Fried rectifier bridge in channel relay in d'lite..

[mlp-mx6]

Bob-I, wouldn't you need the center tap on the xfmr with that full-wave setup?

[daz]

You need to lose the centre-tap on the heaters or the DC connection to ground to get this to work without shorting out the transformer through the bridge.

In this instance I'd lose the DC connection to ground, whereas if I were using DC heaters for the preamp I'd lose the centre-tap connection to ground and use the DC ground as the reference.

You can't have both at the same time unless you create a virtual ground for the DC (the 100R resistors trick) but it shouldn't be necessary for a control circuit such as this.

Your heaters (6.3V AC) should go to the AC terminals on the rectifier. The DC terminals will go to the relay board only. Don't get them anywhere near ground. It'll also require using an insulated (e.g. Cliff) jack for the footswitch.

Thanks for the tips.
So in essence keep the virtual ground from the 6.3V ac heaters, but not ground the relay/ filter caps? Sorry complete newbie with relay stuff.

You might try a different ckt. Use a full wave instead of a bridge. I used this successfully in one amp.

Do you have a basic schemo of something like this?
Thanks!

[Bob-I]

Bob-I, wouldn't you need the center tap on the xfmr with that full-wave setup?

Well, that's why I posted it, he seems to be using a CT.

[Bob-I]

Do you have a basic schemo of something like this?
Thanks!

Yes, I posted it.

[paulster]

So in essence keep the virtual ground from the 6.3V ac heaters, but not ground the relay/ filter caps? Sorry complete newbie with relay stuff.

Exactly. You're there with what you've got; you're just repeatedly shorting out each half of the heater secondary through the bridge rectifier by grounding your DC supply to the chassis. Since the heater secondary can supply lots of current it's kissing goodbye to your bridge rectifiers very quickly.

If you imagine that you have +/- 3.15Vac at the heaters by using a centre-tap (real or virtual) then your DC ground point is actually alternating between these points during each cycle. It isn't actually referenced to the 0V point in the middle.

Just treat the DC ground as completely separate from the other ground in the amp and you'll be fine. That's why you'll need an insulated jack if you're going to have a footswitch connection.

If you have a 5Vac supply available for a tube rectifier and you're not using that, you could always rectify that for DC. 5Vac when rectified will give 7Vdc less the drop across the diodes, so this should be good for 6V. You could happily connect the DC side to ground with that if you wanted to.

[daz]

thanks Paulster for your explanation. So the hoffman diagram is incorrect since it has the dc grounded??
thanks everyone for your help.
will keep you posted on how I go.

Do you have a basic schemo of something like this?
Thanks!

Yes, I posted it.

thanks bob.got it. Sorry attachment didn't show up initially on my computer.

[daz]

Thanks Paulster, after few more fried bridges from miswiring, I separated the the dc grounding and yes it works!
The amp still runs hum free - quiet as a mouse.
Well initially I could switch the PAB on once but not off.
The channel switch works fine however.
So either I've wired the PAB relay funny or possibly not enough voltage to switch both relays on?
From the hoffman diagram the 180 ohm resistor is to drop voltage for one relay. Would I need to lower it alittle to switch two??

[paulster]

Assuming you've wired the PAB and the channel switch one exactly the same then it looks like either the PAB relay has died or you're not getting enough voltage to properly energise the coil. Does the PAB relay work if the channel switch relay is not energised?

What relays are you using (manufacturer, part)? And what DC voltage are you getting across the cap by the bridge rectifier and across the second cap when one relay is energised? From the part we can get the specs for the coil, and the two voltage readings will give an idea of what resistor is actually required to ensure that both coils will energise independently or simultaneously.

[daz]

Assuming you've wired the PAB and the channel switch one exactly the same then it looks like either the PAB relay has died or you're not getting enough voltage to properly energise the coil. Does the PAB relay work if the channel switch relay is not energised?

What relays are you using (manufacturer, part)? And what DC voltage are you getting across the cap by the bridge rectifier and across the second cap when one relay is energised? From the part we can get the specs for the coil, and the two voltage readings will give an idea of what resistor is actually required to ensure that both coils will energise independently or simultaneously.

Hi paulster I'm using 5v Omron G6A-234P-St-US dpdt non latching relays.
Fiddled around with wiring and it both relays work now. Fantastic, one happy camper now.