Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: goldtop1 on April 17, 2012, 07:08:19 am
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Suppose I have a schematic with known PT voltage specs I.E. 200-0-200@ 200ma. The schematic has no reference for the OT type or choke type and size. How would a amp hobbyist like myself choose the correct transformer and choke value? I really couldn't find any info on a thread about this. Is there a magic formula?You guys are brilliant with your amp designs and I have built a few of your designs with great success! I am just wondering if this is a simple solution or do I need to be an E.E. to figure it out! :help:
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many factors will influence the choice of the OT : will the amp be a SE, PP ? What kind of tubes ? What kinda current supply by the PT and drawn by the amp ? Tube or SS RECTO ?
Your pT is 200-0-200 @ 200 mA, let's say it is SS rectified: you'll get approx 280V with no load. Find an amp with that kind of voltage and see if it suits your needs.
Colas
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Colas I don't think that answered the question. For example, how would I choose the components if the amp is push pull uses EL84 tubes and is tube rectified? Where does one find info about this topic. I have seen several amp schematics that list the power transformer with voltages and specs, but the OT and chokes info are not listed on the schematic. So what does one do? What is the math? I don't want to design an amp, I want to use a proven design that works. I just need to fill in a few blanks. :help:
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Suppose I have a schematic with known PT voltage specs I.E. 200-0-200@ 200ma. The schematic has no reference for the OT type or choke type and size. How would a amp hobbyist like myself choose the correct transformer and choke value?
Not enough info. If you actually have such a schematic, please show it.
That PT could deliver 282VDC unloaded with a conventional rectifier, or it could deliver 564VDC unloaded with a full wave bridge.
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Here is a amp with 200-0-200 @ 200ma with no reference to OT or choke options. If the attachment doesn't come up the amp is the SH-11 in the schematics file.
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The output tubes and circuit set the criteria for the OT. In this case, push/pull EL84s and they would like to see a 15 to 20 watt P/P OT with a primary Z of about 8K.
The choke has to handle current for only the little tubes and the screens of the EL84s, so a small low current choke would be fine. More Henries would result in a smoother B+ rail.
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One other issue...
220v * 1.414 = ~311v
12w / 311v = ~0.039A per tube
0.039A * 2 = 0.078A, or 78mA
That's way shy of your 200mA available. It seems like the power transformer would be very under-utilized, or maybe better suited to a 4-EL84 output stage.
Craig's schematic doesn't specify the current rating of the PT. Or maybe you have a 200v @ 200mA PT. A double-sized output stage, with about 4k primary would be good. Or, you could use only 2-EL84's with an 8k primary, simply because you have an available PT and don't want to spend the money on something else.