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Old 31st August 2004, 10:08 AM   #1
SHiFTY is offline SHiFTY  New Zealand
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Angry Fixed bias supply problems...

Hi, I am trying to rig up a fixed bias stereo 807PP amp using diode rectifiers. I have wired the amp so that negative bias/g1 voltage is switched on with the filaments, and then the B+ and screen/g2 voltage is turned on with a second (standby) switch.

With roughly 550V on the anodes, 295 on the screen (g2) and -33V on the grids it works fine, solid AB1, no problems.

The problem i encounter is turnon surge of the bias voltage: When I turn the B+ on, the grid voltage briefly rises from -38V to -10V before settling back to -33V. As a result, the plate current of each 807 rises to about 100mA at turn on, settling back to 35mA. This is pretty high, and I am a bit worried about the cathode stripping effect of this! The 807s are NOS and it happens with a different (good) set as well.

My bias supply is like this: 100VAC secondary winding -> 1N5404 -> 220uF -> 2.2K -> 22uF -> variable resistor to ground (bias taken from wiper). The other leg of the 100V winding is connected to ground. I can adjust the current quite nicely, it works fine, just the massive turnon surge.

I am guessing I need better regulation in this bias supply, anyone got any schematics for this type of thing?
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Old 31st August 2004, 10:41 AM   #2
SY is offline SY  United States
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Have you checked that there is no oscillation as you bang the tube with its B+? That the B+ (or even more important, the screen supply) isn't overshooting? And that the secondary bias winding voltage isn't dipping at turn-on?

One easy thing to try is to temporarily connect a really big cap across the 22uF, like 4700uF or so, then see if the circuit still behaves the same way. If it does, look somewhere else than the bias supply. Also, 100mA isn't really all that huge- I wouldn't worry much about cathode stripping. If it's only running that high for a couple of seconds, dissipation won't be a problem either.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 31st August 2004, 10:44 AM   #3
MIKET is offline MIKET  United States
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Default Other possible solution

If you use an RC filter in the bias supply fairly large resistor in series with a cap which feeds the bias voltage the bias voltage will rise slowly causing a soft start effect. This should help eleviate the turn on issue.
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Old 31st August 2004, 11:22 AM   #4
Yvesm is offline Yvesm  France
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When I turn the B+ on, the grid voltage briefly rises from -38V to -10V before settling back to -33V.
Most probably the time that link caps charge trhu the grid leak resistor.
Use no larger value than necessary.

You may also try to delay (big RC) the 807' G2 voltage.

Regards, Yves.
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