HOT LF-out tube

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LA1HU
Posts: 9
Joined: 29 Sep 2019, 00:28

HOT LF-out tube

Post by LA1HU » 22 Jan 2020, 22:24

Hi all

I find that the LF out tube becomes much hotter than the RF tube, hard to put the fingers on. That's something I haven't noticed until now. Have therefore replaced all the capacitors with new ones, checked that everything else is normal. No immediate errors found. Choke measure to 33H / 885R. I also have cross-changed the 6SK7tubes, as well as tried a new 6SK7. It still gets too hot in my view. Does anyone had that experience ?
The paraset work great but got a little worried about this TOO hot LF tube.

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Parasetguy
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Location: The Netherlands, Velp
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Re: HOT LF-out tube

Post by Parasetguy » 22 Jan 2020, 22:44

Hi Jan
Measure the current of the Plate. It must not exceed 13 mA.
Check Grid#1 voltage. Should be minus 1 V.

Regards
Henk

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LA1HU
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Joined: 29 Sep 2019, 00:28

Re: HOT LF-out tube

Post by LA1HU » 23 Jan 2020, 17:02

Thanks Henk.

Measured here:

Pin 4 grid1 = + 1.1 mV.

Anode current 6.8 mA.

Anode Pin 8 = 210 V

I see on the voltage map that g1 says g1 = 0 volts ?

Do you think the value of cathode and g1 resistance is critical?

Cathode resistance here is 254 ohms, and the

g1 resistance is 258 Kohm

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Parasetguy
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Location: The Netherlands, Velp
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Re: HOT LF-out tube

Post by Parasetguy » 03 Feb 2020, 11:42

As I have problems myself with getting the transmitter to work, I stumbled upon this site:
https://qsl.net/ve7sl/paraset.html
It has diagrams with measurements (read about his PSU) that can be helpful. And interesting mods.
Pin 4 grid1 should be negative to get amplification going. Tube handbook says - 8 V.
Cathode resistance and the g1 resistance are within 10 % tolerance, so not critical.

Interesting to understand the process:
https://www.youtube.com/watch?v=065xngzQzuk

https://www.youtube.com/watch?v=065xngzQzuk

success!

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