Hummmm help

[dwhutchens]

I just recapped an Ampeg Rocket R-12. After replacing all the electrolytics I had a ticking tremolo and this weird thumping hum, kinda like a heartbeat any time the volume was past 4 or 5. I completely rebuilt the tremolo circuit and got rid of the ticking but not the heartbeat. I replaced the rest of the caps one by one and most of the resistors with no luck. After studying the wiring and comparing it to the schematic for a couple of hours I found that the B+ was wired wrong, the voltage was higher on the preamp than the phase inverter. I fixed that and the heartbeat went away.

Now I'm down to a constant 60 cycle hum on the speaker. It measures a little less than 400mv peak to peak on the scope It goes away when I pull the PI and preamp tubes. I can see it all the way back to pin 2 and 5 of the phase inverter. I used a can filter because theres not much room in the chassis. I unhooked that and built the power supply on an eyelet board with individual caps and ran jumpers out to it with no change. I tried disconnecting the filaments on the 6v6 and 6sn7 tubes and running them off 4 flashlight batteries and the hum did not change. Ive been moving wires around with no luck. Any other ideas? Please!
Thanks,
David

[olddawg]

Did you replace the big metal can 4 X 20mf cap? It is the main power supply filter and the first thing to suspect.

[dwhutchens]

Yes,

I replaced it with one from Antique Electronics. The 6v6 cathodes were tied to one section of the can. I put in a dedicated 25/50 cathode bypass cap and tied the first two sections of the can together for 40uf on the first filter.

[dwhutchens]

Progress?

Tonight I hooked a couple of 16uf caps to some jumper wires and tried clipping them across different points on the B+ circuit. When I connected them to the B section that goes to the PI and the 6v6 screens the hum was reduced considerably. How much capacitance can I safely have at that point without damaging the 5y3? Currently Its wired as in the schematic except I have two sections of the filter cap tied together for the first stage going to the center tap of the output transformer.

[olddawg]

I have a 1960 Rocket that I recently recapped because of hum. It is so silent now you have to get very close to even know it is on. Instead of trying to engineer around the problem you might want to find out what the problem is. Do you have a scope? Look at the ripple voltage. Was the filter cap can you replaced a NOS cap or a new one. New ones are available because the old Sprague machines were shipped to China and they are being made again. The new ones are an exact replacement manufactured to original specs. Fliptops has them for $28. A NOS cap may be as bad as the one you took out. The only reason to use a NOS electrolytic is for historic restoration.

[dwhutchens]

I assumed it was new,

I got it from tubesandmore.com Here are the specs:

CAPACITOR, ELECTROLYTIC, 20/20/20/20 µF @ 475 VDC

C-EC20X4-475

Made in the USA, to the same specs as the Original Mallory, on Mallory's original machinery.

4 sections - all 20 µF @ 475 VDC
-10%, +50% tolerance
85° temperature rating
1-3/8" diameter x 2" tall

I'll try and measure the ripple but I probabley won't get to it until Thursday night.
Thanks,
David

[MWJB]

"How much capacitance can I safely have at that point without damaging the 5y3?" As much as cures the hum, if that is what's causing it. I wouldn't go over 100uf but, I would have thought that 40uf would have been enough in most cases.

This seems though to condradict your earlier experience when pulling the preamp tubes & the hum disappearing? Have you tried seperating the grounded eyelets on the board and running individual ground wires to the input jack ground? This had a major effect in reducing hum on my Reberberocket.

[dwhutchens]

I have tried several

different ground schemes but I may try some more. Does a ground loop cause 120hz buzz or 60hz hum? I get a buzz if I turn up the volume but it goes away when I put my hand on the chassis over the preamp circuit.

Olddawg, you don't by any chance have a photo of your chassis you can post so I could see the layout. I think someone had been in there before me a long time ago. Some parts didn't match up to the circuit or the origianl components but were obviously from the 60s or early 70s.

Thanks,
David

[olddawg]

different ground schemes but I may try some more. Does a ground loop cause 120hz buzz or 60hz hum? I get a buzz if I turn up the volume but it goes away when I put my hand on the chassis over the preamp circuit.

Olddawg, you don't by any chance have a photo of your chassis you can post so I could see the layout. I think someone had been in there before me a long time ago. Some parts didn't match up to the circuit or the original components but were obviously from the 60s or early 70s.

Thanks,
David

Mine is also a R-12. Mine was bone stock when I got it with no replacements. It almost exactly follows the schematic of the R-12A drawn by Joe Piazza on 4/18/97.

[dwhutchens]

Ripple

Do you have a scope? Look at the ripple voltage.

I got a chance to fire it up again tonight. There's definitly ripple on the B+. I set my voltmeter on AC and read 7.2vac on the B+ with 40uf on the first stage and 20uf on the next two. I clipped a 33uf cap across the second stage and the ripple dropped to 3.9v and the hum got quieter. I put another 32uf across it and the ripple went down to 2.8 and the hum is even less but its still there. Tomorrow I'm going to play with the grounds some more. I have a half-breed amp I put together from scratch on a $13 Hammond chassis with Weber's cheapest transformers that started life as a 5e3 clone and wound up with a single input Princeton tone stack into the unused half of the 12ay7 and an extra gain control and you have to stick your head into the speaker to hear any noise at all with the volume down. I just hate to give this Ampeg back to my friend with this much hum.
David

[olddawg]

It is sounding more and more like the new 4X20mf cap is bad to me. Did you solder the can to the chassis? The only other thing that could be causing that much ripple would be someting drawing excessive current. Does your amp have a 22K 1 watt and a 1k 10 watt resistor between three of the caps? Look at the R-12A schematic.

[dwhutchens]

No,

the origional one was mounted to one of those insulated things you sick the can tabs trough and twist them so I mounted it the same way. It's only grounded through the power transformer.
David

[dwhutchens]

Ok, so I regrounded everything today.

I pulled the transformer screws out and cleaned them and the chassis and grounded the PT center tap, cap can and mains cord all to the same screw with new star washers. I ran all the circuit grounds to one of the input jack grounds. I ran the OT ground wire to the input jack. No luck

After studying the diagram some more and comparing it to my 5e3 fender diagram I decided to move the PI from the same power supply node as the screen grids to the preamp node. Guess what! No more hum I pulled all the extra caps out so I only have 3 20uf sections of the cap can in use and still no hum. Silence until I turn up the volume and get some pick up noise

Now the side effect to this seems to be a much weaker tremolo. Should I use the leftover cap section for another power supply node to feed the PI separately? If so, should I drop the 22k and use two 10k, or use two 22k resistors. Or will this make any difference? I think I'm getting close.

Thanks,
David