It has been a while since I build Frank’s pre amp with regulated PS, a nice and good sounding project. I build it with the regulator as from Frank’s schematics, but I had some trouble sourcing the tubes used – I admit, they aren’t that hard to find, but can cost quiet a bit when you need more regulators. So I started thinking about an alternative and stumbled upon the regulators based on the ECL85 – I found at least three, but all aimed at a 250VDC, while I needed (at least) 300VDC. Surely it’s possible to make a regulator for 300VDC with a ECL85, but I am also charmed of the idea of cascoding triodes for the error amp (just like in Frank’s regulator) to get more amplification, resulting in a (even) lower output impedance.
In my search for cheap tubes I found, beside the ECL85, lots of ECL84. The transconductance from this one seems to be at least as good as from the EL86, widely applied in regulators. Problem of the ECL84 is it’s relative small plate dissipation, in triode mode about 5W, I believe. Hum…what to do? Use two!! Parallel the pass devices (the pentodes of this tube) and put the triodes in a cascode. Even before consulting any theory on the matter, I realised that paralleling the output of the pentodes could give problems because of the very low output impedance. But I went further: I read the articles from Broskie, from steve bench, some books from Elektor, compared schematics of regulators, but beside a small mention from steve bench and frank’s regulator, I didn’t find anything more using a cascode error amp in a regulator. And my search for cascode circuits wasn’t that productive too. So I decided to go for the trial and error, and the support of diyaudio members.
I designed the regulator using two ECL84 and a OB2. I adjusted the resistor which sets the current for the OB…there is now 7mA running through the OB2 (plus any current coming from the triodes of the ECL84!!??). The OB2 regulates at 105V, that way I settled the voltage at the grid of the lowest triode at 100V. I read somewhere that the grid of the upper triode must be at approximately a fixed potential of 80VDC. I added the value of the OB2 to this, and to make things easier set the upper grid voltage on 200VDC, allowing the use of three resistors of the same value. The plate resistor for the lowest triode I copied from Frank (the use of this plate resistor allows some more current flow through the lowest triode, giving better results…so I read). For the plate resistor of the upper triode I took a 470k, half of what Frank uses (a lower value here gives less amplification, but some more current through the circuit).
The value of the capacitor from the regulated output to the grid of the lowest tube I defined according through this formula, which I took from www.tubecad.com. Don’t know if it is right (Frank’s regulator uses almost a decade less)
C >> (R1 + R2)/(2*pi*f*(R1*R2)
C>> (66.000 + 33.000) / (2*pi*2*(66.000*33.000)
C>> 3,61*10-6
C>> 3,61muF
(f is the lowest frequencies for which the regulator will be used, maybe 2Hz is to low).
Well, that was it. I greatly appreciate anyone’s input on this.
In my search for cheap tubes I found, beside the ECL85, lots of ECL84. The transconductance from this one seems to be at least as good as from the EL86, widely applied in regulators. Problem of the ECL84 is it’s relative small plate dissipation, in triode mode about 5W, I believe. Hum…what to do? Use two!! Parallel the pass devices (the pentodes of this tube) and put the triodes in a cascode. Even before consulting any theory on the matter, I realised that paralleling the output of the pentodes could give problems because of the very low output impedance. But I went further: I read the articles from Broskie, from steve bench, some books from Elektor, compared schematics of regulators, but beside a small mention from steve bench and frank’s regulator, I didn’t find anything more using a cascode error amp in a regulator. And my search for cascode circuits wasn’t that productive too. So I decided to go for the trial and error, and the support of diyaudio members.
I designed the regulator using two ECL84 and a OB2. I adjusted the resistor which sets the current for the OB…there is now 7mA running through the OB2 (plus any current coming from the triodes of the ECL84!!??). The OB2 regulates at 105V, that way I settled the voltage at the grid of the lowest triode at 100V. I read somewhere that the grid of the upper triode must be at approximately a fixed potential of 80VDC. I added the value of the OB2 to this, and to make things easier set the upper grid voltage on 200VDC, allowing the use of three resistors of the same value. The plate resistor for the lowest triode I copied from Frank (the use of this plate resistor allows some more current flow through the lowest triode, giving better results…so I read). For the plate resistor of the upper triode I took a 470k, half of what Frank uses (a lower value here gives less amplification, but some more current through the circuit).
The value of the capacitor from the regulated output to the grid of the lowest tube I defined according through this formula, which I took from www.tubecad.com. Don’t know if it is right (Frank’s regulator uses almost a decade less)
C >> (R1 + R2)/(2*pi*f*(R1*R2)
C>> (66.000 + 33.000) / (2*pi*2*(66.000*33.000)
C>> 3,61*10-6
C>> 3,61muF
(f is the lowest frequencies for which the regulator will be used, maybe 2Hz is to low).
Well, that was it. I greatly appreciate anyone’s input on this.
), consider the voltage divider chain.
