KR-KT88s with Radford amp - not a good match?

[nickbaba]

Having read/heard good things about the (very expensive) KR KT88 valves, I decided to try a set of 4 in my recently restored Radford TT100.

With great anticipation I installed them in the amp and switched it on, ready to re-bias it and get everything in order. But before I could even do that the valves started loudly fizzing, popping and randomly banging through my speakers, to the point that I had to immediately switch the amp off for fear of causing damage to something.

Thinking one of the valves must have been faulty, and not having a valve tester here, I returned the valves. But when they were tested by the dealer (in an amp and by a valve tester) they were found to be 100% fine with no issues.

Is there some reason why the valves would have been unhappy in my amp? Wrong current level or something? I assumed all KT88s are compatible across all amps that are made for them? Or could something else have caused the audio nasties, without the valves being at fault?

[WESTLOWER]

Can Mr Coe advise?

[Boyse6748]

Having read/heard good things about the (very expensive) KR KT88 valves, I decided to try a set of 4 in my recently restored Radford TT100.

With great anticipation I installed them in the amp and switched it on, ready to re-bias it and get everything in order. But before I could even do that the valves started loudly fizzing, popping and randomly banging through my speakers, to the point that I had to immediately switch the amp off for fear of causing damage to something.

Thinking one of the valves must have been faulty, and not having a valve tester here, I returned the valves. But when they were tested by the dealer (in an amp and by a valve tester) they were found to be 100% fine with no issues.

Is there some reason why the valves would have been unhappy in my amp? Wrong current level or something? I assumed all KT88s are compatible across all amps that are made for them? Or could something else have caused the audio nasties, without the valves being at fault?

Was considering a set of these for the McIntosh.

It will be interesting to hear what the perceived problem might be. For now, I will detract from purchase for the short term until some pearls of wisdom are forwarded for possible problems.

Peter


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[AD Audio]

I've had a quick look and can find no published spec for these valves. HT on your TT100 is 680v and my guess is the KR valves are unhappy with that HT on the anodes and/or screens. Both the Golden Lion and GECs we tried were happy at those volts so my guess is the boutique valve is more picky about HT and screen v. The spec for the standard valves gives max screen v as 600.

I've seen this before eg with TJ Meshplates which dissipate much less than standard 300bs. Also KT150s redplate well before the published dissipation for Class A operation. There's only two choices really: stick to the tried and tested or lower HT to suit the KRs at the probable expense of dynamics. You should be able to get your money back from a decent supplier if you want to go that route as I was refunded for several pairs of KT150s
David



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[Audio Al]

You could ask Will at Radford revival for his views

[tubehunter]

The HT on a Radford TT100 is 600v

http://www.radfordrevival.co.uk/literature/tt100/
http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2011/11/TT100-Technical-Instructions-P3.jpg

Max Plate DC on the KT88s is 800v
Screen Grid 600v

https://www.audiogon.com/listings/tubes-kr-audio-kt88-tubes-2017-12-03-accessories

Strange one this

[AD Audio]

The HT on Nick's amp is 680v-ive measured it at my place with my 253v mains and discussed it with Will at the time

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[nickbaba]

Thanks David, and everyone, for your thoughts.

I think, for whatever reason, the KR valves are not well suited to my amp, as when I swapped them out and replaced them with the GECs or the Gold Lions everything was once again smiles and rainbows.

No harm done, dealer was happy to refund.

[AD Audio]

Good result! David

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[montesquieu]

Good result! David

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Hmm I just ordered a set for my STA 100 ... hope I don’t have the same issues.

[AD Audio]

Hi Tom,
I'd have a word with Will as I'm sure he would have measured it during the build. Your transformer will be different but the problem is the multiplier effect with a high mains. 240 to 600 v means any increase in mains v is raised by the same ratio.
David

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