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Old 11th November 2006, 05:27 AM   #1
kegger is offline kegger  United States
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Default Funky reading 12B4A?

Just brought up a new build using a 12B4A driver to a 6CG7 phase
splitter doing duty per channel in a push pull 6L6 type stereo amp.

Yah I may not have the best conditions for these tubes but it's what
I had on hand. 375v supply 6.8k load and 820ohm bias resistor.

Was bringing the unit up slowly on the variac while watching my plate
voltage when I see i'ts climbimg rather high and looking like it's going
to be higher then I had predicted of about 210 volts. Shut it down to
check everything over, made some changes in other areas then decided
to put in a new pair of 12B4's I just got in, fired it back up and all was good,
set the bias on the output tubes, checked some voltages and played some
tunes for a few while watching everything. Plate voltage around 208 on 12B4.

All seems to run as it should, for the heck of it I put the other set of 12B4's
in to see how they respond now, sure enough I only get to 75volts on the
variac and plate voltage on the 12B4 is rising over 250v so I shut it down.
Both channls act the same so both of those funky tubes do the same thing.

Pulled out the tester "what good they are" and all 4 tubes read the same,
right around 100 which is what a new tube shows on my tester.

The ones I'm having an issue with are RCA 12B4A's, the GE's are fine, all are
black plates, don't know if there are others I have little knowlledge on the 12B4.

All the tubes I have are marked 12B4A, including the GE's, I'm getting exactly
6.3vac on the heaters if that woulda mattered.

I'll test the few others I have, but wonder if anyone has seen this or may know what the!

Thanks!
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Old 11th November 2006, 06:17 AM   #2
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Hi,

Do you have a schematic so we can duplicate the circuit and maybe see what's up?

If your tube tester is a simple emission tester, they are practically useless other than a quick life test and shorts. I have several dead tubes that tested good on those, but the grids are roasted out of them.
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Old 11th November 2006, 09:48 AM   #3
kegger is offline kegger  United States
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I hadn't done up the schem on the pc, but here's a quick drawing of
the 12B4's section and it's coupling to the 6Cg7, nothing more then
the elements laid out earlier.

Yes I understand the testers deal here in this is why I said for what
there worth. Mine seems to be pretty accurate but I don't trust it as
far as a good or bad tube for sure though.

Thanks for the comments.
Attached Images
File Type: gif 12b4_driver.gif (7.4 KB, 280 views)
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Old 11th November 2006, 10:57 AM   #4
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Hi,

Thanks. I'll breadboard this within the next couple days and see what reasonably competent advice I can share

I noticed though when designing a preamp recently that the 12B4 can do some odd things DC wise if the plate supply isn't adequately decoupled. It drifts, won't settle and all that. No AC instabilities, just DC
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Old 11th November 2006, 11:06 AM   #5
kegger is offline kegger  United States
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Hey thanks.

The power supply here is Rc,Lc for the output tubes then an additional
Rc for G2 on the outputs (no UL taps on the outputs) and the front end.

Power supply seems rock steady, only that pair of 12B4's gives me an
issue on this unit, still haven't tried the rest of the 12B4's I have yet.

Seems weird though that one set does that and the other set doesn't,
really leads me to believe a tube issue here.
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Old 11th November 2006, 11:13 AM   #6
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Humm..

Do all four tubes have the grids connected to pins 2 *and* 7?

I've run across tubes that are "supposed" to have an element connected to two pins,but in some brands of tubes it's only connected to one pin,instead of both.

It's fooled me before..
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Old 11th November 2006, 11:28 AM   #7
kegger is offline kegger  United States
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I was thinking of that as I'm only connected to pin 2, but no grid input
on the possable bad tubes would make the plate voltage rise?

I could test it easy enough but I didn't think that would have any reason
to cause this but I am still learning and open to new things.
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Old 11th November 2006, 12:08 PM   #8
kegger is offline kegger  United States
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Just measured the 4 tubes with a meter and all 4 have pin 2 and 7 connected.
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Old 11th November 2006, 11:48 PM   #9
kegger is offline kegger  United States
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I tested 4 more and no issues with those in the amp, humm!
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Old 12th November 2006, 12:35 AM   #10
kegger is offline kegger  United States
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Well I tested a few more and found a couple that exibited the same issue.

Decided for the heck of it let um run and see how far they go, well they go
to around 275 volts then settle right in at the 200 mark like the others and
they sound fine, seem to work fine so I'm not sure why they do that but look
to be allright and will work in this config.

Not worried about how high the voltage goes as the tubes are rated beyond
that as is all my parts in the circuit, so I guess I shouldn't be worried.



Live and learn I guess.
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