• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

nrs and current reading interpretation

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Jump to Latest
J

jazz

Member
Joined 2001
Hi,

Q: I've build a ktt88 push pull amp using nrs (http://www.triodedick.com/monobill/monobill%20schema%20versterker.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
 
Last edited:
A

Alllensoncanon

Member
Joined 2017
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:
disco

disco

Member
Joined 2006
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%.
 
disco

disco

Member
Joined 2006
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.
 
Last edited:
petertub

petertub

Member
Joined 2015
tubular-well.se
Paid Member
jazz said:
Hi,

Q: I've build a ktt88 push pull amp using nrs (http://www.triodedick.com/monobill/monobill schema 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
Click to expand...
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.
 
J

jazz

Member
Joined 2001
Hi,

Thanks for the replies guys. For now the tubes will stay but the message is to change em sometime this year. As for the coupling caps: russian ky42 at 600V suffice I'd say (i Always rate the coupling caps at hv). Anyway, i checked and the don't see more then 400V at start up.

More worried about the 500V psu cap For the output tubes for that matter... That's what brought it in for service in the first place. I'll increase the ntc at the ac input and up the resistance between that cap and the first stage a bit.

regards,
Joris
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Top Bottom