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[ dmfraser ]

ST-70
with 12AX7?

knowledge base for the classic Dynaco
ST70

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ST-70
with 12AX7?

by
DeathRex
» Wed Mar 10, 2010 5:25 pm

I'm planning on making a ST-70 clone, based on Kara
Chaffee's MKIII mod, like my MKIII + Akido. I was thinking of instead of a
12AT7, using a 12AX7/5751/6AV6/6AT6. What would I have to replace to use one?

I was looking at the DIYTube ST-70. It's very close, except for the
LM334 and the capacitor (C2) between the 5751 and the 12BH7.

DeathRex

KT88

Posts: 833

Joined: Wed Nov 18, 2009 6:13 pm

Location: Cortez, CO

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by
Geek
» Wed Mar 10, 2010 6:35 pm

Hi,

12AX7 will do in there, just...

1)
Adjust the 39K resistor on the 12AT7 anode so the DC operating point is the
same.

2) More gain will equal more overall NFB. You may need to reduce
the 20R resistor inline with the 200R one on the cathode.

Cheers!

-= Gregg
=-
Fine wine comes in glass
bottles, not plastic sacks. Therefore the finer electrons are also found in
glass bottles.

Geek

KT88

Posts: 3171

Joined: Sun Oct 21, 2007 1:01 am

Location: Chilliwack, British Columbia

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