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Old 14th July 2010, 02:22 AM   #1
Cassiel is offline Cassiel  Libya
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Default Using SE OT for PP.

Old German OT taken from tube radio. Primary is 0- 0,8K -1.6K -3,2K. Secondary is 3 Ohms. I'm using the 3.2K primary, secondary load is an 8 Ohms CTS alnico speaker. Build a 5W SE guitar amp with it but sounds bad. Since I don't like it as SE OT I thought of wiring it as PP using the 1.6K as center tap. Maybe in a PP will sound OK. I don't know how much watts can handle but its size tells me it can do 10 watts. Is it worth trying this?
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Old 14th July 2010, 02:49 AM   #2
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Only one way to find out for sure.

Keep in mind that 3K2 (P-2-P) means that each phase will see 1K6 per phase if running Class A, and 800R if in Class AB. That seems to be a pretty steep loadline either way.

Also, since this was intended as an SE OPT, they probably didn't take any special care with ensuring that the capacitance per phase was as balanced as a proper PP OPT design would have.

You might be better off putting the SE OPT on E-bay and using the proceeds to get a proper OPT.
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Old 14th July 2010, 04:37 AM   #3
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8K is probably closer to the center.
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Old 14th July 2010, 11:20 AM   #4
Cassiel is offline Cassiel  Libya
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Originally Posted by Miles Prower View Post
Only one way to find out for sure.


You might be better off putting the SE OPT on E-bay and using the proceeds to get a proper OPT.
Yeah, if I don't try it I will never know. Thing is, I have to do a lot of chassis work to find out. I'm not selling this, don't want anyone wasting their time and money. I have a proper OPT but I'm just using junk box parts to build this amp as a present to a friend. Saving better parts for myself.

I think Kenpenter is right, the reflected impedance will be 8K (4K per phase) with an 8 Ohms speaker. Good enough for 6V6's. Another choice is to use it as a choke and built a guitar preamp for his SS guitar amp. Just wanted to know if anyone here had any experience wiring a SE OT for PP duties. Thanks for the replies.
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Old 14th July 2010, 03:06 PM   #5
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Even if it fails to completely balance, its gapped to take it.
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Old 14th July 2010, 03:38 PM   #6
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If the 800 ohm tap (0.8k) is halfway or close enough, and you connected a 3 ohm speaker to the 3 ohm output, you would have 1.6K anode load per side in class A, 800 ohms in class AB per Miles post.

But you are connecting an 8 ohm speaker to the 3 ohm tap so multiply that by 8/3, so about 4K3 ohms anode load per side in class A and 2K1 in class AB.

Sound like class A is the way to go. Should be interesting to hear about the result.

I suspect the leakage inductance would be too high to run it in AB anyway without a lot of nasty switching glitches.
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Old 14th July 2010, 03:42 PM   #7
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Yeah, if I don't try it I will never know. Thing is, I have to do a lot of chassis work to find out.

No, use "mini grabbers" and test leads. You don't cut metal until you are sure you want the transformer.

Also check that what you think is the center tap is in fact in the center. Simply measure the DC resistance of each half. This is a quick check The next level of test is to put a small AC voltage on the secondary and measure the voltages on the primary (watch out they will be very high) and see if the voltages match. It is hard to measure impedance but voltage ratio is easy to do. If the voltages on the two halves of the primary match when you put (say) 6 VAC on the speaker terminals then there is a good chance the OT will work for you. Double check the impedance ratio by quaring the voltage ratios How it will sound is a different mater. You will have to try it
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Old 14th July 2010, 04:53 PM   #8
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No, use "mini grabbers" and test leads. You don't cut metal until you are sure you want the transformer.

Also check that what you think is the center tap is in fact in the center. Simply measure the DC resistance of each half. This is a quick check The next level of test is to put a small AC voltage on the secondary and measure the voltages on the primary (watch out they will be very high) and see if the voltages match. It is hard to measure impedance but voltage ratio is easy to do. If the voltages on the two halves of the primary match when you put (say) 6 VAC on the speaker terminals then there is a good chance the OT will work for you. Double check the impedance ratio by quaring the voltage ratios How it will sound is a different mater. You will have to try it
Depending on the size of this transformer you may want to place a resistor in series with the 6VAC source to limit the current to a safe value. Something like 22 ohms ought to be reasonable for this purpose. I have heard of more than one small OPT burning up this way when tested without the resistor due to core saturation when excessive power was applied to the secondary at mains frequency..
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Old 14th July 2010, 08:24 PM   #9
ChrisA is offline ChrisA  United States
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Originally Posted by kevinkr View Post
Depending on the size of this transformer you may want to place a resistor in series with the 6VAC source to limit the current to a safe value. Something like 22 ohms ought to be reasonable for this purpose. I have heard of more than one small OPT burning up this way when tested without the resistor due to core saturation when excessive power was applied to the secondary at mains frequency..

Yes. Correct. I forgot to say that because I use those 1/2 amp plug in power supplies for the voltage source. These have so much series resistance built in they can't supply much current. But if you happened to use a 12 amp heater transformer to test a small OPT you might have a melt down.

So just find any old junk wall wort, the smaller and cheaper the better and crack it open and remove the rectifier and you will ave a safe source of AC for all kinds of testing. Many of these cheap suplies already have fuses inside. Most will pop open if you place them in a vice diagionally

This is also a good way to test a new amp's AC wiring durring a build. Put 6VAC from a 500 ma wall wort into the mains socket and check that nothing is shorted and that the switches work and so on. then apply mains power
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Old 15th July 2010, 09:18 AM   #10
Cassiel is offline Cassiel  Libya
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OK, thanks for the good info. I'm working on it.
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