Fender Bassman 100 Head Silverface 1974 - Blows Fuse

[misterc57]

The new diodes are installed. I am testing the amp to see if there are still current issues. Tubes are installed. As the amp warms up (standby OFF) I am seeing the current rise to over 1 amp before I back off on the variac. No issues when output tubes are not installed.

What is the mornal operating current for this amp?

Thank you

edit: 300 watt power consumption, perhaps this amp is suppose to run around 2.5 amps?

[Helmholtz]

What is the normal operating current for this amp?

I don't have the exact value for your amp. But I expect input current to be similar to a Twin Reverb, i.e. around 1.25A at 120V idling. See MEF thread " SF Twin Reverb power transformer: how hot is too hot?"

[tubeswell]

My quick and dirty assessment of the VA at idle is about 128 (assuming 70% bias at 445vdc), so that translates to a smidgeon over 1A on the primary in your neck of the woods. Allowing for a small margin in current draw under heavy signal conditions, that gets to 1.2A on the primary tops. My 73 100W MV quad reverb (same PT, same number and type of tubes) runs in this ballpark.

[misterc57]

bias circuit issues?

I am not seeing any bias voltage. That single diode in the bias circuit, schematic shows -69VDC on the anode.

With the variac dialed up to 70 VAC output this is when I start to see the current heading north (after the tubes warm up);

I see around +15 VDC on the cathode and 0 VDC on the anode. If I continue to dial up the variac the anode does not leave 0 VDC.

That 80uf cap connected to that anode reads a short (while still in circuit). That does not seem right.

[tubeswell]

Those old white Mallory caps (in the bias supply in those amps) are notoriously bad. That one is probably shot.

[Enzo]

Well, a shorted cap across that anode to ground would certainly explain zero volts, eh?

Disconnect the cap and verify it is shorted, then replace it. Watch the polarity. The diode may or may not be damaged, for 5 cents I would replace it to be sure.

[Jazz P Bass]

If you suspect bias voltage issues, may I recommend that you remove the output tubes until you have it sorted out.

If you truly have +15Vdc at the bias diode anode, then right there is your massive current draw.
The output tubes more than likely have already been through more than one red plate period.
I would replace them after the bias issue is resolved.

[misterc57]

If you suspect bias voltage issues, may I recommend that you remove the output tubes until you have it sorted out.

If you truly have +15Vdc at the bias diode anode, then right there is your massive current draw.
The output tubes more than likely have already been through more than one red plate period.
I would replace them after the bias issue is resolved.

Yes, will do. Caps are on order.

The +15 VDC was on the cathode side.

Will I see proper voltages without the output tubes inserted?

Thank you

[misterc57]

Disconnected the cap and it is shorted. Diode tests good but I will replace with a modern one. Thank you!

[Leo_Gnardo]

Will I see proper voltages without the output tubes inserted?

Nearly so. The high voltages should test a little bit above what you expect, because there's no output tubes to draw current from the B+ supply. Bias voltage should be in the range you expect, say between -45 and -55V on pins 5. Before running the amp with output tubes in, it's a good idea to dial bias voltage to its most negative setting. Then make your adjustments from the "cold tube" end of the scale. Sure beats having excess bias current - red plating - and trying to adjust while in a panic.

One of the first things I do in an amp that's 40ish or more years old, just replace the bias filter cap(s). Be done with it. Cheap insurance. Also saves mental strain: "Why this? Why that? Something wrong with these tubes?" We have better things to worry about.

A caution: make sure the bias filter cap is installed in the right direction, with + end to ground. With decades of experience, I still make that error occasionally. These days I'm better at catching it & correcting before powering up.

[misterc57]

Before running the amp with output tubes in, it's a good idea to dial bias voltage to its most negative setting.

So when dealing with negative voltage, what is considered most negative? Say when comparing -40 to -80? My instinct would be -80 is most negative.

Thank you

[tubeswell]

So when dealing with negative voltage, what is considered most negative? Say when comparing -40 to -80? My instinct would be -80 is most negative.

Thank you

Yes -80 is more negative than -40 (which means -80 is a ‘higher’ voltage than -40). 0 is neutral = ground potential. Either side of neutral is electrical charge of a + or - nature. Depends on whether you’re counting surfeit or surplus of electrons (relative to 0, that is). All voltage is relative.

[Justin Thomas]

Yes, best to think of it as a number line. Zero in the middle, positive increasing moving to the right, negative increasing going to the left...

Justin

[Enzo]

A voltage has amplitude and polarity. A sort of vector.

[Helmholtz]

Yes, best to think of it as a number line. Zero in the middle, positive increasing moving to the right, negative increasing going to the left...

Justin

...or even better a vertical scale, where you can visualize the -40V is a higher voltage than -80V.