"Blows Fuses" Debugging

[R.G.]

I came late to the most recent "my amp blows fuses, what could it possibly be??" thread. I thought I'd post some thoughts expanding on the debugging/fixing ideas that are at the "Tube Amp Debugging Page" http://www.geofex.com/ampdbug/ampdebug.htm at geofex.com. This page has been on the net at geofex since the early 2000s, so it gets overlooked by the infinite supply of beginners that the internet provides us.
The central problem with fuse blowing problems is that the fuse blowing makes the amp stop what it was doing, so you can't use your meter. One approach to this is the light bulb limiter, which stops the fuse blowing, so you have some time to meter things. This is good for seasoned techs, as they know where to meter to find out what's going on. It's not so good for beginners who don't know where and how to meter.
There is a more straightforward way to debug fuse blowing issues that uses only a soldering iron and fuses. In general this is to connect only one new thing at a time to the chain of power from the wall socket through to the speakers. The new fuse will blow when you connect the thing that's eating all the current. Here's how that might work, as an example.
1. Put in a new fuse. Take out all the tubes. Temporarily unsolder the primary of the power transformer from the AC power wiring in the amp. Plug in and turn on the power switch. Only the AC wiring before the power transformer will actually be live because the power transformer and everything else is disconnected. If the fuse blows, the AC wiring in the front of the amp is the problem, and needs to be traced out and fixed. If the fuse doesn't blow...
2. Unplug, re-solder the primary of the power transformer into its place. Temporarily unsolder every PT secondary wire, and insulate them with tape, wire nuts, something to keep them from accidentally contacting something (including you!). Turn on the power. This step has added only the PT to the AC wiring, so if the fuse blows, it's a problem with the PT. If it doesn't blow...
3. Reconnect ONLY the heater secondary wires from the PT, not the high voltage secondary. Plug the AC cord back in, flip on the AC power. If the fuse blows, the heater wiring has an issue that needs traced and debugged. Notice - a heater short may not always blow an AC line fuse by itself, but it might. If the fuse does not blow...
4. Reconnect the high voltage secondary to the rectifier and disconnect the rectifier from the first filter cap. Insert a rectifier tube if your amp uses a tube rectifier. Plug in the AC cord, flip the power on and off, check the fuse. If the fuse blew, there is a problem with the rectifier or the wiring to them. If the fuse did not blow...
5. Reconnect the rectifiers to the first filter cap and disconnect the first filter cap from the output transformer and inductor or screen grid dropper. If your amp doesn't have a bleeder resistor wired across the first filter cap, solder a 220K 1W to 2W resistor across the first filter cap to bleed down high voltage, and connect your meter across the first filter cap to watch the DC voltage. Plug the AC cord back in, flip the power switch on and watch the meter. Did you get the expected high voltage? If so, turn off the AC power and watch the DC voltage ramp down. Check the fuse. If you got no high voltage or the fuse blew (or both!) you have a problem with rectifiers and the first DC cap, most likely the first DC cap. If no problem found...
6. Reconnect ONLY the high voltage from the first filter cap to the OT, leaving the screen supply open. Plug the amp back in, turn on power, then off. Did the meter show good high voltage, and the fuse remain OK. If no high voltage or the fuse blew, there's a problem with the OT.
7. Plug the output tube(s) only back in. Plug the AC power cord in again, turn power on and off. Did the meter show good AC voltage and the fuse remain OK? If not, there is an issue with power tubes, possibly a shorted tube, possibly wiring, possibly loss of bias. In a cathode-biased amp, there may be a problem with the cathode resistor and/or the cathode resistor bypass cap. If all is good ...
8. Reconnect the screen supply inductor (or resistor in some amps) to the first filter cap, and disconnect the screen filter cap from the rest of the dropping string by temporarily unsoldering the dropping resistor after the screen filter cap. Re-plug the AC line cord, turn on the AC power, check the meter for good high voltage, then turn off the amp and check the fuse. If the fuse blew, either the inductor has a short from winding to core (I've seen this, only one time, but it's possible), the screen filter cap has a problem, or the screen grid on a tube is shorted. If you suspect a screen grid short, pull the power tubes back out and re-try this test. If the fuse now stays good, swap for different, known-good tubes. Otherwise, check the screen filter cap and wiring. If all is well...
9. Reconnect the dropping string to the screen grid filter cap, re-plug the AC power cord, turn on and see if you get good DC voltage and power. In almost all standard-architecture amps, the dropping resistor string will prevent any signal tubes or wiring short after the screen grid filter from blowing fuses, so you will have found the vast, vast majority of issues by now.

There are always funny, truly rare and strange cases, but the process above will catch and isolate a fuse blower in all but rare cases. What this process amounts to is walking an open circuit from the AC power connections through the amp's power circuits, reconnecting only one section at a time. When the defective section is reconnected, the fuse blows.

[Winder]

Great stuff. Thanks for posting.

[Fritzer]

Thank you Mr. Keen! You are a Legend.

[JMFahey]

Thanks RG

Yes the only way is to go step by step, checking along the way.

I´d add: try to resist the impulse of "now that I´m here, I´ll improve/mod/replace whatever".

Unless you see something gross, say an exploded/leaking/bulged capacitor, a toasted resistor, carbhonized socket or PCB, etc. leave alone for now and focus on the main problem which brought you here.

You will have time later to "improve" but now you still want to minimize variables, not add new ones.

[xtian]

Phil! Sticky this, please.

[pompeiisneaks]

Looks like maybe colossal beat me to this one, it's already stickied.



~Phil

[xtian]

Looks like maybe colossal beat me to this one, it's already stickied.

I just enjoy bossing you around.

[pompeiisneaks]

Looks like maybe colossal beat me to this one, it's already stickied.

I just enjoy bossing you around.

You're here working my fingers to the bone, I see how you are.

~Phil

[jwhousley]

What would you recommend when an amp has a mind of it's own as far as blowing a fuse? I have an amp that seems to blow it when the standby switched is switched from on to off... only sometimes. Also, it may blow the fuse after playing for a while, and then suddenly just blows the fuse. And, sometimes, if you increase the volume and drive the amp harder it blows .... sometimes the rectifier (GZ34) flashes as it blows the fuse....

Very strange....

[brewdude]

I think I would start with a new rectifier.

[jwhousley]

I think I would start with a new rectifier.

Tried different ones so far... 2 new JJ's , Weber Z34 and a New Generic Electron one... same result.

[pompeiisneaks]

Did you ensure hte first filter cap isn't too much for the rectifier, they often have a max filter rating for the first filter node post rectifier.

~Phil

[pdf64]

As well as just the reservoir cap value, there’s the non obvious supply resistance feeding the rectifier anodes. That acts with the cap to limit the max current per half cycle each anode passes.
In the golden age, transformer designers took it as read that a certain degree of winding resistance in the HT winding should be maintained. That protects rectifier valves and causes saggy, hot running PTs. Whereas unless they’re instructed otherwise, modern transformer designers tend to use CAD to ‘optimise’ for minimal losses, hence get winding resistances to the lowest feasible level. Which may cause rectifier valves to become stressed, unless external resistors are added, to make up for the low winding resistance.

What would you recommend when an amp has a mind of it's own as far as blowing a fuse? I have an amp that seems to blow it when the standby switched is switched from on to off...

Far more info and detail is required, in order to analyse what might be happening, eg schematic and PT characteristics (winding resistances and voltage ratios).

The terminology around standby, like the concept itself, is somewhat topsy turvey.
Does the fuse blow when the standby switch contacts are made or broken? And what fuse are you referring to?

[jwhousley]

Did you ensure hte first filter cap isn't too much for the rectifier, they often have a max filter rating for the first filter node post rectifier.

~Phil

Yes, its 49uF...under the 60uF rating of a GZ34.

[mojotom]

I just looked at this thread as I had that fuse issue happening with flashing from the rectifier too.
I was a bit short on time so I had to find the issue quickly (other work to be done inside the amp).
Found out one of the first filters caps (two in serie) was shorted causing too much current draw from the rectifier.

Very nice How-To btw ! Thanks !!