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Old 7th April 2004, 11:40 PM   #11
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by fdegrove
With careful choice of operating points and layout you'll hardly ever have to resort to gridstoppers...
Hello, thoriated. But can you afford probes that maintain that 500MHz bandwidth to the probe tip? (I can't.)

As Frank says, grid-stopper values are heavily dependent on layout. I recently had to resort to 10k on a point to point layout (great for audio, less great for RF), whereas the PCB equivalent got away with a few hundred Ohms...
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Old 8th April 2004, 04:20 AM   #12
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Default 6dj8 vs 6922

Hi Hacknet.

Actually these two tubes are somewhat different, so don't get too confused by them.

The 6DJ8 was designed for RF tuner service in which the gain needs to change with varying signal levels so as not to overload in high signal areas, near the transmitting tower as an example.

As Glass Audio pointed out, the transconductance and gain of the 6DJ8 varies quite alot with voltage and current.

As also pointed out by Glass Audio, the 6922 has the same high transconductance in the "high current region", but the transconductance and gain do not drop off nearly as much in the "low current region". This translates to much lower distortion, as I have confirmed in my own testing/measurements.

The result is a different sound, although brand makes a large sonic difference.

Hope this helps.
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Old 8th April 2004, 08:16 AM   #13
hacknet is offline hacknet  Singapore
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i will hook on a pair of cathode caps and have listen. thanks.


anything else?
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Old 8th April 2004, 01:08 PM   #14
SY is offline SY  United States
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Steve, if the GA article you're referring to is the one called "Is the 6DJ8 suitable for audio?", there has been some talk that what the author was testing were actually 6ES8s that a supplier had illegitimately relabelled. Real 6DJ8s do not have the remote cutoff characteristics that he described.
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Old 8th April 2004, 01:38 PM   #15
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Hi,

Quote:
"Is the 6DJ8 suitable for audio?",
He,he....Why was I thinking the exact same thing?

That article was published by GA issue 2/93 page 1, author was Danzil Danner.

In issue 3/95 page 10 Roger Modjeski replied and it turned out they had indeed run tests on a 6ES8/ECC89.

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Old 8th April 2004, 02:05 PM   #16
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Default my testing

My testing/measurements confirm there is a substantial difference in distortion between major brand 6DJ8s and 6922s.

If these major brands had relabeled tubes then my apologies. I wonder if they knew? And why relabel them? Simply ran out of stock and needed some to sell?

Take care guys.
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Old 8th April 2004, 02:31 PM   #17
SY is offline SY  United States
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Often, yes. Also, it's tempting when you run across a large stock of 6ES8s for a few pennies per to just relabel them and reap the rewards. Not necessarily a stock situation, more like dishonest opportunism. Shocking to say, that actually happens in the high-end audio industry!

BTW, examination of the grid structure can give you a clue about the actual pedigree of the tube. The 6ES8 has a variable pitch to the grid spacing; the 6DJ8 is quite even.
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Old 8th April 2004, 03:39 PM   #18
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Default Yes

Hi Sy,

"Often, yes. Also, it's tempting when you run across a large stock of 6ES8s for a few pennies per to just relabel them and reap the rewards. Not necessarily a stock situation, more like dishonest opportunism. Shocking to say, that actually happens in the high-end audio industry!"

I didn't make myself very clear Sy. I didn't mean in the recent past, audio stashes etc although you are certainly correct.
I mean 60s vintage. I tested old tubes out of TVs (got alot of different tube types), and maybe a NOS or two, have to look closer. Looks like even back then, some pretty big wigs must have been dishonest.

"BTW, examination of the grid structure can give you a clue about the actual pedigree of the tube. The 6ES8 has a variable pitch to the grid spacing; the 6DJ8 is quite even."

I may investigate further, maybe "open" one tube up and inspect closer. Be interesting if that long ago, we had some crooks pulling the same stunts.

Take care Sy and thanks.
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Old 8th April 2004, 04:57 PM   #19
SY is offline SY  United States
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Steve, the other thing to keep in mind is that in any triode, gm changes with ip. More so in high gm tubes, since you're starting at a higher value. But rp also changes, and in tubes that aren't designed for remote cutoff (variable mu), the product of the two parameters stays remarkably constant. That's the other way to separate the wheat from the chaff.
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Old 9th April 2004, 12:13 AM   #20
hacknet is offline hacknet  Singapore
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i`ve knocked in a sanyo oscon 100uf cathode cap and i`ve found the pre to sound much more fluid and warm.

Thanks for the advice!
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