Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JustMike on March 02, 2021, 10:06:25 am

Title: Randall RM-100 Impedance switch
Post by: JustMike on March 02, 2021, 10:06:25 am

Does anybody know where I can find one? I emailed Randall support. Hopefully they'll sell me one, but...

Title: Re: Randall RM-100 Impedance switch
Post by: JustMike on March 04, 2021, 06:48:42 am

OK, so I've decided to find a generic DP3T slide switch. Switchcraft makes them. Then I realized I need to know the current rating and I'm not sure how to calculate it. Is it as simple as A=W/V? But what would the output Voltage be? We're talking about the speaker output of a 100w amp.

Title: Re: Randall RM-100 Impedance switch
Post by: pdf64 on March 04, 2021, 06:57:51 am

A 100W amp can put out up to 200W.
Power x resistance = voltage squared
Power / resistance = current squared

So 200W at 16ohms would require 56V and draw 3.5A

But as switching under load is an extremely bad idea, the switch contacts only need to be rated for ‘carry’, not ‘switching’, currents.

Title: Re: Randall RM-100 Impedance switch
Post by: EL34 on March 04, 2021, 12:31:37 pm

Quote from: JustMike on March 02, 2021, 10:06:25 am

Does anybody know where I can find one? I emailed Randall support. Hopefully they'll sell me one, but...

I never trusted most impedance switches I saw on amps
That's why I used these monster rotary switches on all my amps


I sell them on this page
https://hoffmanamps.com/MyStore/catalog/Switches.htm

I have a shipment coming in on Friday

(https://hoffmanamps.com/MyStore/catalog/images/IMG_2266.jpg)