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Old 30th May 2008, 10:05 AM   #1
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Default Primary impedance output transformer

Hi Folks,

I am trying to work out the primary impedance for the output transformer. Truth is I don't know how to do it! I have consulted Morgan Jones (vol 1&2), searched the forum and googled.

It is made a little more complicated by the PSE (duel300B) design.

My 'gut' feeling is that I need about 1.2 k ohm but this is really based on not much more than a semi-educated guess.

I am planning on using an 8 ohm secondary tap.

I will be happy with just a number but if someone has the Friday night energy for a quick and basic outline of how the value is calculated I would be very interested.

I have attached the circuit diagram.

Thanks in advance,

Rob
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Old 30th May 2008, 10:41 AM   #2
chrish is offline chrish  Australia
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G'Day Rob,

As you know, I am still new to this, and my experience is limited. The WE datasheet lists 3K and 4K output transformers. Other designs I have seen use around 3K. If you are going to have dual 300Bs running parallel, then you would halve that, so around 1.5-2K.

This is a good site for data for valves http://tubedata.tigahost.com/tubedata/sheets3.html

Look forward to monitoring your progress!

Chris
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Old 30th May 2008, 01:07 PM   #3
Tweeker is offline Tweeker  United States
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The 300A and 300B Vacuum Tubes datasheet has data going to 12K ohms in the included graphs. Recommended operating points to 5K in that B+ range.

As impedance goes up distortion and power both drop. 3rd H sees more benefit as impedance climb, diminishing returns on second. See datasheet.

Id suggest 1.2K-2.5K. Do you have a power goal, know speaker efficiency? Max power is at the low end of the range, you lose about a 1/3 of power at the high range.


Edit: Interstage unit on diagram is labeled 10ma unit, but diagram also states 5842 is drawing 20ma?
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Old 30th May 2008, 07:36 PM   #4
Sheldon is offline Sheldon  United States
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Here's a detailed description of the process: http://members.aol.com/sbench102/po-dis.html

As noted already in the previous replies, just account for the paralleled tubes in your calcs.

Sheldon
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Old 30th May 2008, 10:08 PM   #5
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Hi Chris,

Thanks again for the advice - I did everything but look at the datasheets! I am a little way from building this one as I am currently involved in another project. I am making a fender tweed deluxe for my son but thought I should hide such a non-audio monstrosity from this board! Let me know when you move to Sydney, I would love to have a listen to your amps.

Hi Tweeker, I am looking at about 14w, so I guess that I will be looking at the lower end impedance range. As for the interstage transformer, that anomaly caught my eye as well. I looked into it and according to the specs the 10ma transformer can handle a primary current of 20ma when wired 1:2.25 for an SE interstage transformer.

Thanks Sheldon, I have just skim read your article this morning but I will have a good look at it tonight. One of my goals for this project is to understand what I am doing and not just copy a schematic.

Thanks all for the help.

Rob
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Old 2nd June 2008, 03:14 PM   #6
Tweeker is offline Tweeker  United States
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You can probably get that power at about 2K.
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