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    Building, troubleshooting and testing of these amplifiers should only be
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Primary impedance output transformer

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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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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
 
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?
 
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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