Output tubes drawing different currents

[rutledj]

For some reason my matched 6l6's don't seem so matched any longer. One idles at 35 while the other idles at 26ma.

Is it possible for the tubes to change like this? These are practically new tubes and did draw within 2ma of each other when first installed.

Thanks
Rut

[heisthl]

Welcome to the rip off of tube matching, totally normal to be different after a little run time.

[phsyconoodler]

It is a very common thing for this to happen.What does the amp sound like now?Sometimes a mismatched set of tubes sounds better than a closely matched set.The quality of new tubes these days is a problem and reliabilty is an issue.
Some guys like to use dual bias setups to help with this phenomenon and also to purposely mis-match bias to experiment with tones.
Make sure the amp is fully warmed up before you measure current.
there are lots of other things that can make tubes drift apart like that too.Make 100% sure that all solder joints around the tube sockets are secure and check all screen resistors.If this is a new build it can make you go crazy thinking you have a huge batch of bad tubes.Also try swapping tube positions and see what happens.
And your readings may only be as good as your testing equipment.Use fresh batteries in bias meters and make sure all 1 ohm cathode resistors are actually one ohm.Or at least exactly the same from one tube to the next.
Lastly,check the output drive line capacitors and make sure one isn't leaking.

[ampgeek]

Another argument for making individual bias pots a standard practice for us homebrewers.

I am very curious as to why it isn't a standard practice. Sure seems like the small cost of an extra pot and associated items would be more than made up by avoiding what appears to be endemic tube matching issues.

Reliability issues? Anything else that should be considered?

I couldn't live without it in my D'lite build.

TIA,
Dave O.

[talbany]

Who was it that came up with the idea that mismatched output tubes sound good. Try it you might like the tone of mismatched output tubes. Yeah maybe if you are Kurt Cobain and don't really care about how you sound.



VVT

[phsyconoodler]

Nice how you take it out of context by omitting some key words.Like'sometimes'.
Using Kurt Cobain as a reference to anything musical is kind of like saying Kenny Rogers looks good with his new facelift.At least we agree SOMTIMES.
SOMETIMES is the key word here.Not every amp responds to mismatched tubes.But wouldn't it be nice to be able to experiment and find out for yourself?And at the very least you could get those mismatched ones drawing the same current with dual bias.

Hot-rodding an amp doesn't always mean add more gain and put in bigger speakers.It can mean fine-tuning what you already have in there.Blueprinting the stock package.
Kurt Cobain?Wasn't he a whiny brat that won some prize for making teenagers more screwed up than they are already?

[talbany]

Don't get me wrong I am all for independent bias pots for output tubes. I put them in all of my personal builds, especially if you like to use NOS stuff you can squeeze every bit of life out of them. Sorry if I offended anyone with the Cobain reference I read in an interview with him that he had one of his amps serviced and had new tubes installed and didn't like the sound of new tubes the older the better he said. Anyway I respectfully have to disagree with anyone who says mismatched tubes sound better in any amp.


VVT

[phsyconoodler]

No offence taken!
I just like music and to have Cobain and Music in the same sentence is painful!
I had an old Peavey ultra that was a nice sounding amp.It had one power tube on it's way out and it sounded glorious!I loved that amp and then one day I decided to get smart and look at the tubes while playing.One was glowing quite a bit brighter tha the other so I got a matched set of kich-ass tubes and popped them in and optimized the bias.The amp never again sounded as good as when that tube was going south.I popped the old tubes back in and was rewarded with that killer tone again.Of course it didn't last very long until the one tube finally died.Ever since then I have experimented with mis-matching.It dooesn't always work,but when it does the sounds are incredible!

[Structo]

Just like a race car, they always sound the best right before they blow up!

[heisthl]

Try balancing the PI on that Peavey.

[rutledj]

Well, swapping the 2 output tubes makes the reading switch (the same readings follow the tube) so it is the tubes themselves.

Does using dual bias adjustments and setting them to matched essentially give you a matched pair?

Rut

[heisthl]

yup, at least at idle, as an experiment try leaving your meter connected and put a signal into the amp amd see if they match as the volume goes up. If they do - you have a matched pair(assuming your PI is balanced).