Silverface Fender Bassman 100 - Checking Voltage Question?

[BonesMachine]

I have a 70's Fender Bassman 100 head and am a complete novice working on amps. Today I was trying to check the voltage at one of the filter caps and forgot to connect a load to the speaker out and blew a fuse. i immediately heard a couple of clicks (transformer arcs?) and turned the amp off real quick. i connected my speaker and turned on the amp and heard clicks again thats when the fuse blew? the amp was working fine before. i dont have an extra fuse to see if i messed anything up. i know the no load thing is bad. just wondering if the way i was checking the voltage could have caused this. i had my multimeter set to dc voltage, the black lead connected to the amp chassis and the red lead to the + side of one of the filter caps. so whats the word on that? that shouldnt be a problem right? thanks

[Jon Snell]

On a lot of valve amplifiers, touching a capacitive test probe onto the HT makes a minute spark that is seen by the amplifier as noise. This is amplified and if there is no load on the transformer can cause one or a number valves to arc or even the transformer can be damaged.
Remove the output valves, fit a new HT fuse and see what happens when powering it back up.
If all seems OK, one or both output valves have been damaged and must be replaced.
If the fuse blows, possible transformer damage, which is very unlucky. Seek advice from a competent engineer.

[catalin gramada]

an OT didn't blow instantly quite so. Imagine even nothing connected still see some load through nfb resistor. You need to do a little more to blow it. It was happen also for me to start an amp without speaker connected for minute, never broked any OT. You may do a simple test connecting a sine wave generator to output jack and without power tube inserted measure AC voltage on each side of PT primary relative to center tap.If there are equals meant no turns are in short and you get luck

[Jon Snell]

catalin gramada; "Imagine even nothing connected still see some load through nfb resistor".
22k on an 8 Ohm tapping, is not a load. The nfb loop will tame spikes but not stop them.
Probably a faulty valve will be found.

[catalin gramada]

catalin gramada; "Imagine even nothing connected still see some load through nfb resistor".
22k on an 8 Ohm tapping, is not a load. The nfb loop will tame spikes but not stop them.
Probably a faulty valve will be found.

It is more like 820+100 still.

[Chuck H]

It is more like 820+100 still.

Right. So greater than 200X intended load. I'm still going with John on this point.

[g1]

In any case most Fenders have the output jack wired up to short to ground if nothing is plugged into it. Unless that jack was defective, or an empty plug/cord was connected, the power tubes will have been running into a short rather than an open.
So the standard scenario is you will not damage a Fender OT by not connecting a load to the main speaker out jack.

[BonesMachine]

ok i just got some spare fuses. i plan on turning it on again without the power tubes and hopefully i remember a load. before i do that. are there any transformer tests that i can do in circuit with power off to minimize any possible further damage?

[The Dude]

FWIW: With the power tubes out, you don't need a load.

[Helmholtz]

ok i just got some spare fuses. i plan on turning it on again without the power tubes and hopefully i remember a load. before i do that. are there any transformer tests that i can do in circuit with power off to minimize any possible further damage?

Without power tubes the OT is essentially out of circuit. Only a short between primary and secondary or core would blow the fuse.

You can test the OT for winding shorts: neontest-1.pdf

Did you check if the output jack is shorting?

[BonesMachine]

ok thanks ill take a look at that info..so right now im just worried about causing further damage because it doesnt sit well that it continued clicking and blew a fuse after hooking up a load. so just to make sure should i be concerned with turning it back on (with tubes IN or OUT) at this point and causing further damage?

[Helmholtz]

No risk of further damage with power tubes removed (except maybe for fuse).

For tests with tubes, a bulb limiter will be useful.

What about the output jack?

[Chuck H]

What about the output jack?

Third this^^^^^^^^^^^^^^^^^^^^^^^^^ (The Dude mentioned it once earlier)

If the regular speaker jack was in use, but not plugged into a speaker cabinet that would mean an open load. Or if the jack is defective or somehow disabled it could mean an open load. If neither is the case there's a good chance you were testing into a shorted load. Chances then are good that it's just one or both tubes that were damaged.

[BonesMachine]

very interesting... there could have been an unused adapter not connected to anything in the primary speaker jack when i first turned it on with no load so i guess it could have been in use with no load? memory is a little hazy on that but very possible. the primary output jack is a shorting jack. its wired to the ext output which is not a shorting jack ..... by the way i tested all the power tube heater pins and no shorts... UPDATE: just plugged it in with no power tubes and it didnt blow a fuse. i left it on for about a minute. i was using a fast acting fuse just cause initial try. so i guess thats something good but looks like it doesnt test the OT. i havent tried any testing of that yet

[Jon Snell]

If no fuse blows with no output valves plugged in, the transformer is 99% OK. (Shorts to ground).
Measuring a valve with a DVM will not show up a faulty anode to grid leak and an anode to grid leak will not show up unless the correct voltages are applied to the valve on test and heaters are lit.
Even most "Tube Testers" will not show up intermittant short circuits in a valve because the voltages are not correct.