KT120 Tryout as SE

[Paul Barker]

the 66 was a bit bass light compared to the 88 and 120 but had better treble extension and sounded overall abit sweeter.

Base light because it is twice the impedance your transformers don't suit it that's all.

If you had each valve type in it's correctly designed transformers: no base light.

Same transformers, higher impedance valve always will sound bass light compared to a valve of half the impedance.

[colin.hepburn]

Out of interest the feedback in the cathode of the Kt120 is confusing me a bit as Steve is just using the cap I thought the resistor also had to be lifted from ground as well as the cap
As in mike Hs version is this a better way to do it and can i do it this way on my JE labs EL34SE amp

[Cressy Snr]

Out of interest the feedback in the cathode of the Kt120 is confusing me a bit as Steve is just using the cap I thought the resistor also had to be lifted from ground as well as the cap
As in mike Hs version is this a better way to do it and can i do it this way on my JE labs EL34SE amp

If you use the resistor and the cap together, you are passing the output valve bias current through the OPT secondary. This DC offset may or may not be a problem. I prefer it not to be there. Just using the cap in the feedback loop avoids DC on the OPT secondary.

No reason why you can't use cathode feedback on the EL34 amp, but you need to be aware that when using this kind of feedback scheme, it is very easy to end up with positive rather than negative feedback. Unlike global feedback where the amp wll start howling if the feedback is the wrong way round, with this cathode feedback scheme (IME) you don't get oscillation.
What you do get is an increase in loudness for the same volume setting you had before feedback. This tells you the feedback is wrong polarity. You need to reverse transformer primary if that happens, so that you get a decrease in volume.

[Mike H]

As in mike Hs version is this a better way to do it and can i do it this way on my JE labs EL34SE amp

Are you sure it's 'my' version and I didn't just copy it from somewhere else? I'm not

[colin.hepburn]

[quote="SteveTheShadow
If you use the resistor and the cap together, you are passing the output valve bias current through the OPT secondary. This DC offset may or may not be a problem. I prefer it not to be there. Just using the cap in the feedback loop avoids DC on the OPT secondary.

[/quote]
Thanks Steve for the lowdown on this method of feedback I don't use any feedback in the JE amp but may try this thou

[colin.hepburn]

As in mike Hs version is this a better way to do it and can i do it this way on my JE labs EL34SE amp

Are you sure it's 'my' version and I didn't just copy it from somewhere else? I'm not

i took it to be your idea as it came from your website I think

[Cressy Snr]

More experimentation today has resulted in the EF184 being triode strapped.
The reason was that though pentode mode worked well, I felt that triode strapping the KT120
lost some of the detail I had been getting from Ultralinear.
But I couldn't return to ultralinear operation of the output stage and keep the pentode drive as tonally it wasn't right.

I used this loadline put forward in 2006 by Tom Schlangen at ETF of that year.



Fantastic linearity with enough current to drive KT120s in Ultralinear mode.

[Cressy Snr]

The schematic is shown below with voltages and currents:



Because it was less cumbersome to reverse the OPT primary than the secondary to get the feedback in the correct polarity,
the Ultralinear connection is at 57% of primary turns rather than 43%,
and the schematic of the output transformer shows this, but there is no downside, it sounds lovely.

I'm now at a better point soundwise than I was with the MH4 driver.
Tone and detail are excellent with superb bass response.

I like it very much.

[Paul Barker]

That's interesting.

[ed]

yeah.....very interesting...gonna have a go at this 6ej7 into sv811

[JamesD]

Hee Hee Hee....

EF184 strikes again...

Damn! Hush my mouth!

But I am surprised it can take on an MH4 for tone...

not surprised the bass is better or the detail but I would have backed the MH4 for tone..

Fascinating stuff - Thanks Steve!

J

[Cressy Snr]

Hi James,

I don't think it would beat the earlier versions of the MH4 such as AC/HL or the globe MH4 with the bigger anode structure. Steve let me hear an AC/HL and a few other early globe shaped equivalents to the MH4 when I took the amp to his place when it was only a little 3/4 watter.

The tone these valves of Steve's produced was to die for

OTOH I do think the EF184 beats the later version produced after the war, which is the one I have, not by much, but it does. It is also by far, the quietest input valve I have ever used. The silence is total when the amp is idling resulting in very good instrumental separation and the absence of distortion produced by following Tom Schlangen's loadline and op point is very audible, if you know what I mean.

There is also another point to be made in that it seems to me that the KT120 was quite deliberately designed from the outset, to perform at its best when operated in ultralinear mode. There is simply no contest, at least when compared to triode operation there isn't.

The sound it produces when UL configured is in a different league to triode operation. Of course that's only my experience with this power pentode, others might think differently.

[ed]

gonna have a go....I only have the 12hg7 to compare with, which I tried when I first played with the sv valves...Pete Millet had said really good things about the 12hg7/12gn7, but I didn't fall in love with it....

to my ears the ecc83/82 aikido sounded much more sparkly, so I stuck with it....time to have another try.

[Paul Barker]

It appears to be too good for words.

There is one elephant in the room. Oscillation.

Watch out, build defences.

[Cressy Snr]

It appears to be too good for words.

There is one elephant in the room. Oscillation.

Watch out, build defences.

Oh yes!
With my EF184 input stage, I have 100ohm carbon composition screen stoppers right up against the g2 pin. There are also 1K carbon composition control grid stoppers up against the g1 pin.

So far so good.