Hot rod deluxe Loud hum when standby switched on problem

[the great waldo]

I've got an old Made in USA hot rod deluxe which has a very loud hum when the standby is switched on. None of the controls make any difference. It just makes a loud hum and nothing else. I had a quick look inside but couldn't see any burning. Could the output transformer be blown ? or does anyone have any ideas as to what it might be ?

Cheers
Andrew

[Jon Snell]

I would check the output valves and screen resistors.

[xtian]

After you clear the power tubes and screen and grid resistors, next suspect is filter caps.

[g1]

Another quick thing you can try is remove the preamp tube nearest to the power tubes. Does it still hum?

[the great waldo]

After you clear the power tubes and screen and grid resistors, next suspect is filter caps.

I've ordered some new ft filter caps, but I don't think the caps are suspect as the hum normally isn't that high. I'm getting no audio signal through at all from the input.

[glebert]

Make sure you are plugged into the correct output jack.

[the great waldo]

Make sure you are plugged into the correct output jack.

Yup I checked that. I was wondering if a blown output transformer would make such a loud hum. If so I would check it with my meter. These amps are really poorly put together. I'v 2 old bandmasters that are a doddle to work. I must say i'm a guitar maker by profession and have a technician where I work that can service the amp but if possible I would like to be able to fix it myself.
Cheers
Andrew

[nevetslab]

I've ordered some new ft filter caps, but I don't think the caps are suspect as the hum normally isn't that high. I'm getting no audio signal through at all from the input.

If you've already checked as requested those suggestions from Jon Snell, Xtian & g1 (and we don't yet see results from those), what happens if you plug a patch cord into the Power Amp Input jack? A shorting plug is best for that, but just inserting a short patch cord will separate the Preamp stages from the Power Amp Stage. g1's suggestion removed the driver tube from the output stage, which usually stops the hum, unless you do have open screen resistor (most often cause) and grid resistors (seldom fail). If either/both screen resistors are open, chances are you have one or two bad power tubes.

You can also measure the combined plate/screen current of both tubes by measuring across the 1 ohm cathode resistor found on the left corner of the tube PCB, as you're facing the rear of the amp. Should be a 1 ohm/2W resistor, and the terminal closest to the rear is the cathodes. Opposite end is Ground.

[Helmholtz]

I was wondering if a blown output transformer would make such a loud hum.

A push-pull OT will cause hum if one of its primary halves is open. This would leave the amp running on only one power tube. You should still get some sound/output signal, though.

[nevetslab]

Yup I checked that. I was wondering if a blown output transformer would make such a loud hum. If so I would check it with my meter. These amps are really poorly put together. I'v 2 old bandmasters that are a doddle to work. I must say i'm a guitar maker by profession and have a technician where I work that can service the amp but if possible I would like to be able to fix it myself.
Cheers
Andrew

A blown output transformer would normally blow the mains fuse. AND, since you're hearing loud hum thru the speakers, I'd say your xfmr IS performing it's task. If you've already checked the driver tube (12AX7), and the hum doesn't stop, and if both screen resistors are intact (470 ohm/2W), you still may have bad power tubes, or just one of the two (6L6GB/5881). If either/both screen resistors ARE open, you probably have bad power tubes. Tube substitution would work here in both cases. I've had bad driver tubes that caused hum, swapping it cleared that hum.

I've also had hum result when the power supply choke (between Output Xfmr Primary C/T & Screen supplies) have failed. Measure the AC ripple voltage with your DMM in AC Volts mode on the two BLK leads from the choke. Careful....this is HIGH DC Voltage! If you're measuring roughly the same ripple voltage (like maybe 4VAC at the C/T and 3.8VAC on the Screen Supply, you've got a bad choke. Only fix there is ordering a replacement.

Do you have the schematic for this amp?

Hot_Rod_Deluxe_Schematic.pdf

[the great waldo]

A blown output transformer would normally blow the mains fuse. AND, since you're hearing loud hum thru the speakers, I'd say your xfmr IS performing it's task. If you've already checked the driver tube (12AX7), and the hum doesn't stop, and if both screen resistors are intact (470 ohm/2W), you still may have bad power tubes, or just one of the two (6L6GB/5881). If either/both screen resistors ARE open, you probably have bad power tubes. Tube substitution would work here in both cases. I've had bad driver tubes that caused hum, swapping it cleared that hum.

I've also had hum result when the power supply choke (between Output Xfmr Primary C/T & Screen supplies) have failed. Measure the AC ripple voltage with your DMM in AC Volts mode on the two BLK leads from the choke. Careful....this is HIGH DC Voltage! If you're measuring roughly the same ripple voltage (like maybe 4VAC at the C/T and 3.8VAC on the Screen Supply, you've got a bad choke. Only fix there is ordering a replacement.

Do you have the schematic for this amp?

[ATTACH]58347[/ATTACH]

Thanks I've got the schematic. I've tried 2 sets of Valves still hummed (It's a hum similar to putting your thumb on the jack plug whilst having the amp turned full up) I'll check the screen resistors tomorrow, (it's late here) thanks for all of your help so far.
Cheers
Andrew

[The Dude]

Check or resolder the output tube sockets. It's a very common problem on these amps. The output transformer is the last thing you should suspect.

[Enzo]

I was wondering if a blown output transformer would make such a loud hum. If so I would check it with my meter. These amps are really poorly put together.

Transformers should not be the first thing you think of, nor the second. They do fail, but rarely. And your meter could detect an open winding, but when transformers short, it is usually turn to turn, not end to end. In other words it might make a 1/10 ohm difference on your meter.

Ha;f a push pull would not remove all music from output.

When we say check the screen grid resistors, we actually mean their function. In other words instead of just measuring the resistors, measure voltage at pin 4 of each power tube. Is there B+ there? That way you are testing the circuit including the resistor. A good resistor can be on a circuit with a broken copper trace.

While in there, I suggest resoldering all pins on the power tube sockets.

[Mick Bailey]

A quick check is to measure the voltage at the bias point (LH corner of the power tube board viewed from back). Should be 60mv or thereabouts. If half of that, one tube isn't conducting. Those Illinois caps show yellow crud around where the + lead exits the rubber seal when they get really bad. They're usually bad anyhow. Or will be soon.

Did you remove the PI tube as in post #4?