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PostPosted: 26 Nov 2012, 20:23 
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mwhouston wrote:
I know for bigger rec. tubes e.g. 5U4, 20uf is considered max. A couple of 100ohm Rs from the tranni sec. to the plate connections help stacks. It's just the first cap which is critical.


Yes I imagine some 'phool' using a small cap, then a couple of very low value resistors, and then a great big cap. Oh the poor rectifier. You need some resistance after that first cap into the first RC filter to soften the inrush :eek:

It is far better to use several RC stages, than less stages with HUGE capacitors. Each stage filtering the supply that bit more until it's all smooth and quiet. It's also a lot cheaper!!

I did a lot of reading on filtering, and for large currents and power supplies with big swings in current (large power amps) a choke input is much better, but for small relatively stable current demands, it gains you little. I did test chokes in the B+ while designing, there was no change performance, so I went the simple RC route.

Here's an extract from one of my text books:

As load increases the volts drop across a resistor increases. This in the old days meant that you'd need to increase the size and voltage of your transformer, which was costly. Also if you have a load that changes a great deal, say swinging from 100-200ma then the large variations in voltage is undesirable.

A choke has a high reactance to ac (effective resistance). e.g. A 10H choke will have say 6300 ohms reactance to a 100hz ripple, 12 times more reactance than a 500ohm resistor, but has less than half the DC resistance, say 200ohms. Therefore, when selecting a choke the lower the DC resistance the better it seems.


As the OTL HP amp draws little current (52ma), which is very stable hardly fluctuating even when driven full volume as the tiny speakers exert no load, a choke would just be a waste of money/space, and a waste of a good choke. :D

The other thing on my heater smoothing is that even 5mv is very audible in a HP amp, and having to avoid the volts drop meant using very small resistors, leaving all the smoothing to the caps, hence the LARGE values. In a power amp 5mv would be inaudible, and I guess you'd get away with quite a bit more.

Also I am very fussy! :D


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PostPosted: 26 Nov 2012, 20:52 
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Actually it's more than just in rush. In rush current is mostly a concern where someone has mistakenly included a standby switch in their design (Something that is almost never required in audio equipment). The more pressing concern is steady state peak rectifier current and conduction angle.

Some rectifiers, like the 5Y3, have relatively low peak plate current limits (440mA). This means that to use them in higher draw circuits (i.e. greater than about 90mA) one needs to keep the first capacitor fairly small. This lengthens the conduction angle and lowers the peak plate current. A properly sized RC or LC section following the first capacitor will have little effect on this so long as the 20:1 reactance rule is followed (i.e. the series resistance or reactance at the primary ripple frequency is at least 20 times the reactance of the first capacitor). This also has the effect of lowering I2R losses in the transformer secondary leading to cooler and longer lasting transformers.

I will take one small exception to your assertion that a choke would be a waste of money is such a small power supply. One 330Ω/100µf section in the filter provides ≈28.3dBv of ripple reduction. By replacing one resistor with a small choke, say 2H, would provide ≈41dBv of ripple reduction. And two such stages would give 82dBv of ripple reduction. Of course the first capacitor would need to be raised to about 18µf but that is not a problem at such a low current level. I would consider that a good trade for a few tens of dollars in small chokes. IMHO.

Checkout this link http://diyaudioprojects.com/Technical/Tube-Power-Supplies/ for more information on power supply design.

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PostPosted: 26 Nov 2012, 21:15 
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Suncalc wrote:
I will take one small exception to your assertion that a choke would be a waste of money is such a small power supply. One 330Ω/100µf section in the filter provides ≈28.3dBv of ripple reduction. By replacing one resistor with a small choke, say 2H, would provide ≈41dBv of ripple reduction. And two such stages would give 82dBv of ripple reduction. Of course the first capacitor would need to be raised to about 18µf but that is not a problem at such a low current level. I would consider that a good trade for a few tens of dollars in small chokes. IMHO.

Checkout this link http://diyaudioprojects.com/Technical/Tube-Power-Supplies/ for more information on power supply design.


Good point well made :) As I mentioned my knowledge reached it limits to date, but I keep learning.

The only observation I bring in my defense, is that almost without exception, the commercial HP amps use multiple stages of RC filtering. Of course that does not mean it's best practice 8-)

And while the choke would, in your example, give greater ripple rejection, it was not needed to achieve the end result. Granted a choke has a greater reactance to AC, and therefore makes smoothing easier when voltage is already low, due to the lower DC resistance, and when high fluctuations in current are present as it avoids the volts drop problems.

Lifting the cap to 18uf concerns me as a lot of the tubes call for 4uf to 10uf, but you seem to suggest this is OK if the overall current draw is low. i.e. You can exceed the data sheet values?

I also previously printed out that tutorial, and followed it several times, before considering ending my life over the hideous mathematics involved. :eek:
I then turned to PSUD2 http://www.duncanamps.com/psud2/index.html to test my own design decisions, followed by bench testing the circuits with dummy loads (high wattage resistors). This approach was far less stressful :D


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PostPosted: 26 Nov 2012, 22:06 
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Nordic wrote:
I also previously printed out that tutorial, and followed it several times, before considering ending my life over the hideous mathematics involved.
First, my apologies concerning the article. Mathematics comes very naturally to me and I might have gotten a little carried away when I wrote the article. I do have a spreadsheet that follows that tutorial which makes the math much easier; for I too grew tired of forever repeating the "then add A to B" routine. I'll be happy to send you the spread sheet if you so desire.

Nordic wrote:
Lifting the cap to 18uf concerns me as a lot of the tubes call for 4uf to 10uf, but you seem to suggest this is OK if the overall current draw is low. i.e. You can exceed the data sheet values?
This statement confuses me somewhat. With the exception of very small capacity rectifiers (6X4 and 6X5) I could find no examples of rectifier data sheets recommending capacitors less than 20µf. In any case, those "typical operation" cases printed in the data sheets are not limiting conditions; just examples. The real limiting conditions are the steady state peak plate current per plate and the transient peak plate current per plate. So long as you don't violate these limits, you can use any size capacitor you need to.

The reason for the very small capacitors in the typical operations sections of these two data sheets (6X4 and 6X5) is because both examples are for a DC output current of 70mA (the limit for these tubes). The peak plate current limits are 245mA and 210mA respectively for peak-to-DC ratios of 3.5 and 3 respectively. These are very small ratios requiring long conduction angles and, hence, small first capacitors. Personally, I would never uses these two tubes to deliver any more than about 50mA DC. Under these conditions you can get away with a larger first capacitor for lower ripple into the filter.

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PostPosted: 26 Nov 2012, 22:32 
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Suncalc wrote:
I'll be happy to send you the spread sheet if you so desire.


Oh yes please :) PM on the way.

Suncalc wrote:
I could find no examples of rectifier data sheets recommending capacitors less than 20µf.


I ran to grab the data sheet I'd been working from, just to make sure I was not going to embarrass myself in a pants down sort of way, and the RCA 5V4-GA data sheet issue 7-58 states 10uf. It does go on to qualify that by saying, "Higher vales of capacitance may be used, but the effective plate supply impedance should be decreased to prevent exceeding the maximum rating for peak plate current". Now at this point, with us mere mortals who's math is rudimentary, we stay within the advised tolerances :angel:

Image

I will try your process again, this time with the spreadsheet. I have another project in the tentative design stage, so I'll give it a go :)


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PostPosted: 26 Nov 2012, 23:13 
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Nodric wrote:
mwhouston wrote:
@Nodric: I would have gone for a higher voltage tranni and used bigger resistors in the CRC cct. something like 100ohm would be better.


I agree and would have liked more volts, but I had what I had. i.e. The tranni was NOS and save be $80 :) It also had lots of amps :)

The benefit of bigger resistors I guess would be improved smoothing. I'm not an expert on RC filter design so I could be wrong :D

Yes better smoothing but unless you are using directly heated triode I wouldn't worry.

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PostPosted: 26 Nov 2012, 23:17 
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Hmmmm... This is interesting. I don't have that issue of the RCA data sheet. But the RCA Sept. 2, 1941 data sheet for the 5V4-G contains the note "When a filter-input condenser larger than 40µf is used, it may be necessary to use more plate-supply impedance than the minimum value shown to limit the peak plate current to the rated value" (emphasis added). And the Tung-Sol Feb 15, 1940 data sheet contains a similar note. The April 1953 Sylvania sheet (again for the 5V4-G) is even more confusing with no capacitor listed in the typical operation section but a graph added showing expected output voltage vs load current for 8µf and 4µf capacitors. The Bimar 5V4G data sheet (date unknown) specifies a maximum reservoir condenser of 32µf. Finally, the GE data sheet for the 5V4-GA has a 10µf under the typical operation section but no note. And the later RCA sheet you provided for the 5V4-GA says 10µf, with a note.

However, all the sheets are consistent in that they specify the maximum peak plate current as 525mA. Obviously, the condenser values provided all depend on the assumptions going into the example power supply design for each manufacturer. However, the actual driving limit on the tubes is consistent.

I would stick by my recommendation here to abide by the per plate peak current limits and pick capacitors appropriately. Both the tutorial provided or the Duncan Amp tool should allow you to do this fairly simply (well, maybe not quite simply, but without a PhD in circuit theory ;) ).

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PostPosted: 26 Nov 2012, 23:28 
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Suncalc wrote:
I would stick by my recommendation here to abide by the per plate peak current limits and pick capacitors appropriately. Both the tutorial provided or the Duncan Amp tool should allow you to do this fairly simply (well, maybe not quite simply, but without a PhD in circuit theory ;) ).


So, is there a simple formula to calculate the 'optimum' capacitor while being aware of the peak plate currents? OR does it involve the PhD :D

PM sent for the spreadsheet :up:


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PostPosted: 27 Nov 2012, 00:51 
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Simple formulae for working with tubes..... that would be nice. ;)

Matt; I, too, would be interested in that spreadsheet.. PM on the way.. :)

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PostPosted: 27 Nov 2012, 14:27 
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My power trans is actually home wound from an old ups trans which was about 300W.

The windings are 2x 6.3v /150v-0-150v

It heats up to about 60 Celsius in summer but it has been giving me reliable power for about 6 months now :)

Cant argue with that when it costs nothing.

For rectification I am using 2x el91 on each channel.

The setup can run an half a 6as7 and single el91 per channel for high impedance phones.

I use 32-60ohm phones and paralleled 6as7 gives me a better sound mostly on the low ranges.

Over all the sound is breath taking with an unbelievable depth.


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