KT120 Tryout as SE

Cressy Snr · Post by **Cressy Snr** » Tue May 28, 2013 9:45 am

Nice though the Mazda amp is, it simply does not have enough power for my Metronomes, sounding small scaled and frankly anaemic, compared to the EL84 push-pull; beautiful, at times magical midrange, but there's more to music than that, for me at any rate.

I decided yesterday therefore, to do a conversion to KT120s.
The big power supply is well capable of dealing with these tetrodes but the James output transformers limit plate current to 90mA, giving 29W plate dissipation.

KT120s will dissipate 60W so they are being run conservatively.
I am keeping the 4 volt, 41MTL mesh-plates as drivers, feeding them from a separate heater transformer.

At my op point, in SE Ultralinear, the KT120 are good for 10WPC. They'll do 6WPC, triode connected.

I decided to do an interim SE amp just to check them out for sound quality as I have all the bits to hand.
50WPC push pull KT120s will be a big investment in PSU caps, dual power supplies and the rest and I see no point in doing such an amp until I am sure I like these valves. I did use them briefly, driven by a 6N1P cascode driver and they sounded decent enough as triodes, but a couple of years have passed and the auditory memory is notoriously weak.

Should have it up and running today so it will tell me one way or the other, whether it is worth proceeding with the big PP. But I have the 6AQ5 to do first.

I'm hoping KT120 will be a good sounding utility amp to use, while I experiment further with balanced input phase splitter-less PP amps, which, to be quite honest, are much my preferred type now.

JamesD · Post by **JamesD** » Tue May 28, 2013 11:38 am

Interesting! The KT120 is a beast! Do you know what it is based on?

J

Cressy Snr · Post by **Cressy Snr** » Tue May 28, 2013 11:55 am

JamesD wrote:Interesting! The KT120 is a beast! Do you know what it is based on?

J

I think it is based on the KT88/KT90 family.
The plate structure is far bigger than either of these though.
There are four getters within the glass bulb, two top and two sides.

The bulb is thick and heavy and with the large getter assemblies it suggests high vacuum. Construction quality is excellent.

JamesD · Post by **JamesD** » Tue May 28, 2013 3:43 pm

I couldn't find anything except that it was a New Sensor development...

I did find a couple of Sophia curves one triode, one pentode for the beastie and they are attached.

James

Cressy Snr · Post by **Cressy Snr** » Tue May 28, 2013 6:54 pm

Aye, those triode curves are the ones I used to extrapolate for Ultralinear connection.
There isn't a great deal of information around about them.

Here is the inside of my SEUL conversion of the Mazda amp:

Small toroid on the right powers the heaters of the 41MTL drivers.

Here is the amp playing:

It looks lovely lit up though I say so myself

The amp has a big, bold, powerful sound, warmed up a treat by those old British 41MTL meshies, which give it a sweet, detailed tonal palette.

The KT120s biased up spot on where I had worked out they would, at -41V and 90mA with about 400V a-k, so they seem to be accurately specified.

It'll do very nicely whilst I experiment with the pp amps.

Does just what I hoped it would.

Paul Barker · Post by **Paul Barker** » Tue May 28, 2013 6:59 pm

What's the output power Steve?

Cressy Snr · Post by **Cressy Snr** » Tue May 28, 2013 7:08 pm

Paul Barker wrote:What's the output power Steve?

At the moment I reckon about 12WPC in UltraLinear, 6W in triode.
The James output transformers are the limiting factor as they will only allow 90mA of idle current.
I am running the valves at about half the max plate dissipation, and using Ultralinear sans feedback.
No problem with any tipped-up balance or flabby bottom, but then there wouldn't be now I know not to do
silly things, that end up neutralising the effect of the negative feedback around the screen/anode circuit!

If I had some bigger OPTs I reckon over 20W would be attainable in UL and 12W or more, in triode.

Icon Audio claim 27W ultralinear and 16W in triode from their MB30SE KT120 monoblocks, so they are running the outputs a lot harder than me.

Paul Barker · Post by **Paul Barker** » Tue May 28, 2013 7:58 pm

Not at all bad.

Only a matter of time before you put them into the PP amp.

Cressy Snr · Post by **Cressy Snr** » Tue May 28, 2013 9:09 pm

Paul Barker wrote:Not at all bad.

Only a matter of time before you put them into the PP amp.

I think you're right there Paul.

This SEUL amp is more powerful than the EL84 triode strapped push pull amp I am currently using. It seems that the more power I put into the big Mets, the better they respond and the better they sound.

I remember Colin, when he first built them advising me that this would be the case. It has taken me eighteen months to cotton on to what he was talking about. But now I completely get what he meant.

Based on the evidence that has been building over the past few months of experimentation with more powerful amplification, a 40 Watt push pull KT120 will make the Mets sing their heads off

Paul Barker · Post by **Paul Barker** » Tue May 28, 2013 9:31 pm

Yes I am enjoying the extra power too. But Diana runs out da room!

JamesD · Post by **JamesD** » Tue May 28, 2013 9:36 pm

Those are big valves

They seem good value too!

J

Paul Barker · Post by **Paul Barker** » Tue May 28, 2013 9:52 pm

I can beat that.

If it's bang for buck power pentode time:

£13 each.

JamesD · Post by **JamesD** » Tue May 28, 2013 10:13 pm

Indeed!

But I was thinking total cost of ownership (although I didn't say that).

Building an amp to take advantage of the GK-71 is an enormously expensive task - the KT120 has humble requirements by comparision

How about a competition to see who can design and build the best value amplifier with the KT120?

I daren't suggest the GK-71 for such a competition!

J

IslandPink · Post by **IslandPink** » Tue May 28, 2013 10:13 pm

Yes, and my EBay finger keeps twitching when I see things like this :
http://www.ebay.co.uk/itm/NEW-2xGM-70-G ... _676wt_958

Sorry not a pentode .. but you just reminded me about them !

pre65 · Post by **pre65** » Tue May 28, 2013 10:23 pm

Or a push pull amp with these ?

http://www.ebay.co.uk/itm/833C-vacuum-t ... 4d0462ea77

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KT120 Tryout as SE

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