Vox AC15 heater fuses blowing -help!

[Wrongdog]

Yup - happens to the best of us. This amp has about 20 jumpers on it to fix bad spots. Too much heat on a junk pcb. This amp had every mod you can read on the net about the AC15CC.

[Jazz P Bass]

This amp had every mod you can read on the net about the AC15CC.

With all of that going on, how does it perform?

[Enzo]

Yes, you did say that in post #5, my apology for not mentioning.

[Wrongdog]

I actually like the tone of the amp - WHEN IT WORKS! Here is Don Tone-man Butlers site with AC-15 info Toneman Amps

It is not as loud as the several 1974x 18 watt Marshall clones I have built and not as grindy when pushed. It has nice high end shimmer and a big mid bass that is sweet. Very sensitive to pick attack and has a cool sustain/blossom/compression when spanked. Blue bulldog speaker. Nice reverb and tremolo.

I am trying to talk him into a Deluxe Reverb - to keep an eye out on a beat up Silverface maybe. Even a re-issue Deluxe - change a few tubes and speaker and leave the damn thing a lone and play it! He has spent WAY too much messing with this amp.

Here goes a rant: I hate PCB amps. They use crap components and are layed out weird to my old fashioned way of thinking. I am an amateur repair guy and do this to help broke musicians. Its too bad a lot of them seem to use Hotrod Fenders and such which I hate at many levels but have gotten good at fixing the POS (I charge extra for them!).

[trem]

OK, you can look up the amount of current each tube heater uses. Ass them all up, and that SHOULD be what is going through the fuse. Now what is something else is already dragging a lot of current through the fuse, then each tube adds to that, and ALL of them along with the mystery draw is too much for the fuse.

Pull that fuse, set your meter to AC AMPS and plug the probes in the right place for that, and clip your probes to the empty fuse clips. What current draw do you read? With no tubes, still draw?

Unless I have the wrong scematic I see two heater fuses, but 4A slow blow. Is the fuse you are blowing supposed to be a slow blow type? Is there T next to the number, like T4A? T is for time delay, as in slow blow.

Just for my own info.....
With the problem he was having, are you saying to:
Leave all tubes in amp as normal.
Pull fuse.
Attach DMM to each side of fuse holder and set for AC Amps.
Run the amp.
Is that correct.?
Thank You

[Steve Conner]

If you do that, be careful! Cheap DMMs have no fuse for the highest AC range. The shunt is literally about 1.5" of stout copper wire. When you clip it across the fuse holder, you defeat all fuse protection for the circuit. Don't ask me how I found this out, but it involved a big rechargeable battery pack and I still have the MOSFET shaped scar on my forehead. ;-)

If you do it with an expensive DMM, you might blow the internal fuse that costs about $7.50 a "pop" so I really don't recommend the technique at all.

[trem]

That is kind of what I was wondering. But I was not sure if I was reading Enzo's post correctly.
With the fuse out, and the DMM in its place.....
Seems like the amp and/or the DMM are going to get the full force of the problem that is blowing the fuse.
But you guys have WAY more knowledge about this "stuff" than I do
Thanks

[Enzo]

Well, OK< yes, one needs to stay within the capability of the meter. My meter has the 10A range, but I guess not all do. I would feel comfortable putting my 10A scale in the place of a 4A fuse. But that's just me. That is why I always have the high current ranges on meters I buy.

Yes, the meter would face the full force of whatever was going on, that was the point, I want the meter to MEASURE what that full force might be. That would characterise the fault.

Hell, bring the amp up on a variac with it then.

Since he was having no fuse blows with tubes removed, but it blows with all tubes present, and even one tube removed then it holds the fuse. That tells me the current is not waiting for me to clamp a meter on it and it sends 100 A down the road. SO by measuring current with tubes removed, we find if something other than tubes is causing excess current to flow. With no tubes would would expect zero current through those fuses, or darn close to zero. SInce that condition does not blow a 4A fuse, if my meter can take 4A, it would be OK here. SInce it holds a fuse with one tube removed, my measurement that way should offer no risk of blow up either.

In my description I mentioned with no tubes and with tubes.

If that is no good, then mount a resistor there and measure voltage across it. The bottom line was we needed to find out if there was any baselinje current with no tubes, and if there was, it indicats a problem.

[Chuck H]

If you do that, be careful! Cheap DMMs have no fuse for the highest AC range. The shunt is literally about 1.5" of stout copper wire. When you clip it across the fuse holder, you defeat all fuse protection for the circuit. Don't ask me how I found this out, but it involved a big rechargeable battery pack and I still have the MOSFET shaped scar on my forehead.

I thought I was going to break a rib laughing! It just struck me right. Strong visual.

[olddawg]

I thought I was going to break a rib laughing! It just struck me right. Strong visual.

Used to work in a shop with a guy who's nickname was Oonk for similar reasons. Lol.

[Steve Conner]

Yes, the meter would face the full force of whatever was going on, that was the point, I want the meter to MEASURE what that full force might be. That would characterise the fault.

What I do now is, I pull one end of the fuse out and leave it sticking up in the air. Then I clip the meter to the sticking-up end and the remaining clip of the fuse holder.

[trem]

Well, OK< yes, one needs to stay within the capability of the meter. My meter has the 10A range, but I guess not all do. I would feel comfortable putting my 10A scale in the place of a 4A fuse. But that's just me. That is why I always have the high current ranges on meters I buy.

Yes, the meter would face the full force of whatever was going on, that was the point, I want the meter to MEASURE what that full force might be. That would characterise the fault.

Hell, bring the amp up on a variac with it then.

Since he was having no fuse blows with tubes removed, but it blows with all tubes present, and even one tube removed then it holds the fuse. That tells me the current is not waiting for me to clamp a meter on it and it sends 100 A down the road. SO by measuring current with tubes removed, we find if something other than tubes is causing excess current to flow. With no tubes would would expect zero current through those fuses, or darn close to zero. SInce that condition does not blow a 4A fuse, if my meter can take 4A, it would be OK here. SInce it holds a fuse with one tube removed, my measurement that way should offer no risk of blow up either.

In my description I mentioned with no tubes and with tubes.

If that is no good, then mount a resistor there and measure voltage across it. The bottom line was we needed to find out if there was any baselinje current with no tubes, and if there was, it indicats a problem.

You know (for a Michigander) you are a pretty sharp guy.
I should have qualified my question better. I was kind of "thinking out loud", and by no means did I want to single out your advice. What you say makes perfect sense for the case at hand.
I guess what was in the back of my mind was the fact that ONE of my meters has a 2 Amp fuse while another has 10 amp. As you say, most problems will probably not blow the 10.
Maybe I should start using Extra Slow Blo in my DMM.
Thanks again for you expertise and wisdom.

[trem]

What I do now is, I pull one end of the fuse out and leave it sticking up in the air. Then I clip the meter to the sticking-up end and the remaining clip of the fuse holder.

Good idea.
You guys are like an electronics encyclopedia.
Thank You

[R.G.]

Lesson: Always check the previous work done. Even when it its your own work. I had re-heated a cold solder joint done and caused the problem....

For me, that always translates to *especially* if it's my own work.

Here goes a rant: I hate PCB amps. They use crap components and are layed out weird to my old fashioned way of thinking.

I get to stick in my rant too.

You haven't worked on a well-designed, well-thought-out PCB amp, and you attribute bad components and bad layouts to the label "PCB".

It is entirely possible to do a well made PCB amp that is easy to service and reliable. It requires some thought and skill, so it's not all that common to see it, but it's entirely possible.

It is also possible to build tagboard and point to point amps with junk parts and really, really bad layouts too.

[Chuck H]

It is also possible to build tagboard and point to point amps with junk parts and really, really bad layouts too.

+1 to that!
I still build on eyelet boards because my amps aren't too complicated and I've just never rigged up for, or gotten into etching my own boards. But I've been considering it for awhile now. I'd say that within the DIY circle there are wayyy more iffy eyelet board amps than PCB amps. Mesa's have a rep for burned traces. Some amps from the last two decades with board mounted tube sockets suffer broken traces and lifted pads. And there are some models that have resistors that run hot laying right on the board and end up burning a trace or leaving a char on the board. But if you look at it statistically the PCB's that DON'T have obvious design problems perform very well. I wouldn't hesitate to buy or build on a PCB amp only because it's a PCB.