Author Topic: biasing power tubes  (Read 10534 times)

Offline sleepyguy24

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Re: biasing power tubes
« Reply #15 on: January 04, 2013, 07:25:28 AM »
Hi Rich

Thanks for the prompt response. I reviewed things with the Jolida and the Tung Sols I have in there and it turns out the set I have in there aren't fully broken in. The guy I bought the Tung Sol KT-120 tubes from bought them from TC Tubes with them partially broken in. He then told me he only put a couple of hours on the quad before he sold them to me. I believe from another thread here tubes need about 100 hours to stabilize. Also the CV-4024 tubes as well as most of the various 12AX7 tubes I've been rolling in are new and not fully broken in. Too many variables. I secured the tube cage for the Jolida in so this will help me to not roll in tubes.

To your question about the KT-120 tubes I do love them but my experience with the family of 6550 and KT-88 power tubes and tubes in general are limited. I have a quad of Penta Labs KT-88 tubes and when I compared the KT-120 tubes to those I found the bass to be better in my system with the KT-120s in place. I also have a quad of NOS Sylvania 6550 tubes but I haven't listened to them yet. These are expensive tubes to replace so I don't want to use them so much.

Thanks again for the help.

Offline richidoo

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Re: biasing power tubes
« Reply #16 on: January 04, 2013, 09:35:40 AM »
That makes sense. When a new tube is first powered up it changes fast and big, you can watch the bias change drastically in real time on the meter for the first few minutes. Adjusting once a day is wise for the first week to prevent a runaway. You will need to adjust bias on them frequently in the beginning, then tapering off until they are stable. With New Sensor tubes this is pretty quick. As long as they are within 10% of mfg bias spec they are safe, in your case, within 50mV of spec.  You are probably already up to the point of checking once a week. If two weeks require no adjustment, then switch to once a month or two, depending on usage. When they start to change again, plan for a new set within 6-12 months, depending on usage. Does your Jolida bias individual tubes or by pair or quad?