Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: wireman on May 22, 2016, 01:32:35 pm

Title: Hoffman channel relay power supply board problem
Post by: wireman on May 22, 2016, 01:32:35 pm

I bought the relay power supply board from Hoffman, wired it up, turned it on and before I could take a voltage reading the bridge rectifier smoked. To make a long story short, the problem goes away when I remove the ground to the amp. I'm powering it off of the 6 volt filament winding, I have almost 8 volts at the filter capacitor. My filament winding is not center-tapped so I ran the ground to the ground side of the 100 ohm artificial center tap resistors. Anyone have any thoughts on what I'm doing wrong?
Thanks
paul

Title: Re: Hoffman channel relay power supply board problem
Post by: tubenit on May 22, 2016, 01:59:46 pm

Here is an excellent thread in ARCHIVES on relay switching.  Look at the drawings, schematics, layouts and photos. 

http://el34world.com/Forum/index.php?topic=14348.0 (http://el34world.com/Forum/index.php?topic=14348.0)

Look at post #31.  There may be an answer for you in this thread.

with respect, Tubenit

Title: Re: Hoffman channel relay power supply board problem
Post by: wireman on May 22, 2016, 02:33:33 pm

Thanks for the link, I'll bet I need to isolate the ground. This amp is a cathode bias so the transformer neg bias tap isn't being used. Could I potentially use that to power the board without isolating the ground?

Title: Re: Hoffman channel relay power supply board problem
Post by: sluckey on May 22, 2016, 02:51:12 pm

Quote

Could I potentially use that to power the board without isolating the ground?

No. Just don't connect any part of the relay switching circuit to chassis ground.

Title: Re: Hoffman channel relay power supply board problem
Post by: wireman on May 22, 2016, 03:30:58 pm

I isolated the footswitch jack and it works as advertised. Thanks for the help
Paul

Title: Re: Hoffman channel relay power supply board problem
Post by: PRR on May 22, 2016, 04:04:34 pm

A "problem" with taking the 6V AC heater supply also for a 6-8V DC supply is that one or the other system can NOT be grounded.

Since we "need" preamp heaters steady with reference to ground (to reduce hum), that means the relay supply (that whole circuit) must be totally isolated from ground.

If you had a spare 5VAC winding (for a bottle rectifier you are NOT using), that can be a fine 6-7V DC grounded supply.