Loadline Tools

[JamesD]

Here is a good introduction to this style of Sakuma design and eventually an explaination of how the cancellation works...

https://jacmusic.com/kits/amp/gm70/GM70-index.htm

Yes its a GM-70 based design

Yes its the reactive part of the load that open it out to an ellipse often taking the signal into the more non-linear parts of the transfer curves...

Lynn Olsen has written extensively on this and, as so often, Steve (Steffie) Bench's document is probably the best reference with worked examples - its a 33 page pdf so its too large to upload so try this link

http://rfcec.com/RFCEC/Section-3%20-%20 ... Bench).pdf


Hope it works

James

[simon]

Great, thanks James, a bit of reading when I've finished decorating. Undercoating when I finished my drink, then the ceiling. Should be finished by about 8...

[simon]

Just read the first article, fascinating. It's so long since I've seen a Sakuma circuit I'd forgotten he used the same driver valve. And the penny dropped when I realised the driver stage was in antiphase to the output. Of course.

A very interesting approach. I don't really fancy another 200W of filament heating though, or double the number of coppers!

The Steffie link works thanks, will have a read tomorrow.

[Paul Barker]

There’s a KT120 model on the site, and the beauty of this calculator is that you can set your transformer impedance, fiddle with the current until the value of your existing cathode resistor comes up. This way you can experiment with substituting other valves into your existing design and be able make an estimate as to whether that valve it is going to sound good, crap, or whether it’ll blow your amp up.

I tried the KT120 triode model with my HT and fiddled the current until it came up with the cathode resistor value in my amp. It gave a 100mA op-point at -27V of bias, 3.8W output at 5K and the 2nd harmonic distortion wasn’t a lot worse than the KT77. The 3rd and 4th however were far lower than the KT77.

Substituting the KT120 into my amp, did indeed produce 100mA, but 26.5V of bias so well within any margin for error. I’ve not heard my KT120s sound anywhere near this good BTW and this is with puny 6SL7s driving them; so with a more suitable driver, it looks like a good candidate for a more powerful amp, without having to spend a great deal of dosh as the Hammonds can handle 138mA of bias. As it is in my amp at the moment, the KT120 is producing a nice musical sound and the tone is big and relaxed.

I’m not going to put up the pic of the curves because it will come out huge again, but the KT120 model appears accurate and as triode strapped pentodes go, it isn’t as bad as all that; certainly it looks better than the KT88.

The KT150 model looks very, very good indeed in triode mode and a nice 10W single-ended amp could be made with it, if your output transformers are any good. KT150s are a bit expensive for me though, which is a pity.

following his link to how the various curves are derived, it looks as if he’s using some sort SPICE modelling setup. Everything I’ve tried in my own amp, working from his models, has come out as predicted, so the models appear to be sound.

BTW, there’s an Osram PX25 in the list of valve models.

Stephie has put up a post on Spicing PP Loadlines . Went flying over my head like a lightning chasing a US stealth bomber at the edge of earths atmosphere. (If for no other reason than to show that Britain Out of all the world had a fighter that could hunt it down.

But I’m sure you all that are capable @ Spice dot clever git’s dot co dot UK. Will enjoy a read.

[Paul Barker]

Oops, http://www.intactaudio.com/forum/viewtopic.php?t=1640