You could go insane running down all the suggestions in this post. First thing is to replace main filter caps which I think you've done. Then measure resistors in output circuit, especially cathode resistors...should be matched to 2% and tubes must also be matched (but how do you know tubes are matched...cuz they were sold as such, or do you have tube tester?). Install a cathode balance pot ( 1 watt) if in doubt..can always remove it later....not reliable long term. 120hz hum on output should be measured and noted. Measure and note (write down) ripple on B+ supply, also measure and note filament voltage.
Next isolate that part of the circuit you want to re-use, for instance V3,V4,V5 and resolder pot side of C9 to grounc point of pot R17. Does that quiet output sufficiently? If not then it's time to sub in some power supplies (filament and B+) Substitute voltages have to be well grounded to circuit ground. But where exactly is circuit ground? Chassis is of course a good ground to some degree, what I mean by that is power transformer eddy currents and ripple currents in chassis make some points on chassis a good ground, others hummy-buzzy ground. If parts of original circuit use chassis as ground wherever convenient that could be a problem or not...and it could be manufacturer went to the trouble to check on that when laying out build. Or they didn't check except to choose a later stage ground point that was suffiently out of phase with earlier stage ground point to effectively more or less cancel cum in final output. So it may be necessary to interupt B+ feeds to any circuit upstream from the output stage so their ripple does not go into chassis.
Circuit ground for sub'd in power supplies should be where V4 and V5 cathode resistors connect to ground or where or where C12 is grounded (connected to chassis), and ideally these will be the same point or only separated by an inch of chassis, cuz eddy currents or ripple current running in chassis between those two point could be a sosurce of hum in output.
Subbing in filaments supply (I think you did that) is easiest to do. If doing so does not fix adequately note the hum/buzz on B+ and output circuit and compare with previous notes. Maybe there was no improvement at all in those measurements....anyway just write it all down for future comparisons. If improvement from filament voltage sub gave no or inadequate improvement then it's time to sub B+, cuz what else is left at this point? If you don't have a good B+ supply for testing can you buy one...even a les expensive switching test bench supply from China will likely (maybe) be adequate for testing. Otherwise and anyway, while listening to output at sufficent level (on speaker you can sacrifice if that happens), and with filaments running on sub'd in dc filament supply, and upstream B+ power rails lifted, turn off the power to unit (but not to filaments). B+ voltage will take a couple secs to decay. You should observe the hum in output is gone for those couple secs if prob is from B+ supply or power transformer eddy currents or chassis ripple current. If that is the case then next step is to try co-locating all circuit ground points still relevant ( so not upstream stuff since B+ lifting previously mentioned)such as C12 grounds and V3,V4 and V5 circuit grounds. Doing so will eliminated as suspects the chassis ripple currents but maybe or maybe not power transformer eddy currents. If co-location does not fix sufficiently then only thing left to check is power transformer eddy current by it's removal and distancing from chassis by 6 inches. Note that if power transformer eddy currents are source of problem that might be revealed merely by removing it's mounting bolts and lifting it away from chassis by a 1/16" to 1/4".
If testing reveals power transformer eddy current problem then one will want to re-bolt power transformer to chassis and pick a chassis co-location ground point well away from power transformer, maybe at least 6 inches but only experimentation will know for sure. The ground of C12 is a good, and more or less mandatory ground co-location point. Having C12 (main filter cap) grounds on an arm of a star grounding design will only rarely work....barely even worth trying.
If you have or aquire a B+ test supply then sub it in and still the recommendations of previous 3 paragraphs hold but you will have more certainty and you won't have to deduce things during the 2 seconds waiting for B+ supply to decay to shutoff.
Bear in mind that even though HP was good test gear it might nothave had really low hum/buzz cuz some hum/buzz ws considered normal in those days and was just accepted....it sould have been hard to find a hi-fi amp that could amplifify signal into a speaker without adding it's own hum/buzz.
The above procedure is a pain in the a** and so time consuming it's hardly worth it but anyone who goes through this will have a new knowldege and skill others lack. This is akin to the torture one must endure to perfect the quad axel in figure skating as shown here:
Pain of perfection
Good luck.
That quad axel struggle was real!
Thank you for this. It helps a lot, every bit of it.
I do have a nice HT power supply unit, restored / calibrated. It is the Precision Apparatus Company B-12, which i purchased from a very trusted individual. So I can do power subbing.
Yes i did replace all caps basically. Didn’t touch a couple small values in Osc but am deleting that anyways.
Yes the C9 input is grounded to its R17 pot ground. Doesn’t change hum. Severe high pitch squeal when nothing is connected to C9 input, but audio is identical when either grounding C9 to its ground or just hooking up a line input from external unit.
Yes i did sub the A/C heater supply with a 6V DC (actually measured 6.3 ironically, unloaded) lantern battery with lots of juice. The unit operated as normal, and no improvement in hum. I don’t think the heater supply is the hum.
I do not have a tube tester but it seems i am on the cusp of realizing i need to have one around since this isn’t the only project i have going on. The same individual above has offered a good price on a restored / calibrated one.
I only have a pair of known “great testing” 6v6’s. They are not guaranteed to be matching. I put them in and the hum was, by ear, identical.
I would like to at some point install an output tube cathode pot and resistor to ground. Basically just create the R48/R47 circuit, which is not in my unit. My unit apparently had “change no 12” implemented, which is displayed below from the end of the service manual. There is no longer a “pair” of cathode resistors to measure and compare.
In all of these suggestions, i was expecting to see one in particular but didn’t….. I understand everything you’re saying about chassis currents from transformer and also C12 negative, and am going to inspect the layout a bit further, but what I’m starting to be most curious about is air-space distance from transformer to the tubes and EMF radiating. It seems stupidly obvious. You know, the kind of EMF noise you hear when you walk over to equipment with electric guitar in hand and bring guitar close to a power transformer. Isn’t this a factor inside tube amps? The power transformer is sitting just a couple inches from one of the output tubes, and maybe a few inches from the 6SN7 driver / inverter tube. You can see this in one of the earlier posts in this thread. But maybe you’re already referring to this and I’m not just not yet familiar with the terminology.
So my reasons for hooking up an external supply are vast, haha.. All of the physical contact noise/hum factors you mention are maybe going to be revealed, but maybe also just plain air-space distance between the transformer and the tubes will be revealed to be an issue. I could even try just disconnecting the factory PT and extending its leads, and sit it somewhere a couple feet away, then bring it closer to see if that’s what’s happening.
It’s really helpful to hear about the ground point for the external supply though. On the 201C, there is only around in inch between the output cathode ground point and the C12 ground point (which is also pin 9 of power transformer). So maybe that’s actually healthy. In any case, i will hook ground from supply to one of those, and they are close by each other as just said. I suppose i can choose one, so maybe that’s the transformer signal ground which is same point as C12 ground, that I’ll go with.
When i do hook up the external supply, I’ll be better able to isolate the audio section from the Osc. I’m concerned about rail voltage rising with the factory power supply, if i disconnect power to Osc section tubes. The PACO B-12 supply is regulated, so i won’t have that concern. If i wasn’t worried about voltage rising, i would already completely removed all Osc tubes and components.
I do have connection questions.. what about the PT’s own chassis ground? It’s mounted directly to 201C chassis. So, in the case of extending the factory PT on longer leads, i should connect the PT chassis to 201C chassis with an extra lead? And in the case of the B-12 regulated supply… The regulated output’s ground is fastened to the front face plate. So i guess in that case I’m just using one ground connection from B-12 for both “pin 9 ground” and chassis-to-chassis ground.
Lastly regarding connection.. these regulated supplies are supposed to…
1: Insert a bipolar +300/-300/ground to the 5Y3?
2: Insert a +350/ground to C12B?
3: Insert a +310/ground to C12C?
This isn’t a lot of work. This is absolutely worth the time because it already sounds musically excellent. It’s already teaching me more about tube layout and i have other projects i will apply this experience to.
