Hi everyone, longtime, I mean looooong time lurker, first time caller here.
I'm looking to put together a supro 1624 circuit and need help figuring out what I need for a transformer.
B+ voltage shows 350 on the schematic.
It's running a 5y3, 2-6973's and 3-12ax7's.
I've found a few, but I'd like to know how many milliamps on the secondaries I'm going to need(or general area to look at. Here's specs for a Hammond I saw:
The unit features a 115 VAC, 60 Hz primary and two secondary filament windings, one 5.0V for tube rectifiers, plus a 6.3V for heater filaments. Leads are a minimum of 6" in length. Class A insulation (105ºC). These transformers are conservatively rated and CSA certified. VA: 182, Secondary RMS: 350-0-350, DC mA: 175, Fil. #1 Rectifier: 5.0V/3A, Fil. #2 Heater: 6.3V/5.0A.
Thanks again for any help. Ed
Need milliamp rating for xformer
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Need milliamp rating for xformer
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Re: Need milliamp rating for xformer
At 120mA you would reach the limit of the 5Y3. Probably 80mA to 100mA is adequate for this amp. On the 5V and 6.3V you only need 2A each with the stock tubes. The 6.3V needs to be 2.5A or higher if you want to change the power tubes to EL84's.
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David Root
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Re: Need milliamp rating for xformer
Adding to LoudTHUD's good advice, I would say that, in the Hammond 200 series, the 276X is about the right voltage (320-0-320). It has 3A at 5V and 5A at 6.3V, which is overkill in this case, but they're not expensive, and it's rated 150mA DC which is fine too.
If you want a laydown type (Z-mount), I would go for the 273CZ.
The PT you reference is too high voltage at 350-0-350. Save some money.
How did I figure out 320-0-320? A surprisingly simple but also surprisingly accurate SWAG-type calculation which I picked up somewhere, as follows:-
1) Take the desired B+, in this case 352V
2) Add the tube rectifier voltage drop at approx. idle conditions, in this case about 95 mA draw, for a 5Y3GT about 50V. 352 +50=402
3) Divide by 0.9, which gives 447 (This takes care of the average tubes loading).
4) Divide by 0.7, which gives 638. This is the AC CT voltage.
5) Divide by 2, which gives 319V
6) Select nearest available, which is 320-0-320 VAC.
I checked this method on the last amp I built and it was within 3% on the calculated VAC.
Another good one that has the advantage of an internal screen between primary and secondary windings that you ground to reduce hum, plus a copper flux band too, would be the Allen Amps PT25. I have used this PT and it is dead quiet and a very good buy, quite superior to the Hammond IMHO. This is a laydown style, but if you used the Mojo knockoff Princeton reverb chassis it fits perfectly. Then all you need is a BFPR cabinet with the baffle modified to accept a 12" speaker, and an Eminence Red Fang speaker and you have one hell of a great (and very loud!) amp.
If you want a laydown type (Z-mount), I would go for the 273CZ.
The PT you reference is too high voltage at 350-0-350. Save some money.
How did I figure out 320-0-320? A surprisingly simple but also surprisingly accurate SWAG-type calculation which I picked up somewhere, as follows:-
1) Take the desired B+, in this case 352V
2) Add the tube rectifier voltage drop at approx. idle conditions, in this case about 95 mA draw, for a 5Y3GT about 50V. 352 +50=402
3) Divide by 0.9, which gives 447 (This takes care of the average tubes loading).
4) Divide by 0.7, which gives 638. This is the AC CT voltage.
5) Divide by 2, which gives 319V
6) Select nearest available, which is 320-0-320 VAC.
I checked this method on the last amp I built and it was within 3% on the calculated VAC.
Another good one that has the advantage of an internal screen between primary and secondary windings that you ground to reduce hum, plus a copper flux band too, would be the Allen Amps PT25. I have used this PT and it is dead quiet and a very good buy, quite superior to the Hammond IMHO. This is a laydown style, but if you used the Mojo knockoff Princeton reverb chassis it fits perfectly. Then all you need is a BFPR cabinet with the baffle modified to accept a 12" speaker, and an Eminence Red Fang speaker and you have one hell of a great (and very loud!) amp.
voltage
I found on the magnatone-valco users group that they recommend 250-0-250, or 275-0-275 @ 90-100ma, 5.0v at least 2.0A, and 6.3 1.5-2.0A. This is for a Supreme that runs 2-12ax7's, 2-6973's and a 5y3.
Seems a little weak? These old 6973's that I've saved for the project are a pain to find. Just don't want to burn them up
Seems a little weak? These old 6973's that I've saved for the project are a pain to find. Just don't want to burn them up
Re: Need milliamp rating for xformer
Although a bit simplistic, lookup up the power tube and see what it typically draws in plate current + screen current. Allow another 2-3mA per preamp tube. Add it all up. That is your minimum mA rating. You will be better served by getting more mA. Single ended amps are very inefficient, so you need to derate by a factor of 3 or 4. Push pull amps are much more efficient.
For example, Ia for a pair of EL84's is 92. Add maybe 10mA for screens, and 6mA for 3*12AX7 (guessing on the 12AX7 you should check). Something in the 120mA range should work fine.
The other rating you should consider is the VA rating. This is volts x amps. It is a measure of how much demand the primary winding can manage and includes the demands by all secondaries taken together. For this you look at the HT winding + any filament windings. If the demand is too high, you will either melt the PT, experience excessive sag, or both.
For example, Ia for a pair of EL84's is 92. Add maybe 10mA for screens, and 6mA for 3*12AX7 (guessing on the 12AX7 you should check). Something in the 120mA range should work fine.
The other rating you should consider is the VA rating. This is volts x amps. It is a measure of how much demand the primary winding can manage and includes the demands by all secondaries taken together. For this you look at the HT winding + any filament windings. If the demand is too high, you will either melt the PT, experience excessive sag, or both.
Re: Need milliamp rating for xformer
most 18 watt marshall trannys are 150ma rated,and run the same amount of tubes etc.. I think some have a 5v winding too,or you could use the 6CA4 with the 6v winding. Also should put you in the range of voltage your looking for.
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CaseyJones
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Re: Need milliamp rating for xformer
6973s are rugged little bastards, they'll be happy with around 400 volts on the plates if you really want to get the most out of them.
The super duper cheap-o transformer for budget Supro clones would be the Weber VST W021530. It's tapped 270-0-270 and 340-0-340 plus 5V/3A, etc. On the low voltage taps you'll end up around 310-320 volts on your plates with all the current your application will ever need. If you want the little sumbitch to stand up and pay attention use the high voltage taps for voltage up in the over-360 range. You can fine tune your voltage by swapping to a 5AR4, it's a better tube than that junker 5Y3 anyway.
The super duper cheap-o transformer for budget Supro clones would be the Weber VST W021530. It's tapped 270-0-270 and 340-0-340 plus 5V/3A, etc. On the low voltage taps you'll end up around 310-320 volts on your plates with all the current your application will ever need. If you want the little sumbitch to stand up and pay attention use the high voltage taps for voltage up in the over-360 range. You can fine tune your voltage by swapping to a 5AR4, it's a better tube than that junker 5Y3 anyway.