OK. So far, the lowest hanging fruit for testing is to short across the choke to take it out of the equation and see if it's stimulating something in the rectifier tube. At present (at least at the volumes I run the system), there is zero audio hum. Just the high pitched ringing that hangs over everything from the tube that builds over the course of an album, and resets as I tap it every time I flip or switch so that it can build again. If shorting the choke works, I'll mess around with some external shielding to see if I can get it back in.
Next lowest hanging fruit is to add another capacitor ahead of the choke. Adding a bleed resistor and uprating cap voltages seems like cheap insurance on the side to flesh out a parts order.
What would a "swinging" choke be called in modern parlance or look like on a spec sheet if I were to get down the flow chart enough to need to source one? The choke that's in there is a Hammond audio choke. Nothing special as this was/is my first project of this sort, but also not something random out of scrapped equipment.
Next lowest hanging fruit is to add another capacitor ahead of the choke. Adding a bleed resistor and uprating cap voltages seems like cheap insurance on the side to flesh out a parts order.
What would a "swinging" choke be called in modern parlance or look like on a spec sheet if I were to get down the flow chart enough to need to source one? The choke that's in there is a Hammond audio choke. Nothing special as this was/is my first project of this sort, but also not something random out of scrapped equipment.
First of all, if you short the first choke, you will then have a capacitor-input supply and your B+ will jump to at least 400VDC. Your 6DJ8's aren't going to like that. Second, they don't make swinging chokes anymore, or if they do they're hard to find and rather expensive. They are gapped and display variable inductance depending on the current load.
Some chokes will handle the ripple of a choke-input supply better than others. There's no way to really know for sure. I also can't guarantee that this is your problem, but it's pretty common for a non-swinging choke to "dance" when used for choke input. The simplest and cheapest solution, if that is the case here, is to add a .47uF/1000V cap at the rectifier output, and a 20K/10W resistor to ground in parallel with C1, or after the first choke.
This may not solve your problem BUT it should be done anyway!! These are both safety measures that will protect your rig should the 6DJ8s fail to conduct. It's not even certain that they are drawing enough current to maintain the critical current draw for the choke-input supply to work.
Have you actually monitored the power supply behavior at turn-on and afterward? You might hook a meter up to it and see what's actually going on.
Some chokes will handle the ripple of a choke-input supply better than others. There's no way to really know for sure. I also can't guarantee that this is your problem, but it's pretty common for a non-swinging choke to "dance" when used for choke input. The simplest and cheapest solution, if that is the case here, is to add a .47uF/1000V cap at the rectifier output, and a 20K/10W resistor to ground in parallel with C1, or after the first choke.
This may not solve your problem BUT it should be done anyway!! These are both safety measures that will protect your rig should the 6DJ8s fail to conduct. It's not even certain that they are drawing enough current to maintain the critical current draw for the choke-input supply to work.
Have you actually monitored the power supply behavior at turn-on and afterward? You might hook a meter up to it and see what's actually going on.
Hopefully the 6DJ8s are fine at 400VDC long enough to see if the ring disappears, because that's what I'm in the middle of doing. I pulled the choke and replaced it with a jumper while 6mo was napping, and have it running now. If I run it loud (relative to what I normally do, which would almost certainly be considered a very low volume), the predicted audio hum is there, but the ringing does not seem to be happening at the moment. It's my daughter's 5th birthday today, and the house is an unholy din, so tough to tell at the moment.
I believe, and could very well be wrong, that if the magnetic field from the choke is stimulating the guts of the 6CA4, I can make up a grounded shell to go over the transformer and choke that will stop the ringing.
Still need to look into the additional safety measures. If the choke removal does in fact do the job, I'll put the choke back in and start messing around with shielding... I think.
I believe, and could very well be wrong, that if the magnetic field from the choke is stimulating the guts of the 6CA4, I can make up a grounded shell to go over the transformer and choke that will stop the ringing.
Still need to look into the additional safety measures. If the choke removal does in fact do the job, I'll put the choke back in and start messing around with shielding... I think.
Confirmed. Pulling the choke out of the circuit quiets down the rectifier's ring. So it's almost certainly the magnetic field from the choke stimulating the 6CA4's guts. Would adding the cap ahead of the choke fix that, or is shielding my best option?
From the RH84 blog:
Of course they don't include a picture of a completed amp so we can see an ideal LC topology, and I can't find one on the RJM page. But the main choke is working very hard and needs space. I think your chassis is too cramped and the magnetic field of that choke is inducing noise into the tubes.
You need a safe way to bench test how to best position the PT, Choke and Rectifier to minimize that hum.
If you don't wanna drill more holes, add that 0.47uF 100v film cap before the first choke. but it changes the power supply to Cap-loaded, and you'll no longer have the large voltage drop from the first choke, so you'll have a higher b+ and will need to re-bias your 6DJ8 tubes. Or drop 50v+ in the power supply via resistors or MOSFET.
Of course they don't include a picture of a completed amp so we can see an ideal LC topology, and I can't find one on the RJM page. But the main choke is working very hard and needs space. I think your chassis is too cramped and the magnetic field of that choke is inducing noise into the tubes.
You need a safe way to bench test how to best position the PT, Choke and Rectifier to minimize that hum.
If you don't wanna drill more holes, add that 0.47uF 100v film cap before the first choke. but it changes the power supply to Cap-loaded, and you'll no longer have the large voltage drop from the first choke, so you'll have a higher b+ and will need to re-bias your 6DJ8 tubes. Or drop 50v+ in the power supply via resistors or MOSFET.
A .47uF cap should not increase the B+, but it will reduce the ripple on the choke and perhaps avoid the need for moving everything around and adding shielding. This is a trick that Lynn Olson and other professional designers use to maintain the benefits of choke input while avoiding the drawbacks. It's worth a try before rebuilding the whole supply...
I've got steel stock and tools lying around, and I don't have high voltage capacitors lying around. I'll mess around with the shielding first out of convenience.
Just to be sure I'm not making any assumptions though, the .47uF cap is intended to go between the rectifier output/choke input and ground, correct?
Just to be sure I'm not making any assumptions though, the .47uF cap is intended to go between the rectifier output/choke input and ground, correct?
Yes, but Check out this page to learn more about the differences btw CL and LC designs.
There's a lot of mystery and lore around choke-input supplies. I guess caps were crazy expensive back in the day and winding a choke was cheaper. Now cap-input supplies are all the rage and you see very little about choke input designs. But I hear they have great bass, better regulation, yadda yadda. Hard to find a picture of one tho.
There's a lot of mystery and lore around choke-input supplies. I guess caps were crazy expensive back in the day and winding a choke was cheaper. Now cap-input supplies are all the rage and you see very little about choke input designs. But I hear they have great bass, better regulation, yadda yadda. Hard to find a picture of one tho.
Choke-input supplies were used mainly in high-voltage transmitting equipment for various reasons. They improved regulation of the supply but also were pretty much required when employing high-voltage/high-current mercury vapor diodes.
https://www.theaudiofeast.com/blog/designing-choke-input-power-supply-ham-tips-from-rca-1941-feb
The idea was revived during the '90's with the craze for "antique" amplification methods like single-ended triodes and transformer-coupled amplifiers.
In truth, very few commercial audio amplifiers used choke-input supplies past a certain vintage. Cap-input supplies were cheaper, lighter and more manageable.
https://www.theaudiofeast.com/blog/designing-choke-input-power-supply-ham-tips-from-rca-1941-feb
The idea was revived during the '90's with the craze for "antique" amplification methods like single-ended triodes and transformer-coupled amplifiers.
In truth, very few commercial audio amplifiers used choke-input supplies past a certain vintage. Cap-input supplies were cheaper, lighter and more manageable.
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Is it possible to change the orientation of the choke? If you turn it by 90 degrees, its likely that the magnetic flux lines won't bother your tube. In electronic design it's common practice to align inductors to minimise mutual coupling in that way. It might save having to add components or start drilling stuff.
It could also be magnetostriction from the choke/transformer coupling mechanically to the 6CA4 to force microphonic vibration. If it was that mechanism, then modifying the way the choke/transformer is bolted to the chassis may help, as well as alleviating any mechanical coupling to the 6CA4 (decoupling the valve base from the chassis), as well as increasing the mass and dampening the 6CA4 valve itself (as it may be related to the glass envelope and fitting a few silicone o-rings can dampen glass ringing). The o-rings may be the simplest fault-finding action.
Damping rings on the 6CA4 won't help as it it the innards ( plate structure made of steel) that is vibrating.
Move or turn the choke, possibly add a steel or mymetal screen is what would reduce or eleminate the noise.
Move or turn the choke, possibly add a steel or mymetal screen is what would reduce or eleminate the noise.
Worth a try imho, as an internal vibrating structure in a vacuum has to couple to a structure that then makes sound in air.
A simple steel shielding experiment resulted in no change observable on a relatively short timescale. The ring was still audible, but I probably didn't give it enough time to really get swinging. A fail is a fail, so I took it down and moved on.
I'm wondering if some of the ring might not be coming from the choke itself, and the rectifier gets magnetically coupled in a constructive interference sort of manner to join the chorus; only audible with your ear next to the amp to start, and very very slowly builds to where it's ringing over the (not very loud) music. Tapping the tube makes the ring much quieter, but it doesn't completely go away. With the choke out of the circuit, it runs totally silent like it should (albeit with the expected audio hum). While that eliminates the magnetic coupling, it doesn't not eliminate the choke itself being a party to the ringing. Something I read from one of those links back there suggested a potted choke being required, and that's not what I've got. The potted equivalent is pricey, and as best I can tell, much larger to the point that it wouldn't fit on the existing chassis.
The capacitor in front of the choke seems like it might help. What's the math behind the 0.47uF value so I completely understand what it is I'm contemplating doing?
Digging further into capacitor input supplies next...
I'm wondering if some of the ring might not be coming from the choke itself, and the rectifier gets magnetically coupled in a constructive interference sort of manner to join the chorus; only audible with your ear next to the amp to start, and very very slowly builds to where it's ringing over the (not very loud) music. Tapping the tube makes the ring much quieter, but it doesn't completely go away. With the choke out of the circuit, it runs totally silent like it should (albeit with the expected audio hum). While that eliminates the magnetic coupling, it doesn't not eliminate the choke itself being a party to the ringing. Something I read from one of those links back there suggested a potted choke being required, and that's not what I've got. The potted equivalent is pricey, and as best I can tell, much larger to the point that it wouldn't fit on the existing chassis.
The capacitor in front of the choke seems like it might help. What's the math behind the 0.47uF value so I completely understand what it is I'm contemplating doing?
Digging further into capacitor input supplies next...
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I'd suggest there 'is no math' to that cap value. Above a certain value it will start increasing the operating B+ voltage - that value may be estimated using PSUD2 or testing.
Below that value, the cap imho has a few benefits. It softens the sharp discontinuity of the voltage waveform as the voltage of the node is rapidly falling and then rapidly increasing as the diodes commutate. It provides a path for transient currents, where the path can have a relatively small loop area (compared to some of that transient current flowing through the choke's self capacitance and into the load circuitry). But the cap needs to have a suitably high Vac rating, and can get large as cap values get larger, so in practice cap value may depend on what cap is available, or can be accommodated.
Below that value, the cap imho has a few benefits. It softens the sharp discontinuity of the voltage waveform as the voltage of the node is rapidly falling and then rapidly increasing as the diodes commutate. It provides a path for transient currents, where the path can have a relatively small loop area (compared to some of that transient current flowing through the choke's self capacitance and into the load circuitry). But the cap needs to have a suitably high Vac rating, and can get large as cap values get larger, so in practice cap value may depend on what cap is available, or can be accommodated.
Update for anyone coming along behind me or curious to know how it turned out: the cap seems to have worked. Took me a while to get it ordered and installed, then more time to test it out to where I'm comfortable that it's done what it was intended to do, but the amplifier has been quietly doing its job for a while now! Thanks everyone!