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Vincin 25th January 2014 02:17 PM

Trying to understand 6BM8 regulator (audio note clone)
 
Hi,

I've built the following the regulator for my phono and linestage. They are very similar to (if not the same same as) what AN UK used in their current linestage:

CLC raw B+ of 360VDC into 6bm8 triode-wired pentode section as the series pass

120K anode load for triode error amp feeding off the raw B+

Regulated B+ is taken off a 150 Kohm & 43 Kohm voltage divider, this is connected to grid of triode error amp. 150K top resistor/ feedback resistor is by-passed by small 50pf cap.

The cathode of the triode is connected to anode of OB2 VR tube, running at around 15ma, unbypassed, feeding off the regulated B+ through a 15Kohm wirewound resistor.

Triode anode is direct coupled to grid of pass tube.

Regulated b+ is 250vdc there about. There is a 22uf cap across the ouput.

Pretty basic stuff I think, but there are a couple of unsual things in this regulator: there is a 820k resistor connecting raw b+ and grid of triode, and also a 133Kohm resistor connecting anode of the triode and its grid.

Questions:

Does the 820K resistor serve as some kind of feed forward and cancellation of ripple?

Why is there the 133k resistor?

I would like to increase the regulated B+ to 260v - 270v, but cannot do this by adjusting the value of the lower resistor in the output voltage divider. Can someone please tell me exactly what I need to do?

General comments on this type of regulator? And if you have experience with better sounding regulators using different tubes, im all game to try things out.

Apologies for not being able to draw a schematic.

I realise that I may have asked very stupid questions here (my education on electronics is limited to highschool Ohm laws, and what I have picked up from this forum in a few years). Please pardon me for those questions. Tks a million!

kevinkr 25th January 2014 02:25 PM

Please post a hand drawn schematic or a link to a legitimate schematic, it would help quite a bit. :D
(I sometimes draw a schematic by hand and snap a picture with my cell phone and post that - better than nothing.)

Vincin 25th January 2014 02:54 PM

1 Attachment(s)
Thanks Kevin, here you go, hand drawn schematic and taken off the mobile phone. Vi

Vincin 25th January 2014 03:10 PM

1 Attachment(s)
Here it is. Tks Kevin.

kevinkr 25th January 2014 04:14 PM

It does look to me like the 820K and 133K are part of a ripple canceling network. I don't use this in my own designs but have seen people who do. You remove these if you wish. I provide heavily filtered voltage to the pass tube screen in my designs which improves ripple rejection substantially and is unfussy, but at the expense of a large and expensive electrolytic capacitor.

If you need more output voltage you will need commensurately more input voltage as well. If you need 270V the raw input will need to be in the vicinity of (or slightly over) 400V. Once you boost the input voltage you will have sufficient voltage compliance that increasing the value of the lower feedback resistor will boost the output voltage.

I've posted a number of high performance regulator circuits in various threads such as the ones relating to the Muscovite and Muscovite Mini phono stages in analogue source. There is another such posting in this forum..

artosalo 25th January 2014 05:00 PM

That seems complicated circuit without real benefits.

Below is a simple and basic 6BM8/ECL82 voltage regulator.
According to quick simulations it is possible to get min. 60 mA with 100 Vdc accross the pass pentode (360 Vdc in 260 Vdc out).
Ripple rejection is some 46 dB.

R6 at the schematic represents load.

An externally hosted image should be here but it no longer works. Please upload images instead of linking to them to prevent this.

Vincin 25th January 2014 05:18 PM

@artosalo: i've built very similar circuit to the one you posted, it did not sound as nice as the one used by audio note. Before I go built some other circuits (or go back to LCLCRC) you know we just have the urge to understand how things work, hence the questions.

Also, this maybe so stupid but i just have to ask if there is a way to measure the supply ripple without a scope?

brommermartin 25th January 2014 05:47 PM

That is a nice regulator design:D if I need 285v/40ma on the output of the regulator , what do I need to chance ?
Raw b+ 390vdc

artosalo 25th January 2014 05:53 PM

Replace R4 with trimmer pot and adjust to 90k.

brommermartin 25th January 2014 05:55 PM

Thanks;)


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