Speaker Impedance Relation to OT Primary Impedance?

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Speaker Impedance Relation to OT Primary Impedance?

Postby Axe78th » Sun Aug 17, 2008 2:25 pm

Greetings All,

I'm trying to understand how the speaker load or better yet speaker impedance "mismatch" has on the primary of the OT. I have a 103 tranny in a JTM45RI and know this transformer has an 8K primary. Say if I run my amp with an impedance mismatch such as the 8 ohm tap with a 16 ohm cab, does the OT primary go the 16K or 4K? I have some Fender deluxe amps and always seem to like a 16 ohm speaker in them over an 8. Also with some of the vintage EL34 Marshalls I've owned in the past I would usually like an 8 ohm amp setting with a 16 ohm cab.


I am wanting to try some EL34's with my reissue 45 and and read you need to set the amp to 16 ohm with an 8 ohm cab for the right "match". Since all my cabs are 16 ohm I don't want to screw something up and also do not want to rewire and respeaker unless I really need to. I did read in the EL34's on a 103 post here that it was safe to run the EL34's at 8K but what is happening with them at 8K. Less watts? As you see I'm a bit confused. Thanks in advance.
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Re: Speaker Impedance Relation to OT Primary Impedance?

Postby LordAli » Sat Sep 27, 2008 5:09 am

Primary impedance is load impedance for tubes (EL34s). This impedance sets operating load (operation loadline) for these tubes. It depends upon tube which impedance is optimal and upon supply voltage (for example 425V is optimal for 50W amp with 2xEL34s).

Usually amp at this impedance have minimal distortion. It also determine power output. By primary impedance, you set output voltage reflected to secondary, power output and distortion.

If you want to use another primary impedance than optimal or from catalogue, you have to check if your amp with current tubes dont exceed avarage plate power rating. It's very difficult mathematics and for my life (electronic designer) I have never seen clear technique how to count it so I can tell more but this. But if you exceed plate power rating you will damage your tube. :D
JMP50-X1 (JMP50 Combo version loaded with Vintage 30)
JTM50 head (some strange amp of that kind)
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