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Old 18th October 2013, 04:44 PM   #341
mm7 is offline mm7  Canada
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Location: Woodbridge, ON
Quote:
Originally Posted by DF96 View Post
I don't understand. You can't parallel two voltage supplies unless they are absolutely identical in all respects.
Why? What is wrong with following schematic?

Quote:
Originally Posted by DF96 View Post
If buzz occurs as soon as the DC heater supply has a load attached then you have coupling between the heater supply and the main circuit. This could be via wiring or via the mains transformer. You could try a choke input heater supply, as this will reduce charging pulses. The choke may be big, though!
There is coupling of course - heaters are elevated +90V from B+ via voltage divider 200K -- ( 100K || 220uF). BTW before I added that 220uF cap there was lots of buzz. But adding more capacitance here does not reduce it anymore.

Yea, chokes are big and expensive. I am trying to avoid it, if possible. Also I am not sure if it will help for sure. I do not have one to try.
May I replace it with using some (another) power transformer primary?
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Old 18th October 2013, 05:05 PM   #342
DF96 is offline DF96  England
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Quote:
Originally Posted by mm7
What is wrong with following schematic?
If that circuit is what you have built, then one of the secondaries does no work as the other secondary will always present a higher voltage and so reverse bias the diodes from the first secondary.

You may have actually built it with R6 connected to the other end of R2 (top of C5). In this case R2 and R6 acts as current steering resistors or ballasts so both secondaries may contribute provided their voltage difference is not too great.

You may be injecting buzz into a ground. DC PSUs should always be grounded (i.e. connected to circuit ground) at their output end, not partway through as your diagram shows. That is, ground should be at C2, not C9.
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Old 18th October 2013, 05:40 PM   #343
mm7 is offline mm7  Canada
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That ground at C9 is just for LTSpice.
Actually in real heaters circuit there is no ground anywhere. It is somewhat floating being elevated.
Though it is couples with ground indirectly via elevation chain (voltage divider), via lower R9 and C16 of the voltage divider. Look at following schematic that reflects current state.

So what about using a power transformer primary as just a choke?
(in local Sayal they do not have chokes, but have plenty of trannies)
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Old 18th October 2013, 05:51 PM   #344
DF96 is offline DF96  England
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OK. If you apply a DC load to the heater supply, but with the heater supply disconnected from the potential divider do you still get buzz? You need to find what the coupling mechanism is: common ground currents, wiring magnetic induction, transformer interwinding capacitance etc.

A power transformer will make a poor choke because it cannot handle DC. You could rebuild it with an air gap.
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Old 18th October 2013, 11:06 PM   #345
ChrisA is offline ChrisA  United States
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Originally Posted by mm7 View Post
...
So what about using a power transformer primary as just a choke?
(in local Sayal they do not have chokes, but have plenty of trannies)
If you get a big enough power transformer it will work well as a choke. The problem is a 100ma PT is rated for 100ma at 60Hz. If you want to put 30Hz through it the core size doubles, at 15Hz it is 4x larger. It is not infinite at DC but it's still big. All power transformers are designed such that their core is just barely large enough not to saturate at 60Hz and full spec'd current. Some might have larger safety factors than others. So find one with maybe 5X the mass of your PT? You can buy a real choke that is built with a gapped core from Triode Electronics. They ship to Canada which is only a few miles north of them.
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Old 18th October 2013, 11:20 PM   #346
ChrisA is offline ChrisA  United States
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Originally Posted by DF96 View Post
... DC PSUs should always be grounded (i.e. connected to circuit ground) at their output end, not partway through as your diagram shows. That is, ground should be at C2, not C9.
This is right. Two comments.

1) is the ground is just to make Spice happy then place a 100M or even 1G resister in series with the ground. That will tell everyone to ignore the ground.

2) As said above the physical layout really does matter and I suspect some of the noise may be a result of things we just can NOT see in a schematic diagram. In fact coupling almost by definition takes place via paths that are not shown in the schematic.

In any case something is wrong here. I've built many amps that use AC heaters and the hum is acceptable. People usually result to using DC heaters only to remove the last tiny bit of hum. and when they do they usually find they need to regulate the heaters.

The neat thing about using an LM7806 for the heatrs is that the chip rejects noise better than any passive filter. (yes 6VDC is fine, maybe even better then 6.3) If you worry about cost, size and weight, adding one three terminal chip is better and more effective than a choke. But you have to feed it about 9V and heat sink it.

Last edited by ChrisA; 18th October 2013 at 11:29 PM.
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Old 19th October 2013, 01:37 AM   #347
mm7 is offline mm7  Canada
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DF96>OK. If you apply a DC load to the heater supply, but with the heater supply disconnected from the potential divider do you still get buzz?

The elevator was connected to heaters.
When I've tested with battery heating, the elevator was connected through heaters to battery. No touch with heater supply.

When I've test with PS heating, the elevator was connected through heaters to PS. If I disconnect it, buzz gets loud.

DF96>You need to find what the coupling mechanism is: common ground currents, wiring magnetic induction, transformer interwinding capacitance etc.

Is there a way to find it?


Chris, I use star grounding with center at input XLR.
I have regulated PSU and I tried it. It buzzes even louder because it does not have a choke or even a resistor on input. High current into its input 15000uF cap overloads the transformer and creates buzz.
Four stages of RC filter smooths ripple to 0.3mV pp (in simulation).
Ripple does not come to B+ through elevator, it is directed to ground by elevator's 220uF cap.
It does not look as this ripple comes to cathodes from heaters. I added
one more stage with 15000uF. No change in buzz.
But what affects buzz is a load in heater circuit. If there is no load - no buzz. If there is load - buzz, even when heaters feed from a battery.
I've measured there is 300mA current between rectifier and first resistor. It is less than specified 1.5A.
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Old 19th October 2013, 02:53 AM   #348
ChrisA is offline ChrisA  United States
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Originally Posted by mm7 View Post
Is there a way to find it?
....
But what affects buzz is a load in heater circuit. If there is no load - no buzz. If there is load - buzz, even when heaters feed from a battery.
I've measured there is 300mA current between rectifier and first resistor. It is less than specified 1.5A.
This is good information. It seems that if the diodes in the heater DC supply are switching then you get the buzz. Even if the supply is not supplying the heaters.

The only way I can think the heater supply rectifier can be causing this is if the coupling is via the power transformer. The switching noise is coupling either magnetically or capastivily through the PT.

I heard of people having trouble with DC heaters. The DC current is AVERAGE only 300ma but the diodes conduct for only (lets just say 1/100th of the time because your resivour cap is so huge) This means that while the average is 300ma the current goes for zero to 30 amps then back to zero. Yes is might be that large, you need an o'scope to see the conduction angle.


Had you tried an AC heater the current would be drawn from the transformer in much smoother way, certainly not as huge 30A spikes.

What to do? I'm guessing here but a smaller resfilter cap, perhaps 10uf or 22uf would cause the diodes to conduct over much of the AC cycle and stretch out the current spikes. Filter the DC with a second RC stage. I bet you'd get the same effect with a larger size (1K ??) resister in series with the transformer's heater secondary. Anything so the diodes will stay "on" for most of the AC cycle.

Or what the heck, just try using 6.3V AC heaters and see what happens. Yes, elevate it up a few volts with the voltage dividers. Just mack sure the rectifier is disconnected. If going AC helps you have found the problem.

One very good solution is to get a separate heater transformer then there is no way for the heater an B+ supplies to couple.

I have myself only had to solve this problem in the other direction. My B+ supply would conduct only over about 10% of the AC cycle and then of course the current drawn for the AC mains was pulsed, not continuous. So the current in the primary was not constant. The transformer has a 6.3 volt tap I used for heaters. Looking with a scope I could see the sine wive on the tube heaters has a HUGE amount of distortion. I think heater elevation fixed the slight buz.
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Old 19th October 2013, 07:31 PM   #349
mm7 is offline mm7  Canada
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Originally Posted by ChrisA View Post
The DC current is AVERAGE only 300ma but the diodes conduct for only (lets just say 1/100th of the time because your resivour cap is so huge) This means that while the average is 300ma the current goes for zero to 30 amps then back to zero. Yes is might be that large, you need an o'scope to see the conduction angle.
That is why I put 8.4R before the first capacitor. This reduces current to 0.5A pikes. And the more resistance I put the less buzz is. But I cannot put here to high R because voltage will drop too much.

Quote:
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Had you tried an AC heater the current would be drawn from the transformer in much smoother way, certainly not as huge 30A spikes.
No. Because I believe it will hum cathodes of upper tubes that are 40 volts higher than heater.

Quote:
Originally Posted by ChrisA View Post
One very good solution is to get a separate heater transformer then there is no way for the heater an B+ supplies to couple.
Yeas. that will be my last resort.

Today I tried with feeding heaters circuit from 18VAC secondary (it can hold up to 1A). This allowed me to increase the input R to 15R, and other resistors in RCs to 3.3R.
The buzz decreased to -65dB.
Also I tried with my regulator where I put 9.4R before its input. Same, buzz decreased to -63dB.

Yeah. Next step will be a separate PSU for heaters.
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Old 19th October 2013, 07:49 PM   #350
ChrisA is offline ChrisA  United States
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Originally Posted by mm7 View Post
That is why I put 8.4R before the first capacitor. This reduces current to 0.5A pikes. And the more resistance I put the less buzz is. But I cannot put here to high R because voltage will drop too much.
...

It looks like the cause is well understood now, coupling of DC supply back to B+

It would be educational to try elevated AC heaters and measure the buzz with AC.
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