Best Approach to Retubing

[Robert]

Hi,

I may have one or more faulty tubes in an 18 tube preamp. Do people isolate and replace the faulty tube, or do they generally retube the whole unit when 1 or more tubes start to go? Some, like the 5AR4s are very unlikely to be faulty.

What I hear is a rustling/crackling sound that varies, and is more prominent in one channel. It is actually more difficult to isolate that I thought, having flipped pairs of tubes to find the culprit. Now, I wonder if there is more than one tube, or whether the tubes are interacting in a way that makes it difficult to sort through.

Also, I do not have a tube tester, and I am not motivated to buy a tube tester. Is there someone who offers a service to thoroughly check tubes for a reasonable cost? There may be some guitar amp repair places in my town, but nothing specialized for audio.

Thanks

 

[wineslob]

What I hear is a rustling/crackling sound that varies, and is more prominent in one channel.

Have you cleaned your volume pot? I had the same issue with my SS pre.

 

[microstrip]

Although using some common sense and basic knowledge of the preamplifier topology can usually reduce the number of possible guilty tubes, first thing I would ask the manufacturer advice - they usually know their product much better than us and can spare you a lot of time and swaps.

 

[mep]

Robert-It doesn't make sense to replace 18 tubes if just one went noisey which sounds like what happened. The usual culprits in a line stage preamp are the line stage tube(s) and that is where I would start looking. You need to convince yourself whether you are hearing the noise from one channel or both. Both channels will make it tricker if that is the case. If it is one channel, I would start swapping tubes from the bad channel one by one to the good channel until the noise moved to the "good" channel. You should have an "ah hah" moment and determine which tube was causing the problem.

 

[microstrip]

Just found that you have a BAT REX . As I have a system with 16 6h30 I know that, although they last for a very long time, during their first 600 hours break-in period sometimes a few of them become noisy (ARC even ships a spare pair with their Anniversary preamplifier) .

I think you should have some in house testing capability for the 6h30 . Unhappily, available tube testers of reasonable cost do not test noise. My advice - buy a very cheap used tube preamplifer and get someone to modify it to become a two channel single triode tube preamplifier using the 6h30 . Then test your tubes individually just listening to this preamplifer. It will save you a lot of time and effort, and money on the long term.

You can fin many schematics of such circuits using the 6h30 in the DIY audio forums.

 

[Robert]

OK. I think I found the bugger. Sounds like people isolate and replace, rather than just retubing the whole set. Maybe every 3-4 years, it would be good to start with a fresh set.

If I ever talk about getting a high-powered tube amp, please shoot me.