Well a fellow AK'er mentioned the Golden Tube Audio SE-40 the other day in a
thread so I thought what the heck, one of mine is sitting on a shelf right under
the St-70 I've been using and there both rated at the same power, 40wpc and
since I'm using the 8ohm taps on the ST-70 and the SE-40 only has 8ohm taps
it would be a good compairison leaving everything as it was powering some ADS 880's. :music:

Quite a bit different, the SE-40 is a lot more beefier sounding with quite a bit of
grunt and brute force to it where the ST-70 is much more delicate, what I was a
little surprised about was the top end of the 2 amps, the ST-70 has a lot more of
the shimmer and sparkle which I've come to appreciate ecspecially in Jazz to where
the SE-40 was just dull up top and rather thick sounding. I tried a few things even
the front end tubes in the SE-40 but not much was a changing, so I went to the
preamp which at the moment I'm using a little Dared SL-2000A, which is suppose
to run the 12AT7 but I've never thought they were the best choice for sonics in
this piece but usually run something with less gain like a 5965A all the way down
to some 12AU7's, tried quite a few and got improvements but it wasn't until I had
tried the 12BH7 black plate Rca's that the system woke up with a great pressense
on top and tightened the low end to where the system sounds really really nice now.

Allmost any circuit that has called for a 12AU7 and I sub in a 12BH7 has been
improved quite a bit or at least the 12BH7 was the better sounding tube, IMHO.

So any want to give a 12AU7 circuit a shot in the arm should try a 12BH7 in there.
(They are a taller tube and the heater current is .6 verse .3 in a 6.3 volt circuit)

:dammit: Now BH7 prices will skyrocket! All joking aside I have heard of this "trick" but never actually tried it. I have a Fisher K200 int. that could use some sparkle. It has 7- AX7's. I wonder which ones I would want to change. I tried to buy a service manual with schematic for it but it turned out to be for a 202. Hmmmm. Any ideas? I've got some BH7's and would love to give it a try. :yes: Jay

Hey!!!,
The 12BH7 is a nice alternative to the 12AU7. The 6GU7 is it's 6V cousin, and it pinned just like a 6DJ8. I'll agree with Kegger on its performance.
cheers,
Douglas

Jay,

I think that the biasing and loads would be completely different for a 12AU7 vs a 12AX7. I'm used to 12AX7's running 1 or 2 ma plate current, and 12AU7's running as much as 10ma! I don't think the 12BH7 would be a very good sub for your 12AX7's. The gain would be much lower, too!

Richard C.

I agree the gain loss may be to much going from 100 on a 12ax7 down to 20 or so on
the 12au7 or 12bh7. The tube you may want to try Jay is a 5751 or maybe the 5965.

Would the extra heater current be a cause for concern on equipment that runs close to the edge/hot already? :scratch2:

You want to pay attention to what heater current the particular tube draws, and make sure the filament winding/transformer can swing it.

So if the PS xformer is on the edge, swapping in tubes with higher heater current may just make it worse.

I'm guessing the tubes mentioned are all rather close with regards to heater current.

IMHO For your Fisher I would try Groove tubes 12AX7M. I'm running one in a Columbia stereo phono and that tube gives the amp great overall tone. Everything is present highs, mids, lows it's all there. :music: Also running one in a GEC 912 plus 14 WPC amp with the same results. I can usually tell when an amp either lacks or has an abundance of one frequency range and both amps sound pretty equal over the whole audio range. Not too sure about the Columbia as I had to add a cap and resistor to each channel to reduce the highs slightly (I assume they boosted the highs a bit to compensate for the fact that most of your volume controls used with tube circuits back then seemed to attenuate the highs slightly).

You want to pay attention to what heater current the particular tube draws, and make sure the filament winding/transformer can swing it.

So if the PS xformer is on the edge, swapping in tubes with higher heater current may just make it worse.

I'm guessing the tubes mentioned are all rather close with regards to heater current.

What he said! :D