HiFi Tube Pre-amp: Analog Scope Testing

This is the start of the line stage pre-amplifier shootout!
We have 3 different China clone pre-amps, 2 were prebuilt, one I built with quality parts from a ebay board. We look today at basic analog waveforms and all seem to be capable performers at this point. The jury is still out on distortion, noise and sonic quality. Here is a link to some info about my build, this info could change as we play around with these.
www.skunkiedes...
This is a project I am excited about as this is something a lot of folks have asked about: a tube pre-amplifier. My build is a clone of a Conrad Johnson pre-amp that uses four 12AX7 tubes. Zero-zone sells this already built for $260ish, but I wanted to build this myself using known quality components and my own layout.
I plan to cover the other pre-amps tested in more detail in future videos.

[@primepreowned1]

Skunkie I hope you realize what a great teacher you are. You have a gift of communicating things that’s over most people’s head an making it understandable. The way you choose your words are incredible. I love your channel. (Thank You So Very Much)

[@SkunkieDesignsElectronics]

Wow, thank you!

[@larrywhaples1315]

Good watch! Enjoy learning new things.

[@cobar5342]

Great testing session

[@dtg474]

All these videos are great. Im learning a lot from you. Thanks for the hard work Skunkie

[@1875michael]

Just wanted to take a moment and let you how much I enjoy your work. This video was especially entertaining and informative. Absolutely love the detailed analysis. I've been learning a lot through your video work and am looking forward to your next video in this series. The detail, clarity and quality are deeply appreciated. Please keep up the good work.

[@crimsonghost6454]

Great video. My take is that all three look good. I guess one could choose how much gain they want and use an appropriate preamp. I curious which one will sound best and if there is any hum or noise.

[@SkunkieDesignsElectronics]

Yeah these do all show: pretty clean square waves, not a lot of visible distortion and no clipping from a 1V RMS input. The next test will show the THD vs gain, the frequency response and finally a subjective listening test. Far from done :)

[@SkunkieDesignsElectronics]

It's mostly from outside sources. Given the AC heaters and other outside influences, it's pretty easy for a tube audio device to have some hum.

[@sonhouse9636]

Very interesting thanks.

[@johnbaker9393]

I'm curious as to why you chose 3v peak to peak for the input level for testing. I would be interested in what kind of output you could get with a lower input level like .7 or 1.0 volts. Thanks for your great videos!

[@SkunkieDesignsElectronics]

3Vpp is 1Vrms which is around what a music signal from something like a CD deck would put out. Not like max signal but an average music signal. 2Vrms is the peak for most. I like to see an integrated amp be able to hit max power off 3Vpp. The other question, if 3Vpp is amplified to 30Vpp, 1Vpp would be amplified to 10Vpp. You bring up a good point, I probably should test them all at say .5Vpp and see how they do as well :)

[@wakjeman15]

Hi Stephi, how this thing sound? Do you plan to continue on this preamp? I am waiting your sound result because mine is hum at low frequency. I built it 100% using all components supplied as a kit. Thanks.

[@SkunkieDesignsElectronics]

Yes I am! In my latest "Monday monolog" I explain I want to build a simple "Spud amp" that requires a pre-amp to test these with.

[@allenwalters8812]

I have high hopes for the 6sn7 preamp. That square wave on it looks a lot like your 6sq7 amp and you really like the way it sounds. I'm hoping it has that tube goodness to the sound.

[@SkunkieDesignsElectronics]

Fingers crossed!

[@phillipmorris9847]

This is fun, thank you

[@b97209]

If you end up with two outputs for two amps, will the outputs split the voltage (23/2)? If the load was 100k, what would the new load be on the preamp?

[@SkunkieDesignsElectronics]

The output voltage is per channel. If you drive two amps from each channel, the voltage wouldn't be split, the impedance would be 1/2 but as the testing show, going from 100K to 50K load didn't change the output but a tiny bit.

[@b97209]

@@SkunkieDesignsElectronics Thank you! That totally makes sense.

[@joesapereira5539]

Stephi I want to echo what some have said below, you are an amazing teacher. From a Tube newbie, I am learning a lot about these amps/preamps that you have built so far. One question. I have built several Solid State amps and would now love to jump in the tube world, but one thing I have noticed is that there is a lot more test equipment required to build tubes versus solid state. From your perspective, should I even try one of your builds if I do not have the testing equipment you have? I have seen several examples in your videos where you use this equipment, from testing the appropriate voltages to the bad tube you fund on this video. Would love to hear your perspective on this. Keep up the great content you are providing.

[@SkunkieDesignsElectronics]

I think you can "jump into the tube world" with just a good digital multimeter. You make a good point about this "bad tube" and I want to play around and see if this tube also makes the measured voltages go wonky, I bet it does. Most of this test stuff I use when designing, or redesigning circuits and making sure what I am doing helps the circuit.

[@joesapereira5539]

@@SkunkieDesignsElectronics Thanks for the response. Now I just need to get up the nerve to do it and pick out a project. I was hoping to maybe start with something simple first like upgrading an existing amp but I just watched your Reisong A12 video and now I am not sure. I'll just wait for your ext video to see what you come up with.

[@allenwalters8812]

What does that mean when the signal is inserted? How do you correct that?

[@SkunkieDesignsElectronics]

I mean by that term is: using a signal generator, an electrical waveform is applied to the circuit and then using a scope, you look to see what that known signal looks like in other parts of the circuit.

[@allenwalters8812]

@@SkunkieDesignsElectronics sorry I didn't check the spelling. I ment the inverted signal on the 6sn7 pre.

[@SkunkieDesignsElectronics]

@@allenwalters8812 Oh, the inverted signal just happens because of the design/number of stages. It's not something that needs correcting :)

[@allenwalters8812]

Maybe the 6sn7 pre will do better with some better tubes.

[@SkunkieDesignsElectronics]

It seems fine so far to me. I will get back to these pre-amps soon, I did order some new tubes and want to have options when we get to testing these.

[@allenwalters8812]

I'm shooting you an email with the only schematic I could find on the m7.