OP TX impedance

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OP TX impedance

Post by Ampman5150 »

Im looking for a nice easy way to understand output transformer impedance and how it relates to tone and overall sound.
Certain tubes like to 'see' a certain impedance in the OP TX.
So... simply put scenario:
We have a pair of push pull tubes in a circuit. (any tubes)
These tubes like to 'see' 8K
The speaker load is 8 ohms
If we change to an op/tx with a 10K input impedance.. what does that do to tone/volume etc.
We lower the IMP of the TX to 6K.. same question.
Leaving the speaker as our constant .. how does this affect tone?
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Re: OP TX impedance

Post by colossal »


This question is answered by understanding "load lines". Time to crack a beer and hit the books, my friend!

Read this first:

Then this:

To answer your question in brief, when the load line is rotated counterclockwise (by increasing the primary impedance), this results in more distortion at the cost of increased screen current on large signals. Ken Fischer did this with the Trainwreck Express as a means to induce power amp distortion earlier (and less volume than a 50 or 100W cranked Plexi). He doubled the primary to 6k6 for a pair of EL34s, but also took care to reduce both the plate voltage AND screen voltages and padded the screens with 1k resistors. This will make more sense when you read the above and mull it over.
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Re: OP TX impedance

Post by JMPGuitars »

Speaking of tone, you also need to pay attention to the way each transformer is made. The typical 18 watt transformer sounds best at 16 ohm because it's using the full winding, but at 8 ohms, it only uses half.
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