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Old 13th July 2007, 05:48 PM   #1
john65b is offline john65b  United States
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Default Question on what is more important B+ or Plate Voltage

I have built a couple Tube Preamps (12B4, 5687) exactly as shown on the posted schematics - but find in a couple of occasions, to reduce Hum, I had to raise a few resistor values on the PS filter (RCRC). I did not want to change to an LC type filter (too expensive) or change my tranny (Triad N68X)

In one case I had to raise the RCRC filter resistors from 220R to 2KR before the hum was acceptable... This ends up reducing the B+ voltage and ultimately voltage on the plates by a bit.

But reducing the value of the Plate resistor a bit can raise the Plate voltage to where the schematic requires...

Question is what is most important - keeping true to the B+ voltage or the Plate voltage? My guess is the plate voltage is more important and how you get there is irrelevant... as long as you try to stay above 2x to 4x tube plate resistance on plate resistor (depending on who you talk to)...

Sorry for the lame question....

I have 80V on the plates of a 12B4 pre and the schematic lists 90V...wondering if it is worth changing a few things to get that extra 10V is really worth it...
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Old 13th July 2007, 07:14 PM   #2
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Hi John,

First, your question is not lame. From experience, and without knowing your circuit, I would say that an extra ten volts on the plate will not change matters very much. Since B+ and plate voltage go hand in hand, you can't really seperate them too much. But at eighty volts you're within ten percent of design value.

If you're experiencing hum problems you may well have other conditions that's causing it. First you must determine if it's sixty or one twenty cycle. (I'll guess 120) Without knowing exactly what you're working with, I can only mention the many usual suspects.

These include: layout, parts placement, orientation, lead dress, filter cap and parts quality, grounding system, (buss, star, multi point) ground location or loops, shielding, etc, etc.

Also, as was mentioned elsewhere on this board, you should have and know how to use an oscilloscope. They are an indispensable tool if you are going to be working with electronics.

Victor
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Old 13th July 2007, 07:28 PM   #3
john65b is offline john65b  United States
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Thank you for your reply...

I would say 120cps hum...If the hum reduces with the higher value resistor on RCRC, wouldn't that be the problem? Major hum with 220R, next to none at 2KR...

I also forgot to say I have AC on the heaters - both leads referenced to ground with 100R each. Thought about DC on the heaters, but someone advised upping the RC filter resistors first...

I have an osciliscope, but don't yet know how to use it (I got it off ebay for $40) Textronics 5110 I think.

Maybe this weekend I will putz around with it...
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Old 13th July 2007, 07:57 PM   #4
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Ah ha. Next to none reads a lot better then acceptable. Acceptable is a relatively wide term. If it works and sounds good with the higher resistor, then use it that way. But I'm curious about one thing. You say it's an RCRC. Perhaps lower or even loose the first R, unless it's only a very low inrush value. You could also try splitting the 2K with a third C in between. Just a thought.

I'm very familiar with the whole Tek line since I buy and sell equipment for a living. The 5110 is a nice unit for audio. They're no longer supported but there's lots of them around.

Victor
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Old 13th July 2007, 10:53 PM   #5
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If you've decided on a good plate resistor value, a good plate voltage and current, then your B+ will have to be a specific value or you won't meet those three goals.

(Also agreeing with HollowState), have you considered trying a CLC filter?
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Old 13th July 2007, 11:08 PM   #6
SY is offline SY  United States
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Broken record time: since you're not swinging a lot of voltage in a preamp, you can have your cake and eat it too by using a CCS plate load. That removes a LOT of power supply issues while minimizing distortion.
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Old 13th July 2007, 11:34 PM   #7
john65b is offline john65b  United States
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As far as "acceptable" and "next to none"

"Acceptable" hum is noticeable on a pair of high efficiency speakers and a gainclone, but "next to none" hum is on my inefficient (84 - 86db/watt) magnepans and a UCD400 setup.

Since the intent was to stay in the UCD/Maggie setup, I am OK with how it sounds..

I have the 220R/270uf/1KR/270uf arrangement as you mentioned not the other way around.

With a 35mA constant current source, this checks out with PSU designer at 230V for B+. I measured 235V B+ and 82V on plates.

Anyway, I so desperately wanted to see this 12B4 best my Foreplay III (with Auricaps mod), but I have to give the nod slightly to the FPIII right now. Sound is just a more dynamic and precise...Of course the FPIII is $400 and the 12B4 getup was about $75 including the enclosure...I have to admit I went on the cheap on components.

Here is the circuit (Frank's 12B4 I believe - you all have see it before), with only difference is a 2.2uf Bennic Cap, 420R on cathode and 50k Blue Velvet pot...and a B+ of 235V.
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Old 13th July 2007, 11:40 PM   #8
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Here is a pic of the FPIII, the 12B4 in a nice anodized Hammond Enclosure (I love these), and the UCD400 in a cool looking (but expensive) Hexateq enclosure.

SY- - the CSS your advising - with an LM317 on the cathode, right? Just dial in a voltage and Viola?

Edit - whoops, I see you posted CSS Plate load....
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Old 13th July 2007, 11:44 PM   #9
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Well SY, if I read John's original post correctly a CCS is not really an option. Sure it would be beneficial but would require still higher B+. And I'm assuming you mean a tube CCS and not one of those nasty little transistor thingies. (sticks tongue in cheek) So a new power trans would be needed. Or else a votage doubler (ugh) which means more parts and more expense plus a redesign. Or is there something I'm forgetting?

Well on second thought you might be able to make it with the available voltage.

Victor
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Old 14th July 2007, 02:48 AM   #10
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Can't you simply increase the smoothing caps to reduce hum?
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