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142.127.22.126
The circuit draws around 48 ma, the PT is rated 58ma in FW CT with a tube rectifier. All dc voltages are within 5% of circuit values. Lousy transformer or what? Any thoughts? FWIW the PT is a Hammond and I have heard they can be a bit sketchy when used near their maximum values.
Follow Ups:
You say after many posts that the transformer is 'shot'. Prior to that you indicate a fuse blew and the transformer was hot and smelt.
How have you confirmed/tested that the transformer is 'shot'?
I have. I measured the resistance across the HV secondary, and then from the CT to each side of the winding. One side measured 12 ohms and the other measured 58 ohms. I have found a replacement transformer and installed it, it is now working as intended, played for 3 hours this evening and got moderately warm. Best of all did nor blow any fuses:)
I didn't follow your post closely enough ... the transformer shorted?
with regards,
I suppose that to be the case as there is a large difference in resistance between either side of the HV winding. If you have some other interpretation of that I am all ears.
thanks for the reply ... I'll look back through the thread too
and I see that trobbins weighed in with a circuit mod fail safe solution
glad you got it up and running again!
with regards,
I would recommend fitting at least a 1N4007 in series with each 5U4 anode, and preferably a CT fuse, as the best means to avoid collatoral PT damage in to the future.
Good advice. How do you size the CT fuse? My best lesson learned from this is to not be cheap and size the PT with lots of current margin:)
That is my take too. By de-rating the transformer you are more likely to have long life, reliability, and quiet operation. And it does not add greatly to the cost, generally.
Take care.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
It can get a bit technical and there is a link below to a design article.
If you can measure the new PT winding resistances, and confirm the operating voltages and currents and circuitry (I can see an on-line schematic but it has an unidentified series resistor) then I can take you through the PSUD2 process as well as the practical selection process.
Cap input or choke input supply?
Cap input. The circuit is the JE Labs simple 46. My arithmetic tells me the circuit should draw about 46 ma :) Turns out that it seems to be drawing about 65ma :( Since the power transformer is rated to 58ma what happened is not surprising. Now I likely need a new transformer and to figure why the circuit is drawing so much more current than it should. Amazing what a good night's sleep can accomplish. Thanks for all the input.
With cap input the current draw shouldn't be more than about 60 % of power transformer rated current. For a 58 mA rated transformer that is 35 mA. With choke input on the other hand you can draw 90 -95 %.
You are certainly correct about the choke input giving you more current. However Hammond specify their power transformers FW CT and capacitor input for their rated current.
I checked the Hammond catalogue and you are right of course. Hammond does spec it's HT transformers for cap input supplies. Never seen that before.
It was more or less standard in the past to spec the DC current assuming a cap-input filter and an appropriates rectifier tube.
That is more of a problem now that many are using solid-state rectifiers. Hammond more recently has been specifying the rms current, along with unloaded voltage and winding DC resistance. I suppose that's because of PSUD - DIY customers can now model their design and calculate the rms, which was much more difficult in the days of graphs and slide rules.
As an example, the old 270AX was specified at 50mA DC; the new one with the new 115/125v primary is specified at 58mA RMS.
A 58mA RMS rated secondary seems low for 50mA DC, CT, C-input tube rectified power supply. For 50mA DC draw I'd expect to see 70mA RMS, or greater.
The JE Labs design quoted would likely have 65-70mA RMS on the B+ secondary, which exceeds the 58mA RMS rating.
I don't like to run transformers near their maximum ratings; I'd be more comfortable with a secondary rated for > =80mA RMS. I also like to de-rate the power transformer's VAC by 50% (or more) for cooler running and perhaps lower noise and longer life.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
The 58mARMS is for the whole winding. For FWCT you would get 1.414 times as much current, i.e. 82mA. It's complicated, and the current version of PSUD has some issues with how it reports current numbers in this configuration.
Thanks Paul.So, a transformer rated for 58mA RMS for the whole winding would handle 82mA configured as FWCT? And PSUDII does not calc the RMS Iac accurately for FWCT?
My advice: de-rate power transformers and design the PS so values can be tweaked to attain close enough voltages. And I'm kinda happy that I'm moving from tube rectifiers to SiC diodes in bridge config. ;-)
Any help much appreciated!
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Edits: 08/30/20 08/30/20
I'm not sure if I was clear; I certainly didn't say what I meant very well. Let me make another attempt:
If you had a fullwave bridge on a transformer rated 58mA RMS, you might reasonably expect to get 35mA DC output from a FW bridge, at a voltage that is 10% greater than the unloaded RMS voltage of the whole winding. With a FWCT you would get 1.414 times as much current, at half that voltage, i.e. 50mA DC - which is in fact the old DC current rating of the transformer under discussion. So those things are consistent. (I think PSUD is reporting 35mA DC now - but don't take my word on that.)
******************
I just recently did a small study of these issues, and got some interesting results. It provides a way to tweak the voltage or current of a power supply without changing the power transformer.
All the above ignores the effect of series resistance. The above "reasonable expectation" occurs when the series resistance, including that of the rectifier diode, the HV winding, any resistance in series with the rectifier, and the scaled primary resistance (i.e. everything before the first capacitor) is about 8% of the DC load resistance. A smaller series resistance gives a little more voltage at a little less current, and a larger series resistance gives a little less voltage at a little more current. The effect is modest, maybe +/-10% over the range of 5% to 15% series R over load R.
Incidentally, this series R over load R is not the same as the AC regulation as normally defined. In normal "regulation" terms the range is 9.5% to 18.4%. I did say it was complicated!
Thanks for the clarification Paul. And for the extra information - interesting and helpful know.
Take care.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
The choice of a transformer also depends on how much heat development is allowed.
A normal transformer can easily handle a temperature rise of 70 degrees Celsius. Only no one would want such a hot transformer.
A good manufacturer can give the maximum temperature rise at maximum load.
Yes that's true, that ripple current can't be denied.
The circuit draws 48 mA, so where does the rest of the current go?Must be something wrong.
http://jelabs.blogspot.com/2017/10/je-labs-simple-46.html
Edits: 08/29/20 08/29/20 08/29/20 08/29/20 08/29/20 08/29/20 08/29/20
I've followed the circuit exactly, and it played music until it didn't:( So I am stumped. If I had used a larger transformer I would not have noticed my mistake. The output transformers are gapped for 30 ma which is probably what they were seeing. Which means that the tube was probably dissipating 7.5 watts. JE seems to think that the 46s are only good for 5.5 watts but I don't believe that and nowhere have I ever found any specifics about what the actual maximum plate dissipation.
No, the circuit tell us that the current for the 46 is 23mA.
Check if you have the same voltages on anode and kathode as in the circuit.
Edits: 08/29/20
Clearly you did not read my previous post. RCA spec sheet says 22ma and 250 plate volts. And all component parts were correct, I always measure resistors before use. Unfortunately with a burned out transformer kind of hard to measure regardless of what I measured before
Which post? Where did you post RCA data?You wrote that you use 'The circuit is the JE Labs simple 46'
So i follow that circuit and the total current is about 48mA, 23mA for each output tube and about 1mA for each driver tube. Plate dissipation is almost 6W , not 7,5W.
Edits: 08/29/20
Did you read the article accompanying the schematic? JE is following the values given on the tube data sheet for the 46 triode connected, which are 22ma and 250 volt on the plate. Check the data sheet. Anyway 1ma more or less is of no great consequence, this is tubes we are discussing. And it still does not explain away my problem.
They say they try to follow as close and they did....And you are right 1mA does't make the differance.But as i say before measure the anode voltage and kathode voltage, if that is correct then you know that this part is ok.
You must have somewhere a huge leakage current because even de electrolytics (if new/ reliable) have not such leakage currents.
Edits: 08/29/20
Well all very moot as the transformer is shot:( Have not the discretionary income at the moment to get a new one as I have been out of work for some time, COVID-19 eh?
Well, it's something to keep in mind when things improve. Hopefully sooner rather than later.
i hope you used this before you started:
http://www.duncanamps.com/psud2/index.html
Have been using it for years...
Did you noticed that a 58mA transformer cann't deliver 48mAdc in this amp?
btw the voltage drop on the output transformer is huge for a transformer.
Rdc more then 600 Ohm
Edits: 08/29/20
Yes, I noticed that, mine have 280 ohms so I need even lower voltage before the output transformer.
I hope you not have the 270ax but a bigger transformer
https://www.hammfg.com/files/parts/pdf/270AX.pdf
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