loud hum on power up

[Brian Evans]

I'm trying to resurrect a Grommes Little Jewel, to hopefully form the basis of a modified low power guitar amp. the LJ is a mono all tube, two 12AX7's in the pre-amp and a pair of 6V6GT's, with a 6X5 rectifier. I get a loud 60hx hum from the speaker when it's powered on and the heaters get warm, not affected by any control. It has new 12AX7's, the other tubes are original, no obvious faults. I suspect the rectifier tube - but with no test gear to speak of I don't know how to troubleshoot this. any thoughts on how to proceed?

Brian

[bnwitt]

How old is this amplifier? It sounds like the power supply filter caps may need to be replaced. Have you done that? Do you have a schematic or can you post a gut shot of the amp?

Edit: Ok, I found a schematic. It appears in the power supply you have the following filter caps:

40uf 400V
30uf 350V
10uf 300V
10uf 250V

You also have a 100uf 15V on V2 and a 20uf 25V on the 6V6 shared cathodes. If these are over 15 years old you should change them all. I’d use 50V caps on the cathode caps for V2 and the power tubes. Match or exceed the voltages on the power section filter caps.

[Brian Evans]

schematic

http://www.grommesprecision.com/grom...ewelreview.PDF

inside of this PDF is a review and a schematic of the amp. Mine is a factory version, immaculate point to point wiring inside. Caps are original. This was literally a "side of the road" find by my wife.

I find myself totally missing my oscilloscope - all I know about troubleshooting involves looking at stuff...

thanks, brian

[bnwitt]

Ok, well if the caps are original 1958 they need to be replaced. Heck, many hip joints don't last that long. Its almost guranteed they are the source of your hum. You don't need an oscilloscope to work on tube amps by the way. I would replace all 6 electrolytics before I did anything else. When the filter caps in a power supply are dried up they don't do their job of reducing dc ripple from the rectifier output. Think of them as little water balloons on a water pipe with a pulsating pump output. In between pulses of the pump the water balloons push back water into the pipe to fill in the pressure drop and smooth out the pipe's output of water. A dried out filter cap is like a water ballon with a hole in it. It can't build up electron (water) pressure during the pump cycle for the subsequent pressure drop discharge so you get a pulsating or rippled output.

[Brian Evans]

electrolitic caps

I've just been examining the amp, and it appears that the 40uF, the 30uF and the first 10uF 300V are all part of one can, which also has a 50uF 25V which is sub'd for the 20 uF on pin 8 of the 6V6's. All to ground, so the cap has 4 leads plus ground. The other cap has two leads and is the 100uF and the 10uF from the pre-amp section. The 4 cap can is 1.4" diameter, 2.5" tall, screw mounted to the chassis, the smaller cap can is 1" dia and 2" tall. Any idea about finding those parts?

Thank you very much for the help, I appreciate it!

Brian

[bnwitt]

I doubt very seriously that you will find those multicaps in stock anywhere. You will have to put in individual caps for the replacements. Here is Mouser's page for Sprague Electrolytics:

http://www.mouser.com/catalog/629/587.pdf

[Brian Evans]

Hi, another question. I can get a multicap from Antique Electronic supply that would fit, but rather than 50 - 40 - 30 - 10, it's 80 - 40 - 30 - 20. My recollection to college (35 years ago...) is that filter caps on rectifiers are not all that critical and I would think that this cap would work... it is about the right size (same diameter, just a taller. What do you think?

Brian

[bnwitt]

Well, you could probably get away with that. The The 40, 30 and the 20 could be used in place of the 40, 30 and either 10. The 80 wouldn't work for the 100 and if you put it in place of the 40 it would increase your bass output.

[Robert M. Martinelli]

Hi, another question. I can get a multicap from Antique Electronic supply that would fit, but rather than 50 - 40 - 30 - 10, it's 80 - 40 - 30 - 20. My recollection to college (35 years ago...) is that filter caps on rectifiers are not all that critical and I would think that this cap would work... it is about the right size (same diameter, just a taller. What do you think?Brian

Hi Brian,
Be advised that, if those multicaps are NOS ones, chances are they won't work as well....their shelf life is not infinite, and electrolytic caps last longer when operating, as the charge/discharge process helps keep the electrolyte in good shape. All in all, I would suggest to put in modern "separate" replacements as per bnwitt's advice rather than possibly worn out NOS replacements....Yes, you could maybe "reform" them, but it's a long and boring procedure so....why bother?

About your college recollection....talking 'bout tube amps filter caps are not all that critical in Push-Pull power stages, because whatever ripple/hum present there tends to be eliminated "by design" by phase opposition on the OT....usually PP power stages take their +B voltage at the PS "less filtered" point because they don't need good filtering and they' re rather forgiving in this respect.

The preamp stages are a wholly different story, because whatever ripple would be amplified together with the signal, so they need their +B to be filtered much better than PP output stages.

Increasing filter caps value can be done, but care has to be taken not to overwhelm the rectifier tube with an excessively high inrush current due to the caps charging. SS rectifiers have a higher current rating and can tolerate a much higher filter capacitance.

Hope this helps

Best regards

Bob

[Diablo]

Hi Brian,
Be advised that, if those multicaps are NOS ones, chances are they won't work as well....their shelf life is not infinite, and electrolytic caps last longer when operating, as the charge/discharge process helps keep the electrolyte in good shape. All in all, I would suggest to put in modern "separate" replacements as per bnwitt's advice rather than possibly worn out NOS replacements....Yes, you could maybe "reform" them, but it's a long and boring procedure so....why bother?

Hope this helps

Best regards

Bob

You can get brand new and freshly made CE cap cans from Antique Electronic Supply. They aren't new old stock made 30 years ago. They are a little more expensive than separate components, but they work just fine and typically fit into the stock location on the amp.

[Robert M. Martinelli]

I agree....I only wanted to caution him against putting in NOS filter caps.....that's why I said "If...." - that doesn't obviously apply if he manages to find brand new ( and fresh ) multicaps.
Regards
Bob

[guitician]

Power on with just rectifier

Hi, this may not prove anything, but did you try powering the amp up with just the rectifier? If it still hums you've got power supply problems. It's aways a good idea to eliminate any other cause first. I've got a dozen amps from the late fifties/early sixties and they all have the original power supply caps. I guess some caps make it, some don't.

[Brian Evans]

I ordered a new rectifier tube, it turns out that The Tube Store is in my town, so I should get it today (along with the Guitar Amp Book, and a pair of new 6V6's). I will try powering up with tubes removed, because it feels like a power supply issue to me. One thing I didn't mention is that there is no amplifier working at all, no signal getting to the speaker at all, so beyond the heaters working (which is obviously a separate circuit from the main power supply) there is nothing happening at all beyond a hum. We'll see. If nothing else this is a good thing to be thinking about and keeps me out of the garage (I work on race cars mainly).

thanks for all the feedback. Yes, the cap I was looking at is a new manufacture one. It's close enough that I will probably try it, or at least get a 2 or three cap can and only have to add on one or two separate caps inside the chassis.

Brian

[52 Bill]

I will try powering up with tubes removed, because it feels like a power supply issue to me. One thing I didn't mention is that there is no amplifier working at all, no signal getting to the speaker at all, so beyond the heaters working (which is obviously a separate circuit from the main power supply) there is nothing happening at all beyond a hum. We'll see. If nothing else this is a good thing to be thinking about and keeps me out of the garage (I work on race cars mainly).

If you remove all of the tubes except the rectifier and power up the amp, there will be no hum unless there is a problem with the output transformer, tube sockets or something else, as there is no way for the speaker to make sound through an open circuit.

If you have a single high voltage cap like a 20uF at 450v, temporarily wire it in parallel to the first filter cap section of the multi cap can. See what happens to the hum. Also you could try taking an AC voltage reading of the DC buss to see how much ripple is really there.