• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

PP pentode screen supply

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Ex-Moderator
Joined 2004
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.
 

Attachments

  • rwm amp el34 pentode p-p.jpg
    rwm amp el34 pentode p-p.jpg
    48.3 KB · Views: 842
Ex-Moderator
Joined 2004
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.
 
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.
 
jeff mai said:
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.
 
audiousername said:
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.

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:
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! :D

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.
 
SY said:
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.

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?
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.