My New Aikido Cathode Follower Preamp

[Ali Tait]

What's the best choice out of ecc82,ecc88 and 6n6p for the ACF?

[simon]

Mmm, good question. Maybe 6N6P if you have nice ones. I don't and I replaced my original 6N1P/6N6P Aikido which I use in a phono for E88CC/E88CC and it was much better. Within reason, if you set the ACF up with the right cathode resistors (so the current stays within permissible) you can change the valves and see which you prefer.

[Ali Tait]

I take it the nice ones are the '80's non gold grid ones?

[pre65]

Everyone should build an aikido .
6SN7 at 10ma and 6CG7 at 10ma or a little more would be the best options from those, Phil .
Bear in mind due to heater/cathode voltages, you might want to run the upper sections on each channel out of one valve, with the lower sections likewise from another valve . Then you can run the two heater windings at low and higher potential . That's what I did . It makes the wiring a bit more messy, but it's more reliable in the long run - esp. if you are running over 300V B+ .

Aha, I could see no mention in the instructions about raising the heater potential. On the 6CG7/6SN7 looks like the max heater-cathode difference is 90v. Going by the chart the B+ is 240v.

[simon]

I think maybe earlier than that Ali - mine are late 70s/early 80s I think.

Philip, don't forget with the Aikido type circuits that with the sections stacked on top of each other there's roughly half the HT across each valve. So if the HT is 240V then the intersection between the two is at 120V. The cathode resistor will raise the potential a little, but if the maximum heater to cathode rating is 90V then you could raise the heaters to 60V-ish it will roughly bisect the upper and lower triodes. There's a whole load of stuff I wrote on Aikidos somewhere which might be helpful, I'll see if I can find it.

[simon]

Aha, I could see no mention in the instructions about raising the heater potential.

It's about half way down on page 5 of the manual.

[simon]

Here it is if anyone fancies wading through it all again. Most of it isn't relevant as it's about a full Aikido but some of it is relevant.
http://www.audio-talk.co.uk/phpBB2/viewtopic.php?t=4819

[pre65]

Aha, I could see no mention in the instructions about raising the heater potential.

It's about half way down on page 5 of the manual.

So it is.

That could be a use for the two Lehane indirect heating kits I have.

[IslandPink]

To follow up two different issues -
(i) In my limited experience,'nice' 6N6P's are anything other than the early-90's gold-grid ones !
( people say the 70's or even 60's ones are better than the 80's ones I believe, haven't tried any yet )
(ii) the heater high/low issue was me thinking in terms of when I built the Aikido 300B driver. There I had 455V B+ . With 250 or less you can find a compromise position, I expect.

You may get good results from quality E88CC's in comparison to 6N6P, but beware of the pinout differences between the types - for instance 5687's are not the same as 6N6P, annoyingly .
Generally for a pre-amp it's worth going for a valve with a decent current ie. over 10mA - otherwise you're not gaining anything over running direct from the phono or CD player !

[Ali Tait]

Cool, I have some good 88's.

[pre65]

I got some 25mm thick MDF from a free-cycler the other day, and I've cut a bit out for a breadboard ACF preamp.

I've sanded it and given it a coat of sanding sealer. Breadboard it may be but I like it to look reasonable.

There is enough MDF left to do the GK-71 project as well.

[IslandPink]

Shame you're not closer, Phil , I can hardly get around the house without falling over pieces of 25mm and 18mm birch ply at the moment .