What's the cause of this electrical hum in my cheapo tube amp?

January 16, 2015 9:15 AM

video of the issue, details within.

I've got a brand new epiphone tube amp, and it's very noisy, as you can see in the video linked above.

I've got a nice multimeter, though I don't know much about using it. Electronics is not my field. What I've found so far:

The power in my apartment doesn't appear suspect. I've tried a number of outlets on a number of different circuits, all with the same results. Using a fancy surge protector/line conditioner or not doesn't make a difference. I get a steady 120.6 ~ 120.8 VAC across both the hot and cold and hot and ground in all the outlets I've tried, so I assume everything is well grounded.

Using the debugging ports on the amp chassis (the two red ports you can kind of see in the video, and a black port between them you can't see so well) the temp of the circuit is about 400°C. I don't know if that's normal or not, but my gut feeling is that it would be. Also about 134Ω and 17 mV DC across whatever those ports measure. I have no idea at all if this is useful or relevant information. Interestingly, the multimeter also says ~17 µamps, which is different from what I'd calculate using Ohm's law, expected more like 12~13 µamps. Again, this could be completely normal for reasons I don't understand.

Knowing very little about this, and based on the fact that the connection to the speaker seems involved, my gut feeling is that if the connection for the speaker were a balanced TRS arrangement that were properly grounded, that might do the trick, but I'm extremely loathe to open up the chassis that houses that connection.

Any ideas beyond "return it" or "find someone to repair it" ?

posted by colin_l to Sports, Hobbies, & Recreation (10 answers total)

Have you tried simply pulling and re-seating the tubes?
posted by Thorzdad at 9:45 AM on January 16, 2015

I'm with you on the notion that it's the speaker-cable connection, and a couple of posters on this thread suggest that the source of amp buzzing is a cold solder joint on the speaker. The OP, though, doesn't have quite the exact problem you have. You may have to go ahead and open it up.
posted by puritycontrol at 10:10 AM on January 16, 2015

Response by poster: Thanks. @Thorzdad - I did see somewhere on the intertubes about swapping the tubes, and after giving that and your suggestion a go, no luck.

@Puritycontrol - that's probably the next step. I found an XLR - 1/4" TRS I could cannibalize, but I'm out of alligator clips, so I'll report back after some shopping and whatnot.

Related - even though I'm not messing with the connector inside the amp (yet?), what would you recommend I play with connecting the sleeve (ground) part of the TRS connector to? I could try a screw on the chassis, like I'd do with a car, I could just attach it to the ring (cold) lead on the speaker, since that's what the current unbalanced arrangement does, or I could try dropping it into the black (cold? ground?) debugging port. Thoughts?
posted by colin_l at 10:43 AM on January 16, 2015

I've worked on a LOT of tube amps. It's kind of hard to tell from the video, but doesn't sound that horrible to me. seems like a typical modern (made in china) tube amp with bad lead dress - "signal" wires too close to or crossing "power" wires, bad shielding, etc.

*** IF YOU ARE COMFORTABLE around voltages that can definitely kill you, and may be present even when unplugged (In other words, don't do this unless you know what you're doing, okay?) *** I'd look at the guts and possibly rewire, starting with twisted pairs on the heaters. I don't think it has anything to do w/the output jack or speaker connection

One major tip: you NEVER want to pull the speaker cable with the amp on. Running a tube amp with no speaker means that the circuit sees infinite impedance, so your tubes just pour it on... This can lead to killing your output tubes, or possibly your output transformer!
posted by sluggo at 11:12 AM on January 16, 2015 [3 favorites]

Response by poster: Here's some follow-up. I was poking at some things with a continuity tester to get a sense of how things are grounded and to where, and I wound up pulling the amp out (I'm *terrified* of amplifiers for exactly the reasons sluggo mentions) after I had trouble with a grounding screw.

I found some interesting things.

This photo has two items of note:

The first is that the connection for the speaker jack looks like it could be converted from unbalanced T/S to balanced TRS by simply inserting connectors into the middle part of the jack housing. The input jacks are the same housing, but have all three rungs instrumented.

The second is that potentiometer next to it (below it in the picture). I didn't read that as a potentiometer from above. This picture shows what that pot looks like from the top, and also has what I've been calling the "debugging ports".

So any thoughts on what that potentiometer might be responsible for, and how I could play with it with confidence I could put it back at the original setting?

And how crazy would it be to try to find the metal tongues needed to insert into that jack, move what's on the bottom now up to the middle ring, and then just ground the sleeve?
posted by colin_l at 12:05 PM on January 16, 2015

That's a bias pot. the idea is that you can set the bias for the power tubes with it, which would typically be done when you change the power tubes at some point in the future. The three jacks are probably for banana plugs, and you would use those to monitor & compare the bias current values you adjust w/ the potentiometer. The current would be in the tens of mA's range, and you'd compare measurements between #1 red pin/black pin and #2 red pin/black pin. (More on bias; Bias calculations ). I wouldn't recommend that you do anything with any of these at the moment, it probably has nothing to do with the issue at hand.

I'm not following why the speaker jack is suspect, or what you're trying to solve by changing it to TRS - could you fill me in?

Can you get some wider "gut shots" of the amp? My suspicion is that the buzz would probably have a lot more to do with sloppy lead dress, and I could speak directly to this if you can get shots of the whole thing (showing the tube sockets if possible). Thanks!
posted by sluggo at 12:24 PM on January 16, 2015 [2 favorites]

Sluggo is on the right path here, and I agree with him -- that's a pretty typical hum for a 2 x 6V6 amp with standard fixed bias, push-pull topology.

My own take is that your tubes could be biased a bit too hot, and there is a good possibility the hum could be minimized by a proper rebiasing, which you would accomplish with a bias probe (or a multimeter applied to the red and black test ports) and a careful adjustment of that internal potentiometer you showed in your pics. But that is not an operation that someone who doesn't know what they're doing and is afraid of an amp's internals. Take it to a good local shop and ask a few questions, This should be a trivial matter for a shop to check and adjust your bias. If you're local to me in the SF area -- I'm near San Mateo -- drop me a MeMail and I'd do it for you ;-)

It is also true that tube amps hum. Some hum more than others, and yours does not seem to hum excessively, but the hum is is a byproduct of amplifying a signal with tubes. It's just what they do. In addition to verifying the bias of the power tubes (the two 6V6 tubes), you could also try one or two lower gain preamp tubes. Your amp came with two 12AX7s, which are the highest gain preamp tube. You could try changing the V1 tube to a 5751 tube, which has 70% the gain of a 12AX7, but this will also reduce the total gain you have available. Depending on the type of music you play, this might be a good thing, or it might take the amp away from the tons you want.
posted by mosk at 1:14 PM on January 16, 2015 [1 favorite]

Response by poster: Sweet, thanks for all the good info.

I guess I suspected the speaker jack because the noise went away when I pulled it, though you've set me straight on that account, and because one of my internalized prejudices is "noise? look for unbalanced connections that could be balanced."

Maybe I'm just playing the thing too softly inside my apartment, but noise feels crazy high to me. Maybe my belief that I can mic this, stick a compressor/limiter behind the mic, and make the hum go away doesn't apply at that low volumes I'm trying to keep myself to.

@mosk - I am in the Bay Area, and I've got all of next week off of work anyway, so I'll send you a mail and see if there's a good time to poke at this and maybe go pick up some tubes to play with (happy to buy from you anything you've got laying around).

@sluggo - yeah, I can get more gut shots, but not in the next few days. My six year old will be with me most of the weekend, and his electronics know-how is currently at this level, so the guts are staying well inside the cabinet for now :)

Thanks
posted by colin_l at 1:40 PM on January 16, 2015

Thirding that that seems like a pretty normal hum for a small tube amp. When you unplugged the speaker (also seconding don't do that) I think the hum you were hearing was most probably just the hum of the transformer, again, totally normal.

because one of my internalized prejudices is "noise? look for unbalanced connections that could be balanced."

Sure, but speaker connections are never "balanced" - a speaker is really just a coil of wire suspended inside a magnet, and the "hot" or "positive" connection is one end of the coil, the "negative" is the other end of the coil.

And how crazy would it be to try to find the metal tongues needed to insert into that jack, move what's on the bottom now up to the middle ring, and then just ground the sleeve?

Pretty nuts, honestly, since you can buy balanced jacks for not a lot of dough. (A balanced version of the one in your amp is 75 cents.) Also, that wouldn't do anything, since 1) the sleeve of the jack is plastic, so no electrical ground connection is possible, and 2) since the plug connection from the speaker is TS (Tip-Sleeve, or "unbalanced"), moving the metal bit up to the middle ring is still making a connection with the sleeve of the plug.

Maybe my belief that I can mic this, stick a compressor/limiter behind the mic, and make the hum go away doesn't apply at that low volumes I'm trying to keep myself to.

To reduce noise, you actually want pretty much the opposite of compression - you want expansion and/or gating. See this page from the SAE. Note that this will only really help reduce noise & hum in moments when you're not playing. You'll have to get into some fairly complex EQ'ing & maybe some "magic box" stuff inside whatever software you're recording with to reduce any noise that occurs while you're playing.

Maybe I'm just playing the thing too softly inside my apartment, but noise feels crazy high to me.

Maybe - that's one of the catches with those kind of "one-knob" amps, your signal-to-noise-ratio sweet spot is a function of amp volume, as opposed to amps with a "gain" knob to set the basic signal level and then a "volume" knob to set how loud the amp actually is.

Start with your guitar volume knob(s) all the way up before you set the amp volume, and it's always worth checking to see if noise is caused by a bad cable or a problem with your guitar's electronics, or by environmental noise sources, like fluorescent lights.
posted by soundguy99 at 5:11 PM on January 16, 2015 [3 favorites]

Also, if you're interested in poking around this or other tube amps, I've found How To Service Your Own Tube Amp and The Tube Amp Book to be invaluable resources over the years.
posted by soundguy99 at 12:05 PM on January 17, 2015 [1 favorite]