Anyone for 6AU5's in a G*S*G?

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Anyone for 6AU5's in a G*S*G?

Postby jwhitmor » Sun Oct 30, 2016 11:18 pm

I scored some absolutely pristine 6AU5 (National brand) tubes at an estate sale. I put a pair into the (6AV5 wired) sockets of my G*S*G, and had a listen. They worked, but had an objectionable "gritty" sound. I am not talking "Golden Ears" gritty, it is readily noticeable. Considering how well the 6AV5 output tubes sound, I expected similar sound from the 6AU5 tubes. I cannot find anything by comparing both tube type's data sheet that would make me think they should not work. I believe the only difference is one more watt additional dissipation for the 6AV5 vs the 6AU5, and the smaller T9 envelope on the 6AU5's. Has anyone else tried using 6AU5 output tubes. I would be interested in your experience with them.
Thank you,
J.W.
If it is not making X-rays, your B+ is too low.
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Re: Anyone for 6AU5's in a G*S*G?

Postby Vapour » Wed Nov 02, 2016 7:13 pm

See this post: http://www.diytube.com/phpBB2/viewtopic.php?f=14&t=1853&p=51478&hilit=6au5#p51478

Quote:
Best to use the larger envelope 6AV5GAs, and Sylvania or Dumont brand are definitely the best for this application, some of the RCAs and GEs have wimpy, undersized plate structures, more akin to those found in the smaller and weaker 6BQ6 or 6AU5. Those tubes don't like to be pushed too far beyond 11 W PD, 15 W is like skating on very thin ice...
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Re: Anyone for 6AU5's in a G*S*G?

Postby jwhitmor » Wed Nov 02, 2016 11:13 pm

I had already read the comment you referenced, and I agree, but I was using the 6AU5 tubes at low levels, around 4 watts maximum, well within the tubes rating. No hot spots on the plates or any other signs of distress, just crappy sound. I just downloaded the curves for Triode strapped 6AU5 tubes of various brand names. When I get a chance to study them, I may find something amiss with plugging them into a mostly stock G*S*G. Then again, there was that old TV commercial that said, "Your only problem may be a bad tube!".
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Re: Anyone for 6AU5's in a G*S*G?

Postby EWBrown » Fri Nov 04, 2016 4:45 pm

6AU5s will need a different value cathode resistor, in order to change V G1-K, and operate at their normal plate dissipation.
4 Watts is low, the tubes are running too "lean". Remember that in triode strapped mode, the dissipations of plate and G2 combine,
and in Class A audio service, they can be pushed 40-50% harder than when serving as vertical deflection drivers. Also, keep the plate voltage
between 250V and 300V max, they aren't as rugged as 6AV5GAs.

I found that 6AV5GAs run at 11 watts sounded thin and tinny, I pushed them up to 16W, and later 20W, and the sound quality was vastly improved.
And the tubes took it all in stride, no signs of early demise.

I later plan to try out 6BQ6GTs, I have LOTS of these, their ratings are same as 6AV5GA, (though I'd keep the max PD at 15W due to smaller plate structures),
but have base pinout similar to 6V6 / 6L6, except that the anode is not pin 3, but is brought out to a plate cap. These could be used on a "Clementine" board very easily.

/ed B in NC
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