Hey Guys
I recently changed one of the caps in the MM2*10 Sixty Five with the preamp. It was one of the big 100uF 450V ones that's in its own section with the preamp caps and I changed it due to a high ESR. Now whenever I plug the amp in and turn it on the light goes on for a second and the fuse blows. I don't think its an electrical short with the chassis. The two Mallory caps are now the same ratings but are different models. What could be the issue?
Thanks
Levi
CAP
Are you SURE you put it in in the right direction? Maybe the other cap is shorted and it's a massive coinkydink? Does it do it with the doghouse OFF? Suggest doing the light bulb limiter and remove one section at a time to find the short.
Coinkydink
Hi mm210, That is a valid point. I will try it without the doghouse to check for a short. I don't see any reason for the other cap shorting though other than having a wire touching the house since its ESR is fine. I will also try putting the old cap back in. There really shouldn't be any reason to replace both right? Do I need to rebias anything after a replacement of the same values? What is a lightbulb limiter?
YES there is a reason. THEY
YES there is a reason. THEY ARE 30 YEARS OLD! Preventive maintenance. REPLACE THEM ALL! No real need to re-bias. LIMITER-
After Changing Caps
Well I changed the caps and the amp was working. Then I flicked the standby switch and the fuse blew again!
?
Fuses
First off, you need to stop blowing fuses! That's what a light bulb limiter does. Build it, hook it up and if a 60 watt light bulb glows bright, you have a massive current draw. If you hook it up and it glows bright for a second or two and settles down to a dull glow, it's ok. You could use a variac and measure the current draw but this is cheaper and easier to get. MAYBE you have a crap tube. Pull the tubes out, fire it up without the tubes and see if it does the same thing. LIMITER FIRST. Mike.
Lightbulb tester
Oh I see. The tubes are all fine, I've been using them for a while and they're glowing all good and it still bombs out without them. I'll build a light bulb tester either tomorrow or on the weekend. No more blown fuses. Thanks Mike.
Does this amp have the
Does this amp have the protection diodes on the power tubes? One was shorted on mine, and drew excessive current. It is easy enough to pull the tubes, and check.
Progress
Hello!
I've got all the components to build the light bulb limiter. The way I understand the diagram is that the neutral goes to the silver screw, the positive goes to light bulb and then the brass screw and the ground goes to the lightbulb ground and then the ground screw which is the chassis. Is this the correct configuration?
Hi Bill. There is no protective diodes in this model... it is an earlier export model.
Levi
Good News!
it ended up being the polarity of the caps after all! The amp is now so ear bleedingly clean its crazy!
Caps
Glad you found it. Mike.
Cap change
Thanks Mike. I've been changing the rest of them out and its all working fine. Its a bit more colourful actually. I have on hand some 220uF (50V) caps available which are a really good brand. Do you know which 150uF caps it be acceptable to replace with higher values?
Caps
Uh-have you looked at the voltage ratings of the caps there are to replace? They are rated MUCH higher than that.
Caps
I'm referring to the 4 aluminium caps in the main enclosure which are 150uF 50V.