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Bias supply [message #31709] Sat, 21 January 2006 06:30 Go to next message
PakProtector is currently offline  PakProtector
Messages: 935
Registered: May 2009
Illuminati (2nd Degree)
Hey-Hey!!!,
The amp's requirements are for a negative rail capable of supporting front end current, and supplying fixed bias. Even with 2A3's we can get away with less than 100V negative. Good thing, since it is a convenient line in the component ratings. 100 PIV for diodes, and 100V caps.

For the CCS in the front end, it is good to have at least 20V to dictate operation where the capacitances have stopped changing due to delta-voltage. Too much voltage requires troublesome heatsinks.

The supply uses a 30VAC input to a voltage doubler. Two CRC stages, one full voltage, and the other from the midpoint and also dropped through the decoupling resistor.

One could also terminate the two supplies with chokes. From Mouser, a 6.8, or 10 millihenry high current choke ( J.W. Miller 5800 series, PN 542-5800-682 and -103 ). They're $1.48 each. For the cap, a good film or film/foil unit. The ASC X387 of 7.5 or 10 uF from the bias pot wiper to ground will do nicely. The split-CT grid chokes need the two center ends AC coupled to ground anyway, and this cap is a very good choice there.

More drawings to come...:)
cheers,
Douglas

the Schematic [message #31710 is a reply to message #31709] Tue, 24 January 2006 09:55 Go to previous messageGo to next message
PakProtector is currently offline  PakProtector
Messages: 935
Registered: May 2009
Illuminati (2nd Degree)
hey-Hey!!!,
Here is the link to the schematic:
http://www.audioroundtable.com/GroupBuild/Projects/Galahad_Bias_Doubler_Schematic.pdf

The unloaded AC voltage of the supply TX should be very close to 30. AC voltge * 2 * root2 needs to be always less than the PIV of the diodes used. There are Schottky of higher voltage, so uprating is fairly easy.
cheers,
Douglas

Re: the Schematic [message #31711 is a reply to message #31710] Tue, 24 January 2006 15:21 Go to previous messageGo to next message
Damir is currently offline  Damir
Messages: 1005
Registered: May 2009
Illuminati (2nd Degree)

Good work! May I suggest a schematic for bias pots?
Pots are 100k/lin, (50% on the scheme just means that program needs % of "opening" the pot...); in the middle position we have about -65V out.
Resistors R2 & R3 (between the wiper and "input" leg of the pot) are not necessary, but it`s a precaution if wiper gets discontinued. The useful range is about -50...-80V.

Re: the Schematic [message #31712 is a reply to message #31711] Tue, 24 January 2006 16:36 Go to previous messageGo to next message
2wo is currently offline  2wo
Messages: 44
Registered: May 2009
Baron
Won't c1,c2 short the signal to ground...John

Re: the Schematic [message #31713 is a reply to message #31712] Tue, 24 January 2006 17:03 Go to previous messageGo to next message
PakProtector is currently offline  PakProtector
Messages: 935
Registered: May 2009
Illuminati (2nd Degree)
Nope...remember, this low impedance stuff is at the other end of a high impedance device. The grid resistor, or grid choke. For most power tubes, 100k is good. It is a reasonable compromise between loading and isolating.

For valves like the 1619/1624 or triodes like the 2A3, some makers suggested inductive coupling to ground. Either the secondary of an IT or a grid choke. For a pure resistive element would load the previous stage too much.

Hence Galahad, with its 10k plate load. Even a 2A3( which seems to accept over-spec grid circuit resistance values ), a 60 or 70k might be a better idea...:)
cheers,
Douglas

Re: Bias supply [message #31714 is a reply to message #31709] Wed, 25 January 2006 10:01 Go to previous messageGo to next message
Manualblock is currently offline  Manualblock
Messages: 4973
Registered: May 2009
Illuminati (13th Degree)
Say Douglas; man it would be great to follow this 2A3 thing but let me illustrate how this discussion is absolutely not for the fainthearted or uninitiated builder.

What does the sentence negative rail supporting front end current mean?

good to have at least 20V capacitances have stopped changeing due to delta-voltage?
Why are the heatsinks troublesome?

Thanks; if this kind of questioning is not productive let me know.
The rest I understand.


Re: Bias supply [message #31715 is a reply to message #31714] Wed, 25 January 2006 13:49 Go to previous messageGo to next message
Damir is currently offline  Damir
Messages: 1005
Registered: May 2009
Illuminati (2nd Degree)
Well, we have a CCS (sink), common for both tubes (diff. phase splitter/driver). DC voltage between the cathodes (connected together and "CCS-ed"), and the ground is only a few volts, and for good work CCS "needs" more, at least 20V. We must use a "negative rail", or -20V referred to ground. Heatsink must "deal" with P=U*I dissipation, and if we use large neg. voltage, then we apparently need large heatsink:-)


MOSFET D-S voltage [message #31716 is a reply to message #31714] Wed, 25 January 2006 14:42 Go to previous messageGo to next message
PakProtector is currently offline  PakProtector
Messages: 935
Registered: May 2009
Illuminati (2nd Degree)
Hey-Hey!!!,
I think Damir answered all but the capacitanceone. Go to the Fairchild Semiconductor site, and download the data sheet for the IRF820. Scroll down to the chart with che capacitance. The caacitance is greatest at low drain-source voltages and levels off after about 10V or so. Hence the 20V recomendation.

On heatsinks, anything is a bit ov a pain compared to bare TO-220's for me. But I am a bit on the lazy side. It shows everywhere: simple PP circuits, simple linestage circuits...nothing too complicated anywhere...:)
cheers,
Douglas

Re: Bias supply [message #31717 is a reply to message #31715] Wed, 25 January 2006 16:11 Go to previous messageGo to next message
Manualblock is currently offline  Manualblock
Messages: 4973
Registered: May 2009
Illuminati (13th Degree)
So where does that neg 20 v come from then?

Re: Bias supply [message #31718 is a reply to message #31717] Wed, 25 January 2006 23:28 Go to previous messageGo to previous message
Damir is currently offline  Damir
Messages: 1005
Registered: May 2009
Illuminati (2nd Degree)
The new schematic - 30V AC PT and voltage doubler rectifier with two outputs. Larger neg. voltage for the 2A3 bias, and lower for the CCS.

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