dsj1188 Posted February 24, 2019 Posted February 24, 2019 Hi. I bought 8 above tubes for my Audio Research VT100 from the US and they have max 100 hours. They came biased. One of the tubes has blown. How do I get a replacement wirh same bias but not having to go back to vendor in the US. Thanks
frednork Posted February 24, 2019 Posted February 24, 2019 Hi, when you popped the tubes in did you adjust anything on the amp?
andrewinbriz Posted February 24, 2019 Posted February 24, 2019 They wouldn't have come 'biased'. They may have been matched (tubes with similar testing characteristics selected), and unless your amplifier has auto-biasing, you need to bias them. See your amps owners manual.
VanArn Posted February 24, 2019 Posted February 24, 2019 (edited) Amplifiers using fixed bias require adjustment of the cathode current when replacing output valves. The owners manual should tell you if can be performed by a user or if it should be performed by a technician. Edited March 6, 2019 by VanArn typo
VanArn Posted February 24, 2019 Posted February 24, 2019 As andrewinbriz suggests, the KT88 valves most likely have been emission tested only which is not a guarantee that they will be matched running in an amplifier.
dsj1188 Posted February 24, 2019 Author Posted February 24, 2019 (edited) Thanks. So I need to buy a replacement and get it matched to the other 7 by a qualified TECHNICIAN? They were bought as matched octs. Edited February 24, 2019 by dsj1188
frednork Posted February 24, 2019 Posted February 24, 2019 As mentioned above the valves do not come pre-biased. Biasing can only be done on the particular amp you are using the tubes with. Biasing on your amp needs to be done when you change tubes. if this is not done the tubes may not last as long as they would have with biasing and your amp may not operate optimally. So, was the amp biased after the tubes were replaced, and how long ago were the tubes replaced? Tubes with "only 100 hours" bought from overseas can mean anything and many people are duped by unscrupulous sellers. Are they vintage valves or new production valves? Read this http://www.audioresearch.com/ContentsFiles/VT100 MKII_BiasAdjust.pdf if it makes no sense to you you will need a tech. If you want to match the valves you have you will need to get them tested on the same model of calibrated machine the seller you buy from uses as different testers give different results. These are a few things to consider...
frednork Posted February 24, 2019 Posted February 24, 2019 Also if the tubes have been used for years and 1 has gone it may be time to replace the whole lot, they still need biasing though. I suspect you may need to take the amp to a tech who knows about tubes and take it from there.
VanArn Posted February 24, 2019 Posted February 24, 2019 Experienced technicians do not use emission valve testers. A high power valve amplifier should provide independent adjustment of fixed bias to each valve and also a means of adjusting the push pull balance. This is because valves cannot be relied on to age equally despite being matched by some sort of valve test.
dsj1188 Posted February 24, 2019 Author Posted February 24, 2019 Afer I bought them I has Eddie Lee biased them for me in 2013. I had them for 2 years with little use and then had it stored until now when 1 blew up. I will try and contact him again to source and fix it for me.
Mat-with-one-t Posted February 24, 2019 Posted February 24, 2019 1 hour ago, dsj1188 said: Afer I bought them I has Eddie Lee biased them for me in 2013. I had them for 2 years with little use and then had it stored until now when 1 blew up. I will try and contact him again to source and fix it for me. You’ll need to look up what “biasing” refers to, as mentioned by members here. Perhaps have a read here: https://wallofsound.ca/audioreviews/amplification/output-tube-biasing-an-introduction/ 1
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