Art Audio Kit on Common Ground Magazine

[sonata149]

In the late nineties there was a British 'underground' magazine "Common Ground". Although a subscriber I only received 3 issues. In issue 3 there was a feature on an Art Audio EL34 triode PP kit, with circuit diagram.

There are two connections marked -v and +v from 2 resistors in the first stage cathode line. I'm not sure where they go to. I'm assuming they are part of the NF circuit and should go to the secondary of the OPT - but not sure.

I tried contacting Art Audio but their email address is dead. I'm trying to build this circuit so, what's the best a-a primary impedance for EL34s in triode PP?

Does any one have a copy of the 2nd part of the article presumably published in issue 4 - if it ever went out!?

Thanks in advance.
Joe A

[gingertube]

Joe,
This says 5K a:a for triode mode push pull EL34
http://tdsl.duncanamps.com/show.php?des=EL34

The +V and -V terminals shown as definitely the global feedback connections. -V should go to speaker common and +V you'll have to experiment to see if 4 Ohm or 8 Ohm etc tap is the best.
Get the amp going without the feedback connected first and then hook it up.
If the amp oscillates when the global feedback is connected swap the two feedback leads.

The circuit suggests that 1 12AX7 is shared across the 2 channels and ditto for the 12AU7. Thats always a bad idea.
Have a look at a 12DW7 which is half a 12AX7 and half a 12AU7 in one envelope and use one of these for each channel.

Cheers,
Ian

[sonata149]

Hello Ian,

Many thanks for your reply. It confirmed my gut feeling about the +v and -v connections. Also, didn't realise Duncanamps had this level of data and I could have searched for the Data Sheet. Sorry and thanks for the trouble to attach the link!

I didn't know it's considered bad practice to use a double triode shared between channels. Tks for the tip, as well as the ready made solution in the form of the 12DW7. Never heard of this valve or seen it in circuits.

Best regards,

Joe A