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Old 13th May 2015, 01:36 PM   #1
max1979 is offline max1979  Italy
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Default Stabilizing DC heaters [newbie]

Hi all,

That's my first experience with tubes and I'd like to design a linear regulator but it seems I cannot stabilize the voltage for the heaters to the desired 6.3V

My preamp is based on 3 tubes per channel and I can provide the needed anode voltages:
- 310V@4m to the phono stage (2xECC83+2xECC81 - SRPP config)
- 350V@6m to the line stage (2xECC82 SRPP config)

My problem is providing the 6.3V (DC) to all the heaters - in attach the schematic.
In order to simulate the tubes, I added 6 current source (300mA), one for each tube.
It seems I cannot provide more than 2,5-3V, at least this is the voltage measured between H+ and H-.

A quick explanation about the source used to simulate the power transformers. The assumption is to have 230V for the primary and as secondary:
1) 270V@100mA for the anode
2) 6.3V@3A for the heaters
I simulate this adding two sin sources with the following features:
1) 270*1.41 = 380 VacRMS (the amplitude used) and 135Ohm
2) 6.3 * 1.41 = 8.8 VacRMS and 105mOhm
Hope this is correct.

Now, what's wrong with my simulation or my design? how can I get my 6.3V?
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Old 13th May 2015, 01:46 PM   #2
jgf is offline jgf  United States
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Max,

I'm not sure what problem you're having as you don't show or mention the dc heater voltage you get out of the circuit.

If you're resulting voltage is low (and maybe even if it isn't) consider heating with 12.6v at half the current (using a voltage doubler) as all those tube types you mentioned can be wired for either heater voltage.
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Old 13th May 2015, 01:56 PM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by max1979 View Post
2) 6.3 * 1.41 = 8.8 VacRMS and 105mOhm
Hope this is correct.?
This is not correct: 6.3V is already an rms voltage, if you specify rms voltages in your simulator, you don't need to change anything.

Anyway, the voltage should be even higher that way.
Here is what I get with your incorrect voltages.
Did you wait long enough?
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Old 13th May 2015, 01:58 PM   #4
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Why do you want so much smoothing? There is no need, 2200uF is more than enough. Just a waste otherwise.
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Old 13th May 2015, 02:28 PM   #5
max1979 is offline max1979  Italy
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I think there is something bad in the params I'm using for the simulation.
Follow what I can see running a simplified version of the circuit. The problem comes when I attach the current source that pushes down the voltage to 1V.

@JonSnell you are right, but as soon as I decrease the cap values I get a huge amount of ripple - maybe same simulation issue
@jgf voltage doubler is a good advise I was not thinking about, I'll check it as soon as I will reach a graph close to the one presented by Elvee
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Old 13th May 2015, 02:48 PM   #6
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When you use a full wave bridge rectifier your output voltage will be
AC voltage X 1.41 MINUS the voltage lost across the rectifiers which
in your case will probably be about 2 v. So in your case closer to 6.8 v dc.
That is if we ignore any voltage drop across the 2 seriese resistors you have.
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Old 13th May 2015, 03:20 PM   #7
max1979 is offline max1979  Italy
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Let's assume the AC coming from the transformer is Vac = 6.3V and the power transformer is able to provide 3A - I'm asking for 1.8A.

The voltage drop on R7 (105mOhm calculated by PSU Designed II) should be
V1 = .105*1.8 = 0.189V
and the voltage lost across the rectifiers is Vb = 2V
So the voltage (RMS) we should have after the bridge should be
V2 = (Vsac - V1 - Vb)*1.41 = 5.8V
Now I added the RC filter including R6 = .22 Ohm and we have a drop of voltage here too (Vf).
Vf = .22 * 1.8 = .396V

Final voltage should be
V3 = V2 - Vf = 5.4V

The simulator says I have something like 1V. This is what I don't understand...
Changing the diodes (using 1N5401) I can reach 3-3.2V

I'm a bit confused, there must be something that I don't see here
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Old 13th May 2015, 03:50 PM   #8
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by max1979 View Post
Follow what I can see running a simplified version of the circuit. The problem comes when I attach the current source that pushes down the voltage to 1V.
Try to replace the I source by a 3.5Ω resistor, it is equivalent and maybe the simulator will be happy with it
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Old 13th May 2015, 05:24 PM   #9
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Max1979, I believe that there's something wrong in your calculation or the values you're entering in the program. I tried PSU Designer II with the components of your scheme and it gave me from 5 to 6 V DC at the output, depending mainly on the type of diodes. Best choice in your case would be Schottky diodes which have less tension drop ( SB540 or similar). Those values will also change a bit depending on the regulation of your trafo. I used 5%, but it might be 10% or more.
Anyway, 6.3V/3A AC is a bit tight for supplying 6.3V/1.8 A DC if you want to use regular diodes and also an intermediate resistor. You should either use a lower value for the resistor R6 and use Schottky diodes or else a trafo with a winding of at least 7 - 7.5 V AC
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Old 13th May 2015, 08:34 PM   #10
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Artur5 View Post
Max1979, I believe that there's something wrong in your calculation or the values you're entering in the program. I tried PSU Designer II with the components of your scheme and it gave me from 5 to 6 V DC at the output, depending mainly on the type of diodes. Best choice in your case would be Schottky diodes which have less tension drop ( SB540 or similar). Those values will also change a bit depending on the regulation of your trafo. I used 5%, but it might be 10% or more.
Anyway, 6.3V/3A AC is a bit tight for supplying 6.3V/1.8 A DC if you want to use regular diodes and also an intermediate resistor. You should either use a lower value for the resistor R6 and use Schottky diodes or else a trafo with a winding of at least 7 - 7.5 V AC
I wouldn't be that categorical: it is true that a 3A AC winding for a 1.8A DC output is too tight, but it doesn't mean it won't be able to deliver 6.3V DC with 6.3V AC input. The rms current will be somewhat higher than 3A, that's all. If there is a single transformer is dedicated to heating, this could be a problem, because it will overheat. If it is one of a winding of an otherwise underutilized transformer, a few tens of % excess on a single winding is not too much of a problem.
Will Max be able to extract 6.3V DC from his winding, that is the real question. Based on statistical data I have extracted from many real transformers, this will mainly depend on the construction: a toroidal will easily do it, and will probably need some increase in the 0.22Ω resistor, an old style EI tranformer (with superposed windings) will be spot on, but a more modern EI having side by side windings will probably fall too low. This is the result of leakage inductance.
Schottky diodes can improve matters by a few hundreds mV.

That said, the individual variations can be very large, and overwhelm any statistical evaluation completely. The best option is to make a real test with the intended load and rectifiers: it takes very little time and effort and gives a true picture of the actual situation
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