100 watt PT in 60 watt Mark IIB(Blowing fuse)

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MK IIB
Mark III
Posts: 191
Joined: Tue Sep 20, 2011 11:15 am

100 watt PT in 60 watt Mark IIB(Blowing fuse)

Post by MK IIB » Mon Jul 30, 2018 7:50 am

Hi Folks.
My Boogie Mark IIB is blowing fuses sporadically.
It's been recapped and has new Mesa power tubes with hardly any use at all.
This amp is a 60 watt model but has a replacement power transformer.It is a 100 watt Fender Export PT from a dual Showman I believe?It also has a Fender choke too?
Could this 100 watt PT be putting out too high voltages?
I've been playing it for a couple of weeks now and it's been fine but a couple of weeks ago blew 3 fuses in one night?I got a batch of slow blow fuses online.Bad fuses??
I'm interested to know if any damage could be done or any affect on tone from running this PT in the amp?

woodbutcher65
Mark III
Posts: 170
Joined: Fri May 04, 2018 10:36 pm

Re: 100 watt PT in 60 watt Mark IIB(Blowing fuse)

Post by woodbutcher65 » Mon Jul 30, 2018 12:32 pm

A larger transformer is going to dictate using a larger fuse. It's about the inrush current when you first turn it on.

Use a 3A slow blow fuse. That should work just fine. It's what's found in a 100 watt Mark III with export transformer.

MK IIB
Mark III
Posts: 191
Joined: Tue Sep 20, 2011 11:15 am

Re: 100 watt PT in 60 watt Mark IIB(Blowing fuse)

Post by MK IIB » Mon Jul 30, 2018 1:13 pm

When I got it back after being recapped it had a 3amp fuse in it.I thought the tech accidentally put in the 3amp and I put in the 1.5 amp as it says on the amp.I never thought having the the 100 watt PT would require a higher rated fuse.
Any thoughts on the voltages and what difference it would make to the sound/feel of the amp?
Thanks a lot for your help.

woodbutcher65
Mark III
Posts: 170
Joined: Fri May 04, 2018 10:36 pm

Re: 100 watt PT in 60 watt Mark IIB(Blowing fuse)

Post by woodbutcher65 » Mon Jul 30, 2018 1:21 pm

I'd expect higher supply voltages, and with the increased current capacity, the amp should feel stiffer and more dynamic. If you like a soft, spongy feel, you're not going to get it.

MK IIB
Mark III
Posts: 191
Joined: Tue Sep 20, 2011 11:15 am

Re: 100 watt PT in 60 watt Mark IIB(Blowing fuse)

Post by MK IIB » Mon Jul 30, 2018 1:43 pm

I should mention that I'm in the UK(240 volts)It says on the export switch to use 1.5 amp for 200-240 volts?

woodbutcher65
Mark III
Posts: 170
Joined: Fri May 04, 2018 10:36 pm

Re: 100 watt PT in 60 watt Mark IIB(Blowing fuse)

Post by woodbutcher65 » Mon Jul 30, 2018 4:42 pm

At 240 volt, 1.5A should be the right fuse size. That does imply there may be something going on with the amp.

So, put a 1.5A fuse in it and see if it blows the fuse if you turn it on with no power tubes installed.

Then put the tubes back in and see how it goes.

You may have a shorted power tube. In some cases they can cause fuses to blow.

Another distinct possibility is the filter capacitors. Have they been changed? By now they're certainly long overdue for it, if they're original to the amp.

MK IIB
Mark III
Posts: 191
Joined: Tue Sep 20, 2011 11:15 am

Re: 100 watt PT in 60 watt Mark IIB(Blowing fuse)

Post by MK IIB » Wed Aug 01, 2018 2:46 pm

It's still working well with the new 1.5amp slow blo fuse.I notice there is a bit of a thump when switching stand by on and the valves flash blue.Like a rush of current.Is this normal?

woodbutcher65
Mark III
Posts: 170
Joined: Fri May 04, 2018 10:36 pm

Re: 100 watt PT in 60 watt Mark IIB(Blowing fuse)

Post by woodbutcher65 » Mon Sep 10, 2018 9:09 am

Yes, that's common enough to call it normal. No worries.

MK IIB
Mark III
Posts: 191
Joined: Tue Sep 20, 2011 11:15 am

Re: 100 watt PT in 60 watt Mark IIB(Blowing fuse)

Post by MK IIB » Mon Sep 10, 2018 10:34 am

Well it's still working with that last fuse?Hopefully it was weak old fuses??

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