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  #1  
Old 5th October 2018, 02:26 PM
Garyk Garyk is offline
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Default KT5881 valve change

Hello all
I am not electronic orientated, but still trying to learn a few things.
I have a lovely KT5881 MK1 thankfully built by An Other.
I received my new valves the other day Shungang treasures KT66 z
Straight out of the box they were lovely and lush and already have the edge on my Sovtek 5881s.

But Boy do they run hot!!
So hot i have now move the WAD to the top of my Hi Fi rack as the KT66s were starting to blister the finish on the underside of the shelf above.
Now the heat is free to heat my listening room.

I have read on here some where, that the heater element runs at 1.3ma as opposed to 0.9ma for the 5881s... is that going to be bad for my amp?


Thanks
Gary
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  #2  
Old 5th October 2018, 03:08 PM
John Caswell John Caswell is offline
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Default Re: KT5881 valve change

Hi Gary,
This is not a substitution I would recommend, purely on the grounds of heater current consumption.
The 5881 is rated at 6.3v 900ma whereas the KT66 is rated at 6.3v 1.3A. This is well above the 10% overload that most transformer manufacturers suggest, and I venture to suggest that if you keep the KT66 in place you will likely suffer power transformer failure due to overheating.
I would also suggest that there is an possibly an output transformer mismatch as well.
Not a good idea!

John
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  #3  
Old 5th October 2018, 03:55 PM
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Phil Y Phil Y is offline
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Default Re: KT5881 valve change

Hello Gary,
May I ask why you thought the KT66s would be a suitable replacement, were you told this by someone?

John/anyone,
Recently I joined and then left a valve and valve amp group on facebook.
I was surprised and dismayed by the attitude and lack of knowledge of most of the people on there. Most of them seemed to think it was normal to change valves on an almost daily basis ( I did suggest that they will probably wear out their valve bases) and also the general view seemed to be that as most of the common octal pentode/beam tetrode output valves have the same pinout, this makes them interchangeable. "My amp has 6L6s, I think I might try some KT150s" sort of comments.

I quickly gave up and left the group. There was a couple of people with some knowledge on there but they were mostly ignored.

Regards, Phil.
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  #4  
Old 5th October 2018, 04:12 PM
John Caswell John Caswell is offline
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Talking Re: KT5881 valve change

Phil et al,
Built my first amp when I was 9 albeit with a bit of help, I'm now 77 and still learning, but apparently still "don't know nuffin" as I am constantly contradicted by self professed "starters" because the have read ...... etc. In the end I just keep quiet and do my own thing and wait for the c**p to hit the fan.
When I joined this forum there was a great lack of knowledge so I tried to impart hard earned knowledge and information, ways to do things, etc., mostly successfully, hence my original "title" Dr John, which I thought to be a bit OTT so soon dropped it.
There are now quite a few knowledgable people on here and on other forums and I like to think that we all have imparted our knowledge to others to help them along the route to happiness, cos nirvana doesn't exist!!!!!
What do they say 'A little knowledge or fools rush"

John

Last edited by John Caswell; 5th October 2018 at 04:14 PM. Reason: Additions
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  #5  
Old 5th October 2018, 04:37 PM
Garyk Garyk is offline
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Cool Re: KT5881 valve change

Aah. I was not advised by anybody on any forum.
However I took the information from the advert of a valve trader that I had successfully purchased from in the past, (for a previous amp)

It was actually a Quad I bought but this is the headline from the description.

Matched pair 2 pcs Shuguang Treasure KT66-Z Vacuum Tube Re KT66 5881 6P3P

As I'm new to this valve game, and still learning, I took this on face value that the KT66 / 5881 / 6P3P were interchangable.
Ive only recently found this forum.... after I bought the new valves, kinda bad timing really
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  #6  
Old 5th October 2018, 06:38 PM
colin.hepburn colin.hepburn is offline
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Default Re: KT5881 valve change

Hi Gary/John
And welcome to the fourm Gary yes you are in good hands. there with ower very own Dr John and others. who will be happy to help you

Now the very valves you talk about i use in my single ended J.E Labs amp. but whare i differ is that i built the amp with a switched cathode bias resistors of 680/470 ohms so i can run the KT66 / 5881 / 6P3P at about 54mA.

And switching to the 470ohm resistor i can run my EL34s at about 61mA all valve are running into a 5K output TX

Heater TX is 4 amps however if anyone sees this as wrong pleas chim in and comment on it as i am still learning as well thanks
colin
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  #7  
Old 5th October 2018, 06:46 PM
bob orbell bob orbell is online now
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Default Re: KT5881 valve change

Hi Colin and all, John was talking about the heater current, the 5881 transformer is rated at 4 amps MAX, Gary is drawing 1.2 amps over MAX, this will do the transformer no good. Bob
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Old 5th October 2018, 07:04 PM
John Caswell John Caswell is offline
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Default Re: KT5881 valve change

Hi all,
Yes, apologies, I did not make my intention clear that it was heater current I was discussing.
Gary, Yes the Quad amplifier will accept KT66, 5881, 6L6 and 6P3P because its heater current supply is adequate for 2 off KT66 ie 2.6A so as the 5881 take 0.9A =1.8A for the two so that is not a serious problem. However, and there is always a however, as the current drawn is less, the voltage will rise - maybe not excessively, but possibly enough the exceed the valve manufacturers 10% so a check on heater voltages is wise in the Quad.
The other way around ie more current draw can be very serious with certainly overheating and possible transformer failure.
Modern transformers are fairly forgiving and can reach internal winding temperatures of 180C but I do not recommend it. If this is tried on a Quad pitch will start to leak out - not a pretty sight!

John

Last edited by John Caswell; 5th October 2018 at 07:05 PM. Reason: my usual typos
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  #9  
Old 6th October 2018, 02:39 AM
Garyk Garyk is offline
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Default Re: KT5881 valve change

Oh John I'm so sorry this is a bit embarrassing for me.
when I said A quad I meant 4 valves instead of a pair.
not a quad amplifier
so I have a WAD kt5881 mk1

and I have 4 x KT66 in it.
I hear the advise and will put my 5881s back in the morning
I should be able to get some money back on Ebay for a quad of shugange treasures kt66 with 6 hours on them.
thank you all very much for the advice and saving the best power amp I have ever heard.
yes I've spent a fortune on power amps and always returned to my KT5881....... the one that sounded magical with KT66s

thanks

Gary
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  #10  
Old 6th October 2018, 11:16 AM
Richard Richard is offline
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Default Re: KT5881 valve change

Hi Gary,

Some valves are interchangeable and some backwards/forwards compatible but you need to check very carefully.

5881 is a later family member of the 6L6 family. 6L6 was designed as an audio valve and the power ratings and construction changed over the years.

Your amp is designed around the 5881 which is a military ruggedised version of the 6L6GB with 23W max plate dissipation.

The series ended with the 6L6GC with 30W plate dis. Thus 5881, 6L6GB or 6L6GC would be compatible with your amp. Svetlana SV6L6GC is a favourite of many here if they're still available.

KT66 is a different valve, the heaters draw more current and the output impedance requires different transformers for best matching.
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