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Old 1st October 2018, 09:34 PM
Garyk Garyk is offline
Junior Member
Join Date: Oct 2017
Location: Farnham
Posts: 10
Default Kt5881 bias

This is my first post here.
I'm not really worthy, as I cheated and bought my lovely KT5881 2nd hand and fully built.
it was my first dabble into valves and it has changed everything.
I know nothing about electronics.....I can solder some plugs on interconnects and that's my lot.
Currently the power valves are sovtek 5881 WXT
then 2 x mullard EF86 and 2 x sovtek 12AX7WA
I recently swapped the 12AX7WA for groove tubes.....and tried to run them in but couldn't wait to get my sovtek back in their rightful place
I have ordered a quad of valves to try on the back line.
after all that babble, my question is

does my amp have a self biasing mechanism or do I need to get my hands dusty.
I understand the principles / necessity of biasing etc and happy to have a go with my multimeter.
but equally happy to leave well alone.
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Old 2nd October 2018, 08:11 AM
snowman_al snowman_al is offline
Join Date: Nov 2016
Location: West Wales
Posts: 60
Default Re: K5881 bias

Morning Gary and welcome.
Your K5881 amplifier is a type known as Cathode Bias. (Better called Self Bias or Automatic Bias as well.)
The output valves use a fixed value cathode resistor to set the bias and cannot be adjusted. You just replace the valves. A matched pair in either channel might give very slightly better performance, but do not worry if they are not.

I like the Sovtek 12AX7 too, it is my go to ECC83 / 12AX7 if I need a replacement these days.

Last edited by snowman_al; 2nd October 2018 at 08:15 AM. Reason: typo
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Old 2nd October 2018, 08:22 AM
bob orbell bob orbell is offline
Senior Member
Join Date: Nov 2009
Location: kettering northants.
Posts: 1,786
Default Re: Kt5881 bias

Hi Gary and welcome, from your post, it seams that you do not like the new Groove Tubes, but you don't say why, I personally think they are very good, but do take some time to, if you like, run in, my advice to you is , if you like what you hear, then leave it alone, you can spend far too much money on valves as many on here will tell you. BOB
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Old 2nd October 2018, 12:02 PM
Garyk Garyk is offline
Junior Member
Join Date: Oct 2017
Location: Farnham
Posts: 10
Default Re: Kt5881 bias

Thank you very much for the bias info, that suits me nicely.

yes, re the Groove Tubes, I was experimenting with the groove tubes and maybe I may sound mad but, but they sounded ....gritty? like they are made for a heavy metal guitar. you know that sharp silence between the notes was gone. It just my taste and sometimes you cant explain it other the result of involuntary toe tapping or not.
I was surprised there was that much difference.
I know I needed to burn them in but I missed my old sound.

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Old 2nd October 2018, 01:37 PM
Phil Y's Avatar
Phil Y Phil Y is offline
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Join Date: Jul 2006
Location: Bristol, UK
Posts: 737
Default Re: Kt5881 bias

Hello Gary,

Different valves, and all components come to that, make a difference. Sometimes very subtle, sometimes more obvious. If you prefer the Sovtek then they are better for you. I have never used Groovetubes myself so I can't comment on them. If the amp is fairly new to you and you are enjoying the sound, then as BOB says, leave it alone for now. I know it is tempting to swap valves as there is so much talk about it on the web but in my personal view, it is generally more about fine tuning the sound to your taste rather than making huge leaps forward.

Commission for Dark Skies (CfDS)
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