newb here! Q: mismatched tubes

[km-r]

hello its my first PP amp build and im wondering if mismatched tubes will kill my amp (or me)...

i bought one NOS philips and one NOS **** el84/7189 for use with the 18watt.

is this not a problem with cathode biased amps?

im very excited in building this amp, and ill be back in a while...

[CurtissRobin]

The matching of tubes is nowhere near as important as tube sellers would have you believe. If you're going for something specific then matching could be the ticket but even extreme mismatches won't physically hurt your amp. It may just sound a little off from what you want but the components won't much care. OTOH it may sound just the way you want it to sound if the power tubes are really distant relatives rather than twins or kissing cousins. I say use what you bought and if it's off, analyze what you're hearing and make appropriate changes.

KennyO

[Wicksy]

You dont want them too far off otherwise they'll draw dissrent amounts of current and things could go nuclear....on the other hand you dont want them too matched either otherwise they sound very sterile and boring

[CurtissRobin]

You dont want them too far off otherwise they'll draw dissrent amounts of current and things could go nuclear...

Define "go nuclear" please. Randomly selected tubes are unlikely to be so mismatched as to cause any harm. The worst case with a pair of good but wildly mismatched tubes is that the tubes are biased differently and one half of the OT primary works a bit harder than the other. It's extremely unlikely that the bias difference will have one tube cranking and the other redplating. With a mismatch, if the OT runs warm enough to cause damage then it's surely the wrong OT for the application. Separate (as has been often recommended here), properly sized screen grid resistors will take care of the rest.

I emphasize that I'm talking about properly working tubes. If a tube is bad then all bets are off.

KennyO

[Wicksy]

Thats general consensus (that i posted before). Funny though because i got off a German Ebayer about 8 RFT EL84s for about £1 each a few years back. They were all untested and unmatched. I discovered that all but one tube worked perfectly in any combination but one tube kept redplating. Likely that it was a bad tube and just overly stressed and died.

I would of thought that fixed biased power supplies would suffer with bad bias? If one side pulls too much current then they're going to have to dissapate more power surely? Plus it makes biasing a bugger.

No-one biased their gear much back in the day though did they? From what i gather you could just plonk in a pair of tubes bought at your local pharmacists without any matching....as long as the date codes were the same you never got problems.

[CurtissRobin]

I would of thought that fixed biased power supplies would suffer with bad bias? If one side pulls too much current then they're going to have to dissapate more power surely? Plus it makes biasing a bugger.

Yeah, one tube will dissipate more than the other but the difference between randomly selected tubes (as opposed to matched tubes) will be more in the sound than in the life of the tubes. Even the character of the sound isn't likely to diverge all that much as you plug in different tubes.

No-one biased their gear much back in the day though did they?

Well, yes they've always biased tubes or the circuits wouldn't work. They just didn't bother pairing up tubes with certain matching characteristics because it wasn't deemed terribly important. It's a lot less important today than some of us make it out to be.

KennyO

[Colin_D]

If you search the forum you'll find plenty of threads on tube matching. Here's one where some particularly highly regarded amp builders/techs chimed in... http://www.18watt.com/modules.php?name= ... sc&start=0
Look at sgtoverdrive's first post in particular. That guy knows what he's talking about!

[ajkimba]

Yes, but isn't it a different story for fixed biased amps?

I know, in the link from the past, that they where talking about fixed bias and just popping in a tube....?

I have just popped tubes in a say a 65 bassman head and sometimes it didn't sound good till I biased the tubes or sometimes the balancing pot did the trick. I almost always changed the balancing pot to a fixed bias pot, is this wrong?

No difference betweeen fixed and cathode as far as matching tubes?

[zaphod_phil]

Not really any real big difference in principle between fixed-bias and cathode-biased amps here IMO - as long as the tubes in a pair aren't a long way out from each other, so you get one red-plating or the tone sounds poor. I agree it's a good idea to go with what your ears tell you.

[Wannatone]

Yes, but...
in an 18 watter the output tubes are running close to meltdown...

So, if they are not biased (selected) correctly, you're asking for trouble.

To see what's the difference between fixed and cathode bias, just try to pull one of the EL84's to see what I mean...

Accidental mixing aint no good I tell ya.

There is nothing wrong with mismatching within reasonable limits.
Just make sure none of them exceeds 100 percent of the specified plate dissipation and you'll be fine.

[ajkimba]

From experience, I think Phil's let your ears tell you is the best and I agree that too far a difference from each other can cause excessive hum, poor sound, etc.

I usually buy my tubes in sets, therefor they are closely matched, hopefully. I'm a NOS pull guy, and I have a lot of tubes that are USA old school "killer tubes" but unmatched. I don't have tester to match so I usually use an amp that has been set-up to bias with a voltmeter and try to get reading close to the same mA draw. This seems to work fine for octal tubes. As far as 9 pin power tubes cathode biased, I just pop them in listen and go from there. Maybe changing the cathode R if anything close to red plating or try another tube.

I will say, as far as poping them in, sometimes you get a killer sounding unmatched pair with all kinds of good harmonics. The only down fall is the wear and tear on the sockets. The best quality sockets are in order.

[Brewmaster]

Maybe changing the cathode R if anything close to red plating or try another tube.

I will say, as far as poping them in, sometimes you get a killer sounding unmatched pair with all kinds of good harmonics. The only down fall is the wear and tear on the sockets. The best quality sockets are in order.

I agree with this.
I went to a 150Ohm Cathode R and eliminated red plating on some of my NOS tubes. My RFTs in particular were not usable until I did. Changing the 3w metal oxide power resistor from 100ohm to 470 ohms helps as well.