Replacing Tubes

shamsteak

Hi all,

I'm not too well up on tube replacement but i was wondering is it possible to replace just one 6L6 GC STR tube without having to rebias the amp? I think one is blown or damaged as theres a build up of white powdery spots inside the glass..It's a decent amp, a Hughes & Kettner Statesman Dual 6L6 only a few months old and it started this crackly screaming noise with a big loss of power in the middle of a gig on sunday night!! There is less than 50 hours of use on the amp and it is well taken care of..

Any advice much appriciated!!

Paolo_M

Couple of things.

You should not replace only one power valve ever (unless of course it's a single ended 5 watter! ). Preamp valves are different.
Power valves operate somewhat like the tyres of a car. Put a brand new tyre on one side will cause the other to wear even faster, leaving you with the same problem in 3-6 months time, so what have you really achieved?
Power valves that are not reasonably closely matched will hum more.
Power valves that are not reasonably closely matched are prone to more over-current conditions and blowing fuses.

When replacing power valves you should always recheck and adjust, if necessary, the bias level on the amp. Again, to use the tyre anaolgy, bias is a bit like tyre pressure. It drifts over tme naturally of time, and too much or too little is not a good thing. Different valve sample require different bias levels to give the same performance.
With incorrect bias range; at best your tone (think cars general performance) will suffer, at worst you'll blow fuses and burn out.

My advise is to spring for a new matched set of power valves and have someone check the bias levels.

FarmerGreen

Just a couple of points.
Not a good idea to take a hot amp outside into the cold air, it can crack the power valves.
Have a good look for dry joints and signs of discolouration on the valveholder.

darrenw5094

Paolo_M wrote: »

Couple of things.

You should not replace only one power valve ever (unless of course it's a single ended 5 watter! ). Preamp valves are different.
Power valves operate somewhat like the tyres of a car. Put a brand new tyre on one side will cause the other to wear even faster, leaving you with the same problem in 3-6 months time, so what have you really achieved?
Power valves that are not reasonably closely matched will hum more.
Power valves that are not reasonably closely matched are prone to more over-current conditions and blowing fuses.

When replacing power valves you should always recheck and adjust, if necessary, the bias level on the amp. Again, to use the tyre anaolgy, bias is a bit like tyre pressure. It drifts over tme naturally of time, and too much or too little is not a good thing. Different valve sample require different bias levels to give the same performance.
With incorrect bias range; at best your tone (think cars general performance) will suffer, at worst you'll blow fuses and burn out.

My advise is to spring for a new matched set of power valves and have someone check the bias levels.

What would happen if you had two brand new power valves at home. And then bought two more, put all 4 into the amp?

Would they still be matched, as they are all new??

Organeer

The answer to the last question is NO, they will not necessarily be matched. Valves are made within certain tolerances and individual valves can vary quite a bit in terms of their outputs. Different factories make similar versions of the same valves, but again output varies. Matched sets cost more because someone has to test the output from each valve, & team it up with a valve of similar output, a time consuming exercise, as each valve has to heat up properly before it can be tested. Tube Amp Doctor sell excellent valves. There is probably a stockist here in Dublin. If they are any good they can probably bias them as well for a small charge. Then you'll know it's right.

darrenw5094

Organeer wrote: »

The answer to the last question is NO, they will not necessarily be matched. Valves are made within certain tolerances and individual valves can vary quite a bit in terms of their outputs. Different factories make similar versions of the same valves, but again output varies. Matched sets cost more because someone has to test the output from each valve, & team it up with a valve of similar output, a time consuming exercise, as each valve has to heat up properly before it can be tested. Tube Amp Doctor sell excellent valves. There is probably a stockist here in Dublin. If they are any good they can probably bias them as well for a small charge. Then you'll know it's right.

Even if they are all JJ valves? Or at least the same brand??

Organeer

Sorry, yes, even if they are all the same brand.

FarmerGreen

.

Eoin Madsen

darrenw5094 wrote: »

What would happen if you had two brand new power valves at home. And then bought two more, put all 4 into the amp?

Would they still be matched, as they are all new??

As already said, they would not be matched, unless by some fluke. The brand is not really relevant, nor is the newness of the valves.

But, if the two you had were a matched pair, and the two new ones were also a matched pair, you could still balance the two sides of the output. Put one pair on the inside two sockets and the other on the outside sockets. Biasing will take a little while longer cause you want to be sure one pair isn't going to melt down while you set bias to the other. But once you figure out which needs the cooler bias, you just set the bias voltage to suit that. Depending on how different the pairs of valves are, you might end up with the other pair biased a little cold but that's not really a huge problem. Unless you find yourself disliking the sound, of course.