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nrs and current reading interpretation
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Old 14th January 2020, 06:10 PM   #1
jazz is offline jazz  Netherlands
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Default nrs and current reading interpretation

Hi,

Q: I've build a ktt88 push pull amp using nrs (http://www.triodedick.com/monobill/m...versterker.GIF

after four years i've got it back for service. Been used dailey for about 4-6 hours on average. back then i set it to about 45-50mA per tube, nrs voltage was about -51Vdc.

Now this Original setting gives about 38-39mA per tube. Setting it back to 50mA reguires about -48Vdc. How should i interpret this reading? I guess it's time to change output tubes. But how fast?

Need to give me dad an indication. Is it sensible enough to change the tubes now when it is in service anyway? It's my dad's amp so doing it next year is no problem. What would you advice?

regards,
Joris
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Last edited by jazz; 14th January 2020 at 06:18 PM.
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Old 14th January 2020, 07:47 PM   #2
baudouin0 is offline baudouin0
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Difficult one to answer they have done 5000hrs. If you want to check for low emissions you would need to pass more like 200ma (at lower plate voltage) and a valve tester.
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Old 14th January 2020, 08:03 PM   #3
jazz is offline jazz  Netherlands
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Thanks,

Unfortunately I don't have a tester so can't check that. No immidiate contacts that have it either.

Guess i'm asking what you guys would do. I Know self bias but nrs settings are not in my experience.

Joris
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Old 14th January 2020, 08:10 PM   #4
Alllensoncanon is offline Alllensoncanon  United States
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nrs and current reading interpretation
I would just bias all four tube to 50mA and leave it as is. If you look at a KT88 ultra linear curve at ~400V (plate to cathode voltage), it looks normal still for -48V grid bias to achieve 50mA. No tester is better than in circuit testing which you just did.


The schematic your link posted is fix bias, not self bias.

Last edited by Alllensoncanon; 14th January 2020 at 08:13 PM.
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Old 14th January 2020, 08:32 PM   #5
baudouin0 is offline baudouin0
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Agreed leaving would be fine.
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Old 14th January 2020, 08:40 PM   #6
Alllensoncanon is offline Alllensoncanon  United States
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nrs and current reading interpretation
Correction, it should be ~450V (plate to cathode) for your circuit, my reading mistake. Regardless, at that voltage, -48V to get 50mA is still looks fine. 100% is -49V for ~52mA.
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Old 15th January 2020, 11:20 AM   #7
disco is offline disco  Netherlands
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38/50mA is 76% so there should be some life left. Because characteristics change with tube life (higher ri) it will probably have more distortion because the load is relatively lower than optimal now. Output power will be lower too. Discard the tubes when emission has dropped to 60%.
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Old 15th January 2020, 04:21 PM   #8
Alllensoncanon is offline Alllensoncanon  United States
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nrs and current reading interpretation
Quote:
Originally Posted by disco View Post
38/50mA is 76%

Since the change of plate current is the result of change of Vgrid, there needs to be the tube's "gm" added to the equation. I think the tube is still 90+ % emission from spec (via KT88's Ultra Linear V/I curve).
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Old 15th January 2020, 04:48 PM   #9
disco is offline disco  Netherlands
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Hmmm, I have a different definition for emission than you. For all voltages equal, one compares the DUT to average new values.

"nrs voltage was about -51Vdc. Now this Original setting gives about 38-39mA per tube." So, keeping all voltages equal a drop from 50mA to 38mA leaves only 76% emission.
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Last edited by disco; 15th January 2020 at 04:50 PM.
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Old 15th January 2020, 05:20 PM   #10
petertub is online now petertub  Sweden
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nrs and current reading interpretation
Quote:
Originally Posted by jazz View Post
Hi,

Q: I've build a ktt88 push pull amp using nrs (http://www.triodedick.com/monobill/m...versterker.GIF

after four years i've got it back for service. Been used dailey for about 4-6 hours on average. back then i set it to about 45-50mA per tube, nrs voltage was about -51Vdc.

Now this Original setting gives about 38-39mA per tube. Setting it back to 50mA reguires about -48Vdc. How should i interpret this reading? I guess it's time to change output tubes. But how fast?

Need to give me dad an indication. Is it sensible enough to change the tubes now when it is in service anyway? It's my dad's amp so doing it next year is no problem. What would you advice?

regards,
Joris
First a fault in the schamatics, and if implemented in the amp it must be fixed.
The coupling caps are marked 450V, this will be exceeded at start when

ECC99 does not conduct but bias is about 40V. These caps must be 600V


Second, it might be time for new tubes. 4 years * 4h/day will be about 5000h
Do replace the ECC99 at the same time. Use matched powertubes.
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