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Old 13th January 2005, 02:49 AM   #1
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Default PP pentode screen supply

Hi,

I have converted my triode-strapped EL34 P-P stereo amp to pentode mode. I have arranged the power supply, shared between the two channels, as follows:

* Power transformer secondary 340v@700mA
* SS diode bridge 4x MUR1400E
* 100uF smoothing capacitor
* 5H 40 ohm 600mA choke
* 470uF reservoir capacitor - to CT of the OPTs
* 10H 260 ohm 100mA choke
* 235 uF decoupling capacitor - to screen grids via 220 ohm
stoppers and to 6SN7 differential drivers
* 4.7k resistor
* 50uF decoupling capacitor - to 6SL7 LTP splitters

I would be grateful for comments concerning the best way to connect the screens, if not as above. Is there a better way, based on the chokes I already have? (I'm aware that the screen voltage needs to be stable but I don't want to get involved in a building a stabilized power supply.)

Please see schematic of amp.
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Old 13th January 2005, 02:54 AM   #2
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Default The power supply

Please see schematic of power supply It's a bit complicated, because it uses a 6D22S for slow start, which is bypassed by a relay after 35 seconds to avoid loss of B+ voltage.
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Old 13th January 2005, 05:36 AM   #3
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Bleh, too many caps. Could do the job with 4 times less, but I digress.

The screen is connected fine. If you see its voltage dropping a lot at full output, it might be worthwhile to add a regulator of some sort. Depends. If it IS soft, it'll reduce max Po slightly and compress the volume a bit (reduced gain) at these high levels.

Tim
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Old 27th January 2005, 02:19 AM   #4
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Thank you for your response. One reason for so many caps in the PS is limited working voltage, so I'm using them in series with resistors across them to share the voltage equally. Another reason is that I want the B+ and screen supply to be reasonably stiff, to cope with transients.
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Old 27th January 2005, 02:47 AM   #5
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It's so easy to regulate the screens, I'd just do it out of principle. A 317-based regulator (Curcio's adaption of the Maida reg, for example) will work beautifully in this application and has a low parts count and cost.
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Old 27th January 2005, 04:13 AM   #6
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Quote:
Originally posted by SY
A 317
*Cough choke sputter* yeesh, why would someone use something that dies over 40V? That's less than legal line variation for a 400V supply.

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Old 27th January 2005, 04:26 AM   #7
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Go read the Maida app note so you understand why the 40V limitation is moot. I've used them in a dozen or so amps over the past 20 years and haven't killed one yet.
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Old 27th January 2005, 08:04 AM   #8
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Since this has been coming up a bit lately, here is Linear Brief 47, High Voltage Adjustable Power Supplies, March 1980 Gee, it's older than I am!
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Old 27th January 2005, 08:31 AM   #9
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Hey, what about a stack of VR tubes?

Easy, cheap, and they look really cool. And don't let somebody talk you out of trying them because they say they're noisy.

You'd sacrifice a little power because of the dropping resistor required. Maybe that's not OK.
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Old 27th January 2005, 09:59 AM   #10
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Quote:
Originally posted by jeff mai
You'd sacrifice a little power because of the dropping resistor required. Maybe that's not OK.
Yes, you'd sacrifice some efficiency because you're sending current down through the dropping resistor (or CCS) and VR valve, but how do you sacrifice output power for a given screen voltage? If you're able to hold the screen steady, I would have thought it possible to increase maximum output power if previously the screen suppy sagged under load.
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