Power valves, are they suitable?

antonio66

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I am using a 'cheap Chinese valve intergrated' RFTLYS A2 KT88 Tube integrated amplifier with headphone amp with Bluetooth input : China-hifi-Audio online store,  and to say I am pleased with it would be an under statement, many might find the headphone out to be worth the price alone.  Since I'm new to valve amps  I have purchased some 12AU7's and 12AX7's/5751's to try out after reading they influence the sound more than the output tubes.  I missed out on a bargain quad of EH KT90's, but what I'd like to know is, how do you determine what power tubes are suitable for use with your amplifier.   I have been told KT90's would be ok, as well as 6550's but what else would work ie 6L6 variants or something else?

 

Cable Monkey

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The issue you have is the words ‘might work’. I would stick to finding well regarded versions of KT88’s or their drop in alternatives the 6550 or possibly KT120. The KT120 draws more heater current so that would require confirmation before trying them. I don’t know about the KT90 so couldn’t comment.

 
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Psilonaught

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I am using a 'cheap Chinese valve intergrated' RFTLYS A2 KT88 Tube integrated amplifier with headphone amp with Bluetooth input : China-hifi-Audio online store,  and to say I am pleased with it would be an under statement, many might find the headphone out to be worth the price alone.  Since I'm new to valve amps  I have purchased some 12AU7's and 12AX7's/5751's to try out after reading they influence the sound more than the output tubes.  I missed out on a bargain quad of EH KT90's, but what I'd like to know is, how do you determine what power tubes are suitable for use with your amplifier.   I have been told KT90's would be ok, as well as 6550's but what else would work ie 6L6 variants or something else?
Did you buy that from China hifi store?

I've been unsuccessfully trying to get the company to respond to a pre-sale question I had about one of their preamps and I get no response. It has totally put me off buying anything from them.

 

pmcuk

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It's quite a hobby to start tube rolling when you have a tube amp. But be careful. Circuits are designed around a particular tube.

- the operating point will have been selected for that tube, and the resistors that go with the operating point

- the heater current should not be exceeded, since it will stress the transformer providing it

- the current through the tube (determined by operating point) should not be exceeded since it will stress the mains transformer and possibly the rectifier. 

The only way you can properly check all this is by looking at the data for each tube. You'll find this on https://frank.pocnet.net. This will tell you the heater current and maximum operating conditions. It won't tell you the operating point, however, and for that you need the schematic and the ability to calculate the voltages and currents using ohms law. Even then you might have to use a voltmeter on the circuit to test what voltages you actually have. And measuring inside a tube amp requires the knowledge to do this safely. That's the only way to do it properly. 

So don't be taken in by posts saying "this tube sounds better than that tube" and assume you can switch between one type and another type. The output tubes are central to the amp and it is far safer to use the type of tube the amp came with. 

The input tubes will probably have the most influence on the sound, but again, stick with the specified tubes. Switching from 12AX7 to 5751 is safe enough but i wouldn't go beyond that unless you know what you're doing. Tube rolling within each specified type of tube is OK, but you can end up spending a lot of money for not much gain in sound. Plenty of threads on tube shootouts, especially on Audio Asylum Tubes forum. 

 

rabski

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you can end up spending a lot of money for not much gain in sound
This, more than anything.

Pmcuk, myself and a few others on this site have each accumulated what I think you might sensibly term 'too many valves'. Some of us have been messing around with valve stuff for many decades and the one thing I can guarantee is that when anyone talks about 'night and day' differences with swapping valves, either the originals were faulty or out of spec, or someone is trying to justify how much they've spent.

With regard to different types, I wouldn't risk it without having the amp on the bench first. A lot of Chi-fi is far better than you could expect, but the big 'however' is that the vast majority have mains transformers that are rated close to their maximum, and often not designed to take the potential UK voltage. They may be stressed a touch 'as is' and swapping anything that puts more load on is a bad idea.

Unless something is a definite drop in replacement (and don't just take the word of people who sell valves, as plenty of them suggest things can be swapped that are quite different) then stick to what it came with. In the case of 12AU7, 12AX7 and KT88, you'll have enough varieties around to keep you busy for years.

 

pmcuk

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I've certainly accumulated far too many valves, for sure!! Some of them have been very good investments, like the DHTs. Some of the more uncommon types will be hard to shift, good as they all are. I bought a lot of European tubes before Brexit, and that was a wise move. Tubes like EL12 and EL11 which have a following in Europe, but little known over here. And some less common types like EL41 and E80L. All brilliant sounding valves. And within the UK I've been collecting EL33 and PL33 (got 25 of those). EL33 is a particular favourite - beautiful sounding valve, very like EL11. Customs and VAT charges now make European vendors uneconomic, which is a real shame since I love the European valves more than the US valves. The German Post valves are legends in Europe.

But I digress...... I don't use any of the common types like KT88, 12AX7 and 12AU7. If you collect these common valve types in an intelligent way your investment may be fairly safe. I used to buy the US types in a big box to make the shipping economic - the US types are cheaper over there and if you know your dealers you can get good prices for tested valves. Back in the day I had an "insiders" list of small dealers who still had stocks of desirable valves they were willing to sell cheaper than the big dealers. But they've long since been cleaned out by collectors. I myself cleaned them out of a lot of 01A, 26, 46 and 10Y. But you can still get good deals on the common valves. The RCA types are usually very good, but they all can be collectible. 

So collecting NOS or good tested (80% plus) US valves is still a potentially decent investment. Prices have remained quite stable since the pandemic, maybe because people in lockdown are paying attention to their sound systems. The days of massive profits, however, are gone. Triple mica 5751s are a good example - silly prices now. 

 
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rabski

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I have a fair few 'silly money' valves in the pile, but that's because I bought tons in bulk before they were worth anything other than scrap value. Mainly complete stock from old repair shops and the like. I never did so because I predicted any value, simply because it seemed a criminal waste for them to end up in landfil. Over the years, I've got rid of all the stuff that's purely TV or radio, mostly to other 'addicts'. Endless tea-chests have gone, but I still have enough to keep me in fun and games for years yet, and some that are genuine NOS unobtanium stuff that'll go towards the retirement fund.

The best thing is discovering and adapting, as always. If I look at a schematic, I won't go and buy valves, but will try and see what I can use that I have.

Unfortunately, it's as much an addiction as a hobby. Despite constantly promising myself I'll get rid of stuff, I usually end up buying more. It's inevitably the 'I have never tried one of those' scenario when something pops up and the datasheet looks nice. Plus I was brought up on IDHT stuff and have more recently discovered DHTs. I didn't have enough (still don't) so I'm still doing the 'ooh, that looks decent' thing late at night, which inevitably finishes up with odd boxes arriving a few weeks later that I'd completely forgotten buying. The Russian milspec surplus was like that for a while, but a lot of the decent stuff is long gone and they're starting to change 'Western' prices for a lot.

It's a fun hobby...

 
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antonio66

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Thanks to all for some great and helpful posts. I will say the only valve types purchased that did not come with the amp are a couple of 5751's, Brimar (very cheap) and GE Black Plate (good price and because I wanted it).  I did check with the supplier who told me KT90's would be ok to use although I am a little worried about biasing. Having never done it before and seeing there was no holes and screw adjustments on the top plate I thought great auto biasing  :D only to find out you have to remove the bottom plate to bias the valves and to what I've seen and read the amplifier should be warmed up before biasing.  Any tips?

 

rabski

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Tips? Without wanting to sound like your granny, if you need to ask, then I'd get someone else to do it. If it's set internally, then you'll be probing around the inside with a multimeter and there are voltages a few mm away from the test points that could easily spoil your day. or possibly, your potential widow's day.

If you're experienced enough with working on high voltage stuff, the instructions should be clear. If not, then it's strightforward for anyone who knows valve amps.

I'm happy to encourage DIY, but poking around the inside of a valve power amp isn't the best place to start. I always feel a bit guilty making comments like this. I mean, like most of us I started off with zero knowledge, but frankly I was incredibly lucky on a couple of early occasions. I was also young enough to have very quick reactions, self-aware enough not to do things twice if they hurt a lot the first time, and just about bright enough to realise I needed to find people to learn from, and proper books to read.

 
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rabski

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Incidentally, there is highly unlikely to be any sonic benefit from using KT90s. They won't magically make the amplifier have a greater output, because that's determined by the circuit. I've never tried any, though other people aren't full of praise.

 

antonio66

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Unfortunately he amplifier is not being used in the uk, I'm roughing it in the Far East, so don't know of anyone who could help with the biasing.  I will have to take the bottom plate off and have a look, will also contact the supplier again.  I was not wanting greater output from picking KT90's, only a sonic benefit which you claim is unlikely  :/ :D

We have obviously read different reviews, I remember reading a long and detailed review of all the KT88/KT66/6550/6L6 ect. type valves and you've guessed it the KT90 came out on top. I know 6550's are suitable and obviously KT88's, I'm in no hurry to change the tubes, the stock ones sound just fine, but you know how it is when reading these 'damn' forums, giving you ideas on how to reduce your bank account.   :)

 

pmcuk

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antonio - if you see a future for yourself with valve equipment then teach yourself how it all works. I taught myself mostly with Morgan Jones "Valve Amplifiers" to start with. Then a lot of stuff on the Internet. And I've been a regular member of DIY Audio and Audio Asylum forums since around 2005. I've learned a huge amount through reading threads and asking questions. I also keep voluminous notes in a Word file and an Excel file. These days there's a lot of tutorials on YouTube. 

You don't need to get to know valve equipment - it's an addictive hobby which takes a while to learn and you need to be cautious and respect higher voltages. But it takes you a lot further than tube rolling and switching cables. 

 

dannybgoode

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Without wanting to sound like your granny, if you need to ask, then I'd get someone else to do it. If it's set internally, then you'll be probing around the inside with a multimeter and there are voltages a few mm away from the test points that could easily spoil your day. or possibly, your potential widow's day.
It would be unfair of me to name names but some complete idiot prodded around inside their hybrid HF transceiver that has a valve power output stage back when solid state was making the receive end of HF rigs really good but it was still cheaper, easier and gave better transmit audio to use valves on the transmit power stage.

Anyway said idiot hadn’t turned the radio of when they stuffed it in the finals cage that was hovering around 1kV. Next thing they knew they had been fired half way across the room and their arm properly ached for over a week. They have learned their lesson so yes, your warning is more than appropriate…  :$

 

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