Hoffman Relay PS Capacitor Failure

[sds1]

Hello,

First time using the Hoffman relay boards. Starting getting some erratic behavior (rapid on/off cycling?) today out of one of the relays when playing thru the aggressively.

Thought I fixed a cold solder joint and was testing again when I heard a hissssss:



I have since located this forum thread:
http://el34world.com/Forum/index.php?topic=14348.0

And the warning here:

Be careful here. If your filament winding has a CT connected to chassis, DO NOT CONNECT THE NEGATIVE SIDE OF THIS RELAY POWER SUPPLY TO CHASSIS! Doing so will poop the bridge or PT, maybe even both. Better to be safe and float this relay PS. There is no reason to connect the negative side to chassis. That means to also isolate any chassis connectors used to connect to a FS.

I used the chassis as a return path on the relay PS circuit, and my filament winding has a CT, but is connected to power tube cathode (elevated).

So would an elevated CT still apply to the warning above?

I'm not sure what went wrong here and I want to be sure that I fully understand the nature of the trouble here before proceeding to replace that cap and possibly rewire the return path of the relay PS so as not on the chassis.

Thoughts?

Thanks!

[sluckey]

So would an elevated CT still apply to the warning above?

Yes. Don't connect the relay supply negative to chassis. Float it.

[sds1]

So would an elevated CT still apply to the warning above?

Yes. Don't connect the relay supply negative to chassis. Float it.

So with that correction and a new capacitor I should be OK to proceed?

Just to be clear, my DC circuit goes relay PS+ -> relay -> footswitch -> chassis -> relay PS-

I used the chassis as a matter of convenience to go from the footswitch jack all the way back to the relay PS. I'll need to isolate the footswitch jack and run a wire from jack ground back to relay PS- , correct?

Thank you for your time.

[EL34]

I would have never thought to use the chassis as one of the wires in a separate DC circuit with a bridge rectifier.
If this is something that other people are doing, I can add notes to my relay hookup pages to not do that
Run two wires from the relay power supply to the relay coils
Red DC+
Black DC-

[shooter]

Float it.

pretty sure you got it from the above posts, If you're not clear on float it, It's totally independent and isolated from whatever you're putting it in as far as DC volts goes. 

[sluckey]

I'll need to isolate the footswitch jack and run a wire from jack ground back to relay PS- , correct?

That's correct. Alternately, you could use a separate transformer for the 6.3VAC input to the relay power supply. Then you can use the chassis as ground for the relay supply. Seems much easier to just float it. There is absolutely no reason to use the chassis.

[sds1]

Thanks to all for getting me back on track. I still want to learn how and why the cap popped. I am lacking the electrical theory behind the issue and it's frustrating to have a component fail like this. I feel like I should know better.

Here's a gut shot of the build. It's Sluckey's Tweed ODS w/ reverb:


Holy balls what a great amp.

Thanks again and have a good weekend.

[sluckey]

Here's a gut shot of the build. It's Sluckey's Tweed ODS w/ reverb:

Nice build, but... That ain't mine. I never built such an amp.

[sds1]

Here's a gut shot of the build. It's Sluckey's Tweed ODS w/ reverb:

Nice build, but... That ain't mine. I never built such an amp.

Oops, confused again! Well you know I think I did use your improved 18W layout a couple of builds ago...

[sluckey]

Tweed ODS sounds more like tubenit.

[sds1]

Tweed ODS sounds more like tubenit.

Yeah, sorry.

[shooter]

Here’s a page that gets you the basics of both full wave and full wave bridge.  Once you get the jist of both, doodle on paper using both a CT ground AND the bridge grounded, the typical results are 2 diodes immediately fail – shorted, for awhile, putting AC on the cap, which then usually fails.  It can get really ugly the bigger the AC volts on the rectifier

https://circuitglobe.com/center-tapped-full-wave-rectifier.html

[sds1]

Here’s a page that gets you the basics of both full wave and full wave bridge.  Once you get the jist of both, doodle on paper using both a CT ground AND the bridge grounded, the typical results are 2 diodes immediately fail – shorted, for awhile, putting AC on the cap, which then usually fails.  It can get really ugly the bigger the AC volts on the rectifier

https://circuitglobe.com/center-tapped-full-wave-rectifier.html

Thank you! I will keep reading until it makes sense.

It's just now sinking that I double-grounded a rectifier. I understand the different rectifier configurations for HT and grounding there (I've built both and know the difference is the CT), it took this long for it to translate. I still couldn't have predicted HOW it would fail but in retrospect I feel like I coulda have noticed this had I been paying more attention to the relay PS circuit. Instead I kinda just painted by numbers and forged ahead without understand what I was building.

[shooter]

until it makes sense.

I give before that and just write notes to myself 

[EL34]

New - relay page update
http://el34world.com/projects/relay_switch.htm

[sds1]

Thanks Doug!