which radiates the skill of its maker is only
for those who fall asleep when they see a mass made article, like say a
fibreglass boat, or a modern house that resembles a plastic chook pen.

I cannot understand why bass would be better with Garter biasing though.
Effectively each output cathode has a cap to 0V. But there is twice the
normal Rk used, so Iadc regulation would be better. Its OK for class A.
And if one tube begins to conduct more Iadc than the other as it ages,
then its rise in Ek causes half that rise on the other tube's grid, and
that raises the Ek and then the Eg of the original tube. I'm not sure
sure the Garter is worth the extra heat and B+ lost because you have a
higher Ek than normal, but in this case the waste is negligible because
its only a 2W amp. The Garter is at least better than having a common Rk
and Ck for both cathodes IMHO, although Williamson didn't think so. But
then he had some adjust pots to trim his amps dc and ac. These went up
in smoke when a KT66 or an 807 decided to slowly die. Leak just used
individual Rk and Ck. I like this the best because its simple, and bias
change in one tube doesn't change anything very much in the other tube.
But because we now have had solid state sinjce 1947, we can use a few
devices to make elegant bias management circuits which stop smoke
occuring and allow visual monitoring of bias balance condition.

Patrick Turner.



dre7 wrote:
Hey Raymond:

Congratulations! My server won't let me access binaries anymore. Do you
think you could email a pic? I'm really happy with my transformer input,
interstage coupled 6J5>PP 6B4G. Standard cathode bias, though. Bass is
decent through my T-Line speakers, but I've downloaded the Blumlein circuit
as I admire it's elegance and might put it to use in some other project.

Andrew


Raymond Koonce <rkoonce@xxxxxxxxxxxx> wrote in message
news:49fe2496$0$23746$bbae4d71@xxxxxxxxxxxxxxxxxxxxxx
Hi RATs,

I finally got my 71A amp up and running and in a proper chassis. It's a
push-pull design with transformer coupled 56s driving the 71As. It has
transformer input with a pair of Newcomb microphone transformers.
Really nice sound, zero hum in Klipsch Cornwalls. I used, at Al Marcy's
suggestion (thanks Al) Blumlein Garters. I found the bass to much
improved over a standard bias method. Nine tubes and sixteen
transformers for 2 watts/channel :-). I posted a photo on
alt.binaries.pictures.radio. I will have a web page up tomorrow.

Happy listening!

Raymond

.

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