Kustom tube amp not working

[gdamato]

Hello all,

I have a Kustom Defender 112 tube amp which is not working. I've listed some info on the amp and what I've noticed/tried thus far.

Facts -
- The amp has 2-EL34 power tubes and 3-12AX7 tubes (2 preamp, 1 phase inverter)
- When the power switch is turned on...
  -The pilot light on the control panel does NOT illuminate
  -Both EL34 tubes illuminate and heat up
  -I do not hear any noise/hum even when dials are turned

So far I've done the following, in an attempt to fix...
- Replaced the 2-EL34 power tubes
- Replaced the power fuse located below the power plug
- Swapped the power cord

Any ideas on what the problem may be or any logical next steps to try? Any help/guidance would be appreciated!

[sluckey]

Do the little tubes light up?

[gdamato]

Sorry sluckey, I'm just now seeing your reply. The little tubes do not appear to be lighting up.

[acheld]

From what you are reporting, look at several items:   Fuses F2 and F3; at R80 (a large elevated resistor); and the traces around the ferrite inductor L1.   

It could easily be something else, but your amp has a power supply for those three tubes as a standalone circuit -- the odds are that the problem may be lurking there.

[gdamato]

From what you are reporting, look at several items:   Fuses F2 and F3; at R80 (a large elevated resistor); and the traces around the ferrite inductor L1.   

It could easily be something else, but your amp has a power supply for those three tubes as a standalone circuit -- the odds are that the problem may be lurking there.

I think you nailed it! I just pulled those two fuses and checked them with a multimeter and they appear to be blown. Question for you on the replacements... the fuses read "T1AL250VP" which I understand is 1A 250v. My question is, there appear to be different varieties of these such as "fast acting" or "time delay". Would you happen to know which one I should use?

[sluckey]

T indicates time delay (slow blow).

     https://www.amazon.com/5Qty-Littelfuse-T1AL250VP-Slow-Blow-GLASS/dp/B07FQLNB89

[acheld]

Why did the fuses blow?   Any ideas?   Usually that means an closed (or short) circuit -- whether a transient problem, like a screwdriver poked in the wrong place, or a failed component.

[mresistor]

or a tube short

[gdamato]

*UPDATE* I just received the fuses in the mail and replaced them. Once I turned on the power, the pilot light came on for about a second and then went off. I haven't opened the amp back up yet, but I'm assuming it just popped the fuse(s) that I just replaced. Any idea why or anything that I can try/test?

[thetragichero]

unfortunately, the prevailing wisdom among much of the guitar playing public is "it blew a fuse so i must replace it" rather than "it blew a fuse so what's causing it to draw more current then it's designed to"
can be annoying when dealing with tube amps but with solid state amps can cause more and more damage every time it's run while failing. at the very least you'll want to construct a lightbulb limiter while testing where the problem is to save you replacing fuses. I'd see if the problem persists with no tubes in it, with just preamp tubes in it. possibly disconnecting secondaries, etc

[tubeswell]

*UPDATE* I just received the fuses in the mail and replaced them. Once I turned on the power, the pilot light came on for about a second and then went off. I haven't opened the amp back up yet, but I'm assuming it just popped the fuse(s) that I just replaced. Any idea why or anything that I can try/test?

Fuses are designed to blow when too much current goes through them. So, look for the cause of 'too much current'. Its a process of elimination.


The first thing you can do is check the PT is not shorted internally. Pull the tubes out and disconnect all PT secondary windings and (making sure the winding ends are not shorting to the chassis, or to each other) pop a new mains fuse in and switch the power on. If the fuse blows, it is likely a shorted PT. If it doesn't blow, then you'll know the short is in the circuit(s) after the secondary windings. Hook up the heaters next, and if the fuse still doesn't blow, then hook up the bias supply and check you've got negative voltages on the output tube grid pins. If that's okay, and the fuse still doesn't blow, then hook up the rectifier and the reservoir cap (without the tubes in) with amp off standby. If that blows, you'll know its a shorted rectifier and/or reservoir capacitor. If it doesn't blow, put the output tubes in and switch standby on see if the fuse blows (in which case it's shorted something else, like a shorted turn in a choke (in which case, disconnect the choke and see if fuse still blows), shorted bleeder resistor or output tube. If it's any other shorted filter cap, other than the reservoir cap, the fuse may not blow straight away (although supply resistors will probably cook and smoke). continue looking for shorts between power supply voltage sources and the ground etc etc


I'm assuming you're smart enough to take all safety precautions when changing hookups around with the power off, caps discharged and the plug disconnected from the mains etc etc . If you don't know this, then leave the amp for someone else to fix

[acheld]

Which fuse blew?  They are labelled. 

If it's F2 or F3, by the schematic, it really limits the components which may have failed, so you should be able to track down the failure point.

[mresistor]

Why not put this amp on a light bulb limiter, then remove all the tubes, replace the fuses and see if there is high current. If not check the -bias voltage  If everything is ok then add tubes, power tubes first and then the 12AX7s one at a time.  At least with the limiter you could save some fuses.

[gdamato]

First off, thank you all for the advice. I'm slowly realizing that I'm getting a little beyond my ability. Both F2 & F3 fuses have seemed to blow. Attached is a link with a few pictures in the event you may be able to clearly see an issue, such as a bad capacitor or something. I may have to try to find someone that is more qualified to repair this.
https://photos.app.goo.gl/VL5u8Jv2hoEqdPTN9