Hello all:
I am making an RH84 version2. I have a question about my power supply.
I am attempting to salvage a used transformer for the B+. This transformer has multiple windings. The B+ winding is 630vac with no centre tap. My target voltage is +- 315vdc. I have read from the hammond site I should expect ~ 45% of AC with lots of ripple from a halfwave rectifer. So I thought this would work and have room for smoothing.
I am trying to use a beautiful 5U4gb I have in a nice ST bottle as the rectifier. For some reason I get 500+ volts DC.
I have one leg of the transformer going to both pins 4 and 6 of the rectifer. The other leg is (-). The heater transformer is connected to pins 2/8 and floating with CT not connected. B+ is drawn from pin2 into 35uF>13ohm>330uF with a 220kohm bleeder. B+ went up to +- 530vdc before I shut it down, I did not wait for it to stabilize as I am using 400v caps. Next I put a pair of 5408 diodes between the transformer and the rectifier thinking it would help, same result?
I am perplexed with what to do? or what I did wrong? Is the problem with using the 5u4 as it is directly heated? Any thought would be helpful.
Note:
I am pretty new at this so try to be gentle with your comments
I have already completed the chassis work. (I got a bit ahead of myself with that one. It looks really nice by the way.)
I am hopeful this can be sorted out with a little help.
Thanks in advance
I am making an RH84 version2. I have a question about my power supply.
I am attempting to salvage a used transformer for the B+. This transformer has multiple windings. The B+ winding is 630vac with no centre tap. My target voltage is +- 315vdc. I have read from the hammond site I should expect ~ 45% of AC with lots of ripple from a halfwave rectifer. So I thought this would work and have room for smoothing.
I am trying to use a beautiful 5U4gb I have in a nice ST bottle as the rectifier. For some reason I get 500+ volts DC.
I have one leg of the transformer going to both pins 4 and 6 of the rectifer. The other leg is (-). The heater transformer is connected to pins 2/8 and floating with CT not connected. B+ is drawn from pin2 into 35uF>13ohm>330uF with a 220kohm bleeder. B+ went up to +- 530vdc before I shut it down, I did not wait for it to stabilize as I am using 400v caps. Next I put a pair of 5408 diodes between the transformer and the rectifier thinking it would help, same result?
I am perplexed with what to do? or what I did wrong? Is the problem with using the 5u4 as it is directly heated? Any thought would be helpful.
Note:
I am pretty new at this so try to be gentle with your comments
I have already completed the chassis work. (I got a bit ahead of myself with that one. It looks really nice by the way.)
I am hopeful this can be sorted out with a little help.
Thanks in advance
I assume this is where you got your idea from: Design Guide For Rectifier Use - Hammond
But the 0.45 x Vac is only valid when the load is pure resistive (and not too low in value). As soon as you add a capacitor for smoothing, the voltage will be much higher. On the Hammond site it says 0.9 x Vac (as a rule of thumb; the actual voltage depends on the resistance of the load, and, to a lesser degree, on the value of the capacitor used for smoothing). So you measuring 500 Vdc is in line with this.
But the 0.45 x Vac is only valid when the load is pure resistive (and not too low in value). As soon as you add a capacitor for smoothing, the voltage will be much higher. On the Hammond site it says 0.9 x Vac (as a rule of thumb; the actual voltage depends on the resistance of the load, and, to a lesser degree, on the value of the capacitor used for smoothing). So you measuring 500 Vdc is in line with this.
A half-wave rectifier produces a waveform with a peak voltage of 1.414 times the rms value. A capacitor-input filter with a large enough capacitance will therefor produce a voltage equal to the peak value.
The average over a full cycle is 0.45 times, so a choke-input filter would give the right voltage, but the half-wave rectifier draws a net DC current, which will saturate the transformer, reducing its impedance, leading to overheating, transformer damage, and blown fuses. Not recommended!
You need a center-tapped transformer for the standard tube rectifier circuit. Usually the data sheet for a rectifier will have a graph showing the DC voltage given the transformer voltage and load current, for cap-input and choke-input filters.
The average over a full cycle is 0.45 times, so a choke-input filter would give the right voltage, but the half-wave rectifier draws a net DC current, which will saturate the transformer, reducing its impedance, leading to overheating, transformer damage, and blown fuses. Not recommended!
You need a center-tapped transformer for the standard tube rectifier circuit. Usually the data sheet for a rectifier will have a graph showing the DC voltage given the transformer voltage and load current, for cap-input and choke-input filters.
Okay thank you for your replys:
I read the guide wrong and should not have expected a 45% B+. A capacitive load leads to ~ 90% Vac
As Kwadjo pointed out I got about 90% of VAC.
I did try the inductor first and ended up at the same point.
The Hammond guide suggest I should get 45% with a resistive load, but what value of resistance and should I put the resistor before or after the inductor. I do not know how to calculate the resistor? Is it simply ohms law or are there other considerations?
I read the guide wrong and should not have expected a 45% B+. A capacitive load leads to ~ 90% Vac
As Kwadjo pointed out I got about 90% of VAC.
I did try the inductor first and ended up at the same point.
The Hammond guide suggest I should get 45% with a resistive load, but what value of resistance and should I put the resistor before or after the inductor. I do not know how to calculate the resistor? Is it simply ohms law or are there other considerations?
Get Duncan Amps PSUD2 software for free and you can try every type of rectifier circuit you will ever need. You just plug in the type of rectifier and the values of cap and whatever. Very good tool.
Thanks for the help I ran a few simulations.
It looks like I am going to make a lot of heat with a resistive load.
Maybe as Paul suggested an new transformer is a better option?
It looks like I am going to make a lot of heat with a resistive load.
Maybe as Paul suggested an new transformer is a better option?
yah I know, I was just trying to use up a closet full of stuff that seems to getting bigger rather than smaller.
As I have been told not every idea is a good idea!!
Here goes another $160.
Oh well it will look good and sound good so it might as well be expensive I guess? Seems like a lot for a transformer
Thanks for the help
As I have been told not every idea is a good idea!!
Here goes another $160.
Oh well it will look good and sound good so it might as well be expensive I guess? Seems like a lot for a transformer
Thanks for the help
Get an Antek inc. toroidal transforme. They are good and less experience than most. Lots of different voltages and currents too. If you need a 5v for the rectifier just put a resistor on one of the 6.3v windings.
Thanks.......but,
I already made the chassis and the options are limited to top mounted bell ended EI units. So it will be a Hammond 273bx methinks.
I already made the chassis and the options are limited to top mounted bell ended EI units. So it will be a Hammond 273bx methinks.
Is it a dual primary transformer? Feeding a 230V primary with 115V would give you half the voltage ...
Do you have 115V in Ca?
Jan
Do you have 115V in Ca?
Jan
I think that's going to give you too much B+. 272FX looks better. Did you look at the blog page? There are some PSU examples. http://rh-amps.blogspot.com/2013/02/rh84-amplifier-revision-2_26.html
Here's a screen shot of a PSU sim using a 300-0-300 trans. With an RC section up front, you can get bang on 315V. Without the power resistor, B+ is ~335.
jeff
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Thanks Jeff:
I changed the diagram a bit. The DCR of the transformer is listed as 102R I added a 100R power resistor and used a 120ma CC as the load and changed to caps to match what I have on hand.
Seems pretty good to me thanks for the help.
I will order the transformer tomorrow.
I am still going to play with the other design while I wait for the transformer.
Lots of fun so far.
Thanks again
I changed the diagram a bit. The DCR of the transformer is listed as 102R I added a 100R power resistor and used a 120ma CC as the load and changed to caps to match what I have on hand.
Seems pretty good to me thanks for the help.
I will order the transformer tomorrow.
I am still going to play with the other design while I wait for the transformer.
Lots of fun so far.
Thanks again
I'm in New Zealand so we have 240V. I used the 370FX and it did not get warm. The 270EX will be big enough imho. To give you an idea: I'm using the 372FX at 155mA. The voltage and current rating that Hammond gives you is after rectification (taking into account a tube rectifier - voltage is higher with solid state). You can find older Hammond data from around the turn of the century and that is even more conservative than the current ratings, e.g. what rated today at 173mA was in 2003 150mA. (370FX).
FWIW: A couple of years ago I ordered a transformer expecting to see 173mA on the label yet it mentioned 150mA. I questioned Hammond on this and was told that the rating in 2003 was too conservative and that I recieved old stock.
Be carefull with the size of the first capacitor, you do not want to have that bigger than what is recommended for the tube. And with solid state recitification you do not want to go much higher either as it will put higher peak current on the transformer heating it up unnecessary.
Re heating: Measure the HT resistance cold and meaure it after about 4 hours, then you can work out how hot the windings get. The Hammond is rated for 105C, take away 10C for hot spotting and another 10C for some tolerance.
Formula attached. Hammond data 2003 attached.
FWIW: A couple of years ago I ordered a transformer expecting to see 173mA on the label yet it mentioned 150mA. I questioned Hammond on this and was told that the rating in 2003 was too conservative and that I recieved old stock.
Be carefull with the size of the first capacitor, you do not want to have that bigger than what is recommended for the tube. And with solid state recitification you do not want to go much higher either as it will put higher peak current on the transformer heating it up unnecessary.
Re heating: Measure the HT resistance cold and meaure it after about 4 hours, then you can work out how hot the windings get. The Hammond is rated for 105C, take away 10C for hot spotting and another 10C for some tolerance.
Formula attached. Hammond data 2003 attached.
