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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 14th January 2019, 02:36 AM   #1
cbutterworth is offline cbutterworth  United States
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Default pin 1 KT88 SimpleSE

I love my SimpleSE and have run it for a number of years now, with very little issues.

One thing about my build that irks me somewhat:

The holes for one of my KT88 tubes is slightly off-position on the upper chassis. It means that the metal base of the tube sits very close to the chassis (less than 1mm). I tend to tilt the tube slightly to open up that distance.

I had thought about black insulation tape, but worry about it melting due to the hot tube, so I have never done it.

Interestingly, in my KT88s the metal base it tied to pin 1.

My question is: what does pin1 tie to on the SimpleSE board? If I measure resistance from pin1 to ground, I get the same as my cathode resistance.

KT88 specs from Tung-Sol say that pin1 is not connected.

Any explanations, please.

Charlie
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Old 14th January 2019, 03:59 PM   #2
cbutterworth is offline cbutterworth  United States
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Ok, without looking at the board to verify, it seems that pin 1 should be connected to ground, probably via the cathode resistor.

In an EL34, pin 1 connects to grid 3, while in a KT88, grid 3 connect to the cathode directly.

Thus, having pin1 connected in the SimpleSE allows for EL34 valves to be used.

Am I correct in this reasoning.

If I was being really really paranoid, could pin1 of the KT88 be removed, thereby severing the connection between base and cathode resistor?

Charlie
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Old 14th January 2019, 04:25 PM   #3
Tubelab_com is offline Tubelab_com  United States
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Yes, pin 1 in the SSE is tied to pin 8 (cathode) so that EL34's will work in the amp. Some KT-88's and 6550's do have the metal ring connected to pin 1, and metal 6L6's have the entire metal case connected to pin 1. If the metal part of any of the previously mentioned tubes touches ground it will make life very unhappy for the tube and other parts.

If there is a possibility of such a short you can cut the trace between pins 1 and 8 on the bottom side of the board. The board will no longer work correctly with EL34's.

It is possible to connect pin 1 to ground (after cutting the trace) with a short piece of wire. This will make the amp work with EL34's, but at a slightly increased distortion level.

Quote:
If I was being really really paranoid, could pin1 of the KT88 be removed
Yes. I would cut the trace on the PC board, since it's reversible. You can't put the pin back on the tube.
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Last edited by Tubelab_com; 14th January 2019 at 04:28 PM.
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Old 14th January 2019, 06:19 PM   #4
cbutterworth is offline cbutterworth  United States
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George,

Thanks, your answer confirms my worries. The most useful solution is to enlarge the chassis holes somewhat or tape the bases of the KT88s with a high-temp electrical tape.

I suspect that the metal bases of the KT88's easily exceed the 80C max for most electric tapes. I'll have to measure the temp and see.

Any idea of polyimide or Kapton tapes provide sufficient electrical insulation?

Careful use of a dremmel could enlarge the chassis holes sufficiently.

I have also rocked the KT88 tube all around the socket, and I cannot get the metal base to touch the chassis. It get to within a hairline, but doesn't make contact. Still, I'd feel a bit easier if I could see more of an air gap.

Charlie

Last edited by cbutterworth; 14th January 2019 at 06:29 PM.
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Old 15th January 2019, 02:16 AM   #5
Tubelab_com is offline Tubelab_com  United States
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Kapton is what I use for high voltage stuff, and it doesn't mind heat, even soldering iron heat.....it does puncture easily, so avoid sharp things, and maybe use two layers.
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Old 15th January 2019, 04:09 AM   #6
cbutterworth is offline cbutterworth  United States
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Great, Iíll get some ordered. Itíll sure be easier than any other fixes.

Thanks, I appreciate your input and wonderful sounding design!

Charlie
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