Pultec clone - grounding problems

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Winston OBoogie

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View attachment 82080
someone sent these to me, saying they would help with ridding hum from an ac heater supply on a 12ax7 preamp. Does anyone know what they are?

What did the sender say was the recommended use of these three thing-umy-jiggys for getting rid of hum?

As divining rods?

At first approximation - I suppose you could use the metal bit at the yellow end to poke a few holes in your woofer so you can get at the wires and rip them off. That would get rid of quite a bit of low end hum.

Maybe with the other two you could use the black plastic end to plug up your ears (please don't do this). That would lower the remaining hum even more.
 
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Nbtone

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What did the sender say was the recommended use of these three thing-umy-jiggys for getting rid of hum?

As divining rods?

At first approximation - I suppose you could use the metal bit at the yellow end to poke a few holes in your woofer so you can get at the wires and rip them off. That would get rid of quite a bit of low end hum.

Maybe with the other two you could use the black plastic end to plug up your ears (please don't do this). That would lower the remaining hum even more.
I think they might have come from this place. I don’t know. Entreq - Grounding Solutions | Official Dealer Futureshop.co.uk
 

Nbtone

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You could try floating your heater ground with .047uf 600v polypropylene capacitors instead of 100 ohm resistors. This worked out for me pretty well, but there is still some low level hissing when you push the volume. I tried everything before I resorted to trying this.
 

Brian Roth

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I ran across those boxes awhile back (read the "description" part way down the page):


I think a better idea is to combine that with another sort of box found in many homes, the kitty cat's litter box! That way the consumer will have a real reason to criticize the music or system with "WELL, that stinks!"

Of course, replacing the cat litter often will keep the sound clean.

Apologies for the veer.....

Bri
 

morls

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I've been looking at ways to get this bridge rectifier happening. I found this thread.

I've copied this from post #3 (thanks to CJ)

rectifier circuit 2 copy.jpg
I'm using a Hammond 369JX PT, which gives 6.3VAC @ 2.5A.

I'm still trying to work out how to make the bridge rectifier, looking at schottky diodes. This is the limit of my current knowledge, so I'm in need of help!

Does this circuit look like it will do the job?
 

Winston OBoogie

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Yes, looks fine. You shouldn't need the 50 ohm trimmer though.
Two sets of RC filtering work better than one big one so, using schottky's, and with the heater load you have, you might find you can add a second stage of filtering with the series resistor of that second filter being an "adjust on test" resistor to set for 6V3 DC at the tube heaters.

A quick fingers and thumbs mental exercise tells me a pair of 1 ohm resistors in two RC filters would get you in the ballpark.
Ground the negative of your last filter cap rather than using that 50 ohm wire wound pot.

Edit: one other idea might be to use a balanced filter right after the diode bridge with a 0.5 ohm resistor in both legs to your first filter cap. Then use a single "adjust on test" to your second cap. This would help keep out any diode switching nasties and ripple current from your heater supply.
 
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Winston OBoogie

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Yep. I underestimated the 6X4 heater current a little, your total current for all tubes is 1.2A so, a 1 ohm series resistor would drop 1.2 volts with that load.
So, using that balanced first filter with 2X 0.5 ohm resistors from the bridge, your second filter resistor would probably also be about 0.5 ohms.

Whichever way you do it, looks like about 1.5 - 1.6 ohms total resistance would get you there.
With the balanced filter idea, you could also put a first cap right at the bridge, try it both ways, with and without.
You'll want to use 3W resistors, unless you go with a single 1.5 ohm resistor, in which case I'd up it to 5W.
 

Winston OBoogie

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The main thing to do would be to buy a few values of the resistors you're going to use. They can be inexpensive 3W metal oxide types so, not a big outlay. Get a few values either side of my recommendations.
That way you'll be able to play around with getting the voltage correct, either by swapping out resistors, or paralleling another with what's there if the voltage drop is too much.
I haven't accounted for any losses other than the diode drop, I'm not familiar with the Hammond transformer enough to remember what to exactly expect.

So, just cover your (my 😄) arse so that you can get it up and running quickly without having to wait for another order of $2's worth of different value resistors.

Draw out your heater supply schematic and post it here first if you're not sure of what's what and what goes where.
 
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radardoug

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Nbtone

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302ECCC6-82A3-42D3-80FD-5836E00AA0D4.jpeg
okay, I have not tried Schottky diodes in my preamp. Maybe I can learn something here, too. I’ve drawn out schematics for what I understand of what Winston says will work. Am I close?
 

Nbtone

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I was trying to interpret this paragraph:
“Edit: one other idea might be to use a balanced filter right after the diode bridge with a 0.5 ohm resistor in both legs to your first filter cap. Then use a single "adjust on test" to your second cap. This would help keep out any diode switching nasties and ripple current from your heater supply.”
 

morls

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I've had a play around with PSUD. I'm still feeling my way here, but I think this design (simulated) gives me a heater voltage of 6.31VDC, with ripple of 26.3mV. I'm not sure what a target figure is for ripple...

I went down to the local electronics store and used the parts they had in stock as a guide. The only resistors I could get were 1W or 5W, and the values were 0.47, 0.51 and 0.56 ohm.

I'm not sure about the settings for transformer and load, I made a best guess using the figure of 1.2A you provided Winston. Just wondering, what is the formula/procedure for calculating load on the PSU?

Anyway, here's what I came up with in PSUD.

Cheers
Stephen
 

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Winston OBoogie

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I was trying to interpret this paragraph:
“Edit: one other idea might be to use a balanced filter right after the diode bridge with a 0.5 ohm resistor in both legs to your first filter cap. Then use a single "adjust on test" to your second cap. This would help keep out any diode switching nasties and ripple current from your heater supply.”

By "balanced filter" I meant that where the series resistor is located in a standard RC filter, you could halve the value that was there and put the other half in the negative side of the line. So one resistor feeds the +ve terminal of your first filter cap, the other resistor feeds the -tve terminal of your filter cap.
Both are in series with the supply rather than how you have it drawn with one series and one shunt R.




I've had a play around with PSUD. I'm still feeling my way here, but I think this design (simulated) gives me a heater voltage of 6.31VDC, with ripple of 26.3mV. I'm not sure what a target figure is for ripple...

I went down to the local electronics store and used the parts they had in stock as a guide. The only resistors I could get were 1W or 5W, and the values were 0.47, 0.51 and 0.56 ohm.

I'm not sure about the settings for transformer and load, I made a best guess using the figure of 1.2A you provided Winston. Just wondering, what is the formula/procedure for calculating load on the PSU?

Anyway, here's what I came up with in PSUD.

Cheers
Stephen

I can't play with PSUD as I'm on Mac or I'd enter your circuit for you, but, good stuff you're having a mess around with it yourself :)

TBH 26mV seems a bit on the high side to me but the actuality is that the acceptable amount of ripple is the amount that gets the job done.
I would see what happens with playing with the cap values and positions. Double upon the last cap for instance. I don't think PSUD allows for trying a balanced filter after the rectifier and reservoir cap into the first filter cap, could be wrong. But I think this would help ripple.

For the load, I simply added up the heater current specified for each tube: 600mA for the 6X4, 300mA each for the 12A*7's = 1.2A

This is the load for calculating the loss across the series resistors. For transformer rating though, there is a power loss factor when rectifying for DC which means that it's good your power transformer is rated for 2.5A as, otherwise, it would be loaded down.
 

morls

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Ok, I’ll keep tweaking and see what I can do to get the ripple down. I’m learning a lot here, and it’s really good to have a simulator to see what changes occur with each tweak.
 

Nbtone

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By "balanced filter" I meant that where the series resistor is located in a standard RC filter, you could halve the value that was there and put the other half in the negative side of the line. So one resistor feeds the +ve terminal of your first filter cap, the other resistor feeds the -tve terminal of your filter cap.
Both are in series with the supply rather than how you have it drawn with one series and one shunt R.






I can't play with PSUD as I'm on Mac or I'd enter your circuit for you, but, good stuff you're having a mess around with it yourself :)

TBH 26mV seems a bit on the high side to me but the actuality is that the acceptable amount of ripple is the amount that gets the job done.
I would see what happens with playing with the cap values and positions. Double upon the last cap for instance. I don't think PSUD allows for trying a balanced filter after the rectifier and reservoir cap into the first filter cap, could be wrong. But I think this would help ripple.

For the load, I simply added up the heater current specified for each tube: 600mA for the 6X4, 300mA each for the 12A*7's = 1.2A

This is the load for calculating the loss across the series resistors. For transformer rating though, there is a power loss factor when rectifying for DC which means that it's good your power transformer is rated for 2.5A as, otherwise, it would be loaded down.
FB430877-325D-4416-ADF8-F148F680E032.jpeg
Something like this then. Thanks.
 
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