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yet another hum elimination question

After a hiatus I'm back on my hum elimination project. I've ruled out a number of things, and am now checking resistors inside the amp.

For the most part, everything checks out except for a pair of 3.3K and 1.2K resistors, one on each channel, on the circuit from the tube directly to the speaker jack. All four resistors are measuring in at just under 1K, which has me wondering if something else, such as bad tubes, could have caused such a symmetrical degradation. That, and the 60hz hum was primarily in the right channel, not both. For reference, the left and right are linear in this amp, only the bass is push-pull.
 
I am confused.
If you removed all tubes, and still hear the hum IN YOUR SPEAKERS, then it has to be either a PS or heater related problem, or induction leak which I think we can rule out as the PT is too far from the OTs.

In fact you have 3 amplifier channels in your amp.

I would trace with a scope first if it's on the 3 channels and trace from the speakers upwards. With no tube of course. Maybe a tube socket is leaking heater DC somewhere, something like that.
The fact that you hear a quiet loud hum with no tube is odd to me.
 
Some equipment built in the 1950s and 1960s without three prong ground wires used the chassis as ground. Therefor, there are many grounding points and ground loops. Installing a grounded plug and properly grounding it to the chassis will still create hum and can even create hum where there was none before. So the proper procedure for installing a grounded three wire cable is to remove all the extraneous grounds from the chassis, select one point for chassis ground and re-route all the previously grounded components to the new chassis ground with a piece of wire. This amounts to a "star" ground. It is imperative that the RCA input jacks are not grounded to the chassis where they are mounted, but are star grounded to the chassis ground.
 
Thanks so much for all your input; unfortunately I keep finding red herrings and other bits of misleading info to post, but further investigation changes the clues entirely.

I've discovered the hum is affected by the position of the audio input cables from the tuner pre-amp into the amp. If I drape them a certain way over the main power transformer, the hum subsides to the low level heard when the audio input cables are disconnected. Move the cables around by the tubes and the (disconnected) cap can, and the hum gets quite loud. Something in the amp is inducing the 60hz hum in the input cable, and I wouldn't be surprised if is also responsible for the rest of the hum.

What could be inducing the hum? A bad tube? A bad transformer? Could the old disconnected cap can be responsible? How might I shield the input cables?

Thanks again; these hums are a real mystery!
 
Sounds like bad cables to me. Just yesterday I got a bad hum in one channel of one of my amplifiers and as it turned out, one of the RCA cables was bad. If I grabbed it and moved it around, it would come and go. Replaced the cable and no more hum.

Might not be your problem, but just something to think about! :cool:
 
Curiouser and curiouser! I just "swapped" out the audio cable with a modern one (I added an extension just to see if the new one would behave the same way in the same area). It was a bit better, but what I discovered was that its the proximity to the primary power wiring harness to the preamp that affects the hum.

Attached is a photo of the amp. On the left is the audio input, on the right is the wiring harness to the tuner/preamp. Since the power switch is in the tuner/preamp, full 60hz AC direct from the outlet is traveling back and forth on that cable bundle. I wound the audio cable around the other cable and the hum subsided, but did not completely disappear. I guess the next step is to disconnect and bypass the switch on the tuner/preamp entirely and hear what happens.
 

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Hmm, out of balance output tubes could cause the hum. Could be out-of-spec resistors associated with the preamp or output tubes.

Is it possible to upload a sound clip?

And, if you say that the hum is there with no tubes, then maybe it is some kind of induction going on.

Using any compact fluorescent lights in close proximity?
 
Yep! Switching out a bad cable with even a working "cheap one" is an easy test that resolves a lot of issues, or at least rules things out.
 
Brice said:
I am confused.
If you removed all tubes, and still hear the hum IN YOUR SPEAKERS, then it has to be either a PS or heater related problem, or induction leak which I think we can rule out as the PT is too far from the OTs.
Click to expand...

I'm confused too. The speakers are producing a humming sound even though the amplifier has no tubes in it??? I fail to see how there can be any current flowing through the output transformers with no tubes in place, so what is driving the speakers? How can they hum?
I'm baffled.
 
Ever thought of trying ferrites to knock down the AC hum?

Palomar Engineers carry quite the selection. Give them a call and discuss it with Bob and see if a solution can be had.

Cheers
Hermit
 
Hermit418 said:
Ever thought of trying ferrites to knock down the AC hum?

Palomar Engineers carry quite the selection. Give them a call and discuss it with Bob and see if a solution can be had.

Cheers
Hermit
Click to expand...

Ferrites are only good for high frequency noise (EMI/RFI). They won't do anything for 60Hz hum.
 
rothwellaudio said:
Ferrites are only good for high frequency noise (EMI/RFI). They won't do anything for 60Hz hum.
Click to expand...

Really well when the ferrites are made from the correct composition, tested to work at the frequency necessary they perform very nicely for choking off the offending issues. Thanks for pointing out why not to use something that doesn't meet the above standards though and if you continue to believe your theory then you haven't tried the ones I mentioned.

People who have used the cheap junk (from China)of compressed iron filings with no concern for intended frequency range will find they are not effective. However a properly made ferrite employed in the correct fashion can often eliminate or at the very least reduce the annoying problem to so low a level it becomes unnoticed.

Cheers
Hermit
 
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Hermit418 said:
when the ferrites are made from the correct composition, tested to work at the frequency necessary they perform very nicely for choking off the offending issues.
Click to expand...

The OP has something fundamentally wrong with his system. The cause of the problem needs to be found. A constant hum that appears before the tubes have warmed up?... and is even still present with the tubes removed???
Ferrite rings are not the answer.
 
Success, mostly! I slapped a single good-sized ferrite choke from the local surplus store (I love you, Ax-Man!) on all the cables and the bulk of the hum went away. There's still a slight hum in the noise floor, but it's manageable. That doesn't mean I'm not going to try going after it as well; there are still four off-spec resistors that need replacement, and I could probably use a new set of matched power tubes as well, but for the most part, the worst has been vanquished. Thanks everyone!
 
That's interesting. I did not suspect that either as such low frequency. Do you have pictures of your chokes? Thanks.
 
ugh. My frustrated eagerness to kill the hum had me drawing wrong conclusions. The choke on the wiring bundle worked not because it was a choke, but because it was keeping all the wires bundled tightly together around the transformer and rectifier. Simply bundling all the wires in a cable management sleeve did pretty much the same thing. I still have the other hum that's bugging me.

This thing is full of red herrings.. so here's what's up for this weekend's hum-busting:

The worst of the remaining hum is mostly in the bass channel. It's a 60 hz hum that has about a 1 hz beat (wow-wow-wow-wow-wow). The bass channel in this amp is push-pull, so my first inclination is unmatched tubes. Following a hint found on another board, I took my IR thermometer and saw one tube running consistently hotter than the other, further supporting the unmatched hypothesis. However, after switching the tubes around, it was the same socket (the phase inverter), not the tube, that was running warm. I'm hoping the matched tubes I have on order will fix this, but I suspect its the slew of out-of-spec resistors (and perhaps a pair of mylar caps) that will need replacement.
 
Looks like the one filter cap is reversed ? Oops I just saw you said you had it connected to a terminal strip. :)
 
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Folks,

The OP has hum without any tubes being inserted. Thus, the cause cannot be any amplifier component, power supply filter cap or normal amplifier electronics. What is the argument that such could be the cause?

I am puzzled that grouping closer together of the input cable(s) has an audible influence. Those of us with an oscilloscope would have tried some sigmal tracing with that. As said, the salient question is: What is causing enough 60 Hz ac to reach the loudspeaker(s), either directly through the voice coil or via the output transformer? What effect does physically wriggling the input cable have if any? How is the amp switched on, via a remote switch with leads running through the input cable? What happens when you remove the input cable connector? A test (and with due care regarding power transformer high voltages!): Safely, try shorting the output terminals themselves (should eliminate the hum), or short the anode to anode terminals of each output transformer. (Leave the tubes out of course.) Also (if I haven't missed it): What change if any is there with tubes in, when removing only the rectifier tube?

What do you find?
 
lootheer,
Did you change the power transformer on that chassis? The pics I saw has a huge one for such little amp.
It yes, maybe that's the culprit.
 
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