Voltage regulated Grid Bias supplies.

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Dave the bass
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#1 Voltage regulated Grid Bias supplies.

Post by Dave the bass »

Supposing you wanted to drive the grid of a valve ~30VDC +ve via the secondary winding of and IT.

Have I got this next bit right... If the grid bias PSU was voltage regulated, (I'm thinking 0-30V DC using a variable volt reg chip or summat) wouldn't the regulator be struggling to hold the output voltage stable as there's a big AC component (the audio signal) developing across the same winding thats coupled to the grid of the valve we're trying to hold at ~+30V DC?

Voltage reg'd bias supplies, although smooth and ripple free might sound like a good idea but the AC AF signal superimposed on them can cause reg problems, yes?

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pre65
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#2

Post by pre65 »

I'm interested in this question, cos the grid of my CV2799 is driven by a bench PSU which is 0-12v DC, but I want to experiment with higher positive grid voltages.
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thomas
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#3

Post by thomas »

Hmmm decouple it with a cap, maybe?
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#4

Post by IslandPink »

Are you talking A1 or A2 Dave ? Slightly different issues .
Also there will also in general be a resistor or a transformer secondary ( or a grid choke ) between the supply and the grid too .

The demand on the supply is more of an issue with the A2 case as there is significant standing current and more fluctuation in current than in the A1 case .
thomas wrote:Hmmm decouple it with a cap, maybe?
Caps aren't much good for holding a DC bias voltage ! :D
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#5

Post by Mike H »

What Thomas said, if you've got a winding to deliver AC, it's got to be AC coupled to ground at the other end at minimum impedance. :?:
 
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#6

Post by Dave the bass »

Sorry, I should have said, A2 Mark, I'm aiming to drive the grid +ve for part or all of the cycle.

Yep, I understand the basics of it where the grid becomes a conductor where current flows out of it too in addition to the anode but its how that changing current flow out the grid is going to affect the (say) 30V +ve DC grid bias supply if I'm trying to voltage regulate that +30V DC.

The next valve I'd like to work with is an 830B, it can handle its grid being driven +ve and the anode has a 60W plate dissipation. I'm think of driving it from a fixed bias 2A3 with an IT as the 2A3's anode load. The 2A3 will have a stage before it natch, I'm undecided here (basically becuase I'm learning as I go!). A choke or CCS loaded 76 appeals, 2 linear valves at the front end gently building gain to feed a big lump of graphite in a glass bottle that lights up.... :)

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#7

Post by Mike H »

Appendix ~ if you don't like the idea of a cap, another idea is something like a TDA2050 power-amp chip, which will hold its output constant, and regardless of any superimposed AC.

Just think of it as a unity-gain op-amp (preset on the input to set the DC), but which has a high current output capability (compared to your more usual type op-amp).

:?:

As a matter of interest, I've been toying with this idea as a possible means to replace cathode resistor bypass capacitors ~ anybody familiar with the concept of a capacitor multiplier? :D
 
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#8

Post by thomas »

Caps aren't much good for holding a DC bias voltage !



Ummm I meant from the interstage secondary to ground to decouple ac from the bias dc / regulator do dah...
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#9

Post by IslandPink »

thomas wrote: Ummm I meant from the interstage secondary to ground to decouple ac from the bias dc / regulator do dah...
Umm ( I'm thinking aloud here as I'm not an A2 expert )
.. so doesn't this mean you have to apply the bias voltage to the other ( now signal ) end of the trannie ? If you connect this directly to the supply, you will lose the signal as this supply needs to be low-impedance if it's for A2 . Normally I've seen two ways of using bias supply - for A1 you create your nice clean low-current supply and connect to the grid with a suitable grid-leak resistor , or a high-value grid-choke, and the cap coupling from the driver stage has to be sized to get a decent low-end high-pass as per normal coupling ( though maybe a bigger cap typically for fixed bias ) ; and the cathode is connected to ground .
For A2 you need a supply that can supply more current , up to tens of milliamps, but be stable in voltage within this current swing. This connects to one end of the transformer secondary and the other end now generates the voltage signal on it as a result of the signal coming from the primary . You connect this end to the power valve grid directly , with no grid leak or cap . The transformer secondary will be a lowish resistance which retains the bias's supply's low impedance and keeps the power valve's grid in control .

I'll go away and look for some A2 circuits for a bit. Let me know if there's another way ...
What was Dave's question again ? - I've forgotten :D
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#10

Post by IslandPink »

Dave, I just recapped on your question and it's basically about how you design a bias supply . It's a bit like a small SE power supply where you're trying to get a stable-ish voltage for a small current swing . In this case the current swing is pretty small , probably no more than zero to 10-20mA , but you need lots of smoothing in the supply because it's basically adding to the signal - so high-value small current chokes are useful, probably CLCLC or something .

I'm going to look up Ciro Marzio's 'Euterpe' and 'Katelelo' amps first because I know they have bias supplies and one of them at least is A2 .

Obviously Nick or Paul might drop in and give you proper advice !
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#11

Post by IslandPink »

Oh look there he is, what a nice system !

http://www.videohifi.com/magazine/numer ... articolare..

Sorry, slighly off topic !
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#12

Post by IslandPink »

OK there are some passive ones here that are typically for 211 or 845 :

http://www.bonavolta.ch/hobby/en/audio/211_2.htm
http://www.diyparadiso.com/proj/ongaku.htm
http://www.diyparadiso.com/proj/ankoru.htm

- Adjustable just by means of a pot.
Not sure how these would fare with A2 , I guess you would need lower Zout ..
OK Steve ( now Stephie ) Bench did this one for 841's- look for the -108V supply :
http://diyaudioprojects.com/mirror/memb ... tmlHowever you need +30 V which will need a bit of thinking as you can't use any VR tubes I know of even when you've inverted the rectifier connections and the caps . Hmmm... time for Paul to tell us how it's done .
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#13

Post by Mike H »

IslandPink wrote:For A2 you need a supply that can supply more current , up to tens of milliamps, but be stable in voltage within this current swing. This connects to one end of the transformer secondary and the other end now generates the voltage signal on it as a result of the signal coming from the primary.

What was Dave's question again ? - I've forgotten :D
That's basically what I was suggesting. :D

Think we've established Dave is going to use a supply regulator. And yes will have to be on the AC ground end of the IT secondary (opposite to grid end). Thing is, it will also have to supply current in the opposite polarity when you want the grid to be driven negative (ideally), ergo a 'normal' reg (a series pass element type that 'only' supplies positive current in one direction, as a way of putting it) may not work, so needs either a cap across it to provide a low impedance AC path to ground, regardless of polarity, or, what I said, like a p-p power output stage that provides current in both polarities so also looks like a low impedance AC path to ground. Albeit it's elevated at some plus Volts.
 
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#14

Post by Mike H »

Oo-er "The document name you requested .... could not be found on this server"

Anyway the Onkaku & Ankora (?) both have caps to ground for the AC path.

So what Thomas said :lol:
 
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#15

Post by pre65 »

I used a TDA2050 to drive a 1626 into A2.
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