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HK Citation 2 with KT150's!

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mjalazard

mjalazard

Well-Known Member
Just to push the envelope a little more, AmpMedic has finally completed the monster rebuild of my notorious Citation II amp. He has made many changes to allow this amp to drive KT150 tubes. You can personally hear this amp at the 2016 THE Show Newport Beach in June way up on the 13th floor at our NFS Audio room!
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I have not heard a consensus on this but is it OK to run KT 120s and 150s with the Citation II PT? Or do you need to do circuit mods?
 
I hope his work is better than his webpage. That is some straight up geocities single page horribleness. :biggrin:
 
His work is excellent. There have been numerous progressive mods on this amp.
Give his website a break! He's not a web nerd, just a obsessive tube electronics guyl:thumbsup:
 
I do not see why someone will use these tubes unless output transformers are replaced too. And when they are replaced the only thing from "Citation II" left is a label.
 
for_p1 said:
I do not see why someone will use these tubes unless output transformers are replaced too. And when they are replaced the only thing from "Citation II" left is a label.
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I don't know about others, but I'd want to use KT-120 or KT-150 in my Citation for the increased "body" these tubes would lend to the music. It's not about increased output power, but tighter bass response and musical depth. They would also allow running the amp at spec'd bias currents without stressing the output tubes as with the KT-88's that I am using. I'm not worried about whether or not the PS can handle the slightly increased heater currents because they're built like a tank. When I get around to rebuilding mine, I'm definitely going to replace the outputs with KT-120.
 
Mjalazard, what is the purpose of the added pot and what looks to be a voltage regulator on the PCB?
Also, is the added toroidal transformer for the output tube heaters?
 
babblefish said:
I don't know about others, but I'd want to use KT-120 or KT-150 in my Citation for the increased "body" these tubes would lend to the music. It's not about increased output power, but tighter bass response and musical depth. They would also allow running the amp at spec'd bias currents without stressing the output tubes as with the KT-88's that I am using. I'm not worried about whether or not the PS can handle the slightly increased heater currents because they're built like a tank. When I get around to rebuilding mine, I'm definitely going to replace the outputs with KT-120.
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The only case when using KT-120 or 150 makes sense - if you increase idle current accordingly, biasing them heavily into class A. But output transformers will not like current over 100mA, which will be needed. In low current conditions these new type of tubes have no advantage over KT-88 or KT-90.
 
babblefish has got it right. Once you hear a properly set-up amp that has the ability to run KT120's or KT150's, there is no going back to KT88's (although very nice indeed)! LOL!

BTW, the operating specs that are posted for 120's and 150's are all advertising hype and wishful thinking, unless you want the tubes to fail & set fire to your amp. LOL!
If you can/do get them to run at the posted specs, be sure to have a fire extinguisher nearby... LOL!

In addition, you should not use the standard mains filament supply to run both signal & p tubes..
(and frankly not w/the standard set either)...

The bias supply too, needs to be reworked.

In my most humble opinion, the B+ is also barely adequate to operate KT120/KT150's to real satisfaction.
Improving the B+ would also benefit the use of KT88's...

Also, there are several other improvements to be made no matter what p tube you install..

I hope to see all of you at Dr. Mike's NFS Audio room..
 
pjfarano said:
BTW, the operating specs that are posted for 120's and 150's are all advertising hype and wishful thinking, unless you want the tubes to fail & set fire to your amp. LOL!
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It's not the first time I heard about the bigger KT's limitations. This is very good advise, thanks!
 
I'm defiantly in the crowd that using the larger wattage tube like the KT-120/KT-150 is a good thing in a properly running citation II,
(otherwise they really stress the KT88/6550 family, and no you would not want to change the output transformers in a Citation II) :rolleyes:
 
KT-150? And I thought 88s were expensive ?? That's more than my stash of empty beer can returns. Well, I have to lie to the wife again. If I get caught look for a large yard sale drinking Angry Orchard.
 
jazbo8 said:
It's not the first time I heard about the bigger KT's limitations. This is very good advise, thanks!
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Allow me to clarify my statement about advertising hype... While I do like GL KT88's, to my ears (& others), in a properly and thoughtfully modified/upgraded Citation II, KT120's & KT150's do offer a sonic improvement over KT88's.

What is NOT true are the max setup parameters as specified by the datasheet for plate/watt dissipation, mA, etc.

DO pay attention to filament voltage and the current draw specs..

In addition, to those pessimists and glass-half-empty naysayers, don't knock it until you've tried it.
Yes, it does cost additional money to buy these tubes and to make the requested changes to your amp to be able to benefit from these tubes and safely operate your amp,
but don't piss on the parade of those people who want more and CAN afford it....!!!...
Sterling autos are more money than Subaru's...
Neither one is to be ashamed of...

Besides, it may also have something to do with bragging rights! LOL!
 
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The Citation OTs support 150 mA idle current as recommended in the specs for those monster tubes?
I am surprised.
Cool experimentation.
 
I think some people are thinking that one needs to run the KT-150 at their rated specs while installed in the Citation. In my mind, not at all, but instead run them as if they were KT-88's with their bias and voltage settings. Lower overall stress on each component with the advantage of the KT-150's increased plate area and reduced impedance.
 
Brice said:
The Citation OTs support 150 mA idle current as recommended in the specs for those monster tubes?
I am surprised.
Cool experimentation.
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150mA to 180mA is what is actually posted for the average! LOL!

While this Cit II is capable of delivering on such a spec, the tube, in actuality is not...
 
babblefish said:
I think some people are thinking that one needs to run the KT-150 at their rated specs while installed in the Citation. In my mind, not at all, but instead run them as if they were KT-88's with their bias and voltage settings. Lower overall stress on each component with the advantage of the KT-150's increased plate area and reduced impedance.
Click to expand...

Of course, you can operate the KT150 at the same setup specs as the KT88, and there is a modest sonic improvement, but you will not be able safely and reliably operate the KT150 in a stock Citation II even with what is considered to be a generally accepted restoration.

The KT150 loaded Cit II pictured above and my own Cit II, safely operate at a higher mA than a KT88 and benefit from doing so.

But, there are many thoughtful improvements to the circuit and ventilation to allow the KT150's to do so..

Most listed above...

Happy listening!..
 
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One more change was made to the original pic above just yesterday.

The toroid trafo under the power cap on the R side (L channel) in the pic next to the choke, was secured along the R side chassis (L channel) on the same side as the choke.

Yes, in the pic, the original choke was moved from the left side, as pictured, to the right. In addition, both R & L inputs were moved to the R side as pictured (L channel side) instead of being separated on either side...

This Cit II & my own w/KT150's, pull more mA than KT88's.
 
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