Hi team!
Well, I did it. I built one. Admittedly not a Skunkie design but, I am darn happy!
I'm generally new to vacuum tube amps (you may have seen some of my other newbie posts), but I decided to take the DIY plunge.
I watched lots of Skunkie videos (hours of them), took lots of notes, did a lot of other research and freshened up my electrical engineering chops a bit, and I settled on a design based on a EJ Jurich design. It's a 3 stage amp using 6SN7 for input a driver stages, and a 6L6 for power stage (I used Tung-Sol 7581A in place of 6L6 - awesome!).
I kinda flew blind using only a DMM and a Variac (and a little AC socket amp/volts meter thingy from Amazon), but it worked out. I have an oscilloscope and wave generator arriving later today so I will then look at it that way. I built and tested (via DMM) the power supply portions first, then wired up the signal stuff. Then I fired it up with dummy loads, and then "old" speakers.
After listening to this for several hours now (I can't stop), I have to say I am amazed!! It sounds absolutely fantastic. Crystal clear, warm, spacious - all that you'd expect (as a newbie anyway) - and super quiet. Absolutely no hiss or hum that I can detect (aside from some minor physical hum from the Edcor power transformer, which Sunkie alludes to in one of her videos, but it's not a problem).
My speakers have 92 db sensitivity and, with a turntable preamp (from Schitt), it has plenty of power/volume for normal listening. More than enough really.
I've attached a couple of pictures and final schematic. The interior isn't the neatest work, but it's my first one, so I was learning as I went about ways to arrange things. I wasn't striving for interior beauty, but I did respect the locations of AC, etc.
I'm planning to add an input selector switch, perhaps a power LED, and may even add an externally accessible pot to tweak the resistor on the 40% ultra-linear tap (just to play around with that).
I didn't go cheap on parts. Several Mundorf and Nichicon caps, Hammond output transformers, Audio Note volume pot, and almost entirely carbon film resistors (old school).
Anyway, I'm super happy and look forward to the scope testing!
All the best,
DM
looks great - i also have the Jurich book and its a great starting point that actually gets you building - I have also built the same as you have but i chose the monoblock version - i have also built the kt66 monoblocks in that book albeit recently sold - heres the "Katy sixty six"
Ok gang, see attached. I took the plunge on the Audio Analyzer suite and the Digilent Analog Discovery 2, and tried it out on this amp. Here's what I got on the first (and only, thus far) pass (attached). The screen captures are a bit truncated so I added some text to help with that.
So, a few things:
1) any input/feedback at all is appreciated
2) question: on the THD test, there is notable difference between L and R channel. What gives? I certainly can't hear any difference. Tubes maybe? (Although my output tubes are "matched" supposedly.) And, what do you think the cliff at 10kHz is..?
3) frequency response curves are pretty amazing... is this real? (I will say, it jives with what I'm hearing, as best my ears can tell..)
4) THD vs. power... once again, L channel is higher. Thoughts?
Thanks! I love this forum!
I'll bet Skunkie will give you a DYI Champ badge for your 1st successful build.
your p-p voltages should look the same being on either 4 or 8 ohm tap- its more for impedance matching the speaker load - because neither have a load across them and the 8ohm dummy load on either will work for your p-p readings.
Edit - wait.... I missed you were using a 4ohm dummy load?? then your putting out around 5watts!! For a SET 6L6 tube amp 5w is right on the money. The lower powered SET tube amps are imho the best sounding - only requirement being you need efficient tube friendly speakerswhich you seem to have.
aha, I see - thanks Danny! Yes, that was just before clipping. (I was using the old "x .707" rule, but forgot that is based on peak voltage, not p-p...)
I have a related question: the OT on this amp has multiple impedance sec taps, so I installed speaker jacks for both the 4 ohm and 8 ohm taps. In the case of this particular test, I think I had my dummy loads attached to the 8 ohm tap, but it's actually a 4 ohm dummy load. So, how does that confusion play into this math?
thx!
No need for a RMS voltmeter, or a voltmeter at all - simply read the p-p value off the scope screen - then divide by 2.828 to get the RMS value, square that, and then divide by 8 to get the RMS power to 8 ohms.
13 / 2.828 = 4.5 vrms
4.5 x 4.5 = 20.25
20.25 / 8 = 2.5 watts
2v scale - are you at the max just before clipping? If so your at 13vpp - are you using an 8ohm load? if so it would equate to 2.5 Watts
Oh, one more note... I accidentally had the input signal probe set on 10x, so it's really 2vpp, not 200mv.
Initial o-scope tests look pretty good to me... (Kinda wondering why input and output are out of phase, but I have two thoughts: 1) so long as both channels are the same it doesn't matter (?), 2) it's a 3-stage amp, so perhaps this is expected..?)
I'm getting approx 11W RMS output, I think.
Nice 1st build- see you have a waveform gen coming in ... just an fyi there are many function generator apps free for your phone .. just need to get a headset cord to rca's to use. they work very very well.