DUAL Showman

[macguy]

Greetings,

My '67 Fender DUAL Showman:
Fuse blows
Remove all 4 power tubes
Turn amp on, standby off...amp stays on
Insert tubes one at a time
Fuse blows at varying points in the procedure
Install a set of what I felt were known good tubes
Amp played for over an hour
Next day, fuse blows
Now any tube I insert glows like a volcano almost immediately and looks like it's about to erupt
First question: with the amp unplugged from the wall, does operating the power switch a couple of times safely discharge the caps?
Second question: any suggestions as to the problem?
Many thanks,
MG

 

[GilmourD]

Re: DUAL Showman

OK, so you're redplating fairly quickly and blowing fuses. Sounds like the bias circuit went weird on ya.

'67 is AB763, right? Is that what it says on the tube chart inside?

http://www.prowessamplifiers.com/schematics/fender/showman_ab763_schem.pdf

 

[macguy]

Re: DUAL Showman

Yes...that is the one.

 

[GilmourD]

Re: DUAL Showman

Yeah, unless you're savvy with the guts of electronics and well versed in the fact that THE INSIDES OF A GUITAR AMP CARRY LETHAL VOLTAGES AND COULD FRY YOU TO A DEAD CRISP, bring your amp to a well-trained technician. Print out the PDF from the link in my last post in case he needs the schematic.

 

[macguy]

Re: DUAL Showman

Thanks for the info and the warning!

MG

 

[GilmourD]

Re: DUAL Showman

Let us know how it works out.

I used to have a '67 Dual Showman... Man, I miss that amp something fierce.

 

[Gearjoneser]

Re: DUAL Showman

Don't have the amp ON while installing tubes! Even though only the heaters are being activated in that scenario, it's still a bad idea, and is probably why fuses are frying.

Before you ruin the transformer or tubes, get the amp checked out. I realize it costs money, but it's par for the course when you're dealing with a 48 year old amp.

You could have current arcing between tube socket pins, fried screen resistors, or sockets arcing to ground, or any number of things. Let a pro look at it.

It's always good to list your location here, so local forum members who are techs can help you personally, or direct you to a local tech.

 

[Boogie Bill]

Re: DUAL Showman

Do not attempt to repair this yourself!!!

It is tech time. Period.

Bill

 

[zzmoore]

Re: DUAL Showman

You may have a few problems going on.
Elytic caps
Bias Circuit
Rectifier
PI coupling caps
etc etc
As mentioned above, Do not insert power tubes into a live amp.....not in this situation anyway.
Do you have the right Slo-Blo fuse.?
Do you live near me.? You probably have more than one problem, including (possibly) screen resistors.
good luck