# Answer to Buddha about KT-77's



## Wild Bill (May 3, 2006)

_*Hello Bill,

first off, I would like to thank you for sharing your knowledge regarding tube amps on this board. It's always nice to have someone who knows what's what around .

I hope you weren't expecting some techy question, because this one is far from it haha - would you happen to have a spare KT77 in your shop? I had a KT77 in my JSX go microphonic, and I've been using the stock EL34's in it again, but I really miss the KT77's! I don't think it's possible to buy a single online, and when I put an ad up on our buy/sell, I just got a link to the tube depot (which I checked).

If you or anyone you know has a spare KT77 to part with, I would love to get it! Unfortunately I don't know what value the KT77's I have now are - matched quad from eurotubes, and my amp has not been biased (been meaning to get that done, i believe Island amps in london could do it..).

Thanks for any help, have a good one!

Justin

PS: I'd love to learn all about the circuitry side of gear, but to be honest I'm just not terribly interested 
*_

Actually Mr. Budda, this IS a techie question!:smile: Normally I won't answer PM's from this board. I get so flippin' many that I'm always hitting my msg limit! I prefer to answer questions in public forums. That way everyone gets to see them and get some benefit. Also, hopefully it might cut down answering the same question repeatedly. I make my living fulltime from building and repairing amps. I enjoy helping out with tech answers but my time is always being strained. What's more, I'm older and fatter than I used to be so I value what little free time I get even more these days!:smile:

However, your main question is whether I have just ONE KT77 for you! The short answer is no but the long answer is, I woudn't do that to you! My conscience wouldn't let me!:smile:

You see, you mentioned you don't know anything about the other 3 tubes. That means its a total crap shoot as to whether the new one matches the others. If the mismatch is severe it could cause you expensive problems.

The reason tubes are matched is so that when you use them together as a set they will all share the load more or less equally. If one tube is much colder than another it will carry very little of the load. The other tubes have to pick up the slack. So they get overloaded and burn out early, while the cold one lasts forever.

Even more, your amp is designed to have the tubes on one side of the output transformer winding handle roughly half the power and the same with the tubes on the other side. If one side is running much hotter or colder than the other then you get big swings of excessive current in the output transformer, which is inviting it to burn out.

You CAN buy just one KT-77! Just contact http://www.thetubestore.com and ask them! However, this still doesn't solve your problem about getting one matched to the rest you have. Mailing one of the other tubes to thetubestore is an option. They're good people and could test the tube and sell you another one with the same bias number. This will cost you some postage but much less than buying a new quad of KT-77's. 

Sorry if I'm being long winded when you had said you weren't interested in the techie stuff but in this case it would be in your interest to learn. You don't want to cheap out over a tube and cause an expensive repair bill to the amp.

Good luck with it!

Wild Bill/Busen Amps


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## mrmatt1972 (Apr 3, 2008)

I want to start this off by saying that I agree with Bill 100% Buy a matched set from the tube store, have them biased correctly and after that you can use the number system to replace singles.

However, Kevin O'Connor of London Power amps thinks that tube matching is hogwash. See this: http://www.londonpower.com/faq.htm

scroll to the bottom. It's an answer that you might prefer in that it is the cheaper option.

matt


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## Wild Bill (May 3, 2006)

mrmatt1972 said:


> I want to start this off by saying that I agree with Bill 100% Buy a matched set from the tube store, have them biased correctly and after that you can use the number system to replace singles.
> 
> However, Kevin O'Connor of London Power amps thinks that tube matching is hogwash. See this: http://www.londonpower.com/faq.htm
> 
> ...


Well, here's where Kevin and myself disagree! Kevin is partially right but I don't think he has thought deeply enough.

It's true that in the old days guitar amp makers like Fender never worried about matched tubes. However, tubes made in the Golden Years were MUCH closer to the datasheet specs! Modern tubes can be all over the place as to what bias voltage produces how much plate current. I submit that he hasn't taken that into account.

Fender obviously did! Why do you think he put a bias adjustment pot in the amp in the first place? And later on something for bias balance?

Aside from that, I would be the first to agree that many folks today are too "anal" about close matching output tubes. Maybe it matters in hifi circles but even then I doubt if the ear could detect any advantage. A few ma. difference in plate current from bias differences sounds better to me in a guitar amp, adding more "swirl" and harmonics.

What Kevin seems NOT to have considered is that sometimes today tubes are WIDELY mismatched! Not just a few ma but 10, 12, 15 ma or even more. This DOES cause the problems I've described! Here we are out of opinion territory and into fact. Just try it! Test it by experiment! Anyone can see the problems. There is no discussion in such cases. You can't argue that something exists when you can reach out and touch it!

Ask my customer from the band Stone River with the DR. Z amp that hummed BADLY from the day he bought it! Being suspicious, I checked the EL34's and found that they were about 18 MA. different in idle current! This was a severe imbalance in the output transformer. No wonder there was so much hum. A new pair of tubes totally cured the problem. The good Doctor Z's amp sounds fabulous now, as of course it should.

If Justin wants to just spin the wheel and jam in any old KT-77 he'll probably not blow his amp up but then again, people do win on lottery tickets now and then. It's not that hard to get a tube tested. I doubt if thetubestore.com is the only store that would offer this service, although others may not have the test equipment. Why take the chance?

So my advice to Justin stands. How lucky does he feel?

:food-smiley-004:


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## caaustin02 (Nov 1, 2007)

Thanks Guys,

You have me thinking, and I thought of a few questions:

1. What wears out in an O/P transformer? This is important to me because if I have an amp that has old transformers that are not available anymore then I'm stuck.

2. If I have bought a used old amp, and was biased with new O/P tubes straight away is there a cumulative effect if the tubes prior to biasing were very unbalanced?

3. Can a beefier O/P transformer such as a big Partridge type handle this imbalance better by taking more abuse?

4. Does playing loud make the problem worse, or is the imbalance of the O/P tubes always hard on the O/P transformer?

Good Thread, thanks guys.


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## Wild Bill (May 3, 2006)

caaustin02 said:


> Thanks Guys,
> 
> You have me thinking, and I thought of a few questions:
> 
> ...


Transformers are very rugged and relatively simple devices. They are essentially coils of copper wire wrapped around some iron. They don't wear out, per se. I suppose if they were in a wet environment after a century or two they would rust away but that would be it.

What burns them out is excessive heat, caused by too much current flowing through the wires. Think of trying to boost your car battery with cables made of thin little wire. PFFT!

As long as the excessive heat was never enough to actually burn out a wire it's not likely that you could get much of a cumulative effect over the years. The copper is just fine after it cools down. If it stays within its safe temperature range then it would look and perform no different!

Bigger transformers can handle heat better but again, if they don't get too hot then it doesn't really matter.

If you have a bad imbalance in your output tubes then yes, playing louder does cause more heat. Imbalances cause big gobs of excessive current to flow in the transformer windings, causing more heat. Playing louder means more current. That's where the energy for the speakers comes from, after all. This would mean that playing softly would likely never burn out the transformer. Playing for hours on 10 would be asking for trouble!

Again, if nothing burned out and the imbalance was corrected I wouldn't think the transformer would have much cumulative damage.

Another point with transformers is that you can have a scary amount of rust on the outside and they still work perfectly! I've used transformers in my junk box that are nearly 70 years old and they still work just fine!

:food-smiley-004:


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## Budda (May 29, 2007)

Bill, I greatly appreciate your response!

I will swing by the music store downtown and ask if they would happen to have a tube tester handy in their store (they sell some high-end equipment, i hope they do) and have the KT77's tested. They're supposed to be a matched quad, but I don't have a tube tester to verify that myself.

Once I know what the value is, I'll email the tube store (canadian based woohoo) and ask for the KT77.

Ya learn something new every day!

Thank you very much!


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## Wild Bill (May 3, 2006)

Budda said:


> Bill, I greatly appreciate your response!
> 
> I will swing by the music store downtown and ask if they would happen to have a tube tester handy in their store (they sell some high-end equipment, i hope they do) and have the KT77's tested. They're supposed to be a matched quad, but I don't have a tube tester to verify that myself.
> 
> ...


Sorry Buddha, it doesn't work like that!

There are tube testers and there are tube testers. A bias current checker is an expensive device. Only a major tube seller is likely to have one.

Regular tube testers don't measure bias current. For the average serviceman in the golden years of tubes it wasn't necessary.

I would be VERY surprised if a music store had a tube tester of any kind but by all means you can check with your local one! Music stores are there to SELL equipment and NOT to check tubes! They also usually don't have good techs. Why would they? A good tech would want to much money!

Also, bias number matching equipment gives numbers only good for that machine. That means that you could have a number from the machine at thetubestore.com and it could have absolutely no relation to a different machine used by a different store.

I think you're going to have to mail them one of your good tubes.

I know it's a pain but Mother Nature has Laws of Physics. She really doesn't care if that causes us extra work!:smile:

:food-smiley-004:


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## Budda (May 29, 2007)

I asked them about mailing them the tubes I have to find a KT77 that matches them, as my quad was matched (and it damn well better be or someone's getting an angry email). I don't mind mailing a tube, so long as it comes back in one piece!

I figure asking the local store is worth a shot lol, and i dont mind the excuse to go in and look around .

I never thought I'd have to go through all this just to get a replacement KT77, haha. Makes me just want to retube it and call it a day LOL


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