-ve grid voltage

puginfo405 · 28th October 2009, 03:22 AM

wondering if i can derive the grid voltage of about -50V from the same winding as the HT. im using the 0-400V for the HT now and looking at using the 0 to 60V for grid.

is this possible? i use a bridge diode pack for the HT and i find that when i use another bridge pack on the same winding, the first HT voltages seem to vary some times even blowing the 500mA main fuse. any ideas? thks

puginfo405 · 28th October 2009, 03:27 AM

Quote:

Originally Posted by puginfo405

wondering if i can derive the grid voltage of about -50V from the same winding as the HT. im using the 0-400V for the HT now and looking at using the 0 to 60V for grid.

is this possible? i use a bridge diode pack for the HT and i find that when i use another bridge pack on the same winding, the first HT voltages seem to vary some times even blowing the 500mA main fuse. any ideas? thks

sorry the pic did not upload. thks

aardvarkash10 · 03:49 AM

yes. Pictures soon unless someone beats me to it...

Here it is...

The bit you are interested in is the stuff below the earth rail in the upper secondary winding. Its a bit complex in this one cos I needed a whole lot of voltages, but you should get the idea.

In essence, the 0.1uf cap and the blocking diode "pump" the 47uf 'lytic down to the voltage set by the zener(s) to the right of the 10k resistor. Change the zener changes the voltage obviously...

BTW, don't use the SCR soft start as shown - it was a dismal failure!

puginfo405 · 29th October 2009, 06:37 AM

thks varkash. i was thinking of something very simple like this one. would this work for -ve grid voltage?

the reason there's single diode is that, if another bridge is put there, it upsets the B+ voltage.

cheers

DigitalJunkie · 29th October 2009, 10:11 AM

Yea,that should work fine.

kevinkr · 04:15 PM

Quote:

Originally Posted by puginfo405

thks varkash. i was thinking of something very simple like this one. would this work for -ve grid voltage?

the reason there's single diode is that, if another bridge is put there, it upsets the B+ voltage.

cheers

No it won't as you've shorted across the high voltage windings with the two ground connections shown via the bridge rectifier. Take a look at what aardvarkash10 has done - that should give you some clues.

aardvarkash10 · 06:54 PM

thanks kevin!

Pug - take a look at what the circuit I posted does. Its VERY simple in its own right, and really can't be simplified further.

The 0.1uf cap isolates any DC component in the winding from the bias generating circuit.

The diode half-wave rectifies to give a voltage that is negative in relation to the earth by the value of the zener diode string, and the 47uf is a storage cap to keep it stable.

The 10k resistor limits current through the zener chain and the 1meg bleeds the 47uf cap circuit down, while the 47k provides a bleed circuit for the 0.1uf cap.

Its stable, reliable, cheap as chips and adaptable - just follow the recipe, I did! None of the values are too critical either. EG, if your first cap is a 0.33uf, nothing significant is going to change. If your nearest storage cap is a 56uf, no sweat.

EDIT - Actually, I'd be inclined to go 0.33 or 0.56uf in the first cap as my circuit takes a little time - around five seconds - to reach voltage, which means... no bias during that time. Leave that to your imagination.

Use what you got!

Geek · 29th October 2009, 10:21 PM

Out of curiosity, what's that transformer?

The voltages/current looks a lot like the one I recently scavenged from an ancient rack mounted Motorola VHF tranceiver.

Cheers!

puginfo405 · 30th October 2009, 05:57 AM

well i got that made out and now kinda stuck with the -Ve bias grid voltage. its clear now that you sould need separate winding for HT and grid if not use varkash' cct.

cheers

DigitalJunkie · 30th October 2009, 06:27 AM

^^ What Kevin said...I should learn not to post when I'm half asleep.

28th October 2009, 03:22 AM	#1
puginfo405 diyAudio Member Join Date: Aug 2009	-ve grid voltage wondering if i can derive the grid voltage of about -50V from the same winding as the HT. im using the 0-400V for the HT now and looking at using the 0 to 60V for grid. is this possible? i use a bridge diode pack for the HT and i find that when i use another bridge pack on the same winding, the first HT voltages seem to vary some times even blowing the 500mA main fuse. any ideas? thks

29th October 2009, 06:45 PM	#7
aardvarkash10 diyAudio Member Join Date: Mar 2007 Location: Auckland, NZ	thanks kevin! Pug - take a look at what the circuit I posted does. Its VERY simple in its own right, and really can't be simplified further. The 0.1uf cap isolates any DC component in the winding from the bias generating circuit. The diode half-wave rectifies to give a voltage that is negative in relation to the earth by the value of the zener diode string, and the 47uf is a storage cap to keep it stable. The 10k resistor limits current through the zener chain and the 1meg bleeds the 47uf cap circuit down, while the 47k provides a bleed circuit for the 0.1uf cap. Its stable, reliable, cheap as chips and adaptable - just follow the recipe, I did! None of the values are too critical either. EG, if your first cap is a 0.33uf, nothing significant is going to change. If your nearest storage cap is a 56uf, no sweat. EDIT - Actually, I'd be inclined to go 0.33 or 0.56uf in the first cap as my circuit takes a little time - around five seconds - to reach voltage, which means... no bias during that time. Leave that to your imagination. Use what you got! __________________ Yes, conservatism thrives on low intelligence and poor information. But the liberals in politics... continue to back off, yielding to the supremacy of the stupid. It's turkeys all the way down. - George Monbiot, guardian.co.uk, 6 Feb 2012 Last edited by aardvarkash10; 29th October 2009 at 06:54 PM.

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29th October 2009, 10:11 AM	#5
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 4	Yea,that should work fine.

29th October 2009, 10:21 PM	#8
Geek diyAudio Member Join Date: Sep 2004	Out of curiosity, what's that transformer? The voltages/current looks a lot like the one I recently scavenged from an ancient rack mounted Motorola VHF tranceiver. Cheers!

30th October 2009, 05:57 AM	#9
puginfo405 diyAudio Member Join Date: Aug 2009	well i got that made out and now kinda stuck with the -Ve bias grid voltage. its clear now that you sould need separate winding for HT and grid if not use varkash' cct. cheers

30th October 2009, 06:27 AM	#10
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 4	^^ What Kevin said...I should learn not to post when I'm half asleep.

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