Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jim on March 05, 2016, 11:50:08 am

Title: use of MOVs
Post by: jim on March 05, 2016, 11:50:08 am

I am repairing a Bugera 333XL-212 100 watt combo that wont power up.  Found the fuse is OK but there is an AC power board with IEC, fuse, MOVs, primary leads for transformer, power switch leads and ground.
Someone had repaired a burnt trace and resoldered the fuseholder that has now lifted again off the board.  Bugera wont talk to me so I can't getanother board.    Easy--I will hand wire the AC power input like a Fender and ditch the board. My stupid question is how to correctly place MOV's for this rewire.  These 6 MOVs appear to be all in series pairs.  I have never learned to use them--are there specific values I should use here? Can someone shoot me a diagram? Jim

Title: Re: use of MOVs
Post by: HotBluePlates on March 05, 2016, 04:28:24 pm

Assuming this is the correct schematic, it looks like they're all in series.

This diagram shows all 6 in series, but notes 3 of them are replaced with a jumper in EU models. It looks to me like the MOV's and cap are mostly there to condition the incoming a.c.

You could keep them, or not...

Title: Re: use of MOVs
Post by: PRR on March 05, 2016, 05:03:41 pm

Not "MOV" (over-voltage protection).

NTC thermistors (current surge control).

Title: Re: use of MOVs
Post by: trobbins on March 05, 2016, 05:25:35 pm

With so many of those NTC's in a local confined space then I can see why the pcb has had a hard time.  The NTC normally operates at a high temperature - it needs to to function properly.  But co-locating many of them is very risky, especially with a fuse that is also going to be generating heat. 


You could continue to use the existing pcb, but use tinned copper wire links to overlay the pcb traces to allow a solid connection.  But also a good idea to reduce the heat load - so perhaps short out the NTC connected to the fuse (the NTC then acts as a heatsink), and perhaps short out the 'middle' NTC' in the remaining string so that it also acts as a heatsink.


When cold, the inrush current will be higher - so the risk then moves to the fuse, but that risk is mainly due to a hot switch on (NTC's still hot but internal caps discharged) - make sure you have the correct fuse.


CL60 can be found in datasheets for NTC.

Title: Re: use of MOVs
Post by: jim on March 05, 2016, 07:17:23 pm

Wow, I was offtrack -- thanks for the help. I finally found out that Bugera/Behringer parts can be purchased from Pro Audio Services, Broomfield, Colorado 303-438-9335.  Jim