johnheath

Odd behavior or normal voltages in heater supply?
« on: August 02, 2018, 11:10:51 AM »
Hi all...

I am looking into a home-brew vari-mu comp and there is a heater arrangement with mixed AC and DC for different tubes.

The power transformer is a decent model capable of delivering 3.5A @ 6.3VAC... according to data sheet.

The mix between AC and DC is that the V1's are supposed to have DC and the rest AC. To get the DC there is a rectifier bridge (1000V/ 8A) that is brand new and all new capacitors.

According to the tube data sheets the tubes getting AC are two 6AL5's and two 12AT7's... a total draw of 1.2A @ 6.3VAC and the tubes getting DC are two ECC85 with a total draw of 0.87A @ 6.3VDC. All according to the attached schematic.

And there is also a 200mA/6.3V light bulb connected to the VAC.

When all hooked up it proves that the transformer delivers 6.0VAC but after the rectifier bridge it is only 6.15VDC which makes it hard to filter with a "normal" C-R-C filter.

My question, before tearing the supply apart, is if the total current draw is too big or if I should look into the rectifier bridge or the capacitors... or what else? Is it normal behavior or not?

Any advice is helpful I guess.


Best regards

/John






Nothing is impossible - It just takes some more time


pucho812

Re: Odd behavior or normal voltages in heater supply?
« Reply #1 on: August 02, 2018, 11:30:58 AM »
what vari-mu design?

Why not run all the heaters of DC or AC?
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

scott2000

Re: Odd behavior or normal voltages in heater supply?
« Reply #2 on: August 02, 2018, 11:39:08 AM »

When all hooked up it proves that the transformer delivers 6.0VAC but after the rectifier bridge it is only 6.15VDC which makes it hard to filter with a "normal" C-R-C filter.

My question, before tearing the supply apart, is if the total current draw is too big or if I should look into the rectifier bridge or the capacitors... or what else? Is it normal behavior or not?

Any advice is helpful I guess.


Best regards

/John

There's supposed to be almost 9v after the bridge?? nevermind..... :-[
« Last Edit: August 02, 2018, 04:15:33 PM by scott2000 »

johnheath

Re: Odd behavior or normal voltages in heater supply?
« Reply #3 on: August 02, 2018, 11:40:01 AM »
Well it is a home made machine... cannot say exactly but it is a mix of a lot of former vari-mu's I guess?

Yes, the idea of mixing it is that it could be possible to test other tubes (V2 that is) to be able to fiddle with gain and side chain. I guess that I could go for AC all the way but firstly I would like to sort this problem out.

I have done this in other comps and even in guitar amps and it usually works just fine.

I have my eyes on the rectifier bridge at the moment but since it is brand new and they seldom are broken... well, just saying :)


Best regards

/John
Nothing is impossible - It just takes some more time

johnheath

Re: Odd behavior or normal voltages in heater supply?
« Reply #4 on: August 02, 2018, 11:42:11 AM »
There's supposed to be almost 9v after the bridge??

Thank you sir.

Yes, but more realistic it usually ends up around 7.5VDC when using this arrangement in other builds and that was what I expected in this case too... but no :)


Best regards

/John
Nothing is impossible - It just takes some more time

johnheath

Re: Odd behavior or normal voltages in heater supply?
« Reply #5 on: August 02, 2018, 11:43:00 AM »
... and also I have seen that this rectified voltage can vary depending on the total current draw?
Nothing is impossible - It just takes some more time

scott2000

Re: Odd behavior or normal voltages in heater supply?
« Reply #6 on: August 02, 2018, 12:05:48 PM »
right....

A 6.3Vac transformer supply, after rectification and accounting for losses and diode drop, will yield about 6.5Vdc. You can of course adjust this to something closer to 6.3V with a dropping resistor, but this is rarely necessary. Another important thing to be aware of is that rectifying AC to DC in this way introduces a power-factor loss of about 0.5, which loads down the transformer more. In other words, to supply a heater with 300mA of DC the transformer actually has to deliver about 600mA of AC, So be careful not to overload the transformer.

http://www.valvewizard.co.uk/heater.html

johnheath

Re: Odd behavior or normal voltages in heater supply?
« Reply #7 on: August 02, 2018, 12:31:41 PM »
Thank you sir

Yes, the transformer is supposed to deliver 3.5A @ 6.3VAC and the total current draw is 2.27A @ 6.3VAC so it should be safe.

My biggest concern is that the rectifier bridge doesn't increase the voltage more than 0.15V (DC)


Best regards

/John
Nothing is impossible - It just takes some more time

scott2000

Re: Odd behavior or normal voltages in heater supply?
« Reply #8 on: August 02, 2018, 12:47:23 PM »
Well from reading that link, if the ac current draw  is more to make dc , looks like you would be getting towards  the transformer limits from what I can tell??

Is a crc really necessary for  heaters??? Big caps for DC heaters is what my understanding was enough after getting the voltage right???

Sorry I'm not able to give a qualified opinion.......

johnheath

Re: Odd behavior or normal voltages in heater supply?
« Reply #9 on: August 02, 2018, 02:26:53 PM »
Thank you sir

Yes, according to that math it would... but still under the rated 3.5A. So, the question is if it would cause a reaction mentioned in my first post?

Best regards

/John
Nothing is impossible - It just takes some more time


scott2000

Re: Odd behavior or normal voltages in heater supply?
« Reply #10 on: August 02, 2018, 02:42:37 PM »
Thank you sir

Yes, according to that math it would... but still under the rated 3.5A. So, the question is if it would cause a reaction mentioned in my first post?

Best regards

/John

It seems normal from what I'm reading from the link I posted??? mentioning from 6.3 to 6.5 post bridge seems in line with your scenario......

Hopefully someone can answer... This is interesting....

Sorry for the distraction...you did say any advice is helpful (I guess)  ;D

johnheath

Re: Odd behavior or normal voltages in heater supply?
« Reply #11 on: August 02, 2018, 03:04:11 PM »
Haha - yes of course every advice is useful... I need to figure this out anyway. I mean I can still change it all to VAC anyway. I am just concerned that I might get some hum from not using DC. I know that AC works just great in many cases, but unfortunately not in every situation though.

Best regards

/John
Nothing is impossible - It just takes some more time

scott2000

Re: Odd behavior or normal voltages in heater supply?
« Reply #12 on: August 02, 2018, 03:10:08 PM »
Is the heater winding only 6v without a load???

Der..

What is the heater winding without a load??





« Last Edit: August 02, 2018, 09:21:10 PM by scott2000 »

johnheath

Re: Odd behavior or normal voltages in heater supply?
« Reply #13 on: August 02, 2018, 03:17:56 PM »
To tell you the truth I haven't measured that.

I know that it might vary from transformer to transformer, but I must say 6.0VAC is a bit on the low side really.
Nothing is impossible - It just takes some more time

johnheath

Re: Odd behavior or normal voltages in heater supply?
« Reply #14 on: August 02, 2018, 03:21:40 PM »
Yeah, but the question is why the voltage drop in the first place? I should get at least some 7V after the rectifier bridge... or???
Nothing is impossible - It just takes some more time

scott2000

Re: Odd behavior or normal voltages in heater supply?
« Reply #15 on: August 02, 2018, 03:25:24 PM »
Well from what I understand, you have to take 2 volts off AC before the bridge for voltage drop.... then multiply the 1.414......

The voltage drop depends on the load too though so, it's more complex at the moment for me to understand totally.....

johnheath

Re: Odd behavior or normal voltages in heater supply?
« Reply #16 on: August 02, 2018, 03:27:02 PM »
Well from what I understand, you have to take 2 volts off AC before the bridge for diode drop.... then multiply the 1.414......

The voltage drop depends on the load too though so, it's more complex at the moment for me to understand totally.....

Ok, but what is the reason for the 2V drop before the bridge? Have not seen that before in any type of tube amp that I have built or repaired?
Nothing is impossible - It just takes some more time

scott2000

Re: Odd behavior or normal voltages in heater supply?
« Reply #17 on: August 02, 2018, 03:31:06 PM »
Maybe the transformers weren't the same??? IDK...

You have to subtract the voltage drop before multiplying ..... makes a difference....

Seems it's different when talking low voltage windings.... more critical for drops..... using two diodes would be half the drop taken into calculations (i.e about 1v) guess it depends on the diodes but 1v is average in this....? ..... 4 diodes double the drop....(2v)

if you had less current being used, you'd probably see more??

Wonder what the voltage is unloaded..... and how much current it actually is able to supply...

Still sounds right for what you have going on..I'm thinking it's putting out the 6.3v loaded from my math....(you have 6.3v at the ac heaters???)....then you have your dc rectification with it's drop........ 6v for heaters is really ok??


you only are getting 6v at the ac heaters???? 
« Last Edit: August 02, 2018, 07:11:24 PM by scott2000 »

scott2000

Re: Odd behavior or normal voltages in heater supply?
« Reply #18 on: August 02, 2018, 07:31:13 PM »
Ok, but what is the reason for the 2V drop before the bridge? Have not seen that before in any type of tube amp that I have built or repaired?
I see I may have mis-worded what I was saying.... I meant you have to subtract the voltage drop before multiplying...... not that the voltage is lower before the rectifier.....
..
« Last Edit: August 02, 2018, 09:22:42 PM by scott2000 »

abbey road d enfer

Re: Odd behavior or normal voltages in heater supply?
« Reply #19 on: August 03, 2018, 02:11:53 AM »
Ok, but what is the reason for the 2V drop before the bridge?
Voltage drop is due to 2 diode in series (1.4V), plus ohmic and inductive loss in the transformer. This must be put in regard with the peak voltage, that is supposed to be 6.3x1.414.
The resistive and inductive voltage loss happens at the secondary and clips the waveform; that reduces the peak voltage.
The diode loss is almost fixed at 1.4V.
Then the smoothing cap tries to fill the gaps between the charging peaks (they happen only when the input waveform is high enough to make the diodes conduct). You never mentioned the value of the smoothing cap, however that's an important factor in the design. Too small a cap results in low voltage and inadequate noise. Anything less than 10 000uF is a waste of money. Now there should be a small resistor in series to drop the excess voltage.
You cannot properly troubleshoot this issue with just a voltmeter, you need an o'scope.

Quote
Have not seen that before in any type of tube amp that I have built or repaired?
You probably never noticed this 2V drop when you deal with 300V B+.
« Last Edit: August 03, 2018, 05:45:46 AM by abbey road d enfer »
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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