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Old 30th December 2010, 11:58 PM   #1
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Default A Newbie Tx Question

Do you select the mA based on idle current or peak?
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Old 31st December 2010, 12:01 AM   #2
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what application ie power tx or output?

Meh - in either case, select with an easy 20% over-capacity, more if you can afford it. A loafing tx is a happy tx...
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Old 31st December 2010, 12:15 AM   #3
tomchr is offline tomchr  United States
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I'd select based on the RMS current. Allow a good margin.

~Tom
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Old 31st December 2010, 01:00 AM   #4
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Quote:
Originally Posted by Troncones View Post
Do you select the mA based on idle current or peak?
The mA of what? Please define "Tx" and the context of your question. It would then be easier to answer your question. There are many different definitions for the generic "Tx".
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Old 31st December 2010, 01:25 AM   #5
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Based on the max power handling, input or output regardless. This will depend on the class of amplification and maybe the efficiency. Unfortunately you will have to think.

If you think in terms of the power this should keep you from going too far astray.

w
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Old 31st December 2010, 02:06 AM   #6
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Sorry I thought it was a simple question. Newbie mistake!

I'm working on a SE 6BQ5 amp that I hope will put out about 5 watts. The idle current, power stage plates+screens + gain stage plates is about 100mA.

The peak will be about 200mA.

I have two Power Transformers to choose from. 275-0-275, 150mA and 275-0-275, 200 mA.

When selecting the power transformer do you use the idle load of 100mA or the peak load of 200mA?

A short comment on why would be appericated.
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Old 31st December 2010, 02:34 AM   #7
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just go with the big one. The cost difference will be negligible and the performance difference will be appreciable
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Old 31st December 2010, 03:45 AM   #8
20to20 is offline 20to20  United States
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Quote:
Originally Posted by Troncones View Post
Sorry I thought it was a simple question. Newbie mistake!

I'm working on a SE 6BQ5 amp that I hope will put out about 5 watts. The idle current, power stage plates+screens + gain stage plates is about 100mA.

The peak will be about 200mA.

I have two Power Transformers to choose from. 275-0-275, 150mA and 275-0-275, 200 mA.

When selecting the power transformer do you use the idle load of 100mA or the peak load of 200mA?

A short comment on why would be appericated.
Your plate voltage is going to be around 300v. That means you need to keep your plate current to 40ma. to stay under the 12W PD. Total I about 45ma. I have a Magnavox SE 6BQ5 that uses a 140ma PT. It gets pretty warm but not worrisome hot. Those ratings are RMS so you don't need to calculate peaks. Not sure how you got 200ma from 100ma. The 150ma tranny will be all you need. No reason for overkill unless you want to spend more, have a heavier amp and a larger chassis. Keep in mind you don't want to crowd the all trannies together. You need an adequate margin of space to eliminate magnetic coupling and hum.
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Old 31st December 2010, 11:34 AM   #9
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Quote:
Originally Posted by Troncones View Post
... will put out about 5 watts.
I have ... 275-0-275, 200 mA.
55 watts in, 5 watts out. Sounds about right for valves.

You didn't say, but this is mono?

w
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Old 31st December 2010, 04:13 PM   #10
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Thanks everybody for your patience and replies.

I think I got off on the wrong foot by trying to keep my original question brief so let's try this again.

I'm working on a two channel SE 6BQ5 amp that I hope will put out about 5 watts. The idle current power stage plates+screens + gain stage plates is about 100mA (which most of know by heart).

I'm struggling with a little confusion about current requirements. From what I have read and from studying load lines it seems clear that when you increase the grid voltage the current goes up. I assumed this increase would come from the power transformer. Then to further my confusion I read in "Valve Amplifiers" from the chapter Choosing Caps and Transformers that for class A amps "the quiescent current equals the peak current required at maximum power output". The conclusion is, I missed something along the way.

To short cut my problem, for now, I ask you.

I have a 150mA and a 200mA 275-0-275 transformer, which should I use?

The answer would be helpful. Why would be very helpful.

In addition, a tip on entry level reading about power transformers would be appreciated. The tube books I have are short on explaining Power Tx's.
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