Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tdelucia on May 10, 2017, 07:53:43 am

Title: output transformer
Post by: tdelucia on May 10, 2017, 07:53:43 am

I'm using a Hammond 1760e output transformer on a 5e3 build.  What's should the resistance be on the output side of the transformer?  I'm getting maybe .7 ohms.   Seems reasonable to me.  I'm told it should be round 50 ohms.  Just tell me what should be a reasonable value using an ohm meter.  Thanks.

Title: Re: output transformer
Post by: kagliostro on May 10, 2017, 09:50:18 am

Not 50 ohm

Franco

Title: Re: output transformer
Post by: sluckey on May 10, 2017, 09:52:38 am

The secondary side will be close to zero ohms just like you are getting.

Title: Re: output transformer
Post by: tdelucia on May 10, 2017, 11:49:52 am

Thanks.  Getting about .5 ohms, almost nothing.  I thought so.

Title: Re: output transformer
Post by: PRR on May 10, 2017, 11:30:50 pm

The DC (ohm-meter) resistance "should be" 5% to 10% of the nominal audio impedance.

If it were any more than that, it would waste power. Like a too-long too-thin (too many Ohms) extension cord on a heavy wall-power load.

Much less means an extravagant design (very unlikely).

*Sometimes* practical winding matters force round-off of gauge so the turns will fit. In OTs, the high-impedance primary tends to use fat (for the small current) wire and gets relatively low Ohms, so they let the secondary run thin so it will fit.

So an "8-Ohm" winding should be in the 0.4 to 0.8 Ohm range. You appear to be spot-on-target.

Do not trust sub-Ohm readings with common bench meters unless you really know your stuff. Some meters include their own stray resistance. Others round-off small numbers to hide that error. Around 1 Ohm and less, contact resistance becomes a problem.