| Electronics > Beginners |
| Tracking back 100Hz hum in a tube guitar amp |
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| Andreax1985:
Hi guys, thank you very much, you put a lot on my plate... a lot of things to try as well. Please don't stop adding your thoughts, in the meantime I'll try some of the things you suggested and I'll keep you posted. |
| IanMacdonald:
Don't overlook the possibility of transformer-to-transformer magnetic coupling. Is the hum still there with the output valves pulled? (If it's this it will also be present during warm-up) |
| rf-loop:
--- Quote ---i) 0-15 seconds: no hum. ii) 15-20 seconds: hum quickly builds up to quite high levels. iii) 20-35 seconds: the hum gradually subsides to zero level (not audible anymore). I wish it stayed like that forever. iv) 35-45 seconds: the hum slightly builds up again and stabilizes at a low but well audible level (~7mV pk-pk at speaker terminals, see first scope screen-shot in the OP). --- End quote --- Investigation and finding problem need take this timetable to account. This is perhaps miss but least good to think if.... You told these EL84 are matched pair. But do they have also matched filaments? (personally I suspect that matching do not include filaments) Very different cathode temperature also may lead to some imbalance. And this your timetable about hum.... at least not completely incompatible with cathode heating related residue imbalance. (im not sure if you can also watch it with eyes looking glow color when you turn it on, in worst case you may see other EL glow fast bright and after then decay and other EL filament brightness rise (if difference is big). ) Least check it, connect two meters (or scope channels) to record voltages over both filaments when it start and warming. I suspect that in start from gold they run very different and even other one may go totally overvoltage ( in worst case) until also other filament warms up in worst case. As we also know when filament warms up its resistance drop and then other get low power... and again think your time table. Even if this is not any part of problem in this case but this kind of design is not "best possible" at all. As someone have previously noted, transformers magnetic fields can also be source of problems, this is why we many times see that specially E type transformers have weird positions and angles. If transaformers are toroidal then it is more easy. Then, filament current is also near 0.7A so this need keep away from sensitive signals, so it need least be twisted pair and better is far away from sensitive parts as also someone have previously noted. Preamp filaments are DC and end amp is AC. But why designer have selected series mode for drive filaments. It can do, specially for 12AX7 but it is not necessary and with EL84 end tubes this method is - imho, bad. World is full of good and top class and state of art tube amplifiers. Why this designer has drawn his own magpie's nest. It is intersting if can see how it have mechanically assembled and how these all GND's are connected. |
| Andreax1985:
Yes, I'm starting to believe that hum has something to do with unbalance between power tubes. @floobydust , @eblc1388 : I measured H1 and H2 AC voltage w.r.t. HTR GND using a multimeter. That is, If I'm not wrong, the AC voltage across R18 and R19. Well, I read 12.40V across R18 and only 2.0V across R19. Is this normal? |
| Electro Fan:
Hi Andreax1985, Just wanted to say I admire your tenacity in tracking down the hum and fixing it. Tube equipment (both for musical instruments - like guitar amps - and for hifi systems) can provide outstanding sound, so it would be great to reduce or eliminate any extraneous noise, especially persistent hum. I really like the way you have been diligently trying to diagnose the issue with your study of the circuit, your use of your test equipment, and your Q&A in the thread. I don’t know the answer to the problem but I have a hunch you will figure it out and in the process you are going to come out of this as an accomplished tube amp designer. Personally, I’m intrigued by two theories: 1) that it’s a GND issue, and 2) that it has something to do with tube performance, possibly matching (but it could be something else in the circuit, of course). I don’t know if you have access to a good tube tester but some of the best tube testers ever made were produced by Hickok. Of all their testers, one of their very best models was the 539C. The 539C has become a bit of a collector’s item and it’s therefore somewhat pricey - but just in case you are interested in learning about them and how they work (in case the 539C or one of Hickok’s other models might help you with tube testing) below is some info. Good luck and please keep us updated as you get the issue resolved. https://static1.squarespace.com/static/57ae25c75016e1690c41b053/t/57ae303803596ed24fa8f9c2/1471033401729/Hickok+Tube+Tester+539C+Manual.pdf https://stevenjohnson.com/hickok/data/hickok-539b-539c-test-data.pdf https://www.ebay.com/itm/HICKOK-MODEL-539C-TUBE-TESTER-Tested-in-excellent-condition-/183654239996 |
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