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Old 14th October 2007, 01:59 PM   #1
horace is offline horace  United Kingdom
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Default Replacing KT88 with KT90 - Beginner's questions

Hi

i have at last fixed my valve amp, thanks to the advice I received here (details of amp etc here:

http://www.diyaudio.com/forums/showt...71#post1324871

I am now thinking of putting some better tubes in it. first on the agenda are the KT88s. These are cheap chinese ones (as far as I can tell - they have 'National' on them in red script). I read that Electro Harmonix KT90s are a direct substitute and won't require any alterations beyond rebiasing. Is this the case? Will they give noticeably higher volume levels/sound better or should I stick with KT88s? EH, EI, Sovtek?

Please bear in mind that I am a beginner and much of the tech talk on this illustrious forum goes straight over my head! I asked a question about equivalents for the input valves on the amp (6N2, 6N6 and 6N11) and ended up more confused than before i started! - Yes, you can replace x with y - no you can't, that valve is not the same, but you could replace it with z if you rewire your big end. 6N6 = 6N6P - no, not exactly true etc etc.
There does seem to be some disagreement when it comes to substitution of chinese valves. This, I think, was as close as I got to a consensus:

6N2 can be replaced with ECC83 or 12AX7, but the valve base will need some rewiring.

6N6 has no direct equivalent, but closest match is E182CC. This might require changing the circuit in the amp.

6N11=E88CC or 6922.

Assuming the above is correct, does this mean that I can simply pull out the 6N11s and replace them with E88CCs or 6922s without any modifications? If so, are there likely to be any sonic benefits?

Likewise, I could get a local valve tech to rewire for ECC83 or 12AX7 to replace 6N2, but again, would it be worthwhile?

As ever, I am grateful for your advice. I am learning how to solder (properly this time!) and one day I dream of having sufficient skills to build a valve amp from a kit......

Cheers

Martin
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Old 14th October 2007, 03:44 PM   #2
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Quote:
I am now thinking of putting some better tubes in it. first on the agenda are the KT88s. These are cheap chinese ones (as far as I can tell - they have 'National' on them in red script). I read that Electro Harmonix KT90s are a direct substitute and won't require any alterations beyond rebiasing. Is this the case? Will they give noticeably higher volume levels/sound better or should I stick with KT88s? EH, EI, Sovtek?
I have 5 pair of KT90's sitting in a drawer doing nothing. I bought these thinking that I would replace the KT88's that I am tunning in my Quicksilver mono block amps. I replaced them with the KT90's and was disappointed with the results as I lost midrange and hi frequency detail and went back to the KT88's and 6550's I had been using. I am told there is a slight power increase to be had by using the KT90's but seriously doubt that it would even be noticeable. My suggestion is to replace the KT88's with a set of good 6550's or KT88's. I personally like the Sovtek brand but there are others out there just as good.
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Old 14th October 2007, 04:33 PM   #3
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Good KT90s (those made by EI before the NATO action) are a thing of the past. Post NATO action EI and all EH KT90s have problems, either in electrical behavior or in sonics.

3 NICE big beam tetrodes are made in Russia. The EH and GEC "reissue" KT88s come from the Saratov plant. The SED (=C=) 6550C comes from the St. Petersburg plant.

If you can afford 'em, buy the GEC "reissue" KT88s.
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Old 14th October 2007, 04:49 PM   #4
horace is offline horace  United Kingdom
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Thanks - that's two votes for sticking with KT88s. I'll have a look at the prices - the EH ones are reasonably affordable here in the UK, but I'll check out the GEC reissues as well.

Further questions: how much of a difference will changing valves make to the performance of the amp - are we talking huge improvements or more subtle ones?

Which valves tend to have the greatest influence on sound quality - signal or output tubes?

Sorry, like I said, I'm a beginner......

Many thanks

Martin
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Old 14th October 2007, 05:25 PM   #5
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Quote:
Originally posted by burnedfingers


I have 5 pair of KT90's sitting in a drawer doing nothing. I replaced them with the KT90's and was disappointed with the results

Copy that.
The Russian new made KT88's SEDS have a decent thick envelope (far thcker than NOS) and are well made for rigours around the bench.

KT90's would be fine for a tasty power supply project, that's about it.

If you insist on using them in audio, expect double the output stage THD compared to KT88's in a standard UL p-p stage. There will also be roughly 15% more output power in LS.

richj
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Old 14th October 2007, 05:53 PM   #6
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Quote:
Originally posted by Eli Duttman
If you can afford 'em, buy the GEC "reissue" KT88s.
I'll assume you are referring to these tubes?
http://newsensor.com/ProductHighLight.aspx?ProId=1149
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Old 14th October 2007, 08:34 PM   #7
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I would agree. Keep the KT88's. They are great tubes.

I only know of one amp that probably runs better with the KT90 - because it was designed with exactly that tube in mind. That amp is a Jadis

My advise would be to cook your tubes as described by Morgan Jones in his very fine book.

Put them in a dish (not covered) in a cold oven and turn on - bring up slowly to 150 degrees celcius and keep there for 4 hours. Then turn the oven off and let everything cool overnight. Don't move those tubes out of the oven until they are cool. This really works wonders with old tubes and doesn't cost you anything. I actually go to 200 degrees celcius for 2 hours, but this may brown the lettering on your tubes...

Make sure the oven is clean before you do this or you will get chicken/porkchop flavoured tubes... Also, Get the OK from your wife/spouse/etc before you start baking tubes...
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Old 14th October 2007, 08:45 PM   #8
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Also, I would keep the 6N11s and 6N2 in the circuit.

Why does it look like it has 4 output transformers?
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Old 14th October 2007, 09:45 PM   #9
Salas is offline Salas  Greece
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My neighbor runs a Jadis DA-30 that came packed with EI KT90s. He has recently got a Jadis red logo JJ KT-88 ultra matched quad and the amp sounds markedly better with them.
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Old 14th October 2007, 09:46 PM   #10
horace is offline horace  United Kingdom
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Thanks - putting tubes in the oven, eh? is this the sort of thing my wife is going to have to put up with if I go any further down the valve road?
I can see her rolling her eyes as I type this.....

As for the output transformers, I don't know. The amp is a bit of an enigma - I've never found any reference to it anywhere.

Thanks

Martin
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