KT88 Tubes

Before you go throwing out tubes considering the HK’s possible age, ensure they’re not valuable GECs. Have you set the bias correctly?

Before buying more tubes, exchange the tubes in the right channel for the left. If the bad tube remains bad, it is a tube issue, however if the tube that you switch into the bad position red plates, it is an issue with the amp. Good luck!

IMHO, replace all output tubes if it is a tube issue.

Good advice from viridian. Always replace the entire set of power tubes unless you had an early failure in the set. If this amp is in original form internally you will need to replace caps at the minimum.

You can use 6550's in the Citation II. They are generally cheaper than KT88's. That's what I use in my pair of Dynaco Mk III 's.

Having tested nearly every quad-matched KT88 tube currently available on the market, none have approached the level of detail retrieval, holographic imaging, and tonal purity delivered by the Ray Tubes Select KT88 from Apos. It stands in a class of its own. Highest recommendation. https://apos.audio/products/ray-tubes-kt88-select-vacuum-tube?_pos=4&_sid=6e37ebf16&_ss=r&variant=45395661160684

Since you are new to to tubes, I'll make some suggestions:

1) If you hear a pop or hum or change in volume. or anything strange, the amp should not be used until you have determined what the problem is.

2) When purchasing a used tube amp, you want to nail down from the seller *everything* about that unit--the age of the tubes, the last time it was serviced or repaired, and any issues the amp may have had in the past.  With vintage units more than 30 years old, it's essential that the amp has been fully serviced and restored, all capacitors replaced and any other out-of-spec components updated.  Insist on documentation.

3) Tube amps need continuous care and feeding.  That means knowing how to check and adjust the bias of the output tubes and doing so once a week.  

That said, viridian makes a good suggestion.  Swap the KT88s between channels to make sure it's the tube and not the amp.  If it's the tube, since you don't know the age of the tubes that came with it, a new matched quad would be a good idea.  I would replace all the other tubes as well.

If the problem stays in the same channel, you need to take the amp to someone for service, preferably someone who is experienced with vintage amplifiers.

I noticed on the Apos tube site they claim that 5% of these tubes when tested meet their criteria. This means that whoever is manufacturing these tubes is so bad at it that 95% of them are rejected by Apos (sorry about the redundancy). Rest assured that I'll never buy anything from these guys. I've used lots of different tubes in my amps over the years and Gold Lions seem to be the most reliable and best sounding over all. 

Apos - yeah no, not for me either. I use NOS 6550 by GE, RCA or Winged C in my KT88/6550 amps. Have had nothing but good luck with them. Best new issue are Good Lion KT88s but I'll spend more for the NOS mentioned. Just my experience & opinion 

I am new to tubes but not troubleshooting electronics.  Occasionally in my career I have used the swap method on some similar adjacent instrument only to kill that one also as the component itself was the issue.  Is that a possibility with a bad tube in this application?

@bgross , +1, I love Winged C’s, the only tube I enjoy more are GEC NOS. But it’s today and prices are crazy. I think the new GL’s are pretty good. I seem to go between the GL’s and Tung Sol KT-120’s. I recently had Rogue build my M-180’s into the Dark version and all new tubes. I hit about $4k with shipping both ways. What was interesting is they only offered Psvane KT-88’s based on current market reliability issues. I paid $475 for an octet with the plan of going to something better after burn in and setup. But the Psvanes are surprisingly good. I’m looking at new speakers and might need to go KT-120’s for bass depending on what I get. To the OP I’d really consider having your amp inspected and brought up to spec in addition to considering new tubes. My Rogue lost a single resistor in one of the input/phase circuits thus affecting the amp. However I sent both in for a checkup and was told that both had tired bridge rectifiers which effects SQ and would have ultimately failed. That’s a nice amp , fix it feed it , cherish it, play it every day. Cheers , Mike. 

@russbargmann before you insert mew tubes you may want to make sure you do not have a coupling cap leaking DC or a loose pin on a tube socket. These are two of the more common issues for power tubes red plating. 

Everyone, thank you for all the helpful suggestions. Over the net several days I will be trying them out.

Old thread but replace all four at once matched set..  I use PSVane and vary happy

Amazon.com: New Current Matched Quad (4) Psvane KT88-T Classic MKII II Series Vacuum Tubes : Musical Instruments

Gold Lions prob good as well

KR KT-88s & my Prima Luna HP, I couldn't be happier.

@guscreek 

Yes, that's a risk, but often you can catch red-plating tube before it's damaged.  At least then you know not to keep trying new tubes until you can get the amp serviced.

@russbargmann unless your tubes ran more than 5000+ hours, you may swap L <=> R ones, and bias / balance accordingly to service (schematics) recommendation.