Help needed with sudden 120Hz hum in Mission Amps 5e3

[rdh006]

@Jazz P Bass and @Enzo

Thank you for the info ref the 100 Ohm resistors. Unfortunately, I do not have an ohm meter.

Does anybody know the answer to my question above in post #11 -- do I actually need to be looking at the heater circuit given that this is 120 Hz hum?

[rdh006]

@Jazz P Bass
Ref filter caps: I have no experience troubleshooting amps, can I please clarify: you would measure Vac across each cap, with and without the parallel cap jumpered in?

What kind of numbers would you expect to see? What sort of range represents 'high'?

Thanks for the help.

[rjb]

Unfortunately, I do not have an ohm meter.

This would work just fine for your needs.
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$5.99 is the highest price; HF always has "20% off any one item" coupons, and sometimes offers the meters "FREE with any purchase".

[rdh006]

@52 Bill

It's difficult to be precise because the volume controls are interactive on the 5e3 -- because of the way they are connected through the tone control, turning one volume pot affects the other channel. Likewise the tone pot -- it has a large effect on level, and when it is fully counter-clockwise, there is no level. Consequently, turning either volume to zero reduces the hum but does not completely remove it, because there is still some signal path. Turning both volumes to zero removes the hum. Turning the tone pot to zero also removes the hum.

Appreciate the help.

[rdh006]

@rjb thanks for the heads up. I do actually have a DMM. The extent of my inexperience with amp troubleshooting and related tools is such that I did not realise that is what was being referred to.

I can drain the caps, follow safety rules, and I can solder. I can follow instructions and I can apply logic, but I do not have the familiarity with these procedures that you guys have -- I need it all in foolproof everyday language. I'm educating myself where I can, but it's a long way to go yet.

[Enzo]

do I actually need to be looking at the heater circuit given that this is 120 Hz hum?

Never think up reasons to not check something.

[Enzo]

do I actually need to be looking at the heater circuit given that this is 120 Hz hum?

Never think up reasons to not check something.

[rdh006]

Hi Enzo, thanks for the advice.

I'm not seeking to argue with an expert, of course, just seeking clarification. I would have thought the whole process of troubleshooting is a process of elimination -- using logic to narrow down the focus by eliminating things which are not relevant.

So I didn't see my question as an excuse not to measure something, I'm just seeking to increase my knowledge, and to understand -- it would really help if you, or anybody, could answer my question:

(1) is it true that hum from the heater circuit would be 60Hz?
(2) if that is the case why would I be looking there to fix a problem of 120 Hz hum?

[rdh006]

@eschertron - further to reply above, I have also replaced both V1 and V2 with new 12AX7s -- the hum remains.

[R.G.]

I have no experience troubleshooting amps,

I apologize if this is an unnecessary caution, but PLEASE be careful. That thing is full of voltages that can stop your heart.

Amps are valuable, and fun to work on if you have the proper mind set, but they're not worth dying for. If you don't reasonably have the experience and skills to work with high AC and DC voltages without getting shocked, refer it to someone who does. If you do, I apologize for the warning, and carry on.

[g1]

As far as the 60 vs 120Hz thing, the AC we have coming out of the wall at 60Hz is very rarely a nice clean sine wave. Because of the complexities of the 'dirty' waveform and the limitations of our hearing, those 2 frequencies can be confused. It's best to check or measure to be sure.

[Enzo]

I never trust my ears. g1 is right about the distorted waveform, and hte resulting overtones can make it sound higher in frequency. I ALWAYS check with my scope when I am working on hum issues.

I would have thought the whole process of troubleshooting is a process of elimination -- using logic to narrow down the focus by eliminating things which are not relevant.

Sorta. We are trying to decide where the problem is not, but moreso, where it IS. But that doesn't excuse us from verifying all the time that we are not leading ourselves astray. God knows I have lead myself down the garden path from time to time.

[rdh006]

@G1 and @Enzo thanks for the info and clarification.

@R.G. thanks for the concern, no need to apologize. I've read and watched everything I can find on safety procedures. I won't say I'm comfortable around high voltages, but I'm very aware of them and very cautious. I've built a few amps, but building someone else's design does not necessarily result in the deeper understanding required to troubleshoot.

[rdh006]

Ref the heater supply and the suggestion to check the 100-ohm resistors and check the ground connection at that point. Measuring from the junction of the resistors, I see 50 ohms at the other end of each resistor which I believe is what Enzo's post earlier said to expect. The junction of the resistors is connected to the cathode of the 6V6s, instead of ground. That connection shows continuity on the meter.

Have I measured that correctly? Can I conclude that the filament supply is not causing the problem?

[Enzo]

Sounds like a safe conclusion.

My attention turns to ground scheme then. Usually by the input stage there is no ripple on the supply to filter out, however, if the filter caps' ground returns share any copper with preamp grounds, the ripple CURRENTS can impress themselves upon the signal path.