I had the same issue with KR and Coincident, one of tubes broke after 150 hours or so. Coincident refused to replace the tube and asked to talked to KR. They said I should ask Coincident. After several e-mail exchange I gave up.
A few months later I met owner of KR personally at Audio Show in my country. She promised me to replace the tube and after a few days I receive the matching tube.
Beware that not every 300B tube would work with Frenkenstein and for some of them its dangerous. There is no information about it at Coincident site. Here are parameters of Frenkenstein, You can consult it with a technician or compare to recommended by manufactures max values.
Plate Voltage 580 Volt
catode Voltage 97.5 Volt
Plate Current 97.5mA
Plate Disspation 56,55 Watt
Anyway, I'm going to sell Frenkeinstein, I still prefer my second amp, ASL Cadenza (modified). For me Frenkeinstein suffers of typical 300b limitations.
I love the Frankensteins. They just flat out please both the audiofool and music lover in me. These tubes were recommended (and sold to me) by Israel Blume himself, and he hasn't steered me wrong as of yet. I can't believe he would make an error and then not rectify the situation, so I have to assume that KR Audio has issues.
Funny though, I'm at 150 hours or so.
After speaking with Eunice Kron( owner of KR) over dinner in Las Vegas, she advised that KR no longer has any intention of selling KR 300Bs to any outside source, instead opting to manufacture tubes solely for use in their own amplifiers. However, right after this conversation, she doubled the price of the 300B tubes over night.
As for reliability and warranty of KR 300Bs, their longevity for those tubes manufactured in the last 18 months has deteriorated. They seem to be more prone to premature demise. Coincident has received back about a half dozen 300Bs that have gone bad within 6 months of use. KR does offer a 6 month warranty. Well this is not really a warranty at all, as it turns out. After sending back to KR all the defective 300Bs, KR denied offering replacements because " the tubes were misused by their owners and therefore not subject to the warranty". In other words, KR offers a warranty but if a tube becomes defective it is the user's fault and not KR's.
In the case of 2 customers who had their KR 300B balloons expire within 2 months of use, we replaced them free of charge inspite of the fact that we only offer a 14 day warranty on tubes.
Due to all of the above, we no longer stock nor sell KR 300Bs. Our standard supplied EH Gold Grids have proven to be extremely reliable( not a single failure in the field) and sound superb. For those who wish to upgrade, we highly recommend the Sophia or TJ carbon plates. They sound great and their manufacturers extend a genuine up to one year replacement warranty. We no longer offer for sale, any 300Bs other than the EHs since we are confident in their reliability and sonic performance.
As for using differing 300B tubes in the Frankenstein amplifiers, all of them will work as the operating voltages and bias current of our amps is within spec for all of them. Stereomojo used the Frankenstein as the reference amplifiers in a 6 tube 300B comparison with no tube failures. If the 300B is properly manufactured and falls within the specs required for a 300B, it will work without incident in the Frankenstein amplifiers. Some tubes are better constructed then others and will last longer but that is a tube issue nothing to do with the amplifiers.
Coincident Speaker Technology
Sounds like KR definitely has some quality control issues. I had two KR 845 tubes go bad in my Art Audio Carissa Signature, I was lucky that Joe covered them as he had mentioned he likely wouldn't be covered by KR. I have TJ Fullmusic 845's installed for over two years without a problem.
I love my Frankensteins. It is the most dynamic 300b amp I have ever heard.I wouldn't sell them for what I paid.I had Kr balloons and one failed as well(aprox 175 hrs.). I've since moved to fullmusic carbon plates and the amps are singing,even better than with the Kr's IMO.
Regarding 300b compatibility with Frankenstein - at least Mesh plate 300B from Sophia Electric does not work. I have purchased them after KR failure and listen to no more then one hour. Strong distortion so I was afraid of failure. Confirmed by Sophia Electric that tubes are not suitable in my amplifier. I sold tubes and they work very well in typical 300b amp.
I also purchased Emission Labs 320B-XLS - no problems and borrowed Full Music mesh for a few hour. Owner of Full Music noticed that anode shines too much and was affraid of failure.
Regarding the sound of Frenkeinstein - maybe I try another 300b - but at the moment I think this amp is for 300b enthusiast, not for me. Experienced 300b audiophiles told me, that limitations I hear are "generic" 300b feature and every 300b SET they heard more or less suffers of slow and "exaggerated" bas not managed to keep the rhythm - and some high frequencies roll-offs. Speaker compatibility is not an issue as I own Coincident Total Victory 3 (great speakers, I like them very much)
"generic" 300b feature and every 300b SET they heard more or less suffers of slow and "exaggerated" bas not managed to keep the rhythm - and some high frequencies roll-offs.
Sounds like any poorly designed tube amp to me, certainly not a characteristic of a properly executed 300B amp, nor the tube itself !
If you want to try what I believe to the best current production 300B tube currently available by a long mile (and I have tried most in my SOTA silver foil output transformers amp)...Check out the EAT 300B, a truly remarkable sounding and super reliable tube IMHE...
I purchased a quantity of matched pairs and have a NIB pair listed in the AudioAsylum classifieds at waaay below retail price.
I had very poor experiences with KR. I had an older KR Enterprise amp that kept burning through 842 VHD tubes (I think that was the name). The tubes died a number of times and I had to replace with no recourse from KR. The amps sound nice, but it's clear that KR wants nothing to do with customers who have issues with their tubes.
I believe their new amps are better designed than the older KR Enterprise stuff.
Sounds like a shady bunch . Thanks for the warning .
Shady bunch is being kind. Stay away and you will be the wiser for it.
I am the US importer for KR. It is hard to know where to begin.
 KR Enterprise has been out of business for years. No printed documentation exists about these amplifiers, other than what we have recreated here in the US.
 No new 300B balloons have been imported into the US since July 2008.
 KR has had issues with dealers selling out of warranty tubes as new. As importer, when these tubes fail, the most I can do is offer new tubes at my cost, because I never received any money for the tubes in the first place.
 There were a bad batch of tubes that came through Coincident when I first started, but these were more than two years past the manufacture date at the time they hit customer hands, so these should have been marketed as NOS unwarranted by factory.
 The KR 845 does have sensitivity to heater voltage. As the tube heater draws substantially less current than a carbon plate 845, unregulated heater supplies will go to almost double the rated heater voltage. This is a recipe for tube failure. We screen all 845 sales to avoid this issue. Further, we have driven some design mods into KR 845 tubes and KR has renamed them 845M. Their reliability is now world class.
 It sounds like there is a lot of concern about the tubes. If you find a KR tube for sale, get the serial number off the crown of the tube, the tube type, and the vendor. Email that info to: firstname.lastname@example.org with KR Question in the subject field. We will let you know if the tube is legit, black market, or NOS and if the vendor is known legit within 48 hours of the receipt of the email.
 Lastly, unless you purchase tubes from me directly, please send in your warranty registration card, with a copy of the the purchase receipt within 30 days of purchase of the tubes. This is required to get warranty service on the tubes.
Unfortunately, by the time that we joined this party in 2008, a lot of damage was done by some importers/dealers with less than scrupulous intentions. This really was out of KR's control because the firms servicing the US and Canada are not owned or directly controlled by KR in Prague.
Since late 2009, we have bi-weekly quality meetings with KR in Prague to advance quality and reliability. The Mark II amplifiers were designed here in the USA and are assembled here in Michigan with the best and most reliable parts we can find.
Lastly, if you have an issue with a KR tube or amplifier, please email me at email@example.com.
We are trying to clean up the chaos, but trust is easy to lose and takes forever to rebuild.
All the KR 300Bs Coincident received were purchased directly from Eunice Kron in her capacity as owner of KR. From the time or order and prepayment, which was demanded, delivery would take a minimum of 60 days and at times, 120 days. Mrs. Kron stated that all our orders had to be manufactured , therefore the delay. At no time was there any mention that the tubes were anything but current manufacture and up to the latest spec. Coincident ordered and received KR 300Bs on at least four occasions spanning a two year period. On one occasion, delivery after prepayment of the order took 6 months to be delivered.
I took this issue and the reliability problems up with Mrs. Kron when we had dinner at CES 2009. During our meeting she admitted to delays in deliveries and quality control problems which she attributed to the turnover in staff from the very experienced previous workers to new employees who had little or no tube manufacturing expertise. She also admitted to serious financial difficulties and offered to sell the tube manufacturing business to me.
The reliability problems were not solved during the entire time Coincident purchased KR tubes. Mrs. Kron, when sent the defective 300Bs simply refused to replace them or offer a refund, rather stating," if they failed, the tubes must have been used incorrectly". In other words, we offer a warranty, unless the tubes fail.
Coincident has since switched to recommending the Black Bottle Treasury 300Bs, which have in over two years of use in the field , not suffered a single failure and they sound superb.
Israel Blume- Pres.
Coincident Speaker Technology
Planning to buy KR845 tubes??
Nagra VPA was launched in 1999 and has an official frequently asked questions site states that KR 845 tubes no longer comply with the original specifications and therefore no longer supply them with their VPA amplifiers. In 2004 Nagras news letter reported trying out KR 845 tubes unfortunately quality issues continue to prevent Nagra from using them. Nine years later, in 2013 being cautious I emailed the dealer in Singapore but I was later referred to Ms Eunice of KR Audio. The subject was my question as to whether the latest KR 845 is suitable for use in Nagra VPA which I own. After a few emails back and forth it was confirmed by Ms Eunice that KR 845 is suitable for Nagra design. With that written commitment and assurance, I bought 4 pieces (2 matched pairs) of KR845 tubes and started using them. In my system, KR 845 sounded excellent as compared to Shuguang 845 and 845B that I use prior to KR 845. Ten months into use (from new) clocking around 500 hours one tube went dead. I wrote to KR Audio immediately reporting about the tube failure. Strangely enough KR Audio responded that their 845 tubes are not suitable for my Nagra VPA which contradicts to Ms Eunice assurance to me prior to my purchase. Some emails went back and forth among the Singapore dealer, Ms Eunice and I. Some days later, I was shocked and frustrated to see two new tubes delivered from KR Audio despite of me informing them against sending. I refused to accept since these tubes are not suitable. Some emails went back and forth again but this time only with the dealer in Singapore where those tubes were purchased. I was adamant that there was no point and risky of me to continue using any of those KR845 since they are not suitable. Fortunately my Nagra VPA survived the tube failure as in the past there were quite some cases of the older series VPA being damaged using KR845 tubes. In my situation, there maybe two issues, quality of the tubes and suitability.
I was helpless and at their mercy, eventually accepted the offer of a refund of about 50% of the purchased price from the Singapore dealer on condition to return the existing four and the two new tubes. I was very unhappy and disappointed as I was deceived into buying those tubes. I was expecting 90% of the purchased price if not a full refund.
Devastated and in need of replacement tubes of good quality. I started searching forums after forums about the much talked about Psvane WE845. I decided to take the risk and bought a set of four. It was blessing in disguise that these WE845 in my system sounded close to the KR845 only costing half the KR price. Some forums suspect the reliability of WE845 but I am prepared to take this calculated risk as I can buy 8 pieces at the same price of four KR tubes.
Please make your own judgement of the above matter and hope more Audiogoners and other readers are aware of my unfortunate experience before throwing your hard earned money. I would not have got into this mess had I read this forum about that many Audiogoners are unhappy about their experiences with KR tubes.