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EQUIPMENT FORUMS >> ZMA >> Question about tube replacement
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Message started by pineman on 04/04/21 at 22:54:55

Title: Question about tube replacement
Post by pineman on 04/04/21 at 22:54:55

I have been enjoying the ZMA for over six years now and recently replaced all 4 output tubes when one of them began to fail. Is it generally advised to replace the input and regulator tubes at the same time, or do those typically last much longer?
Thanks!

Title: Re: Question about tube replacement
Post by HockessinKid on 04/04/21 at 23:06:15

You don't need to replace the input tubes if your output (power) tubes are getting noisey. The stock input tubes usually last a long time.

I suggest you get a new matched quad of output tubes. There are several very decent sounding new production tubes - TungSol, Gold Lion (New Sensor), etc. I really like the sound of TungSol 7027a substitute tubes for the KT66 tubes. A matched quartet can be purchased online for ~$120. Easy swap out, I bias these at ~58 mV.

HK

Title: Re: Question about tube replacement
Post by Archie on 04/04/21 at 23:32:45

I concur about the Tung Sol 7027As. I bias at 64 to 68mA though. My last quad seemed short lived but I only got 8 to 9 months out of the quad before that. Maybe my high bias is the issue? How long are yours lasting? I pull mine when one or more starts to drift.

Title: Re: Question about tube replacement
Post by HockessinKid on 04/04/21 at 23:59:48

Archie,

I've been getting about 18 months on a quad, guessing 1600-1800 hours. I calculated bias at 58mV using the plate voltage. I think your biasing a bit high which will lead to shorter power tube life.

Happy Easter.

HK

Title: Re: Question about tube replacement
Post by Archie on 04/05/21 at 00:12:11

I'm in the "Class A" range which is a 90%+ so I think you are right. I think I'll back it off and see (hear) is there is much difference. I didn't realize there might be a 2X effect on tube life.