[Freo-1]

While chatting with my local amp tech the other day, we were discussing tubes, tube rolling, amp projects, etc. We both more or less came to the same conclusion:

While most of the current production output tubes are pretty reasonable, the big power tubes (KT88/6550) types do not have the same track record for success. None of them sound as good as NOS types (Tung-Sol, Sylvania, GE). Most are not very reliable over a period of time, and even the ones that are still do not last like the NOS types. The SED 6550C is the closest in longevity, but it just does not seem to have the same smooth sound character as NOS types.

KT88 types is even more bleak. Many of the KT88 types made do not even sound as good as the 6550, and reliability is suspect.  The guitar players seem to like the JJ KT88 (it is rugged), but it just does not sound all that hot in audio applications. There have been complaints about pin size and soldering with the JJ as well.

Here is some background information on the Fender 400PS, which goes into some explanation "why you should use the expensive GE 6550A  tubes".   

Why do I have to use those high priced GE-6550A's?

    You don't HAVE to do anything.
    But, when you do the proper thing, it's a nice thing.
    Then... everything works as it should.

    It's like anything else, when specific things are designed to work together to yield the "maximum" in  performance with long service life.  This is the case with the 400-PS with the original Tungsol 6550, as well as with the even higher performance GE-6550A.

    Likewise, the high performance Musicman amps were specifically designed to run the Sylvania 6CA7 tube, because of their well known superior performance and service life.

    In both cases, no other tubes were considered for use in these amps.  Not even as a substitute replacement tube.

    A year or so back I had a 400-PS owner stop in with a set of NEW Russian 6550WE tubes to be installed.  I really had to play games with that 400 to get those tubes to run.  This was their best day right out of the box for these tubes, and the best they could do was 124 watts per load at 100 Hz for a 372 watt total output from a conservatively 435 watt rated amp.

    After 72 hours of hard dynamic signal running time and a re-adjustment, that 372 watt total figure dropped by almost 50 watts.

    It's one thing to print the specs for a tube, and quite another to make the tubes that live up to those specs with LONG continued service life.

    Here are the GE-6550A data sheets for you to see if the other so called 6550's measure up to the high performance GE-6550A standard.

    High performance with long service life is the hallmark of the GE-6550A.

Are all 6550's the same?

    No, for the following reasons.

    With the way tubes are being made in other countries today, there is little chance of any of the same type of tubes ever being made the same from one country to another.

    There are no unifying standards behind what they produce with respect to standards that cover their manufacturing methods, material standards, or operational electrical performance characteristics.

    In the USA we had government specifications, with industry's material standards and testing practice for  the vacuum tube, and an electronics industry's standard to follow.  Not all tube companies were qualified to supply specific vacuum tube products  to the government of the USA.

    The rest of the world's tube companies *individually* brand what ever they think their tube is, for THEIR standard for a tube type.  This most often is different country to country, and can change from production run to production run.

    This is why we have the problems with same type vacuum tube performance being all over the map from off shore today.

    For example, if they can't even make a tube with the right size pins for our USA tube sockets, would you expect them to have the correct electrical characteristics dead-on spec for a Sylvania or GE of the same branded type of tube?

[richidoo]

I have heard nothing but praise for the new Genalex KT88 replica. But I have not heard it yet. People say it sounds as good as the original. But made in eastern Europe/Russia you just never know from batch to batch what you'll get over time.

FWIW, the JJ pin size thing is 5 years old, it is long since corrected. But it was a stupid mistake, worthy or eternal punishment. 
Rich

[Freo-1]

Rich, I can honestly tell you that not all is well with the GL KT 88 re-issue.  There are a number of people who have had to take their amps in for repair after using them for not a long time. They sound good, but too many have shorted or have bias runaway issues. I'm sure people's amps could be modified to address the bias runaway, but as you said, production run to run may be different. 

Of course you will hear good things about the tube. At 60.00 a tube, it would be counter productive (for vendors) not to push this tube (it costs almost double a SED 6550C). Is it worth twice as much??

Keep in mind this tube has no spec sheets, so "it's a KT88 according to New Sensor", not necessarily to the MO spec. 

[jon_010101]

With all due respect Freo, you seem to have had more problems with those tubes than anyone else.  You should post up a schematic of your amp so we can figure out if there's some funny business going on that might lead to tube incompatibility. 

I've had EH KT88s nearly blow up in my amp, but I know exactly why, and it *Is* due to a quirk in my Citation II that I've been too lazy to fix. 

[ZLS]

    I was using GL KT 88"s on my rebuilt Harmon Kardon Citation II.  The good news is that the amp sounded terrific.  The bad news is that my right channel has blown up twice.  I posses no technical expertise, but I do know that even with the offending tube removed  the fact I can still smell something that has been burnt is not a good thing. 
    I am currently debating what to do with the amp, because of this. 

[Freo-1]

    I was using GL KT 88"s on my rebuilt Harmon Kardon Citation II.  The good news is that the amp sounded terrific.  The bad news is that my right channel has blown up twice.  I posses no technical expertise, but I do know that even with the offending tube removed  the fact I can still smell something that has been burnt is not a good thing. 
    I am currently debating what to do with the amp, because of this. 

If I were you, I would replace the grid and bias resistors, and go get some NOS GE 6550A. If that is too rich, then suggest SED 6550C.

Good luck!

[Ericus Rex]

Has anyone here had GL KT88 problems in amps other than Citations?

[Freo-1]

With all due respect Freo, you seem to have had more problems with those tubes than anyone else.  You should post up a schematic of your amp so we can figure out if there's some funny business going on that might lead to tube incompatibility. 

I've had EH KT88s nearly blow up in my amp, but I know exactly why, and it *Is* due to a quirk in my Citation II that I've been too lazy to fix. 

Hi Jon,

Well, if any other 6550/KT88 types were having bias runaway issues in my amp, I would agree with you. However, the fact remains that these tubes do not have the problem:

1) Tung Sol 6550 NOS
2) Sylvania 6550
3) GE 6550A
4) JJ KT88
5) SED KT88
6) SED 6550C
7) Sv 6550C (older type with the round holes in the plate)

I do have one quad of the GL KT88 that works (out of almost 20 I tried and returned/binned , so not good odds).  Besides, what exactly are the characteristics of the GL re-issue KT88 anyhow?  There are no spec sheets out there, so it's just a guess.  That was exactly the point of posting the data from the 400PS.  The fact of the matter is that for the most part, one cannot really trust that the tubes made today are to spec or not.  Did you happen to catch the fact about the Sovtek 6550WE losing 50 watts of power after some use?  Now, who makes those, and who makes the GL KT88?  (Hint: it's the same mob).

I think if I drop the grid resisitor  and increase the coupling cap, that would perhaps correct the bias runaway (but not the light shows)  .

Frankly, while I like the sound of the tube, I don't like it better than the Sylvania or Tung Sol enough to modify my amp to attempt to accommodate a Russian tube that the vendors are getting 60.00 a pop for.  Now, once I run out of the NOS types, I may need to revisit the lay of the land.

 

[jon_010101]

    I was using GL KT 88"s on my rebuilt Harmon Kardon Citation II.  The good news is that the amp sounded terrific.  The bad news is that my right channel has blown up twice.  I posses no technical expertise, but I do know that even with the offending tube removed  the fact I can still smell something that has been burnt is not a good thing. 
    I am currently debating what to do with the amp, because of this. 

The burnt smell is probably the screen and/or cathode resistors for the failed tube.  Have you taken it apart since?  It should be an easy fix!

Did it fail during turn-on, or after having been on for a bit, or after a power interruption?

Do you know if the bias / balance potentiometers were replaced?

[jon_010101]

A few tubes that they do sell are absolutely not like the originals -- the 6973, for example -- but many have had good luck with the KT88s, 6L6s, etc.  If you've gone through so many, I definitely advise you "give up" on them for now. 

But my concern is that if your amp is eating GL KT88s, it might be eating other ones in a slower, more insidious manner.  The GL might just be more palatable for it.

Do the failures primarily occur under load, or while idling, or on turn-on? 

A schematic, especially of the bias supply, and of the grid circuitry, would be very helpful to assess this.