Concerns about tube replacement

Pure_Brew

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#1
I have a stereo push/pull tube amplifier (2 x 2 6550) and one of the power tubes needs to be replaced, to me it makes sense to replace all, given the usage is 2k-3k hours on all 4. However, that also means that I'd be pulling 3 tubes that are still actually working.

The one tube in question has gotten dim and I cannot crank up the bias to give me more then 11mv on it, and it should be around 50mv.

My guess is that even if I bought 1 new tube, I would need an entire suite of measurement equipment to see how to match them up. Even then I'd still be trying to match to the used ones which I'm not sure is even possible.

Are the 3 used and functioning tubes good for anything? Any testing suggestions?

Thanks
 

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#2
Joe: Short of a tube tester that's all I can think of. There has to be a shop near you which has a tube tester. That service is offered free of charge at Innovative Audio. I have some tubes, but don't know if I will have 4 of the same.

Nando.
 

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#3
If you can bias each tube by itself you should be able to change the bad tube only. Also does the problem follow the tube or position
 

Pure_Brew

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#4
Thanks guys. I did swap positions and it is indeed the tube.

Now I haven't tried to rebias all the tubes for awhile. Is it possible that they will not be able to be biased at the recommended voltage (50mv)? I'm not sure what goes on when tubes get old, or how important matching them is. Would a single new tube just get biased on the amp at a lower voltage to match the older 3?
 

Skratch

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#5
What kind of amp is it? I have 50 year old tubes that perform as good or better than the reissued version of the same tubes. The old Mullard, Tungsol and Genelex Kt88 and 6550 as long as they were not underbiased test and perform perfectly. I an a firm believer of tube rectifiers as they do not shock the cathodes as SS rectifiers do.
 
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Pure_Brew

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#6
It's not tube rectified. I wondered about that - and I suppose that might just be a cost cut to use some cheap diodes to get the job done. Power tubes are Tung Sol 6550 reissues. I've heard tales that original tubes were longer lasting when tube production was in full swing.

Still, I've got what got.

Here is a schematic I found.
 

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Skratch

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#7
It looks like you can bias each tube so I would just replace the bad one. You can buy single reissue Tungsol 6550's on E-Bay for around 30.00 I would check R17 & R18 before putting in the new tube, They are 1 ohm. Also check resistors 9,10,11,12,13 and 14. If they all check out fine you should be good to go.
 
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#9
Any pics of the amp, upskirt please... ;)


If you are worried about fast start just tack in a thermistor and this will give you slow start with out the quick rush to your tubes.
 

Pure_Brew

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Any pics of the amp, upskirt please... ;)


If you are worried about fast start just tack in a thermistor and this will give you slow start with out the quick rush to your tubes.
One of these days I'll open it up. Not impressive imho as I recall....

As far a start up goes, there is a warm up period for the tubes at turn on, then the relay kicks in. Not sure of the function, might be a delay to just the speakers.

I ended up just buying another set of tubes recently. If anything, I'm glad to have it back up and running finally.
 

Pure_Brew

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#11
I remember now that the manual that came with the amp stated power tubes would last around 3000hrs. All things considered, that's pretty much what I got out of these before one went.
 
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