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Old 12th December 2012, 10:54 PM   #1
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Default tube amp hum........normal or suggested fix?

Hi
I'm an electronics newbie and this is my first post so please excuse me if this is not the correct forum for this question:

Background: I've only ever had solid state audio equipment but have always been fascinated by tube amps and their eclectic qualities. So I took the plunge and bought a "starter" used Quicksilver GLA (with four GD EL34 tubes estimated life 200 hours) power amplifier. Loved the sound so I started building other components around it including a Hagerman Cornet2 phono tube preamp, a Uriah Daley lighternote passive preamp (OK not tubes but with the addition of an arduino-based touchscreen input selector and remote this thing is awesome) and lastly a Broskie Aikido octal 6sN7 preamp (love it as well). I probably didn't need Aikido line stage preamp but I learned so much from this forum and from building it that it was worth it!

Ok the problem is that with the new additions I became aware of a persistent low (but noticeable and annoying) hum. Remember I have only had solid state equipment.

I assumed that the hum was coming from the home made stuff and have been pulling my hair out trying to diagnose the source. Lastly, I plugged each piece of the homemade equipment separately into my old solid state amp. Nothing dead silence. No audible hum (just a bit of hiss from the Hagerman cornet preamp)

Its related to the one piece of equipment I didn't build....the Quicksilver GLA tube amp. I must have not noticed it originally? Without anything attached to the inputs the amp is dead quiet. However, once I connect just one piece of homemade or commercial source equipment to the RCA inputs on the amp (even with different RCA cables) the hum appears (with or without signal). The problem is in both channels. If I unplug the R input the R channel output has no hum but the L output still has hum and vice versa


Curiously the tube amp doesn't have an earth ground (class I?; two prong power cord). I added ground safety loop breaker circuits to the source equipment but this had no effect on the level of hum from the Quicksilver amp (BTW which doesn't vary with level of the source(s) volume). I also temporarily connected the amp chassis to earth ground, but this made the hum worse when a source was connected to the inputs.

Any suggestions on how to get rid of the hum problem with the tube power amp?

1. Would changing tubes help? Which one(s) should I start with; the output tubes (EL34s) , the single 12Ax7 input tube or the 6DJ8 driver?

2. Circuit modifications? I cannot find a schematic for the amp but could trace the circuits. But what potential modifications would get rid of the hum?

3. Live with it? Are my expectations for no hum completely unreasonable?
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Old 12th December 2012, 11:30 PM   #2
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The quicksilver should really have a ground wire. I'm not familiar with the schematic but someone here should be able to tell you if any modifications are needed.
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Old 12th December 2012, 11:55 PM   #3
cotdt is offline cotdt  United States
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It's a ground loop somewhere. Try star grounding scheme.
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Old 13th December 2012, 12:21 AM   #4
12E1 is offline 12E1  United Kingdom
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Quote:
Originally Posted by cotdt View Post
It's a ground loop somewhere. Try star grounding scheme.
It's more likely the fact that there is no proper grounding implemented. The lack of a ground makes the construction of a ground loop rather difficult. However, without a schematic and ideally a photo, it is hard to give solid recommendations.
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Old 13th December 2012, 02:23 AM   #5
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Originally Posted by cotdt View Post
It's a ground loop somewhere. Try star grounding scheme.
sorry for being dense but do you mean star grounding within the amp or between the amp and source? The hum is present even if the amp is connected to a single source (no other components in the network). What would a star ground scheme look like for the amp (with no earth ground connection) connected to a single source (with or without an earth ground connection)?
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Old 13th December 2012, 02:37 AM   #6
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I haven't found a schematic for the amp yet but I did find several photos of the amp and its inards on the web. If this isn't sufficient I try to trace the circuit this weekend
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Old 13th December 2012, 08:10 AM   #7
Doz is offline Doz  United Kingdom
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Nice ... no ground and a switched neutral ...

Needs a three core grounded mains lead fitted urgently, the ground taken to the chassis and bolted down nicely using a tag and a star washer. This will give you some safety, although won't fix your hum (in fact, may make it worse, until we get to the bottom of the problem)

I don't know what the regs are in your part of the world, but switching the neutral is a pretty daft idea in my book. Double pole switching both live and neutral is preferred here.
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Old 13th December 2012, 08:40 AM   #8
cjkpkg is offline cjkpkg  United States
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You might try lifting the heaters to B+ / 4. Also looks like all the heaters are off one xformer tap seems like a lot of current necessary.
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Old 13th December 2012, 08:49 AM   #9
12E1 is offline 12E1  United Kingdom
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Quote:
Originally Posted by sybednar View Post
I haven't found a schematic for the amp yet but I did find several photos of the amp and its inards on the web. If this isn't sufficient I try to trace the circuit this weekend
That suggests that the photos are not of your specific amp but of another of the same type.... Apart from the very serious safety issues already identified, the photo of the underside shows some less than ideal power supply grounding (which may contribute to hum), and also some strange differences in the components used between the left and right channels. The non-symmetrical placement of some of the parts adds to the confusion, but I only see a cathode bypass capacitor on one of the pairs of output tubes, for example. If that's not a photo your own amp, then can you take your own photo so that we are making correct recommendations?

Last edited by 12E1; 13th December 2012 at 08:51 AM.
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Old 13th December 2012, 09:06 AM   #10
AJT is offline AJT  Philippines
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do not rely on the chassis for grounding, all power leads should have a corresponding ground return wire....a star grounding connected to the chassis at only one point is also good...
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