Using 12ax7 on Stereo-70, Mark III and Mark IV

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Using 12ax7 on Stereo-70, Mark III and Mark IV

Postby tubularfluxus » Tue Apr 26, 2016 1:35 pm

Hi,
I recently came in possession of a Stereo-70, two Mark III and a Mark IV. I am restoring them and they are currently in their stock form.

I do have few spare RCA 7199 tubes, but I also have many many more 12ax7, mainly old european kind (mullards, telefunken, philips, mazda, siemens, marconi and more...).
I have old good 12au7s as well, from the same brands, but my collection of 12ax7 is larger.

I am planning to switch from the current amps I'm using (mainly modern Chinese 12ax7 + el34 / kt88 amps) into using these Dynaco amps. It would be nice to be able to rotate all these old 12ax7, so I searched for mods who could run 12ax7 in Stereo-70, Mark III and Mark IV.

By googling I found the Poseidon board and the KTA-HiFi ST-70.

Are there any other board which takes 12ax7?

This community around the Poseidon seems to me very active, while I can't find much information about the KTA, or other 12ax7 compatible boards.

Some of my Dynaco are already using the SDS board for increased filter capacitors. I would also like to upgrade to separate bias adjustment for all 4 power tubes, either via
Dynakit Bias Balance Control Kit, or through a board like Poseidon that allows to separately bias the tubes.

Do you think that older exotic 12ax7 mullards and philips would perform better then RCA 7199 and 6AN8, either boards running on amps with separate tube bias and upgraded filter capacitors?

I found on this forum links to purchase the Poseidon for Mark III and Mark IV, but seems sold out for ST-70, right?

What options do I have left then, if I decide to use 12ax7 on the ST-70?

Thank you all in advance
tubularfluxus
 
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Re: Using 12ax7 on Stereo-70, Mark III and Mark IV

Postby TomMcNally » Wed Apr 27, 2016 6:10 pm

A 7199 is basically half a 12AX7 and half a 12AU7 "dissimilar triodes" Most replacement driver boards use a 12AX7 and a 12AU7, sometimes a 6GH8, some other variations also, like a 6AU6 (Half a 12AU7). A 12AX7 won't cut it in both positions. There are 12AU7's all over the place for about $ 12. My opinion is to go with what the board is designed for.

... tom
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Re: Using 12ax7 on Stereo-70, Mark III and Mark IV

Postby tubularfluxus » Wed Apr 27, 2016 11:02 pm

I am fine with using one 12ax7 and one (or two) 12au7. As far as I'm getting some use out of my 12ax7 I'd be happy :)
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