I have a Lundahl 1683 mains transformer, which is usually supposed to put out 250 volts. It also has a 48 volt tap and I'm trying to get 300 (or 298 volts) out of it. The data sheet is: http://lundahl.se/pdfs/datash/1683.pdf
and I'm using Lundahl Transformers - Hybrid power supply having connected the 250, 48, and 250 windings in series. The rectifier went supernova for five seconds and then died quietly. So it seems my math is wrong and did I just put 548V on the tube? How do I get 300V from this thing, and what happened to the good old centre tap??
and I'm using Lundahl Transformers - Hybrid power supply having connected the 250, 48, and 250 windings in series. The rectifier went supernova for five seconds and then died quietly. So it seems my math is wrong and did I just put 548V on the tube? How do I get 300V from this thing, and what happened to the good old centre tap??
Phase up the pair of 250 VAC windings. Connect those windings in parallel and bridge rectify the 250 VAC with 4X 600 PIV Schottky diodes. Use a CLC filter. Start with 1 μF. in the 1st position. With the supply loaded down, add small amounts of capacitance in the 1st position, until the rail voltage is where you want it to be. A 10 H. choke and a large 2nd filter cap. are in order. Slow B+ rise down a tad by installing a negative temperature coefficient (NTC) inrush current limiting thermistor between the bridge rectifier and the PSU filter.
Either tie the 48 VAC winding off or bridge rectify it for a bias supply.
Either tie the 48 VAC winding off or bridge rectify it for a bias supply.
The first time you tried you put 548V into your rectifier. The second time I'm not sure what you did, and maybe you are not sure either. I am concerned by the question you raised in post #1, as it suggests you are out of your depth here. Main HT PSUs are not a good safe place to find yourself out of your depth.
Are you after 300V AC, or 300C DC? They are two quite different things. Eli Duttman has described how to get 300V DC. If this is what you want then follow his advice, and build a PSU which is partway between capacitor input and choke input. If you really want 300V AC then you need to buy a different transformer.
Are you after 300V AC, or 300C DC? They are two quite different things. Eli Duttman has described how to get 300V DC. If this is what you want then follow his advice, and build a PSU which is partway between capacitor input and choke input. If you really want 300V AC then you need to buy a different transformer.
Thanks for the reply DF96. My aim is to get 350VDC. I breadboarded this with 300-0-300 Parmeko iron (which has a centre tap) and everything went swimmingly, and I got about 330 after the rectifier, cap, choke, and cap, which is close enough to the 350 VDC I want to get. I'm baffled by the various windings of the Lundahl, though. I've connected the end of one 250VAC winding to the opposite phase of the other 250VAC winding (as recommended in the datasheet) and I am thinking this will serve as a centre tap -- but I don't know why I'm still getting arcing (perhaps I blew the first cap the first time?).
Another way of getting 300V might be to series a 250 winding and the 48 winding then do the hybrid rectification to get a ground rail? I'll wait to get good advice before trying the valve rectifier again!
Another way of getting 300V might be to series a 250 winding and the 48 winding then do the hybrid rectification to get a ground rail? I'll wait to get good advice before trying the valve rectifier again!
Looks like,
15 connected to 24
22 connected to 17
fullwave bridge off 24 and 17.
or
22 connected to 24 gives center tap power off 15 and 17
for tube with correct value of first cap.
I would forget the 48V to start with and get it working with the 250V windings.
Then see what rectified and smoothed voltage you get.
It seems if you use CT then you cannot use the 48V winding because you will off set the supply on one side this will stuff the rectifier!
Regards
M. Gregg
15 connected to 24
22 connected to 17
fullwave bridge off 24 and 17.
or
22 connected to 24 gives center tap power off 15 and 17
for tube with correct value of first cap.
I would forget the 48V to start with and get it working with the 250V windings.
Then see what rectified and smoothed voltage you get.
It seems if you use CT then you cannot use the 48V winding because you will off set the supply on one side this will stuff the rectifier!
Regards
M. Gregg
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If you rectify with SS you will get 300+
Just for fun here is a link for the RMS Vs DC
AC, DC and Electrical Signals
Your meter will read RMS AC this is not true peak value.
So if you rectify your 250V you will charge to peak. This also depends on current drawn from the circuit. Be carefull do not power your capacitors without circuit connected you may exceed the working voltage due to no current drawn by the circuit!
You will drop more voltage across tube rectifier so B+ will be lower than SS.
Sorry if already know this info.
Regards
M. Gregg
Just for fun here is a link for the RMS Vs DC
AC, DC and Electrical Signals
Your meter will read RMS AC this is not true peak value.
So if you rectify your 250V you will charge to peak. This also depends on current drawn from the circuit. Be carefull do not power your capacitors without circuit connected you may exceed the working voltage due to no current drawn by the circuit!
You will drop more voltage across tube rectifier so B+ will be lower than SS.
Sorry if already know this info.
Regards
M. Gregg
Many thanks for the link, it was very helpful. I figured out the rectifier arcing problem (it was actually very silly of me, really, and nothing to do with the voltages) and am working with the 250-centre_tap-250. B+ voltage is 262 with full load. I've changed the first cap before the choke to a 630V poly, which might be more accommodating of my caprices. Schematic says I can go all the way up to 350V with good tubes, but I think I'll just play music for now 
The great thing about tube rectification with a centre tap is that one can calculate, more or less, what the HT is going to be be. But because I can't leave well enough alone, I will give the hybrid rectification another go when I get around to ordering some Schottky diodes.
The great thing about tube rectification with a centre tap is that one can calculate, more or less, what the HT is going to be be. But because I can't leave well enough alone, I will give the hybrid rectification another go when I get around to ordering some Schottky diodes.
Update to the saga. At 250-0-250 the power's a bit weak and the sound is loose and distorted with the recommended RH power circuit. Following advice from the other members and an email conversation with Per Lundahl I tried a hybrid rectifier scheme, with fast-switching diodes -- tying the cathodes to either side of the 250 V and the anodes together to form a hybrid bridge rectifier.
Mr Lundahl in his email suggested I'd get about 300V on this, but with a full load I got 560VDC going into B+. I tried altering the value of the initial capacitor (47uf) and finally got about 400V with 2uF, but quite a bit of hum. Also experimented with an Obbliggato oil-filled and it sizzled rather like, well, something frying in oil, so that's going into the drawer we all have of 'stuff that might be useful one day'. Finally at 10uF I'm getting 460VDC with tolerable hum.
Any advice on how to keep the voltage down but get rid of the hum. Also wondering if I should ditch the 5U4GB in favour of the specified GZ34... or if there are any other flavours of rectifiers one might recommend? Many thanks in advance.
For reference, the schematic is here: http://www.tubeaudio.8m.com/807/807.html
Mr Lundahl in his email suggested I'd get about 300V on this, but with a full load I got 560VDC going into B+. I tried altering the value of the initial capacitor (47uf) and finally got about 400V with 2uF, but quite a bit of hum. Also experimented with an Obbliggato oil-filled and it sizzled rather like, well, something frying in oil, so that's going into the drawer we all have of 'stuff that might be useful one day'. Finally at 10uF I'm getting 460VDC with tolerable hum.
Any advice on how to keep the voltage down but get rid of the hum. Also wondering if I should ditch the 5U4GB in favour of the specified GZ34... or if there are any other flavours of rectifiers one might recommend? Many thanks in advance.
For reference, the schematic is here: http://www.tubeaudio.8m.com/807/807.html
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Have you tried,
The ordinary simple diode bridge rectifier with the ac in from
15 connected to 24
22 connected to 17
fullwave bridge off 24 and 17.
?
I honestly do not think that the tube rectifier is in your best interests with the level of Voltage /Current you have from this Tx.
Regards
M. Gregg
The ordinary simple diode bridge rectifier with the ac in from
15 connected to 24
22 connected to 17
fullwave bridge off 24 and 17.
?
I honestly do not think that the tube rectifier is in your best interests with the level of Voltage /Current you have from this Tx.
Regards
M. Gregg
Last edited:
Power supply circuts
Hi,
Thats a very clever circuit (2nd link in post 1) A full wave bridge using a double diode valve and 2 sil. diodes. Many advantages,the secondary doesn,t need a tap, only has to be 300v for about 360vdc, the valve slows down the start up phase. I,ve never seen that used anywhere before, very clever design !.
Peter
Hi,
Thats a very clever circuit (2nd link in post 1) A full wave bridge using a double diode valve and 2 sil. diodes. Many advantages,the secondary doesn,t need a tap, only has to be 300v for about 360vdc, the valve slows down the start up phase. I,ve never seen that used anywhere before, very clever design !.
Peter
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