Lafayette LT-100 and 6AQ8 tubes - diyAudio
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Old 15th April 2008, 04:38 AM   #1
rickl is offline rickl  United States
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Default Lafayette LT-100 and 6AQ8 tubes

I just picked up a Lafayette LT-100 tuner to go with my KT-600.

I tested the tubes and the two 6AQ8 test good on emissions, they don't on grid leakage .

I did find one GE NOS in my stash but wondered if I can use a 12at7 or some other twin triode with a mu close to the 57?

I don't have a schematic for the LT-100 (anyone help?) and I have not checked to see how the 6AQ8's are used.

Has anyone used 12at7's for a sub for a 6AQ8? I'll rewire the heaters (4-5 are used on a 6AQ8, 4-5 tied together, with 9 on a 12at7).

Or not worry about grid leakage.

Thanks for any advice. If anyone has a schematic they want to share, let me know.

Thanks again.

rick
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Old 15th April 2008, 03:11 PM   #2
GordonW is offline GordonW  United States
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Be careful of the second triode, and plate dissipation.

On the 6AQ8, the second triode dissipation is FOUR POINT FIVE watts, as opposed to 2.5 watts for the first triode, and 2.5 for BOTH triodes in a 12AT7.

You would need to check for plate current usage on the second triode stage in the unit you're using... if it's wired the SAME as the first triode (same B+, same current, etc), you should be OK with the 12AT7. HOWEVER, if its different, you would need to do the calculations for dissipation from the voltage and current, to make sure you're not going to overheat the second triode in a 12AT7.

Other than that, I'd think it would work.

Regards,
Gordon.
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Old 15th April 2008, 05:25 PM   #3
kevinkr is offline kevinkr  United States
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I don't know which specific locations the 6AQ8 are used for in this particular tuner, but they are commonly used in the front end and in multiplex adaptor circuits which are both sensitive to interelectrode capacitances.

The 12AT7 is not a direct sub for the 6AQ8, filament connections are different. Without checking closer I am sure there are other differences as well beside the lack of section to section shielding in the 12AT7.

Also my experience with leakage tests with most tube testers is that they tend to be wildly inaccurate. I have had many a good tube failed this way and upon testing in a jig I generally find the leakage is within spec.

My recommendation is to continue to use these tubes in their original locations until suitable replacements can be purchased.
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Old 15th April 2008, 08:07 PM   #4
rickl is offline rickl  United States
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Thanks kevinkr and GordonW.

I'll bring it up on a variac tonight and check voltages.
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Old 16th April 2008, 01:04 AM   #5
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I have a zillion of these (ECC85). I'm happy to send some for the cost of postage.

dave
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Old 17th April 2008, 07:54 PM   #6
rickl is offline rickl  United States
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dave,
Thanks for the kind offer. Let me look in my stash again.

Plus I still need to bring it up and see how it sounds.

Thanks again.
rick
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