Unusually high Gm

Drewaudio · 16th October 2011, 04:23 AM

I've got four JJ KT88 tubes and the Gm on one tube is about 2 times higher than the others.

Can anyone point me to a resource or help explain why this might be and what it means for tube operation in an amp. eg Do I though it out?!

Thanks in advance.

Chris Hornbeck · 16th October 2011, 04:26 AM

Is it gassy?

DF96 · 16th October 2011, 02:19 PM

Poor quality control in the JJ factory? There will always be some variation, but 2:1 seems too much.

Do you intend to a) thaw it out, b) throw it out, c) trough it out (do you keep livestock?)?

miklos · 16th October 2011, 02:45 PM

toss

Drewaudio · 16th October 2011, 06:45 PM

Thank guys,

I hadn't thought about gas so I'll test it.

PS As for my poor english....I guess I brought it on myself....perhaps the gas is bovine flatulence..

DF96 · 16th October 2011, 07:15 PM

Would gas increase gm?

kevinkr · 16th October 2011, 09:20 PM

Quote:

Originally Posted by DF96

Would gas increase gm?

I've run into a couple of cases where gassy tubes also had unusually high transconductance, but can't say for sure that such a relationship exists. What I can say is that tubes that measure way above their expected transconductance range often go boom IMLE.. I now usually toss them as suspect..

I will mention that I have had a couple of JJ KT88 that measured well above normal transconductance, and they failed spectacularly and catastrophically within hours of being put into use. The collateral damage took days to repair, and the odor of burned parts weeks to dissipate from my listening room..

TheGimp · 17th October 2011, 12:00 AM

You might try baking it in an oven at 350F (177C) for several hours, let it cool, and see if it changes.

This will give the getter a chance to operate at elevated temp and remove gas.

dgta · 17th October 2011, 12:15 AM

Quote:

Originally Posted by DF96

Would gas increase gm?

I would expect so. Air conducts better than vacuum.

soulmerchant · 17th October 2011, 06:52 AM

Quote:

Originally Posted by TheGimp

You might try baking it in an oven at 350F (177C) for several hours, let it cool, and see if it changes.

This will give the getter a chance to operate at elevated temp and remove gas.

Maybe try a little lower temperature than 177C if you want to keep the phenolic base on the KT88 from bubbling.. I might go to about 150C Max. - but usually just cook at 120C but then leave them in for a good 5-7 hours.

For valves without bases, I go with a higher temperature.

I also try cooking gassy valves more than once for best result - ie. cook -> let cool -> cook again. For some reason this seems to be more effective.

16th October 2011, 04:23 AM	#1
Drewaudio diyAudio Member Join Date: Mar 2006 Location: New Zealand	Unusually high Gm I've got four JJ KT88 tubes and the Gm on one tube is about 2 times higher than the others. Can anyone point me to a resource or help explain why this might be and what it means for tube operation in an amp. eg Do I though it out?! Thanks in advance.

16th October 2011, 02:45 PM	#4
miklos diyAudio Member Join Date: Jul 2004 Location: toronto	toss __________________ miklos

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
EF86 Gm	dsavitsk	Tubes / Valves	7	31st March 2007 07:59 PM
Where can i find Pioneer gm-x624/gm-x524 manuals?	gudrun	Car Audio	1	22nd February 2007 02:01 AM
Gm-120?	longhorn658	Chip Amps	0	27th May 2006 08:43 AM
For GM mostly	Andrewbee	Subwoofers	10	8th July 2005 03:52 PM
8" poly woofer AUD$29. Unusually good.	Circlotron	Multi-Way	2	6th January 2003 02:09 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

16th October 2011, 04:26 AM	#2
Chris Hornbeck diyAudio Member Join Date: May 2011 Location: Little Rock	Is it gassy?

16th October 2011, 02:19 PM	#3
DF96 diyAudio Member Join Date: May 2007	Poor quality control in the JJ factory? There will always be some variation, but 2:1 seems too much. Do you intend to a) thaw it out, b) throw it out, c) trough it out (do you keep livestock?)?

16th October 2011, 06:45 PM	#5
Drewaudio diyAudio Member Join Date: Mar 2006 Location: New Zealand	Thank guys, I hadn't thought about gas so I'll test it. PS As for my poor english....I guess I brought it on myself....perhaps the gas is bovine flatulence..

16th October 2011, 07:15 PM	#6
DF96 diyAudio Member Join Date: May 2007	Would gas increase gm?

17th October 2011, 12:00 AM	#8
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	You might try baking it in an oven at 350F (177C) for several hours, let it cool, and see if it changes. This will give the getter a chance to operate at elevated temp and remove gas.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)