Weird Full Wave Bridge Issue FWB - Blows Fuse

[MarshallPlexi]

I have a 70W amp that I'm playing with. The amp works great with a 350-0-350 @ 300mA winding with a full wave rectifier (two diodes) and is dead quiet. I get about 470V B+ under load and all is right with the world.

So I want to try another PT:
http://www.classictone.net/40-18084.pdf

Obviously I need to use a Full Wave Bridge so I use a BR310 package and I get 500VDC before the STBY switch. That's to be expected. Problem is when I throw the STBY switch the B+ fuse pops instantly.

After popping a lot of fuses I found that I can put a 10R resistor in series with the FWB before the STBY switch and it will not blow the fuse when the STBY is thrown. So to me that means there's not a short but a super high initial current draw that is stout enough to pop a 500mA slo blo fuse.

Why is this happening and is there something I'm not hip to? I did find a Marshall Major schematic where they put the same 10R resistor in series with the FWB (that's where I stole the idea from).

To make matters worse I have a nasty B+ hum now with this amp when using the FWB. I'm completely stumped on this one.

Oh! Just a heads up, I did reinstall the original PT and it's still dead quiet. I was concerned that I had killed a filter cap during testing but the amp seems to be good.

Any ideas on what I'm missing?

[matt h]

(deleted)

[pdf64]

Maybe standby switches are a dumb idea?
What happens if it's not used? With and without the 10 ohm resistor.

To save fuses / stress in such situations, suggest you look to build a light bulb limiter, see sticky.

A schematic of your amp would be really helpful! eg where is the B+ fuse, before or after the rectifier? Power tube bias arrangement / supply?

The surge current with a FWB may be higher than with an equivalent 2 phase FW, due to there being less copper loss / thicker wire.

What was the Vdc B+ with the FWB?

[Jana]

My guess is that you connected the center tap of the transformer HV to ground.

[cbass]

I've never noticed more noise from ma fwb recto though I've read about it.
What knda amp amp we talkin bout here. Really 70watss? If you are running two kt88s or 4 el34 don't think a 500ma fuse is big enough

[Merlinb]

Sounds to me like you've connected the bridge recto up incorrectly...

[matt h]

Sounds to me like you've connected the bridge recto up incorrectly...

If that were the case--how would it be resolved by the inclusion of the 10ohm resistor? (Not arguing, genuinely curious and ignorant)

[Merlinb]

Sounds to me like you've connected the bridge recto up incorrectly...

If that were the case--how would it be resolved by the inclusion of the 10ohm resistor? (Not arguing, genuinely curious and ignorant)

It could limit the current to something just low enough to prevent the fuse popping. I think a schem and photos of your setup are needed.

[matt h]

It could limit the current to something just low enough to prevent the fuse popping. I think a schem and photos of your setup are needed.

I meant how does that suggest the bridge is set up incorrectly? Those modules are fairly plug and play, are they not? It seems like the 10ohm resistor would limit the inrush current either way.

I also wonder if the slo-blo are actually fast-blow fuses in disguise.

[Phil_S]

Removed: posted before morning coffee

[sluckey]

There is no CT on the HT winding of the PT that the OP used with a FWB.

[Phil_S]

There is no CT on the HT winding of the PT that the OP used with a FWB.

Before morning coffee that might not be true

[Merlinb]

Never mind