Thinking of getting into tube amps (and repair)

[tim0101010]

I'm hoping for some wisdom from some of the experienced tube amp repairers/builders out there.

There is a broken Fender Champ clone (but with SS rectifier) available that I was thinking about buying. A Champ seems like a good place to start. I’m no expert with analog circuits, more of a tinkerer. But I've been doing some reading up on tube circuits (also have an EE degree), so I'm hoping that there's a good chance that I can avoid electrocuting myself.

The only info available is that the fault is described as ”won’t power on and the tubes don’t glow”. So it looks like the fuse might be blown, but it would be good to know the most likely causes for this. If its the power transformer then it is probably not economical to buy. However, if it's a solder joint, 6V6 or audio transformer then it could be a good project.

Does anyone want to hazard a guess and put some odds on the likely causes? (The amp is only a year or two old I believe)

Thanks

 

[corliss1]

Welcome to the Forum!

I don't believe in guessing - get it on the bench and find out. It could literally be anything - blown fuse, a tube could have caused the fuse to blow, bad PT, bad OT, aliens. There's no way to know until you open it up and start getting some measurements.

Dead is always much easier to fix than "it works but it sounds funny," at least in most cases. It doesn't mean it's always the cheapest fix, though.

 

[fretknot]

A Champ is a good place to start. Many here will tell you to be cautious with capacitors, and to discharge them before getting your hands in the circuit. With your EE knowledge it shouldn't be a difficult task to sort out that amp. Most likely it will be a pleasant experience.

I've had the same problem come up a few times. Once it was a broken solder joint on the power switch that wasn't visible because it was under a slip-on rubber insulator that covered the terminal.

A fuse is the first place I'd look. If it failed, try replacing the fuse then power up the amp. If it blows another fuse, then try again without the tubes installed. If it doesn't blow a fuse without tubes in place, then you will likely find a short in one of them.

If it blows a fuse without the tubes, it could be a snubber cap on the SS rectifier. Test them for continuity. If that's the problem, then you will be able to identify it quickly. Something will be shorting. I also would suggest looking for poor connections and cold solder joints.

The Champ is simple, so have at it and give an update if you decide to take it on.

 

[Jazzerstang]

I definitely recommend starting with a 5f1 Champ circuit. I built one a few years ago and I have even modified the tone slightly a few times. A couple years later I built a 5f11 clone and it was like 3 times harder. Of course it helps that I have my electrical engineer dad who worked in television for 30 years for advice lol. Both amps are mojotone kits. The Vibrolux is basically exactly what the kit came with.

 

[1 21 gigawatts]

I would hope that whoever is selling it would have been smart enough to check the fuse before deciding to sell it. I wouldn't count on the fix being that easy. Still could be a good pickup and fun project. Good Champ clones aren't cheap, even building yourself.

 

[dogmeat]

if the fuse blew, there is a reason. buy it, bench it.

maybe start here: https://robrobinette.com/Amp_Stuff.htm

Rob is on this forum.

next place might be: https://el34world.com/Forum/index.php

or here: https://ampgarage.com/forum/

 

[tim0101010]

Thanks for the encouragement and all the useful info. This is a great forum.

Let me explain my main worry about the project. If the power transformer is a problem, then sourcing a new one might be rather expensive (eg the MOJO759EX is US$173 including $88 shipping from mojotone.) Is there a cheaper source? Anyway, thats why I was interested to get an idea of the failure rate of the power transformer.

 

[guitar_paul1]

https://www.mouser.com/ProductDetail/Hammond-Manufacturing/290AX?qs=HhoUExdhPFnvTLM0sOFUBw%3D%3D

 

[corliss1]

$88 shipping seems high. I have a PT coming from Germany right now, and 4-day UPS is about $40.

 

[NTC]

The OP is in New Zealand. That might impact shipping some...

 

[AxemanVR]

.
Since you don’t know why it doesn’t work you really have no choice but to consider the “worse case scenario”, which could be: “everything is fried!”.

Okay, perhaps not likely, but who knows, maybe a previous owner cranked it up for three hours straight, overheated the speaker, which cooked the output transformer, which fried something else in the circuit, which blew the fuse.

Thus is the “trying to repair an unknown broken something” risk you face.

It still may be worth attempting though, just for the learning experience (assuming you don’t get electrocuted of course)…

Good Luck!


.

 

[bebopbrain]

Snap it up. How much is at risk buying a small broken amp? At worst you get a chassis and small combo cabinet to play with. You are ahead of the game since you don't freak out if you see a term like "jωC".

Fender Champ clone (but with SS rectifier)

A good sign; tube rectifiers in a Champ clone are idiotic.

 

[Whatizitman]

I started my path on a similar project - SF Champ a friend gave me. Still very much a noob. And definitely no engineer. But IMO nothing beats getting a broken tube amp for cheap and learning how to fix it. Best case scenario you like it once it's fixed and keep it, paying far less than for a working model. Or you flip it and pay for another project. I've done both in the last few years and it's all but paid for itself. Worst case you try and fail, and learn something in the process.

Find out the make and model, and that will give you some preliminary red or green flags. If it's a P2P or turret build clone, then definitely easier to diagnose and fix. If it's PCP, then unless you have experience soldering on PCP circuits, know what you're getting into.

Failure rate on transformers are low in comparison. But even if the PT is toast, if the amp cost is low enough, it's worth it just for the hands on experience of learning how to diagnose problems.

BTW, a blown fuse is a "good" sign. It means the fuse is doing its job of keeping a short from killing off the major components. A blown power tube and screen resistor is a common scenario.

Next to a good DMM and soldering iron, the best tool to have is easy and cheap to make, which is a current limiter. There's no way I would have discovered the shorts in my repair and mod projects without it. Check out @robrob page for info on that. Also Uncle Doug on YouTube has some instructions on building a current limiter. You will need a high wattage incandescent bulb (no CFL or LED), which are getting harder to find. Not sure if available in NZ.



Have fun! And welcome to the Rabbit Hole.

 

[King Fan]

broken Fender Champ clone

All good advice here. But let's focus on the word 'clone' a minute. Is it a commercial clone from a known shop? Or a DIY clone from someone's basement? The odds of a simple repair go way down on the latter. Way way. Of course, the amount I’d pay for a broken DIY clone would go way way down also….

 

[OldPup]

Lots of good takes here. I will add that the hobby is not inexpensive. You may be somewhat kitted out already given your background. If you're getting into it to save money, that's a fun enabler but not a completely accurate assessment of the road ahead. Regardless, I say do it. Buy the Champ, it's a simple circuit. The seller should knock some off the price to account for the your time and the money a tech would charge. Keep in mind hobbies usually cost money.

Depending on answers to King Fan's questions - if it's a clone by a hobbyist, I'd consider dumping the solid state rectifier (which I hear results in higher voltages compared to tube rectifiers), look for blown caps/fuse and bad solder joints. But that's just me. Obviously, some people disagree with at least part of that. Sounds like fun either way.

 

[archetype]

$88 shipping seems high. I have a PT coming from Germany right now, and 4-day UPS is about $40.

Shipping from the US to NZ is brutal. I recently shipped a set of 3 books to NZ and the USPS rate was $90.

 

[Lynxtrap]

What about https://nz.mouser.com
One would think there are other stores as well in NZ or at least Australia that sell tube amp stuff?

 

[screefer]

Welcome!
You are about to have a lot of fun.
Looking forward to your troubleshooting thread!

 

[printer2]

It depends on the price of the amp. No idea what your local market is like, any tube radios around? I am guessing you might not have the opportunities in other places but I have brought home a number of radios and organs to the point where I do not know what to do with the stuff. Mind you, it may mean making up your own chassis and cabinet. Depends if you have the tools and skills to make your own.

 

[jered1011]

Matt at musicalpowersupplies.com sells good transformers for smaller amps