50watter with 1.7k primary?

[studiodunn]

I have a old 100w MM OT with a 1.7k primary and want to try it in a 50 watt build. The build is El34's, so can I just mismatch the speaker impedance...amp @ 16 / cab @ 8?

[pompeiisneaks]

I think it goes the wrong way.

Per this site:

http://www.learningaboutelectronics.com ... php#answer

The turns ratio of your transformer, is sq root of primary imp/secondary imp if you're saying it has a 16 ohm tap for that, it would end up being about a 10:1 turns ratio.

For a 10:1 turns ratio, if you change the primary impedance to 3.4k, you get around 32 ohms. I think you'd want to look at one of the lower taps going UP. I.e. a 4 ohm tap is at about 20:1 turns ratio with primary of 1.7k. if you then change the primary to 3.4k you get about 8 ohms. In other words, if you want to use an 8 ohm cab with that OT you need to use a 4 ohm tap...

Hope that makes sense and my math is right

~Phil

[studiodunn]

Doh...yes and I actually originally typed it the other direction but knew there was no way I could have been right

I wish I could say clear as a bell Phil, but that definitely helped in this educational process. I'm till pretty foggy on windings/ratio logic, but that's no due to a lack of help or available reading.

In my caveman non-math thought process:
The 1.7k is meant for 4x el34s. If I pulled two tubes I then set my amp down from the cab impedance - amp@4 - cab@8.

So In a 50watt el34 amp I could just mislabel the impedance selector and omit the 16ohm secondary tap since I can't make that conversion without doing some wiring trickery.

[pompeiisneaks]

16 ohm would still work, but you'd need a 32 ohm cab attached.

If you want to, you can

You just need to double the impedance on any taps you make available.

Also going the other way around, you'd have to be careful to not over-tax the OT, but you're halving the output so you have a massive overspecced OT for 50W

~Phil

[studiodunn]

massive overspecced OT for 50W

I already feel more boutique