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RE: Help in selecting new KT88 SC

Posted by Kurt v L (M) on June 9, 2014 at 18:09:14

In Reply to: Help in selecting new KT88 SC posted by wiggy924 on May 24, 2014 at 10:35:28:

Asylums..
I am new here and accidently posted with my email address as username ( moniker )..
Boy - what a struggle to change that. I needed to delete the post and are now reposting it with a "proper" username.
Well - it is not any important message , I just didnt like to see my email address at the top of the post..
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In my opinion there is some important issues that needs to be addressed when comparing valves. The tolerance of most valves, referred to the standard valve characteristics as published in the manufacture datasheets are usually up to +/- 20% or more. ( Matching does not change this fact, - a matched pair may be close to one another, but perhaps 15% off the standard/statistical characteristics )
When dropping a set of valves in to a particular amplifier, we expose the valves to the working settings of that particular amp. ( Driver, bias, plate, sg2 Voltages, regulation etc )
Further - but just as important - the OPT turns ratio and the impedance of the loudspeakers are reflected according to these.
Loudspeakers are not 8 Ohms constants. Rather they change according to freq as well as with dynamic signals. A steady signal sine wave freq sweep reveals that the speaker changes impedance as much as -50 to +200%. ( Much more when dynamic music signals are the source )

This means that the output valves will "see" a load changing dramatically following the "chaotic" impedance of the speakers, reflected via the OPT. The particular turn ratio here, as well as the the working settings decides the actual load and the slope to the valves.

The theoretical target load for a set of KT88's, "dialed in" by the amplifier designer, say 5000 Ohms, will as such change from at least 2000 to 15000 Ohms during expose to music. ( or a sine wave freq sweep )

Add to this the merits of the speakers, that will favour some tone characteristics, dump others etc etc...And not least our personal preferences, habitat and so on.

It is in reality an impossible mission to elect an all over best output valve. We can find the ones that suits the particular amplifier design and loudspeakers..
But that is all.

Many years ago I produced some KT88 power amps. ( Mono blocks )
I used British made KT88's. ( Gold Lion, MO-valve )
These were excellent - all of them within 5% straight from the shelves and they did not change much. At the point when I ran out of stock I feared I had to give up this amplifier. I finally got some US made GE 6550A's. They were not as tough, tight ( +/- 20 % ) and die hard as the KT88's, but I modified the circuit, matched the valves and got pretty much the same results as with the MO-valves.
Later I ran out of the GE's - and to make a long story short - I ended with some Svetlana KT88's. ( Thanks to Eric Barbour )
These were however more than +/- 30 % and changed quite a bit during burn in. So matching was vital. Again a little modification was needed in order to get the best out of these Svetlana's.
Sadly they also changed from production series to product series and I finally gave up and discontinued the production of the power amp.
If I today found a set of NOS original MO-valve KT88's and dropped them into that 3'rh generation of this amplifier, it would now be able to get the best out of these wonderful Gold Lion/MO-valve.

I have no experience with the P-Svane KT88, but they sure look good.

What I wish to emphasize with this post is that the choice of valves ( in particular output valves ), depends upon many variables. The design working settings of the amplifier and not least choice of speakers. There is no such thing as an "optimal" specific load point/sweet spot, due to all these variables and unknowns. The best sounding output valves ( tubes ) in one particular set up, are very unlikely to necessarily be the best in all set ups.
In other words it is case sensitive.

A matched pair of valves is good, but it depends as well upon many variables and if we really care about a symmetrical signal through the OPT, it would be a much better approach to make the driver adjustable from an AC point of view just as well.
This is the only certain way to insure the best possible symmetrical signal.
( Personally I do not think this means as much as is sometimes claimed , but that is another story all over )

I am new to this list and may not post the proper way - I am having difficulties sorting the posts in a way I would prefer.
Thanks

- Kurt Lilienthal Steffensen
Kurt Lilienthal

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