tube amp keep blowing fuses

[oomnelson]

Hi everybody !,

I have this issue with a tube amp (a copy of a twin reverb): the amp was working fine, but suddenly, it blowed up the fuse, so I changed the fuse with one of the proper value, 2A slow blow, and blowed up again. I try another, and the same. So, I pull out the power tubes, and same thing, fuses blows up. It´s solid state rectifier.
I checked all the connections with the tester in ohms, and visually, and everything looks fine. I can´t perform a volt reading because the fuse don´t let me.
My question is, what more can I do ?... which one is the next step?.

Thank you !

[R.G.]

1. Go to geofex.com and read the tube amp debugging guide.
2. Construct a light bulb limiter if you have the necessary AC mains power wiring skills.
3. If you don't already have the skills in (2), take it to a tech.

[oomnelson]

Thank you !!!, very clear information about what to do. 

[GibsonGM]

1. Go to geofex.com and read the tube amp debugging guide.
2. Construct a light bulb limiter if you have the necessary AC mains power wiring skills.
3. If you don't already have the skills in (2), take it to a tech.

+1,000,000

The amp is trying to tell you there's something pretty wrong with it, like a short. But maybe something simple and not costly, we hope. Many troubles are like that. 
Bring it in; your safety is more important to us! 

[amptramp]

If a slow-blow fuse blows immediately, then whatever is wrong is not far form the plug electrically and may be any component on the power line side of the transformer or the rectifiers or the input capacitor of the power supply.  I had a very nice H. H. Scott 340B receiver given to me because it kept blowing fuses.  But the fuses looked so totally blasted - fuse material condensed on the glass envelope of the fuse - that I knew it couldn't be anywhere much after the transformer.  It turned out that one diode in a bridge failed short and acted like a short across the transformer.

[oomnelson]

Thank you for your reply !... I have the fuses totally blasted, and all blows up inmediatly. So you say I have to check the rectifier stage, and all the capacitors in that stage first, right ?.
The amp is almost new, and was working fine, I am inclined to think that there shouldn´t be the tranformers the cause of fuse blows. But anyway, with the help received from you, and the geofex web, I have work to do.

[GibsonGM]

Well, oom...listen...yes, the transformer could be blown.  Or the rectifier. Or filter cap(s) after that.   Or a porked tube in V1.
Or a chafed wire touching the chassis.  Or a dead bug inside bridging something.  We just don't have enough info yet, and maybe you don't have the experience yet to get it...which is NO shame, it's something that takes time and small steps.

See what I'm getting at?   You can measure resistances on the transformer to see if it's junk.  You can also disconnect it from the rectifier to see what voltages it puts out if it's NOT junk.   And that can get you killed, like we talked about above.  ANY power-on measurement is potentially dangerous.

Let's try this, to start off easy.
What do you measure for resistance across the transformer primary?   Unplug the power cable, turn the amp on AFTER it's unplugged, and measure across the blades (not ground).   See how many ohms you get.  For ex., my Marshall 18W reads ~4 ohms.   So, it'll be low but existing.    ZERO ohms means a short on the primary side.   Way high ohms....well, what? Burning up?

BTW, you'll need to have a new fuse in for this test, but NEVER have had powered it up (obviously).

[amptramp]

I doubt you would be able to see a difference in the resistance reading of a winding in a power transformer between a good one and a shorted one.  Most have over 1000 turns in the primary and a short between two adjacent turns will blow things but be undetectable by resistance measurements.  But this would probably be too small a rise in current to get the fuse to blow immediately.

[oomnelson]

Thank you so much for all the information, and the advertisement about safety. Tomorrow I will be checking the amp following the indication from geofex webpage. I´ll share the experience here.

[GibsonGM]

I doubt you would be able to see a difference in the resistance reading of a winding in a power transformer between a good one and a shorted one.  Most have over 1000 turns in the primary and a short between two adjacent turns will blow things but be undetectable by resistance measurements.  But this would probably be too small a rise in current to get the fuse to blow immediately.

Yes, you're 100% right, amptramp...I'd like to give him some place to start, however.  1st thing I'd do (at this level: after eliminating things like blown rectifier and the like - the "power off" stuff...tube testing....) is probably open it up, remove the HT and heater connections, and see if the trafo is giving rated voltages.  Nice way to ZAP yourself, though.   OP doesn't seem to have the background to 'go live' at this time, IMHO. 

[oomnelson]

Hi everybody, as I said yesterday, I was working today with the amplifier, following the geofex steps, and I wrote the data about what I did:

- fuse is the wrong rating - replace it with the correct rating - OK
- power tube shorted - OK
- rectifier tube shorted - OK
- power supply filter cap failing - OK
- Carbon trails on the output tube sockets between the plate lug and the other electrodes, especially the heater electrodes - OK
- there is an ac wiring short or high leakage - OK
- AC power wiring or B+ power wiring is faulty/shorted - OK

- power transformer is faulty - Here I found the trouble, the following are the steps I did:
1. Using an ohmmeter, measure the resistance of the primary (usually Black - Black wires) and the resistance of both leads to the chassis. The primary should be under 1K ohms. If it is not, the transformer is dead.

Here the resistance is 6,2 ohms

2. Measure the resistance to the chassis from both ends of the primary winding. It should be more than 1M ohm.
If it is shorted or less than 100K ohms, the transformer is bad. If it is between 100K and 1M ohm, unsolder the primary leads from the terminals they contact and measure again.

Here, I found both ends shorted.

So, this is the cause for the constant fuse blows. I´ll send to re-wire the power transformer. If there is something more to learn, and someone want to share the knowledge, I´ll be very grateful. But I´m very grateful already with all the comments.

[R.G.]

If you have low resistance to the chassis from the primary wires, that is a cause for fuse blowing.

Because power transformers are so expensive, unsolder or disconnect the primary wires from the switches, fuses, terminal strips, whatever, so you can test just the wire leads from the transformer. This disconnects all the AC power wiring from the transformer, and if the transformer still shows shorted to chassis, it is indeed a fault inside the transformer. If it does not, the failure is in the AC power wiring.

And be sure you're testing the primary. The high voltage secondary has a grounded centertap in most of these amps.

[oomnelson]

Thank you R.G. !, I unsoldered the power transformer primary wires from the switch, and tested again, between both ends I have 6,2 ohms again, and between each end and chassis, is shorted.

I tested the choke, and it´s fine.

And now I´ll test the output transformer...