Hi!
The tests which he has done already excluded the 6SL7 heater as the culprit.
Hum is still there when the 2A3 grid is grounded. More B+ filtering is needed
Thomas
The tests which he has done already excluded the 6SL7 heater as the culprit.
Hum is still there when the 2A3 grid is grounded. More B+ filtering is needed
Thomas
A quicky clip lead test with a 20u - 30u 450V cap across those little input caps would probably knock that ripple right down.
I think he said there was low B+ in the first post. Makes me wonder now if the 5U4 is a bummer?
The HV secondary DCR is 110ohms. B+ should be at 322V according to the schematics but I'm getting only 288V (as predicted by PSUDII). Accidentally, that op point (248V plate) is the one recommended in the 2A3 datasheet. Changing the value of the 2uf cap (actually I'm using only one cap of 2 microfarads, a low cost ClarityCap SA) has a huge effect on the B+ voltage according to simulation. Looks like I should not go beyond the 3microfarads. I might increase the last capacitor from 100uf up to 330uf to get lower ripple. Please note that I'm getting the heater 2.5Vac from the same core so may be the 100Hz spikes are present there too.
Mem,
I don't have PSUD to run the numbers. I would appreciate it very much if you would run a simulation with 10uf, 20uf, and 30uf for input capacitors and the 8H choke and give us the change in B+ for each. Very interested for future reference. Thanks Much.
PSUD
Here we go: PSUD simulation Transformer 660Vac no load voltage with CT; 110ohms DCR. Followed by 5ua4g; followed by input capacitor, followed by 8H 60 ohms choke then 100uf capacitor (9mohms ESR). Load current draw 110mA
DC Output at 100uf capacitor
With original 2uf input cap: 287,87V; 913mVpp ripple
3uf input cap: 315,99V; 646mVpp ripple
10uf input cap: 354,52V; 195mVpp ripple
20uf input cap: 358,33V; 96mVpp ripple
30uf input cap: 359,03V; 64mVpp ripple
Adding more filtering in the last cap as said by Vinylsailor reduces the ripple even more. With 3,3uf as the first cap and 330uf as the last one yields 178mVpp of ripple.
Here we go: PSUD simulation Transformer 660Vac no load voltage with CT; 110ohms DCR. Followed by 5ua4g; followed by input capacitor, followed by 8H 60 ohms choke then 100uf capacitor (9mohms ESR). Load current draw 110mA
DC Output at 100uf capacitor
With original 2uf input cap: 287,87V; 913mVpp ripple
3uf input cap: 315,99V; 646mVpp ripple
10uf input cap: 354,52V; 195mVpp ripple
20uf input cap: 358,33V; 96mVpp ripple
30uf input cap: 359,03V; 64mVpp ripple
Adding more filtering in the last cap as said by Vinylsailor reduces the ripple even more. With 3,3uf as the first cap and 330uf as the last one yields 178mVpp of ripple.
Mem,
Thanks for info.
Some thoughts about the situation compared to the original circuit you modeled from.
It uses a 600v secondary but you are modeling a 660v tranny and still coming out with lower voltages. The original circuit, with smaller filter caps, certainly didn't have the hum/ripple you are getting.
There must be a huge difference in the total PS DCR between the original circuit and yours.
There are plenty of common PS designs that use no more than 30uf and a 1.5H choke to provide over 200ma with dead quiet output.
I don't think you will identify and really fix your problem with tons of filtering, it will only mask what's going on and you may never get to the bottom of the problem and continue to believe it was only lack of filtering.
just some thoughts. Thanks for the sim runs.
20
Thank your for your thoughts, 20. I certainly agree with you. The original circuit with 47uF was reported to be reasonable in terms of hum even with Fostex drivers. There is something I'm missing, if a just knew what. I might swap the 5ua4 if I had another just to check whether the forward drop. This is my very first ever experience with tubes, now that I'm paranoid I do hear a light jiggling noise coming supposedly from the rectifier (only sometimes). I use the amplifier in vertical position to check voltages from its botton side; older 5ua4g datasheets specify the mounting position of the tube as vertical only. I must say as well that my 5u4 does not fit so tightly in its socket as the 6sl7 tubes. Could I be getting an increased resistance from this fact or even a false contact producing the jiggling noise? Does this make any sense?
As for the sim runs, 20, you're most welcome. No big deal compared with everybody's help and thoughts.
Thank you everybody. I'll keep you posted.
Hi!
A lot of confusing information. To clarify can you post the PS schematic which you are actually using right now? Along with comparison of sim results and measured results.
As has been mentioned several times: Your filtering is definitely not enough and thats why you get the hum.
Increasing the first cap after the rectifier will increase the voltage. If you want to increase the filtering without touching the first cap, you need to add substantial capacitance to the last cap or add another LC segment.
Thomas
A lot of confusing information. To clarify can you post the PS schematic which you are actually using right now? Along with comparison of sim results and measured results.
As has been mentioned several times: Your filtering is definitely not enough and thats why you get the hum.
Increasing the first cap after the rectifier will increase the voltage. If you want to increase the filtering without touching the first cap, you need to add substantial capacitance to the last cap or add another LC segment.
Thomas
Schematics
Thomas, I'm sorry about the confusion. I'm attaching the "as built" schematics for your reference. Hope my handwriting is clear.
I have a 100uf 450 V spare cap. I could test it ni parallel with the 100uf 550V currently at the circuit. The voltage rating is kind of low since at connection B+ goes up to 400V for a few seconds until the 2A3 cathodes are hot enough to draw current. For testing purposes it would be OK but for permanent use I'd need a higher voltage one.
I'm just trying to understand why the original schematics was so poorly filtered.
The simulation of this circuit yields almost exactly the results I'm getting with the real circuit, simulated with a steady consumption of 110mA, and it is highly influenced by the drawn current. For example, with a consumption of 110mA B+ falls almost 8 Volts to 280V. Ripple in both cases is predicted to be 960mVpp. In the real life I'm getting more than twice that value (2.24Vpp).
Thank you for your help.
Mario
Thomas, I'm sorry about the confusion. I'm attaching the "as built" schematics for your reference. Hope my handwriting is clear.
I have a 100uf 450 V spare cap. I could test it ni parallel with the 100uf 550V currently at the circuit. The voltage rating is kind of low since at connection B+ goes up to 400V for a few seconds until the 2A3 cathodes are hot enough to draw current. For testing purposes it would be OK but for permanent use I'd need a higher voltage one.
I'm just trying to understand why the original schematics was so poorly filtered.
The simulation of this circuit yields almost exactly the results I'm getting with the real circuit, simulated with a steady consumption of 110mA, and it is highly influenced by the drawn current. For example, with a consumption of 110mA B+ falls almost 8 Volts to 280V. Ripple in both cases is predicted to be 960mVpp. In the real life I'm getting more than twice that value (2.24Vpp).
Thank you for your help.
Mario
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