Fender Bassman Rumbling

[mynameisjack2]

I've done a bit of searching and can't seem to find an answer.

I recently got a 70s Fender Bassman 100, I like it a whole lot, but I've noticed that sometimes it produces a hum (sounds a lot like a rumbling oscillation) I've checked to make sure it's nothing in cable (it does it without anything being plugged in) and I've noticed that tapping or moving it causes it to stop for a bit. Sometimes it will stop for 10 seconds, others for hours. The previous owner mentioned that the power cable wasn't properly grounded, could that be the issue? If not, where do I start checking for problems? I'm a newbie to tubes, so I don't know much about them.

Thanks!

[charlie monroe]

Originally Posted by mynameisjack2

The previous owner mentioned that the power cable wasn't properly grounded, could that be the issue?

I don't know if this is causing your hum. It likely is. I do know that you need to put a 3 prong grounded plug on that thing right away. It is not safe. Tube amps use very high voltages.

I have a Bassman 100 also. Dead as nails right now, but it shall live again.

[mynameisjack2]

I'm not entirely sure how to go about doing that. It's a three prong plug right now, but I don't believe the 3rd prong is effectively grounded. He said that the power cable was not original, how do I go about replacing the cable with a new grounded one?

[BogeyBass]

Its hard to tell from your description if its mains 60hz hum or if its self oscillation.

Buying a old tube amp is a investment and usually requires a nice clean up and then its golden.
Aside from retubing, common things done to a tube amp to reduce hum and improve performance is Replacing the large power supply silver capacitors, checking/replacing a few high power resistors. And checking replacing the Bias resistors and capacitors.
Cleaning tube sockets and Control knobs.
this requires a bit more investment over the original purchase price. And requires a fair amount of electronic knowledge.

Self oscillation is common with Bassman 100's because of the poor lead dress of the wiring, or wiring layout.

To address this problem Fender added 2000pf capacitors to the grids of the output tubes.
Often Guitar players or other genius people remove the 2000pf caps because it "affects the tone" usually if no issues are found it seems to not be a problem....until the tubes get old or wiring gets shifted around from general maintenance.

So you need to check if the 2000pfs are there, and if they are bad. any Grid wire going to pin 2 or 7 on the 12ax7s are very sensitive. Its good to keep them away from high voltage plate wires or mains power wires. Also check all connections for loose wires or poor solder connections from previous maintenance.

Alot of problems also come from the phase inverter and everything connecting it. Like the .1 coupling cabs, cathode plate and grind resistors. also Good thing to change is the 1watt 470 ohm screen resistors to 2watt.

but before you get crazy and just start swaping everything its better to save money and time and just measure check whats there.

Otherwise hate to say it but if you have to ask how to change a power cord. most likely everything I said sounds like jibber jabber. and there is no way I can train you over the net to do those things. And their is very important safety measure to take when working with high voltage.

its possible you might just need new preamp tubes, use the proper Ax7 for V1,V2 and a AT7 for V3 or the phase inverter.

[skychief]

And think SAFETY before attempting any repairs/mods to your amp.

Power supply capacitors can store 300 -500 volts - even with the amp turned off and unplugged! They must be discharged properly PRIOR to to any work on the amp.

If you are not familiar with this procedure, I would recommend you have a qualified tech do the diagnosis/repair.

[craig.p]

I agree you should bite the bullet and take it to a tech for a post-purchase overhaul. This should be SOP for recently-acquired tube gear when things don't seem quite right and where the new owner isn't familiar with troubleshooting and rebuilding. Like Bogey says, it's part of the deal when you board the tube amp train -- like it or not. Once it's done and the pain is over, though, that amp should be good to go for years, as long as you run it right. In other words, that tech cost spread out over time really isn't all that bad.

[BassmanPaul]

First thing first is to get your power cable replaced and your chassis grounded. That is a #1 safety issue!!! THEN and only then should you try to go further with the diagnosis.

Start by gently tapping on your pre-amp tubes. A microphonic one can cause the amp to feed back if sitting atop a speaker cabinet.

[4Mal]

Time for a tech. That thing can kill you dead... I'm happy to do lot's of my own maintenance but unless you're qualified, working on an amp like that is not advisable.

As a suggestion, 3 prong cord, power tubes, bias check.

I had an old MM RD-100 redone recently. It didn't cost much to get it reworked and it sounds great through a couple of 3015's. very similar tone to what I used to get from a Showman amp, maybe a little stiffer front end, as you would expect from the Solid State pre. Lovely old school tone.

[mynameisjack2]

Originally Posted by BassmanPaul

First thing first is to get your power cable replaced and your chassis grounded. That is a #1 safety issue!!! THEN and only then should you try to go further with the diagnosis.

Start by gently tapping on your pre-amp tubes. A microphonic one can cause the amp to feed back if sitting atop a speaker cabinet.

I am fairly knowledgable with circuitry, so I'm not oblivious. I'll probably take it to my circuitry class and get some help from my teacher in grounding it. I think it must just be a loose tube, as when I tried it the sound didn't occur, and I think it might have shifted in moving. It is sitting atop of a stack right now, and when I moved it, the sound ceased for the rest of the evening.