KEL84 V Stereo 20

[Cobblers]

Originally Posted by graeme

I will take this up with peter, but only when im sure i cant sort it myself.
I will check all my earthings, just in case, and also try a few different mounting/clamping ideas.
If they all fail i will consult peter.
I will report about everything i try, on one thread or another, so others have the benefit of my experience.

I don't know how much kel 84 runs in class A, it is probably substantially biased into class A (circuit on full time with music in signal or not), in which case it is difficult to totally eliminate hum even in a good transformer, as they are working extremely hard compared to deathly quiet solid state, as it ticks over idling class B.
All the tube amp's I have heard so far (courtesy of my dads previously ever changing hifi -listening to Quad II's got me hooked) hum a bit (not at the speakers, so I recall) , WAD, Quad, Audionote, AI, Border Patrol, Audion, Audio Research, Canary, and others all hummed, it depends on the how much is tolerable. In practice with music on it matters not one jot.

[Richard]

Originally Posted by Phil Y

Hello Rich, thanks for the comments about the sound of the two amps. This ties up with my thoughts on the stereo 20 although it is the treble end that bothers me more as my speakers are not huge (40 Litre bass reflex). I have come close to replacing my treble units(Scanspeek D2008) several times over the years in the hope of improvement. It sounds like i would have been barking up the wrong tree. The one thing that will cloud the issue a little when i try comparing the two amps is that i am building the KEL84 with the components supplied as i don't have the funds to do otherwise at the moment, but my Stereo 20 is fully decked out with high quality passive components. A mix of Holco H4 and tantalum film resistors, and a mix of paper in oil and polypropylene caps(Kimbercaps)
I suppose this does mean that if i like the KEL84, i will have even more to look forward to down the line.


Cheers, Phil.

Yes I don't think you'd be disappointed as Kel84 treble is very sweet as standard. You could make some cheap improvements quite easily. If you're fairly experienced I'd build it with hexfred type ht bridge (see faq) and shottky/pi filter on the heater feed. It also seems to respond well to lowering feedback, again in the faq. Past that it's a matter of taste but I like those yellow axial vishay polyester caps from RS for coupling, very natural if you don't want biting treble. For their lowish cost you could also include Elna Stargets from RS instead of the standard cathode bypass electros. Polyprop bypass caps on the ps electrolytics would be another cheap tweak without blowing the budget on boutique parts.

Rich

hi , i have a leak st 20,and been useing it for the last 20 years, and just built a kel84. all i can say is , the kel 84 stays. its just better. hope you have fun building it, i did.

[Timbo in Oz]

I'd expect a rebuilt st20 (standard parts) to still lack extension and be a bit boomy in the bass.

both of mine sounded that way in good original nick, but the filter caps were farting a bit and MUllard gz34's are not cheap out here!! so I ditched them!

IMO&E a modified circuit using lowmu tubes in place of all 3 12AX7's and much bigger PSU off Good SS diodes snubbed into big bypassed caps can have much lower NFB much deeper and taut tube bass and sounds nicer overall.

Added a CCS to each of the splitter drivers still LTP's - I'm using NoS Aussie 6CG7's like CJ and Audio Research amps do/did (into 6550's) not quite as good as their sister octals (6SN7) but nice tubes. They are idling.

Does the mains traffo in the KEL84 get hot like the LEAK items - or is it overspecced?

THAT would make a real bottom-up upgrade?!