KT5881 valve change - World-Designs-Forum

Garyk · #1 5th October 2018, 01:26 PM

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

John Caswell · #2 5th October 2018, 02:08 PM

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

Phil Y · #3 5th October 2018, 02:55 PM

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.

John Caswell · #4 5th October 2018, 03:12 PM

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

Garyk · #5 5th October 2018, 03:37 PM

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

colin.hepburn · #6 5th October 2018, 05:38 PM

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