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Hummm...

My scope is a little pocket digital model. I cleaned the switches & they all seem OK except for the phase reversal switch.
 
Actually, now that I think about it, I shoudl probably replace or remove the phase reversal switch anyway, since it does not work. I doubt it is causing the hum but it could cause other issues I guess.
 
Is the chassis grounded? Try unplugging the plug, turn it around and plug it back in, if it's not polarized (hot and neutal same size prongs). I don't remember if the plug is like that. I'm not at home right now.
 
The chassis is not grounded to the outlet. The plug is not polarized. And yes, I did try rotation the plug, it did not help.
 
Just an aside, I use a program called Audiotools on my iPhone to check the hum frequency and for many other audio tasks. You can have it overlay text next to the highest peak on the FFT window showing the frequency of that peak, very handy to see the hum frequency. I believe there is still a free version (or 0.99) that would show peaks in the Frequency Analyzer and 60 vs 120 would be obvious. I am in no way involved with the app or the developers, just a fan. I compared mine with a B&K old school analyzer and it is very close, especially for overall A- and C-weighted dB. I haven't purchased any extras, so it is a very inexpensive way to see more of what is going on in your system environment.
 
Scifi, I did play with the hum ballance controls. I was able to add a 60 Hz hum and some noise, but it did not affect the 120 Hz hum I aleady have.

Bob, I will check that out. 60 vs. 120 is easy to find as long as my speakers and/or my phone will go down to 60 Hz without doubling, but that sounds like a useful app if they have it for Android. I also want to try again with my oscilloscope once I get some time at home to work on it.
 
Something that will cause 120hz hum in a push-pull amp even with good caps and proper grounding is badly balanced output tubes. Generally speaking, the power supply at the output tube stage is pretty dirty. It relies on a decent current match on those output tubes in order to cancel out the noise. If yours are really badly off, the hum won't cancel and it will come through the speakers. The worse the imbalance, the worse it will be.
 
Ziradog said:
Scifi, I did play with the hum ballance controls. I was able to add a 60 Hz hum and some noise, but it did not affect the 120 Hz hum I aleady have.

Bob, I will check that out. 60 vs. 120 is easy to find as long as my speakers and/or my phone will go down to 60 Hz without doubling, but that sounds like a useful app if they have it for Android. I also want to try again with my oscilloscope once I get some time at home to work on it.
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I looked at the circuit - it is rather bad design. The only thing that filters high voltage for output stage is a single 60uF capacitor. Output stage does not have balancing option either. Somehow I am not surprised that this amp is humming. My suggestion is either add choke and one more capacitor, or add ripple filter with mosfet. Another option is to use solid state rectifier and at least 330uF of capacitance. The goal is to have less than 1 Volt of ripple at plate when no signal is present.
 
Its a pretty common setup though. You'd be hard pressed to find a vintage amp that filters much more than this at the output stage, and many do less. Not saying its amazing, its just not worse than most things from that time period.
 
gadget73 said:
Its a pretty common setup though. You'd be hard pressed to find a vintage amp that filters much more than this at the output stage, and many do less. Not saying its amazing, its just not worse than most things from that time period.
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If you do not consider boombox grade gear (in 1950-60th terms), choke is a standard element in power supply with tube rectifier. There is no other way around to get reasonably low ripple unless use active regulator (and these were really rare in consumer gear back then). Balancing pot at cathodes of power tubes were standard too.
 
Made a little progress. I removed the non-working phase reversal switch, and that helped noticeably. Turning off the fluorescent lights over eh workbench did too. There is still hum there, but you have to be close to hear it, and it is more 60 Hz now. Unfortunately, I can now hear that there is a buzz in one channel that is volume dependent, and something that sounds like a cheap ocean waves simulator. Pretty sure both of these were there before, I can just hear them better now. Both are pretty low level, you would not hear them under "normal" listening conditions.

Gadget, all I can say on the tubes is that they are all Mullards, 3 are original, the fourth a few years newer. They tested pretty equally (and strong) on a Hicock 800, which is all I have access to. I don't have a matched set to swap out.

for_p1, I do plan to add another filter stage to the power supply once I get all the existing issues corrected. I just don't want to mask a problem by doing that.
 
IF you have this "something that sounds like a cheap ocean waves simulator" - need to check each and every resistor. One of them likely is bad. Generally any resistor that drifter more than 10% off spec should be replaced.

To eliminate possible tube problems, I recommend buy all new set of modern Chinese made ones and do testing with them. If noise is eliminated by that - search for bad tube between your NOS ones.
 
for_p1 said:
If you do not consider boombox grade gear (in 1950-60th terms), choke is a standard element in power supply with tube rectifier.
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Not really. Fisher, Sherwood, HH Scott, Pilot, and lots of others worked like that. Some had chokes, but frequently only after the point where the plate supply tapped out. Many didn't involve bias or balance adjustments either. Maybe its not quite as high brow as you're shopping for, but its not quite hot chassis console fodder either. If things are right, it can and does work just fine. No, its not optimal by any stretch of the imagination, but its not a configuration used purely in garbage-grade equipment either.
 
So far, of all the resistors I have checked, only one was off value & I replaced it when I did the caps. Of course, there are a bunch left that I haven't checked, and I could have over-heated one when soldering stuff. As for the filter set up, I'm pretty confident that the noises I am hearing are not inherent to the design. If I can get everything "fixed", I can then think about improvements.
 
Excellent progress. Remaining (60 Hz) hum could be related to less than stellar lead dress. To some extent it is likely that is at least part of the remaining problem. Remaining hum could also be related to one of the frontend tubes itself--for example heater to cathode bleed through. Might be interesting to try swapping left/right 6AU6's to see if the hum stays in that channel or follows the tube switch. Also same with 6AN8's. You might also remove the 12AX7 (V1B & V2B) to see if hum remains or goes away. For that matter you could remove the 6AU6's too and see if hum goes away.

Just ideas about how to get more clues.
 
kward, thanks for the advice. That should help me narrow it down some.

On the resistors, is off-value the only thing I need to look for? I will probably just replace more of them with modern carbon film ones unless there is a reason not to.I am afraid that unsoldering & then resoldering them will damage one (or more). I will definitely check the values on teh new ones, at least some in the cheap set I bought are mis-labeled.
 
Ziradog said:
kward, thanks for the advice. That should help me narrow it down some.

On the resistors, is off-value the only thing I need to look for? I will probably just replace more of them with modern carbon film ones unless there is a reason not to.I am afraid that unsoldering & then resoldering them will damage one (or more). I will definitely check the values on teh new ones, at least some in the cheap set I bought are mis-labeled.
Click to expand...
If you want a real deal - replace all resistors with modern metal film ones. Use only 1W and higher - this is for them to work at high voltages.
 
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