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I'm trying to puzzle this out so could do with some help. I have a small mains transformer which is specified as "125-0-125 at 50mA".I'm trying to establish the safe current capability with:
1. Full wave rectification (2 phase) with a EZ80 with CLCLC, first cap can be 10uF to 40uF. Voltage off load here is 180v2. Hybrid Graetz bridge, EZ80 plus 2 diodes with choke input. Choke is 15H at 30mA.
I could do with around 200v at 30mA - it's for a 01A preamp so maybe 10-15mA needed.
When the transformer is marked "125-0-125 at 50mA", what current will it actually deliver in these cases?
I've been looking at the Hammond tutorial see here: http://www.hammondmfg.com/pdf/5c007.pdf
So is the "50ma" what Hammond refers to as the "Sec I AC"? if so:
1. Cap input full wave, I DC = Sec I AC = 50mA
2. Hybrid bridge choke input, I DC = .94 Sec I AC = 47mAVoltage out for 1. is .71 of Sec V AC (250v) = 178v, which is exactly what I get off load
Voltage out for 2. is .94 of Sec V AC (250v) = 235v.So if all the above is correct, 235v at 47mA looks better for an 01A preamp than 178v at 50mA. The Hybrid bridge option looks win-win, more volts and the same current capability.
This looks like something for nothing! Is all this correct?
Edits: 09/10/16Follow Ups:
235v @47ma. That transformer is going to run hot. What is dcr of secondary winding? I'm guessing around 125ohms cold. JH
DCR of secondary winding is 332 ohms end to end.
Hybrid Graetz bridge and Choke Input gets you more volts at similar current...?Or does substituting an EZ80 for half of the diode bridge change the result?
Edits: 09/10/16
You stated the transformer was 125-0-125....meaning it has a center tap. Therefore only the last two pics, full wave, NOT the bridge ones apply.
Choke input will get you lower voltage, more current, and better regulation (less difference between peak and average output voltage).
Bottom line is the highest voltage is going to be cap input and even with solid state diodes only about 175 volts.
You stated the transformer was 125-0-125....meaning it has a center tap. Therefore only the last two pics, full wave, NOT the bridge ones apply.> > > > > >
Yes, but if you just treat it as 0-250v and ignore the centre tap, you can then connect it to a bridge, either solid state or I'm suggesting a hybrid bridge with a EZ80 and 2 diodes. Not so?
Try contacting the maker of the transformer. Sometimes you can connect the two secondary windings in series, and do what you want, but at half the current rating.
A center tap transformer is designed to only use one half of its winding at a time, letting the other half cool down each half cycle. The volt-amp rating of a transformer is fixed by wire size. You can't double the voltage out without cutting the current rating by half.
A voltage doubler is likely your best option for what you have....assuming you can get by with 1/2 the amps.
Ah right - so it's a question of wire thickness. So at 250v the current rating would be half of 50mA, so 25mA? That would change things. Is tthat the convention - if there's a centre tap it's assumed you will use it for a full wave bridge?
So in that case a hybrid bridge with choke input would give 225v at 23mA. If that's correct, that's not impossible for an 01A preamp.
The transformer is a very old RS one - Midget Mains. UK sourced.
Clip lead it together and conduct a soak test. JH
Well - for now using full wave rectification with cap input of 40uF. Mains transformer runs cool - no problems. After two chokes the PSU outputs 155v. May or may not be enough for Ale's Gen 2 Gyrator design. Work in progress..
This has been an informative thread.
Hi Andy,
155V is too low for gyrator loaded 01.
I tried (and use) Ale's gyrator with several DHTs (01, 26, 10, 801).
In 01 preamp 105...110V anode voltage requires at least 200-220V B+ voltage for proper working.
I used 220V shunt regulated (Salas SSHV2) power supply.
"Iccs= 4.98mA (cCCS)
B+= 220V (SSHV2)
Ug= -5.73V (22R W22)
R.C. V4 regulator (R1:4R7, Vin:20.5V)"
Thanks for that. So clearly HT needs to be 200v or more, though Ale was of the opinion that the gyrator itself filters well enough not to need another shunt reg. or glow tubes. I'll have to check all this with him.
I don't think anything is overkill in a DHT preamp - I'd use a mesh AZ1 out of choice, and chokes in the filament supply. It's possible to simplify as long as the difference isn't too audible.
Andy, IIRC the transformer secondary was 125-0-125. As I understand things you have center tap isolated and are using the full secondary as one winding into a cap input filter. Don't know what you are using for a rectifier (please elaborate).
I'd like to know what the secondary voltage is (end to end) under load. It should be around 250 volts (AC). I'm guessing it might be a good deal less and that, rather than heat, might be a better indicator of over load. It would be nice if you could input a signal (play real music say) into the preamp when measuring this voltage.
I wired it as full wave with the centre tap to zero volts and CLCLC filter. The boringly obvious. After all the filtering it went down from 178v to 155v. I didn't use the full 250v winding, ignoring the centre tap.
I think I need the whole of the current rating, so what I've done in the past when I'm short of volts is to add another smaller transformer. I would in this instance take a small 24+24v transformer and add 24v on the end of each side of the HT windings. That should take me up to something like 185v final voltage if I needed a bit more. I've only previously done this with a bridge rectifier - I imagine it works with a centre tap as long as you get the phase right. Can anyone confirm?
What am I missing here - the bridge with choke input seems to have the highest "average volts" output in terms of efficiency.
Hello Andy,
Why are you messing around with a 332 Ohm Ohm High Voltage secondary, in audio applications?
WHY waste your time, and your energy??
Jeff
This is a preamp with an 01A tube, which has a 3mA operating point.
What advantages would low DCR transformers and chokes have at 3mA?
This isn't an amplifier.
What advantages did I find this week, when I went from a single 12 AWG wire feeding a 12AX7 at 0.6 mA, of current, the Driver tube in a two stage SET 45 amp,changed to .....a DOUBLE 12 AWG wire, feeding the same 0.06 mA. load, over a UNDER TWO INCH LENGTH.
NIGHT and DAY, a 20% improvement in amp's listenability !! A 70 percent improvement in color, tone, timbre. A 25% improvement in the low end and maybe 10% better dynamics.
In the " after " photo, shown first, you will see two yellow 12 AWG wires connecting to the top od a RF choke, hidden in heatshrink ( blue and red shrink). These two yellow wires are coming off a WIMA DC LINK 50 uF cap, fed by two ( longer yellow wires, also 12 AWG ).In the lower " before" photo, you will see bare 12 AWG wire, feeding two Blue heatshrunk RF chokes, in the same amp, a SINGLE bare 12 AWG wire span, short.
The RF chokes, hidden in heat shrink, are LOW in DCR , uHY rated, BTW, under 1 Ohm, over 1 A. current rated.
So you see, Andy, you and almost everyone else in AUDIO, are unaware of the consequences. A preamp should have the power transformer of an amp, and a GOOD low DCR B+ filter. WE have a LONG way to go in audio and tube audio understanding, world-wide. The TRUTH needs to be known and discussed .
This I write to people with open minds, open ears, who LISTEN and trust their DIY work !!
Jeff Medwin
Edits: 09/11/16 09/11/16
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