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.
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.
Attachments
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
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
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.
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.
SY said:
*Cough choke sputter* yeesh, why would someone use something that dies over 40V? That's less than legal line variation for a 400V supply.
Tim
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.
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! 😀
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.
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.
jeff mai said:
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.
audiousername said:
In the schemo from the original post, the screens are nearly at full B+. The dropping resistor would be significant in size and the screens would run at lower voltage which tends to reduce the power output, all other things remaining the same.
audiousername said:
I've got underwear older than you.
A 317 regulator will allow you to keep a decently high screen voltage with no tendency to sag. If you need to drop the screen rail a few volts (you'll shoot for a voltage that's roughly 10-15 volts lower than the main rail under full load), it might have a minor effect on max power out, but not large enough to be audible. What very well may be audible is the lower distortion and solidity of a tightly regulated rail voltage. In pentode amps, it's the single most critical supply voltage.
audiousername said:
Oh, interesting! I wonder how good regulation is with the reduced input swing to the chip...
Tim
SY said:
Sorry for resurrecting an old thread. I used the search, and the title had all the key search phrases. Can someone provide a link to the schematic for a straightforward regulated screen supply, something like what SY described above?
There's a simpler version on my website- the screen reg for the Red Light District. The fancier Maida regs are even better, but may or may not have a real-world advantage in that service.
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