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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old Yesterday, 06:07 AM   #321
kscience is offline kscience  United States
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Quick question about the Tubelab Simple SE- why such big caps bypassing the driver tubes?
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Old Yesterday, 07:11 AM   #322
mctavish is offline mctavish  Austria
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I believe they were the best testing with lowest ESR...
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Old Yesterday, 01:23 PM   #323
Tubelab_com is online now Tubelab_com  United States
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Yes, at the time the TSE was being I tested a bunch of caps for ESR and ESL since we had a fancy HP component analyzer at work that tested parts from DC to several hundred MHz (500?). The 5842 driver tube used in the TSE liked to oscillate in the 50 to 200 MHz region, so I selected the best parts I could get from DigiKey to get that cathode at a RF ground. The first two prototypes actually used two caps in parallel, an electrolytic and a Tantalum. I still have one of those boards, the other was destroyed.

I went through the same steps for the SSE, but the 12AT7 does not have the Gm that the 5842 has, so it's not nearly as prone to oscillation.

This was over 10 years ago and capacitor technology has improved a lot. I'm sure the criteria I used to pick that part years ago is no longer valid, but I don't have access to Motorola's lab, as I don't work there any more, and the lab no longer exists.

Feel free to use any cap you see fit in the SSE, but I would avoid ultra cheap no name stuff, because the 12AT7 IS an RF tube and it will oscillate if the conditions are right.

The TSE is a bit more critical and I have seen those things oscillate. You are pretty safe if there is a 4.7K or larger carbon COMP stopper resistor on the grid, but do use a decent low ESR and ESL cap on that cathode. The exact value is not critical, and some of the really big ones may have too much inductance and promote oscillation.
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