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I purchased a Jolida 502P amp as an experiment back in January for use with my Magneplanars (1.7's) and it has been absolutely wonderful. Love the sound and much to my surprise my very powerful SS amp (CJ) is not seeing much use these days.
Now to my question. The amp has Tungsol 6550 power tubes and being that I am a tube newby I check the bias every couple of weeks or so. Three of the tubes have held rock steady bias - never varies - 500 mv all the time. One of the tubes tends to very slowly drift down to maybe 495 mv, then I tweek it up to 500mv and then maybe a month latter it's back down to 495 or so. What causes this? is it the tube? is it normal, is it anything to worry about?
Follow Ups:
Hello a 5 MV drift is nothing to worry about. Most tubes drift according to the fluctuation of the electric. My tubes drift a litlle they are el 34 tubes. When it's starts to drift over10 then start to worry about it.
That's a very small drift but maybe that tube is nearing the end of it's useful life.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Thanks Tre, I'll just keep an eye on it. At what level would it become a concern?
All power tubes eventual fail. When you can no longer bring the bias back up to 500mv it is probably time to replace the tube
Alan
It sounds like you are setting DC bias. Tubes have two characteristics that hve to do with matching: emission and transconductance. It sounds like your amp lets you set DC bias, which is related to emission. Transconductance is a separate characteristic.
Ideally the two tubes in a channel should either be matched for transconductance or the amp should have an AC balance control.
Some or all Scott amps allow both DC and AC bias. The downside is that setting bias is a pain in the ass, as the settings interact, so you approach proper bias as an iterative limit. The upside is that any healthy tubes should work well.
It sounds like the tubes in you amp should be transconductance matched. However, your amp probably has enough negative feedback to make even reasonably unmatched tubes work well.
For the reasons alluded to, it's important to know what needs matched in any amp. Tubes sold as matched often means little or nothing. You have to ask exactly what's matched. Most reputable dealers use transconductance testers, so buying a matched pair for your amp should be ok, and don't worry if they don't match well for emission.
Thanks Alan. When it comes time to replace can I replace just the one tube (the amp has bias adjusters for each tube) or should it be in matched pairs?
with the other one in the channel to verify that it's the tube (not the pot or something else).
When you can individually bias one tube you are fine with replacing just the one tube. I would do matched pairs if you got a pricing deal by buying a pair vs just one tube but replacing one tube is fine
Alan
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