Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on December 27, 2018, 04:20:00 am

Title: Current Limiting Resistors for EZ81
Post by: TIMBO on December 27, 2018, 04:20:00 am

Hi guys, This is way over my head, but seems to be needed.

Some info...
Rpri - 35 ohms
Rsec - 150 ohms (half secondary winding)
Vpri - 243v
Vsec - 275v (half secondary winding)

 :help:

Title: Re: Current Limiting Resistors for EZ81
Post by: sluckey on December 27, 2018, 07:01:04 am

Do you think you need current limiting resistors for an EZ81? Why?

     ...OR...

Are you concerned about the difference in voltage ratio and the dc resistance ratio? Don't be. Your voltage ratio is 243:275, or 1:1.13 and your resistance ratio is 35:150, or 1:4.29.

The voltage ratio will always be equal to the turns ratio even if the wire sizes are different. Large gauge wire will have less DCR per foot than small gauge wire. Your ratios above indicate to me that the primary wire has fewer turns and is a larger diameter than the secondary wire.

Title: Re: Current Limiting Resistors for EZ81
Post by: TIMBO on December 27, 2018, 02:44:09 pm

Thanks sluckey, I have never been concerned when it comes to this amount of math.

But seems to be something the local blokes always ask.

I feel the more I get into building amps the more I should know.
I have quite a collection of stuff and I'm slowly moving it on and I would like to feel confident that my creations are not going to fail at turn-on by the new owner.

Most of the components I use are recycled and I always note the valve line up as the PT and OT are not marked with there ratings.

I do try to overcompensate in places if I'm unsure.

Title: Re: Current Limiting Resistors for EZ81
Post by: PRR on December 27, 2018, 04:53:50 pm

Are you asking about a rectifier? Or a transformer?

If a tranny has both a 275V and a 6V winding, the primary has to feed both. So its resistance should be less than ratio of line to high voltage squared.

The high-volt winding is thin, with the same shellac, so there is less copper in a given volume, and the winding resistance may run higher than you'd guess from other fatter windings.

Or are you protecting the EZ81?? The Philips data (http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EZ81.pdf) has suggestions, and you are not far off.

Title: Re: Current Limiting Resistors for EZ81
Post by: TIMBO on December 27, 2018, 06:06:04 pm

Hi PRR, Yes the concern is the EZ81.
https://www.guitargear.net.au/discussion/index.php?topic=51087.0