VOX AC15TBX Filament Fuses Blowing / Made In England / Blue Speaker

[plexi50]

I have a Vox AC15TBX that is blowing the heater filament fuses. I pulled all the tubes and measured 7.48 VAC across the filament leads.Then measured 3.73 VAC each filament lead to ground.
Then i put all (5) 12AX7 preamp tubes in one at a time and got to the last one with no issues. All lite up but weak looking.As soon as i put in (1)  EL84 power tube in the socket the fuses blew. Thinking the power transformer filament winding is bad.Using T4A fast acting fuses. I also tried slow blow for the hell of it. Thoughts? / PT Filament winding??? All tubes are new and test good.

[AlNewman]

What happens when you start with the el84?  Maybe some stray wire or bad connection at the socket?  Are you using a limiter?  Might save on fuses while you're testing.
If it is tube rectified, I know there can be issues with the 6v rectifier tubes shorting to cathode.  Perhaps if there's an artificial center tap one of the resistors is going?  I've also seen lamps with factory defects which had minor shorts to ground, really bringing down the heater voltage.
You could try running your ammeter on your multimeter in circuit to monitor current as you try different things.

Seems funny the transformer would be seriously damaged on a fused circuit.

[pdf64]

.As soon as i put in (1)  EL84 power tube in the socket the fuses blew.

This is a good time to get out your light bulb limiter.
Just that EL84, or does it happen with either?
If just one, maybe it has a fault that the tester doesn’t pick up.

[plexi50]

No dimming with light bulb limiter. I didn't try the second EL84 socket by itself. I have 2 fuses left. Dam i used to have a 5 amp circuit breaker i could clip on the fuse holders. Don't know what happened to it over the years. Think i will go to ACE and see what they have instead of wasting $20.00 on fuses. $10.00 for a (2) pack at ACE. BS. Ok here we go. I'm going to put the last 2 fuses in and see if the second EL84 socket pop's them. Oh the 5Y3 rectifier does get abnormally HOT! Could i just jumper the two fuse holder for a second to see if the tubes light up?

[WimWalther]

~7.5VAC unloaded seems perfectly normal.

You state that the 12AX7 appear dim - what is the fils voltage reading with all of the 12AX7s plugged in?

[plexi50]

~7.5VAC unloaded seems perfectly normal.

You state that the 12AX7 appear dim - what is the fils voltage reading with all of the 12AX7s plugged in?

2.37 VDC Pins 4&5 / 2.47 Pin 9 Only (1) preamp tube lighting up now.

[plexi50]

3.64 VAC On each lead before passing through the fuses

[AlNewman]

Did it come with a blown fuse?  Are you sure it's the right value?

Also, I would maybe check out an auto supply store for fuses.  They usually come in packs of 5 for $3 or so.

[plexi50]

Put in both EL84 power tubes and no light up. Pulled the EL84 on the far right and the one on the left EL84 lights up.This is now with all the Preamp tubes puled.

[plexi50]

Did it come with a blown fuse?  Are you sure it's the right value?

Also, I would maybe check out an auto supply store for fuses.  They usually come in packs of 5 for $3 or so.

These particular fuses are the very small ones. I hate them. I have 100 1"1/4 fuse of all amps 1-10.Yes it came with blown fuses and the customer did not tell me all this ahead of time. He said the amp was working. I don't believe anything i am told when it has come from another shop that could not fix it. I gradually got all of this info over the past 3 days. I have to have people be straight up with me otherwise these are the kind of jobs you do NOT take in.

[WimWalther]

Put in both EL84 power tubes and no light up. Pulled the EL84 on the far right and the one on the left EL84 lights up.This is now with all the Preamp tubes puled.

Measure the resistance of the heaters in all tubes. Compare them to one another and, if possible, to known-good examples from your stock.

Also, measure the resistance of the fuses.. I've come across some that were more like power resistors than fuses.

It sounds like you're chasing your tail. Need to pin down the fault(s)

[WimWalther]

~7.5VAC unloaded seems perfectly normal.

You state that the 12AX7 appear dim - what is the fils voltage reading with all of the 12AX7s plugged in?

2.37 VDC Pins 4&5 / 2.47 Pin 9 Only (1) preamp tube lighting up now.

Nope, 'taint right. Fils should be ~6.8V or 2x ~3.4V with all 12ax7 installed.

[plexi50]

I pulled the power board and re-soldered every single component entire board. All resistors are good and in spec.I will check my tubes filaments in a bit. Just need to pop the board back in chassis. Plastic standoff clips.I am staying positive but really inside it's like 

[plexi50]

Put in both EL84 power tubes and no light up. Pulled the EL84 on the far right and the one on the left EL84 lights up.This is now with all the Preamp tubes puled.

Measure the resistance of the heaters in all tubes. Compare them to one another and, if possible, to known-good examples from your stock.

Also, measure the resistance of the fuses.. I've come across some that were more like power resistors than fuses.

It sounds like you're chasing your tail. Need to pin down the fault(s)

Great idea!

[plexi50]

Preamp tubes measure between 15.5 & 16.5 Ohms pins 4&5. This is really too much. The filament resistors are good. Something is really dragging the filament voltage down. I looked good for any cracked traces on the board. The schematics i have are worthless. 

[AlNewman]

You could try pulling all tubes, (and lamp), removing the fuse and hooking up your ammeter in series through the fuse holder.  see if there's any draw with nothing in the circuit.

[plexi50]

You could try pulling all tubes, (and lamp), removing the fuse and hooking up your ammeter in series through the fuse holder.  see if there's any draw with nothing in the circuit.

Another good idea. I just pulled the EQ board. Looks like it may have gotten wet at the far right end. Was looking for a DC change in some tubes but don't see anything on the board that indicates a DC preamp tube section.

[plexi50]

Pic's

[plexi50]

Crapy white milky solder here and there. I wouldn't wish this on my worst enemy.

[stratomaster]

Rig up a standard fuse holder and temp it into place. Now you can use your fuse stash and save the correct sized one for when you've fixed it.

Like suggested above, put your current meter in series with the temp fuse.  Then put the tubes in one at a time. 

Before all that measure continuity from heater pins to all other pins on the tubes you're using.  Might be a nasty short and the PCB is just fine

[WimWalther]

Preamp tubes measure between 15.5 & 16.5 Ohms pins 4&5. This is really too much.

No, ~16R seems reasonable for 12.6V heaters when cold. The resistance increases as they heat up, and the current drops from the initial 800mA. Check one with a power supply, it should pull 150mA when hot.

[PRR]

6.3V at 0.3A is 21 Ohms. Cold resistance (or with ~~200mV ohm meter) should be 3X to 10X of hot resistance, 60 to 200 Ohms.

So 16 Ohms feels wrong?

[WimWalther]

6.3V at 0.3A is 21 Ohms. Cold resistance (or with ~~200mV ohm meter) should be 3X to 10X of hot resistance, 60 to 200 Ohms.

So 16 Ohms feels wrong?

I believe he said that he was measuring at pins 4&5, which is the 12.6V connection. So 12.6V @ 150mA is 84R (hot).

Resistance rises with temp, so if it starts at 16R cold it would be a 5.25X increase to 84R. This seems reasonable to me.

If you don't agree, I'm ready to listen. Am I just confused?

[WimWalther]

What I don't quite get is the use of (apparently) PFB to control gain.

[plexi50]

Filaments 4&5 on the new power tubes read 1.5r. Again what ohms should i bee seeing on the 12AX7 filament leads pins 4&5 ?The boards and everything is back in place. I Quit! Don't want to but i am still getting conflicting stories that it worked but had low volume. Then i look and see a white smoked up EL84. Understandable at that point. But the previous tech kept the good known tubes and put what i have in here now. They all test good. But my tester is just a basic Emissions/Short tester and may not be seeing something else.

[plexi50]

Solid 3.64 VAC On each lead before passing through the fuses. Load Preamp tubes and get a filament of 2.74 VAC on all sockets.

[sluckey]

Quit mucking around with it! Get a 4Ω (at least 20W) power resistor. Disconnect the PT filament leads from the circuit and connect them directly to the 4Ω resistor. If the PT smokes or the voltage across the resistor is low then its bad. If you can measure 6.3v across the resistor the PT is good. If this is the case, continue mucking around.

[plexi50]

Quit mucking around with it! Get a 4Ω (at least 20W) power resistor. Disconnect the PT filament leads from the circuit and connect them directly to the 4Ω resistor. If the PT smokes or the voltage across the resistor is low then its bad. If you can measure 6.3v across the resistor the PT is good. If this is the case, continue mucking around.

Muckin one more time!!

[PRR]

I believe he said that he was measuring at pins 4&5...

We are all confused.

Does he mean *on* 4&5 or *across* 4&5? (to where?)

4&5 are commonly strapped, then 6V fed between 4&5 and 9. A 6V connectrion. Which may fit the 5V to 7V (and halves of that) he is reporting in many posts.

[plexi50]

I believe he said that he was measuring at pins 4&5...

We are all confused.

Does he mean *on* 4&5 or *across* 4&5? (to where?)

4&5 are commonly strapped, then 6V fed between 4&5 and 9. A 6V connectrion. Which may fit the 5V to 7V (and halves of that) he is reporting in many posts.

13.6 ohms between pins 4&5.   0.73 ohms between pin 4 & 9.  0.73 between 5 & 9. Pins 4&5 together to pin 9  03.9 ohms.These are out of circuit and in my hand tests

[shooter]

 
tubes n sockets be like apples n grapefruits