Pultec clone - grounding problems

[Bo Deadly]

Morls,

As you can see from the numerous posts, the heater traces on those boards are quite simply wrong. If you have any more Drip boards you should probably drip them into the trash can.

But you can salvage this build I think. For now maybe keep it simple and just add heater wires like in your modified pic. But if you're paying attention the critical bit is that you want to minimize the space between the two wires. Meaning no "loops". Loops of opposing currents emit electromagnetic radiation. This is the problem with the PCB traces on that board and it is almost certainly the source of most of the noise. You can see how the two different heater traces have a big gap that make the current run around in a big loop.

So the simplest fix would be to do something like this:



So this is looking at the edge / bottom of the PCB. Make one long section if tightly twisted wires using heavy gauge wire like 20 AWG. Note that the overall length is going to shrink quite a bit if you use a drill to twist as described above so use more wire than you need.

About 6" from one end tease apart the twist enough to create a gap in the insulation with an Xacto knife taking care not to break any of the conductor strands. Pull the insulation off about 1" (I suppose you'll need to un-twist the end temporarily to just pull the insulation away to create the gap and then twist them back together again after) and then put a sharp bend in the bare gap. Solder those bare gaps to the heaters of one of the two tubes with the wires coming away from the board at 90 degrees. Don't lay them flat across other traces. Then bend the short end back up to the other tube, trim and strip the ends accordingly and solder those heater wires. Make sure you use the same wire from pins 4,5 and the same wire for pins 9. So now you have a loop of twisted wire sticking out about 2-3" out from the PCB. Then trim and strip the other end to solder directly to the rectifier tube socket pins but leave enough slack so that the heater wires can come down away from the board at 90 degrees and rest along the bottom of the chassis. You wan the heater wires to actually lay flat along the bottom of the chassis if possible because the steel will actually "absorb" some of the electromagnetic radiation so leave a decent about of wire between.

Then cut the traces after the lamp terminal block like in your modified pic.

 

[morls]

I agree that DC would be best. However, we have a 6X4 H.T. rectifier here which has its heater fed from the same 6V3 AC, so it's not as simple as just rectifying the AC right at the transformer

Would it be possible to run the 6X4 heater on DC? This spec sheet indicates AC or DC for the heater

 

[morls]

Morls,

As you can see from the numerous posts, the heater traces on those boards are quite simply wrong. If you have any more Drip boards you should probably drip them into the trash can.

I'm very close to finishing a build of the Drip Dual STA-Level, so I'm going to have to look at those boards too. It has a different PSU though, which provides 6.3VDC @ 2 amps.

This Pultec board pre-dates the revised PSU.

So the simplest fix would be to do something like this:

View attachment 82018

So this is looking at the edge / bottom of the PCB. Make one long section if tightly twisted wires using heavy gauge wire like 20 AWG. Note that the overall length is going to shrink quite a bit if you use a drill to twist as described above so use more wire than you need.

Thank you for these detailed directions Bo. And thank you to all contributors, this has reminded me of how generous people can be in sharing their knowledge!

 

[Winston OBoogie]

Would it be possible to run the 6X4 heater on DC? This spec sheet indicates AC or DC for the heater

Actually, looking at your board again, yes. I was concerned about something I probably needn't have been.

So if that's possible for you, do it that way then. Don't forget to remove the 6V winding's centre tap from ground.

Once you have your rectified and filtered clean DC, grounding the negative of that supply should be OK without elevating it.

 

[morls]

I would suggest for a bit of simplicity to mount a bridge rectifier and large 'smoothing' cap right at the transformer, or at least well away from anything else and feed with DC. You will need a dropper resistor to 'lose' a bit of voltage and then another large cap. I think schottky diodes may be beneficial as they are a bit less 'noisy' than ordinary silicon and they won't lose so much voltage which then makes the cap/res/cap arrangement better to reduce ripple. Some Marshall guitar amps use this idea where the first couple of valves use DC with the latter and output valves running on AC.

Would a centre tap rectifier be another alternative?

You should maybe elevate the heaters of the audio tubes by about 40V DC too. The original Pultec schematic shows this I'm sure. In your case, you can create a centre tap with a pair of 100 ohm resistors from each side of your heater voltage. Use the junction of the two 100R's to inject your elevating voltage .

I'm not sure I understand this. If the heater voltage is 6.3V, increasing by 40V DC seems difficult given my limited knowledge.

Once you have your rectified and filtered clean DC, grounding the negative of that supply should be OK without elevating it.

Does this refer to the 40V DC elevation?

 

[morls]

Thanks,
I’ve been doing some reading, quite a learning curve happening here!

I’m thinking about a bridge rectifier, and using the centre tap to elevate the DC.

This might not be the right way to go about…I’ll keep reading I think.

 

[scott2000]

I think Ian's power supply info has an example of heater elevation but I don't have the info handy.

IIRC this guy explains it a bit pretty well...

Vacuum Tube Virtual Ground or Center Tap & Elevated Heater Wiring Explained​

 

[morls]

Looks great, thanks!

 

[Winston OBoogie]

As things relate to your project, I don't know that you'll need to elevate the heaters.

But for info's sake, there are two main reasons to elevate them: One is to ensure that the maximum heater to cathode voltage of the tubes isn't exceeded. This would be done irrespective of whether the heater supply was AC or DC.
Another main reason to elevate the heaters is when you're running them with AC. Elevating the 6V3 AC by 40 -60 volts will give lower hum.

Another thing to do with AC for achieving the lowest possible hum is to use a "humdinger" pot across the heater winding. You'll see a lot of times that a heater winding has its centre tap grounded. However, the centre of the transformer might not be exactly the optimum position for cancelling out hum. So a 250 ohm or thereabouts "humdinger" with its wiper to ground allows you to tune the hum to its lowest level.
The wiper of this pot, besides going to ground, could also be where you connect your reference voltage to elevate the heaters to take care of keeping the heater to cathode voltage within spec.

In your case, the audio tubes have cathode voltages only a few volts above ground so no need to elevate for their sake.
And the 6X4 has a large heater to cathode voltage maximum so not really a problem there. Although you do frequently see them elevated to B+ level so there is some benefit.
Anyway, there's most likely no reason to elevate a DC heater supply for your Pultecs. Just ground the negative of that supply after it's rectified and well filtered.

What the issue I was worrying about earlier in regard to sharing the audio tubes heater with the 6X4 when running them DC had to do with was: switching noise from the heater rectifier making its way to the B+.
I haven't bothered with tube rectifiers in 30 years but, if I did, I would want to use a separate winding for the rectifier (as did the original Pultec) and also use a power transformer with an electrostatic shield. Either that or use an entirely separate transformer for the audio heaters.

However, I see that your board has (I think) a series resistor right after the 6X4, before the first cap which will go a good way towards filtering out any noise imposed on the 6X4 cathodes via their heater supply.
This, together with the fact I was probably over worrying about the rectifier switching noise, leads me to thinking that a good clean DC feeding both your audio tubes and 6X4 will be OK.

Too much info? Not enough?
More importantly, anyone see a problem with what I propose or have a better alternative?

 

[morls]

Great amount of information, thanks! I'm learning a lot here

I think I'll look into building a bridge rectifier with cap/res/cap filter.

 

[corgan4321]

I just finished a pair of the more recent Drip build with the revised PSU and DC heater source. My main source of hum was from the power switch wire. I would recommend shielded power cable grounded on one side only for the wire going from PCB to the switch. I also made sure the chassis was making good contact with itself everywhere.
For what it’s worth, my units measured about -96dB self noise in my DAW with inputs and outputs connected.

Does your transformer have separate winding for the heaters? Maybe you could replicate the heater DC source from the revised PSU like my revision has?

 

[user 53957]

It makes no difference: AC vs DC heaters. I've built quite a few from scratch both ways and the issue I see in his pics is not all the grounding points are connected to ground, which would cause an issue. Also what can cause this is the wires, needs to be 450V type and a lot of people I see have issues with power noise use cheap 250V hook up wire.

 

[Winston OBoogie]

It makes no difference: AC vs DC heaters. I've built quite a few from scratch both ways and the issue I see in his pics is not all the grounding points are connected to ground, which would cause an issue. Also what can cause this is the wires, needs to be 450V type and a lot of people I see have issues with power noise use cheap 250V hook up wire.

AC heaters, when properly implemented, can certainly be fine. Lots of equipment out there to show this.

The issue with the boards shown in this thread is that it is not done properly here.

And using 450V rated hookup wire won't eleviate or do anything at all to change the fact that a big ol' loop of heater current is enclosing the signal circuitry on the bottom of the pcb.

 

[corgan4321]

Glad to hear that there's a revised power supply now

I think it might be beneficial to employ a power switch that switches both live and neutral and to use tightly twisted pairs again. Shielding the pair would help with higher frequency noise, but twisting would help cancel the low frequency mains field.

In my case, actually, the shielding of the power switch (also have them twisted) removed my 60hz hum. I didn’t have any high frequency issues.

 

[user 53957]

AC heaters, when properly implemented, can certainly be fine. Lots of equipment out there to show this.

The issue with the boards shown in this thread is that it is not done properly here.

And using 450V rated hookup wire won't eleviate or do anything at all to change the fact that a big ol' loop of heater current is enclosing the signal circuitry on the bottom of the pcb.

I am just pointing out the common things I had found in the past when someone wants me to help them out. Other things I see from time to time in new builds is that a transformer winding is not properly center tap, and heater noise is almost impossible to get rid of it because of the manufacturing flaw. You can find out if this is going on by just powering the tubes with the heaters only, and taking a scope reading on the cathode. If there is noise pulses there instead of nothing, that is the problem.

 

[user 53957]

humdingers or the hum nulling circuit was created to establish a dc reference potential point to prevent interelectrode arching. Any flaw in the ac signal going to the heaters will be detected by the diode junction that is formed between the heater and cathode. Some you can fix by dc the heaters some not. But in all cases its the transformer's winding fault.

 

[Winston OBoogie]

The wiper of a humdinger can absolutely be used as the point to which you apply a reference voltage to make sure heater-cathode voltage isn't exceeded.
But it isn't the only reason to use one.
Using the centre tap of a transformer might well be OK for some amplifiers, but for lowest hum - a pot between 100 - 500 across the 6V3 is recommended for high gain amps, wiper connected to ground.
A lower gain amplifier will be normally be OK if you use equal value resistors of 50 - 100 ohms across the 6V3 with centre connection of the pair grounded. Even this equal resistor scheme will give better null than using all but the very best made transformer centre taps.

RDH4 is your friend, chapter 31. As is looking at how high end amplifiers were built when valves ruled the roost.

 

[Nbtone]

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?

 

[Bo Deadly]

In my case, actually, the shielding of the power switch (also have them twisted) removed my 60hz hum. I didn’t have any high frequency issues.

Non-ferrous shielding like the foil or braid of a shielded cable will have zero effect on electromagnetic noise. Twisting the wires is what causes fields of opposite currents to cancel.

 

[Winston OBoogie]

Exactly right.
Following science instead of voodoo actually works. Funny that!