Best replacements for KT99A tubes?

@_rossbower The guitar industry is really what drives pentode power tube production. It does not want anything other than KT88s because there are so many vintage Marshalls around and guitar players on the road want to be able to find a tube if one fails.

So there are some very good KT88s available now. JJ makes a blue glass (mentioned earlier) which is really quite good. I think you'll find in an amp designed for KT88s, that even though they make a bit less power, KT88s sound better. 

FWIW, I believe that there is little, if any, difference between the KT99 and the KT90. The former's designation as a KT99 was adopted by the tube manufacturer (relabeler?) and sold at a premium price. So I was told. I used the KT99 in some Sonic Frontiers mono's. Nice tube/sound. I replaced them with 6550's and KT88's over the years I had them. 

Do you still have your old KT99A tubes?  I have a ST-120 amp with 4 of them, and unforunately one of them was broken, so I am looking for a replacement.  If you are willing to part with a couple, I would be very interested.  Thanks.

@_rossbower
decooney, Awesome, thank you! I have seen some people recommend the KT-90 or even KT-120 for this amp. I’d feel more comfortable using something written down in the owners manual, but I don’t know enough to really compare these options. Do you have any thoughts?

Only 6550, KT88, EL34, KT99s are noted in the manual. Outside of that, I could not recommend anything outside of those parameters for the SLM-100 amps. You can always call and ask Steve Wytek at Cary Audio. Not sure how many SLM-100 or SLAM-100s they service any more. Amps have evolved since the 100s, and good caps help, a lot.  Unless you need gobs of power, not sure you'd get much more anyhow, but not sure why well-matched KT88s would not work well in these particular amps. Lots of people ran nice KT88s with those. 

Each of these tubes listed in the manual has a different character and presentation. And, its’ "system dependent". In other words, folks will ask what’s in your system now, speakers used, preamp used, input tubes used in preamp and amplifiers, interconnect and speaker cables used, sources...ALL of these impact the signature too. From there, tubes can help you to fine tune with some trial-and-error and expense. And, can its possible get you in the "ballpark" and tune more from there. If it were me, and had to guess not knowing your system, "its system dependent, circuit dependent". I have some ideas on what I’d test but a question for a question may help get you closer...to add more or less salt & pepper to the meal at your house is the question. What’s your taste...and we all hear differently. You’ll get different answers from many.

Question:  
To YOUR ears, what’s missing with the sound of your now-current 6550s? What do you not like? What do you like?

@decooney Awesome, thank you! I have seen some people recommend the KT-90 or even KT-120 for this amp. I'd feel more comfortable using something written down in the owners manual, but I don't know enough to really compare these options. Do you have any thoughts?

@_rossbower

_@decooney This is for a pair of SLM-100’s. You remind me I need to track down an original owners manual... I’m definitely planning to get matched quads. Any thoughts on EL34 vs. KT88? I had the sense that KT88’s were higher quality somehow to the EL34.

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Operators Manual (SLM-100) on the Cary website:
https://www.caryaudio.com/wp-content/uploads/2013/06/SLM-100.pdf

JJ KT88 Blue Glass years back and Genelex re-issue Gold Lyon KT88 were popular as I recall, bias accordingly. Really good coupling caps help a lot in the older amps.   Similar chassis to the older 805 and V12R like I had. 

@clearthink Damn! Could you be any more clear? :P Thank's for the input. Can you elaborate on *why* you have such strong feelings? I haven't seen anyone as decisive as you be anti-6550.

@decooney This is for a pair of SLM-100's. You remind me I need to track down an original owners manual... I'm definitely planning to get matched quads. Any thoughts on EL34 vs. KT88? I had the sense that KT88's were higher quality somehow to the EL34.

@_rossbower

Is this for the SLM-100 or SLAM-100 monos by chance?

If so, shows in the OM they originally came with 6550s. Original OM mentions max bias adjustment of 175ma for all four tubes per amp, with potential use of EL34, KT88, KT99s. Maybe go 160ma to start, 40ma per tube. Lots of old references, some using JJ KT88 Blue Glass back then.  And, I recall seeing several running them with Gold Lyon Gelenex KT88 reissue too, fwiw. Recommends matched quad (of the same type tube) per amp if you replace them.

KT90 maybe but a 6550 is not a proper, appropriate, suitable substitute for KT99A in any amplifier intended for installation, use, or operation in a Music Reproduction System.