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Old 13th December 2012, 10:29 AM   #11
tricomp is offline tricomp  Denmark
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I suppose the picture shows an amp that violates all common sense and construction principles.

Power input cable apparantly not secured against yanking. A few pulls and it may come loose inside and put live voltage on the chassis.
Obviously the mains circuit breaker should be a dual-pole.
The bridge rectifier in the power-supply should be connected directly with both + and - to the main filter-cap. (Left in picture) and the negative connection of the cap should be taken to the (future) star-grounding lug between the cap's.
All connections that are now on the negative pin of the cap should go directly to the star-ground except the parallel resistor 470K.
Float (hoover) a dual, bare thick copper-wire bus-bar, soldered to the star-ground, above the small-signal tubes and splitting it left and right above the EL34's. Do not connect far ends to any existing ground-lugs.
All components and tube connections with separate grounding-lugs should now be moved and soldered to this bar except the electrolytic cap in the upper right of the picture. It's negative lead should be extended and be connected directly with the star-ground.
Filament wiring is hopelessly wrong laid-out. Tightly twisted leads should supply the pre-tubes and have a couple of 100-ohm resistors series-connected across the supply with the center of the resistors connected to star-ground (If the filament transformer doesn't have a centered supply with the center connected to star-ground)
Since the input RCA's are so close to the pre-amp tube I suppose the use of shielded cable from them won't make any difference. However, the RCA's should be floating (NOT connected to the chassis) and the ground-lugs should be connected to the star-ground, directly. Strap from one RCA to the next and on the the star-ground.

There's still a possibility that this re-wiring will not cure the hum-problem.
The power-supply transformer may couple with the output-transformers and introduce hum. Not likely, as they are orientated correctly against each other but still very close.

I adhered to the above described principles when building a tube-amp recently and it has no detectable hum present. You have to put your ear all against the speaker to detect the slightest amount of hum.
When that is said every amp construction is different and it may be a pain to cure any hum-problem.
Guitar-amps may be laid-out very crazy and still present no hum, even with multiple grounding-points. You'll just have to work with it and see what happens.

Good luck,


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Old 13th December 2012, 11:53 AM   #12
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I would wrap the power transformers in copper shim stock, also, like the ones on Peavey amps. It is soldered in a circle around the windings and E-frame. Dynaco amps have the power transformers enclosed in a stamped steel shell, grounded, but that is beyond the capabilities of the amateur. Hammond Clock Co ran their heater wires as twisted pair, as tricomp said. Hammond organs also have stamped steel shells around all the transformers and chokes.
My 1959 and 1966 designed dynaco equipment do not connect the power amp chassis to safety ground on the mains, only to the ring of the RCA jack. My hum is least when I ground the preamp (which also has a grounded RCA jack) to the head shell of the turntable. The tube dynaco preamp is also not connected to the mains safety ground. For safety I have the ground lug of the preamp case connected to the third pin of the power strip with a separate wire and plug, but this doesn't add or subtract hum. That wire to the power strip ground is the trunk of my gounding tree (star). If the RCA connector amp-preamp gets the ground lug spread out where it doesn't connect, it hums pretty badly, so I know the case of the amp is grounded if it is not humming badly.
I thought amps with chassis connected to mains safety ground needed differential input stages with equal amplification on the RCA ring to eliminate common mode hum. The grounded CS800s amp and PV-1.3k amps both have op amps on input with the other input attached to the 1/4 phone plug shaft, the analog of the RCA jack ring. That differential input would obviously be very expensive with a tube amp, requiring a whole extra tube. All of my tube equipment, dynaco or hammond organ, has amp and preamp chassis not connected to mains safety ground as designed. When a safety ground is added to the chassis, hammond insisted a measurement be taken of leakage current between chassis ground (transformer shell) and safety ground of less than 400 microamps, to make sure the transformer wiring wasn't leaky. Actually they required the measurement at the end of every service visit.
In replacing the PAS2 tube preamp with an op amp mixer which uses 100 time less power, I had to isolate the mixer RCA jack rings from the steel case with o-rings to eliminate hum. As tricomp suggested.
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 13th December 2012 at 12:13 PM.
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Old 13th December 2012, 12:11 PM   #13
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The construction looks very amateur for a modern company. Start simple and ground the chassis before going to any exotic measures.
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Old 13th December 2012, 12:19 PM   #14
12E1 is offline 12E1  United Kingdom
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As I pointed out - it is not clear that the photo is of the OP's actual amplifier so we cannot be sure what steps are required. Even then, some simple remedial measures regarding the grounding and heater wiring would normally be enough to take care of the hum issues. The OP says they are new to electronics - describing the use of differential inputs, op-amps, etc is probably jumping in at the deep end.
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Old 13th December 2012, 01:19 PM   #15
DF96 is offline DF96  England
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Does hum appear whether the source is switched on or not?

If you switch off the power amp does hum disappear immediately, or take a few seconds to go? My guess is that the power amp is injecting some hum current down the interconnect ground conductor.
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Old 13th December 2012, 03:40 PM   #16
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Quite right probably best not to rely on images of another, but related amp. The images I have now posted are of my amp including some up close pics. To my untrained eye there are no major differences between the layouts of my amp and the one I posted from the web. But I defer to the combined expertise of the people on this forum

I have certainly learned quite a bit from the comments so far
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Old 13th December 2012, 03:47 PM   #17
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Originally Posted by DF96 View Post
Does hum appear whether the source is switched on or not?
Yes, the hum is present whether or not the source is on

Originally Posted by DF96 View Post
If you switch off the power amp does hum disappear immediately, or take a few seconds to go? My guess is that the power amp is injecting some hum current down the interconnect ground conductor.
Good question. I'll check on this again cause I cannot remember precisely what happens to the hum when I switch the amp off.
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Old 13th December 2012, 03:53 PM   #18
DF96 is offline DF96  England
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I'm sure I must be imagining this, but the ground wires from the input sockets appears to go straight to the ground tags on the two fat electrolytics. In the case of the white (Left?) channel this appears to be the reservoir cap. I could not imagine a better way to inject buzz from the PSU into the signal circuitry. Whoever wired up this amp clearly knowns nothing about grounding. It is truly awful! You are guaranteed hum/buzz as soon as you connect anything to the input.

It is a pity you bought it second-hand, as otherwise you could send it back under warranty and demand a refund.
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Old 13th December 2012, 04:00 PM   #19
kmaier is offline kmaier  United States
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Yea, that's some poor work indeed. Looking at the closer up pics, it appears that the filament winding is 12.6VAC center-tapped and it's likely that a 5-volt rectifier winding is wired to the filament on the lower 9-pin tube. The upper 9-pin tube is wired in with the octal output tubes.

Regards, KM
... just because you're paranoid, it doesn't mean they're not after you...
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Old 13th December 2012, 04:32 PM   #20
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Originally Posted by Doz View Post
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.

Yes, I was previously thinking about replacing the two prong cord with a three wire grounded connection. The power slider switch is also pretty flimsy and really not easy to get to.

However, as a quick an dirty experiment to see if grounding the chassis would help or hurt the hum issue I connected a line to earth ground to a bolt on the top of the chassis. Not surprisingly this made the hum worse
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