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Biasing question and frustration (Read 1599 times)
Samsdad
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Biasing question and frustration
09/11/18 at 13:02:37
 
I'm trying to figure out if there's a rhyme or reason where a set of tubes will bias, and whether I can contact a seller in advance and ask for certain ones. Let me explain:

I ordered two sets of EL34s (Svetlanas and JJs) and a set of Gold Lion KT77s for my Torii Jr. I like how GZ34s sound in the machine, but it it limits the available bias adjustment on the low end of the pot (whereas a 5u4 seems to give much more flexibility).

On the Svetlanas, even with the bias adjustment turned all the way down, the lowest they'd bias was 1.02 volts. That's a little higher than Steve apparently recommends and therefore I don't want to use them at that level.

On the JJs, bias wasn't an issue; there was more than enough room on the low end of the trim adjustment.

The KT77s also had more than enough elbow room at the bottom end of the adjustment.

Of course, when I switched to the ungodly expensive Philips GZ34 that I bought last year, all three sets of tubes could get either just above or just at the recommended .825 volts.

The problem is, is that I'd rather get away from being reliant on that expensive rectifier in the future. Nothing lasts forever except apparently the annual inflation of NOS tube prices. I also have an RCA version of the GZ34, a brown base NOS, and a Gold Lion. Each sets the bottom-end of the bias adjustment differently (sometimes fairly radically), but none of those could get as low on the bias trim as the metal base GZ34 Philips (of course!) until I tried a JJ GZ34 that I bought along with the Svetlanas and JJs a few days ago. Of course the JJ GZ34 blue-sparked on turn on this morning, but that's another discussion.

So with that rambling preamble, is there a way to specify to sellers (who willing to play along) which set of power tubes I need to have the most wiggle room on the bias trim?

For your info, the Sveltanas that I could only get down to 1.02 volts have plate currents in the mid-40mA range, with transconductance in the 5700s.

The JJs that had more than enough wiggle room had plate currents in the low-20mA range, with transconductance in the 4500s.

I don't know if these numbers matter because the KT77s which had enough bottom-end room to get to .825 volts also had plate currents to the Svetlanas in the mid-40 mAs and transconductance in the 6800-6900s.

Thanks for reading.
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Steve Deckert
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If the 1st watt
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Re: Biasing question and frustration
Reply #1 - 09/11/18 at 17:21:06
 

The problem is that a given tube, say an EL34, will have a wide range of tested currents.  You might get a quad that test at around 20mA and on the same tester, a quad that tests around 40mA, and a few lower and a few higher even.

The second problem is that the current is determined by the test voltage, so if a tube tester has a working test voltage of 350V and another tube tester has a working test voltage of 400V, the current of the tube will measure differently on each tester.  

To ask a tube supplier for quads that test at a specific current is usually not a problem if they have lots of inventory.  Knowing what tester they use and the process would also be helpful so that you know every tube set you get from them is measured the same way, and on the same tester.

Steve
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