KT120 Tryout as SE

[JamesD]

Fascinating that ultralinear sounds best!

Yeah, not surprised that pre-war audio triodes have the best tone...damn it!

But there are still EF184s out there for under £2:00 each! Wish balloon valves were the same!

The EF184 is an IF triode so it will oscillate given some encouragement - you have it just right steve.

J

[Cressy Snr]

Fascinating that ultralinear sounds best!

J

Fascinating indeed.

I can only presume that the KT120 was designed to work best using Ultralinear mode, which let's face it, is going to be the preferred operation method in the commercial amps that use it.

I've used EL34s, 6550s, KT88s and EL37s in both SE triode and UL and without exception preferred triode mode. The KT120 is the exception it seems

[Cressy Snr]

Before committing to buying four coupling caps for the EL84 rebuild, I bought a couple to try in the KT120 SEUL.

They are badged by well-known European supplier "Tube Amp Doctor" and are ostensibly for replacement use in 50s and 60s guitar amplifiers, where players want the 'techs to fit traditional value replacement caps (these are 0.25uF, 600V aluminium foil in oil types)



They are really rather good when used in hi-fi amps.
Tone is superb on voices, brass and of course electric guitars.
Treble definition is also lovely and liquid.

Do they better tropical fish caps? Yes they do, giving more of the same type of sound. Are they worth £7 each? I think so because they better any of the Audiophool caps I have tried, by a big margin.

As I have become more experienced in the art of voicing valve amplifiers, I have to say that the musical instrument amp guys certainly know their stuff, when it comes to tone, and it is daft in the extreme, when voicing your own "hi-fi" amp, to ignore the decades of experience the guitar amp guys have.

[IslandPink]

Interesting info on those caps, Steve - good find . I guess the Jensen copper might beat them still ? - but they are a good chunk more expensive. £7 sounds very useful .

I wonder how they compare to Mundorf ZN , which are the same price, approx :
http://www.hificollective.co.uk/catalog ... 8_168.html
I started using them earlier this year, as they beat everything else I'd tried, at line-level , and that was quite a lot of caps over the years .

The PIO's might still be better at power-amp voltages/currents though . I still stick with my Jensen coppers in the power amp .

[Alex Kitic]

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.

Steve,

Maybe it's a little bit late, and I did not read the entire thread: would you consider trying the RH Universal v2 schematics approach in your amp?

1) I am not aware which drivers are you using, but they would most probably fit the two groups the v2 is adapted for (higher transconductance and/or gain - lower transconductance and/or gain);
2) Using a CCS under the cathode of your tubes will indeed lead to perfect control, and allow you to play with tubes (a higher level of tube-rolling);
3) Disconnecting the UL and using a zener diode to set a fixed potential difference between anode and g2 (edit: maybe not clear enough - potential as in pentode mode, not connecting it together as in triode strapped mode but making a fixed difference in potential).

Besides 2), the remaining two interventions seem to be a piece of cake (something you can do in less than 30 minutes).

Would you try it? Check out the simulations on my blog: if power is what you are after, this is a good way to pursue it (comparisons made by those who built the Universal and use it with KT88 are more than favorable - sounding better and seemingly overpowering reasonably hefty PP amps).

90mA current draw is not your limitation with this amp setup, but the available B+, more likely.

[Cressy Snr]

Hi Alex,
Unfortunately the KT120 SEUL amplifier is not working at the moment as it is in bits, so I can't do anything with it.