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Hi there in BC. I just finished the universal 6SN7 preamp using PCB’s, r-core transformer I purchased from you. If there any way for me to check it out before plugging it in to try on my Willsenton R8? I checked for continuity between solder points in traces, checked the grounds, the fuses the volume pot… I did plug it in and verified the voltages. My B+ is over 320 vdc without the tubes plugged in. If I recall, you state +/-290. The heaters work and the tubes get warm. Just don’t want to damage my amplifier, or my speakers. You would plug it into the PRE input… right? Looking forward to a good listen. If everything works out I can send you photos if you like. I used a Hammond enclosure, with 1/4 inch thick plexiglass painted silver metallic,top,and bottom plates like the R8. My wife thinks it looks professional. Looking forward to your recommendation.
Start up testing is covered in the Mellow Tone Kits YouTube Build Series. You mention checking some of the basics which is good. But the most important is to plug the tubes in and check your B+ voltage, make sure it’s close to spec and most importantly stable. If you have a scope and signal generator you can pass a signal through the Pre and check for a clean signal out, otherwise plug the pre into the R8 (Pre IN), turn the Pre volume to zero and turn on the R8. If the R8 is quiet with no volume then turn the Pre volume up to half – still quiet, go full volume – still dead quiet, then congratulations you built to the design and layout of the original Universal 6/12sn7 Kit Preamp. If noise or sound issues go back and see where your build deviates from our design. Hint: ground topology is critical for a quiet design.
Please let everyone know how it went, including pics.
Cheers
JimHey Gilbert! No worries i’ll lend a hand, but unfortunately I can’t provide full support for everyone who just builds from the free schematics. Ok so first problem, you don’t say how loud the 60hz hum is? Is it loud at zero volume or do you need to come up to 1/2 volume to hear it.
Ground topology is critical to a low noise pre or amp. Here are the basics.
Mains safety ground (green wire) from IEC – bolt directly to metal chassis ground point – nothing else.
Power supply ground returns to a star ground point separate from safety ground but close by. I like to take this return from the largest filter capacitor point. Important only one return from each power supply circuit – one.
All RCA, Preamp, Potentiometer ground returns to a star ground point as far away from power supply as practical. Important you must use isolated RCA jacks.
If you watch the build series, you’ll see all of this in practice on the Kit Build.
Now other common sources of hum can be the system grounds interacting – so all audio equipment should be plugged into ONE quality power bar – this will keep all system grounds at the same potential.
I’m presuming you don’t have a scope or I would give some general test procedures to help you isolate the problem.
Never give up, ground loops and related noise are very common in all beginner builds – which is one good reason to start with a Kit – it helps you get oriented to good layout topology, build methods and of course they work perfectly on start up – most of the time!
Good luck!
Cheers,
JimA couple of more thoughts – i’m presuming your preamp is making noise with no inputs connected (open RCA’s), if it only produces hum when an external source is connected then the grounds of the two devices are interacting and often just connecting a quality source will fix this problem.
Also all of our Kits come with quality components – but when it comes to filter capacitors we use the best high quality capacitors we can source – which means Nichicon or other Japanese Manufacturers. Yes they’re expensive – but they’re worth every dollar. Same goes for coupling capacitors – Solen makes excellent quality coupling caps – that are reasonably priced.Just to confirm I understand:
1. When you connect your preamp to the Willsenton R8 Pre IN, you get substantial hum.
2. And when you connect to the R8 CD IN, it is silent.The only difference between inputs on the R8 is that the Pre IN bypasses the volume control – ALL tubes are in circuit – even the preamp tubes.
So most likely you either have a ground loop issue on the Pre IN or you have a circuit issue on the Pre IN, the reason for this diagnosis is simple, if the Preamp was generating hum/noise you would easily hear it on the CD IN.
One simple test is to hook the Preamp up to the CD IN, and slowly bring the volume to max – if that volume level only produces a very low level hiss/noise then you know for sure the problem is in the R8 Pre. Why, because the CD IN and Pre IN will share the same ground return, so it must be a circuit problem in the R8.
I would just use the CD IN and call it good.
Truly,
Jim
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