Johan Potgieter said:
Hey, go ahead and dominate. I'm grateful for the information. I think, based on this, I'm not too far off with this supply. One remaining question: How do I plug the main bias cap into this calculation?
anatech said:
True, but it's in my wiring. What can I say?
Thanks all,
Sheldon
Hi Sheldon,
My intention was to simply say, do not exceed what is on the spec sheet. They specify this cap with an average transformer at X volts. If the max is 50uF, there will not be a big difference by installing a 40uF cap there. You may want additional filtering. That can be provided by using either chokes or resistors and another cap. Possibly a SS regulator.
There is a well known brand that used 100uF after a 6CA4. They felt some of these tubes were of low quality when in fact it was entirely the designer's fault. This same designer had a unit that arced 5U4's on power up. Solution, let's use a Cetron equivalent. Gee, they arc too but not as often. The power supply runs three 6DJ8's and a pair of 5881 shunt regulators. That was stupid. That particular preamp uses two power supply units with a pair of Cetrons in each. One per channel. All it is, is a line level preamp. Even the heaters are shunt regulated individually. It is a service nightmare and completely unfair to the customers.
If reasonable design standards were followed, this would not have occurred.
-Chris
My intention was to simply say, do not exceed what is on the spec sheet. They specify this cap with an average transformer at X volts. If the max is 50uF, there will not be a big difference by installing a 40uF cap there. You may want additional filtering. That can be provided by using either chokes or resistors and another cap. Possibly a SS regulator.
There is a well known brand that used 100uF after a 6CA4. They felt some of these tubes were of low quality when in fact it was entirely the designer's fault. This same designer had a unit that arced 5U4's on power up. Solution, let's use a Cetron equivalent. Gee, they arc too but not as often. The power supply runs three 6DJ8's and a pair of 5881 shunt regulators. That was stupid. That particular preamp uses two power supply units with a pair of Cetrons in each. One per channel. All it is, is a line level preamp. Even the heaters are shunt regulated individually. It is a service nightmare and completely unfair to the customers.
If reasonable design standards were followed, this would not have occurred.
-Chris
Fair enough Cris, especially when dealing with commercial gear. With stuff only for my own use, I'm willing to go closer to the limit. And I couldn't resist the obscure play on words. However, in this case, I don't think I'm anywere near the limit.
Yes, the specs. for the GZ34/5AR4 say 60uf max. (actually it isn't specified on the GE 5AR4). On the GE sheet, the transient permissible current is 3.7 amps and steady state is 825ma. On the Phillips sheet, transient isn't specified, but steady state max. is 750ma. The GE chart III gives a minimum supply resistance of about 100 ohms for a plate supply voltage of 320VAC. The supply resistance is defined by them as secondary resistance + primary resistance x(step-up ratio)squared + series resistance. For my supply, this gives 38+11x(2.67x2.67) +150 = 260ohms.
If I model a simple cap input supply in PSUD, with a 60uf input cap and 100ohms supply resistance into a load of 160ma, I get an initial peak of 2.9 amps, dropping quickly to 0.46amps peak per cycle. My idea of an equivalent model for my supply including the bias cap, is to have the 47uf cap and the 100uf input cap in parallel. Since they both have 150 ohms in series, I just added in the resistance for the transformer to get a total supply resistance of 260ohms. So for 260ohms and 150uf, I get an initial peak current of 1.6 amps dropping quickly to 0.35amps in parallel. That suggests that the original supply is easier on the rectifier than a 60uf input with the minimum recommended supply resistance. Further, that initial peak will be blunted as Johan suggests by the gradual turn on characteristics of the rectifier heater.
Now, this is a rank amateur attempting get past general prescriptions and do some simple engineering calcs. base on something closer to principle specifications (and with the help of some modeling software). I would not be shocked to find I had missed something critical. And I know I'm missing something, because it still doesn't give me a ripple voltage that's close to the actual. But I doubt the ripple current is a problem.
Thanks,
Sheldon
Yes, the specs. for the GZ34/5AR4 say 60uf max. (actually it isn't specified on the GE 5AR4). On the GE sheet, the transient permissible current is 3.7 amps and steady state is 825ma. On the Phillips sheet, transient isn't specified, but steady state max. is 750ma. The GE chart III gives a minimum supply resistance of about 100 ohms for a plate supply voltage of 320VAC. The supply resistance is defined by them as secondary resistance + primary resistance x(step-up ratio)squared + series resistance. For my supply, this gives 38+11x(2.67x2.67) +150 = 260ohms.
If I model a simple cap input supply in PSUD, with a 60uf input cap and 100ohms supply resistance into a load of 160ma, I get an initial peak of 2.9 amps, dropping quickly to 0.46amps peak per cycle. My idea of an equivalent model for my supply including the bias cap, is to have the 47uf cap and the 100uf input cap in parallel. Since they both have 150 ohms in series, I just added in the resistance for the transformer to get a total supply resistance of 260ohms. So for 260ohms and 150uf, I get an initial peak current of 1.6 amps dropping quickly to 0.35amps in parallel. That suggests that the original supply is easier on the rectifier than a 60uf input with the minimum recommended supply resistance. Further, that initial peak will be blunted as Johan suggests by the gradual turn on characteristics of the rectifier heater.
Now, this is a rank amateur attempting get past general prescriptions and do some simple engineering calcs. base on something closer to principle specifications (and with the help of some modeling software). I would not be shocked to find I had missed something critical. And I know I'm missing something, because it still doesn't give me a ripple voltage that's close to the actual. But I doubt the ripple current is a problem.
Thanks,
Sheldon
Hi Sheldon,
Only time will tell. The initial charge will be slowed by the heating cathode on your 5AR4. It might not like it. Steady state pulses look okay. There is the internal resistance of your 5AR4 to add, whatever that figure is.
Anyhow, you are doing better with the design than some commercial designers, so don't appologise. The only way to really tell is to build it. When you have it running, from a cold start, turn it on in a darkened room. Watch the rectifier tube for flashes or arcs. That is the final answer.
-Chris
Only time will tell. The initial charge will be slowed by the heating cathode on your 5AR4. It might not like it. Steady state pulses look okay. There is the internal resistance of your 5AR4 to add, whatever that figure is.
Anyhow, you are doing better with the design than some commercial designers, so don't appologise. The only way to really tell is to build it. When you have it running, from a cold start, turn it on in a darkened room. Watch the rectifier tube for flashes or arcs. That is the final answer.
-Chris
Thanks Cris,
It's built and been running for a couple of months now. I haven't noticed any bright flares going off, but I'll look carefully. I'm using a JJ tube.
As far as the internal resistance of the rectifer, I believe that PSUD has that in their parts models. The program provides a list of various rectifiers (ss and vacuum) to choose from. It would make sense, and if you plug different versions in, output voltages vary according to expectation.
Sheldon
It's built and been running for a couple of months now. I haven't noticed any bright flares going off, but I'll look carefully. I'm using a JJ tube.
As far as the internal resistance of the rectifer, I believe that PSUD has that in their parts models. The program provides a list of various rectifiers (ss and vacuum) to choose from. It would make sense, and if you plug different versions in, output voltages vary according to expectation.
Sheldon
Hi Sheldon,
That's good. If it's been running, then there are no serious problems.
Rectifier tubes have two differences. Effective internal resistance and voltage drop. I haven't looked to see how much drop there is between tubes. Sim at almost no load would tell you what the model has. Sim varying the load from low to max will give you an idea of the resistance in the model.
-Chris
That's good. If it's been running, then there are no serious problems.
Rectifier tubes have two differences. Effective internal resistance and voltage drop. I haven't looked to see how much drop there is between tubes. Sim at almost no load would tell you what the model has. Sim varying the load from low to max will give you an idea of the resistance in the model.
-Chris
I must, from several decades of experience, really echo what Chris said (post #42). I wonder how often I found that folks did not do the basics right. There is sometimes room for footwork within borders, but do not exceed the borders, plus some for safety's sake. I have done both tube and semi-conductor designs, and this is especially so in the latter case. E.g. how often do folks blame negative feedback for many things, when the fault lies elsewhere, etc. (oops, should stop here, I think!) But: YES.
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