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Regulated power supply for my SRRP tube stage

January 19th, 2019

I tried to build my own regulated power supply for tubes SRRP output stage some time ago. Unfortunately, I did not succeed. The reasons? My assembled SRPP stage was buzzing and I could not find the reasons for that 🙁

This time, the story is different. I decided to get a regulated power supply kit  from eBay (via eBay) and tweak it for my needs for using with my SRPP stage. After waiting around one month, I got it 🙂 The board itself looked like this:
Regulated power supply for my SRRP tube stage board

Few words about the board itself. It is combo regulated PSU for tubes heaters and regulated PSU for tubes high voltage. The board itself is really good quality. It came with the capacitors of Vishay brand 47uF 450V and the rest. I found such diagram online:

Regulated power supply with 6Z4 6X4 for my SRRP tube stage

First about high voltage part

Chinese manufacturers/designers chose to use two Chinese 6Z4 (non 6X4 pins compatible, you have either to use a converter or change the pins) tube rectifiers in parallel. Most likely, they wanted to increase the high voltage (HV) current handling. I am doubtful about this choice as it would be enough only one. It is up to you to leave it empty space or do it like me – to change pins and make one socket for Western 6X4 tube rectifier (via eBay). In this case, you could roll different 6X4 rectifiers, including Russian 6C4P-EV (via eBay). The last one should be pin-compatible with Chinese 6Z4.

The diodes D1 and D2 create a virtual center tap, if your your transformer does not have the real one. In my case, I used R-core transformer, which had a center tap. I connected it to the ground of the right bottom connector (as it is shown in the photo above).

Ok, let’s move on. Once we have HV DC, it goes to regulator, driven by a transistor. In the circuit, it is shown as bipolar but it is MOSFET in reality. Btw, provided transistor was recycled. But ok, it worked. That is MOSFET transistor based voltage regulator with Zener diodes. I found another example of similar circuit:

Regulated power supply for my SRRP tube stage

One of the differences, Zener diode for 15V ZD is used here. Its purpose to protect the MOSFET transistor in the situation if the output voltage is bigger than input voltage. Ok, I will apply this to my board too.

I wanted to make a regulated HV power supply and I came across this page. The page is in French but you could use Google Translate to read it as it is worth reading. The guy suggested to build a constant current source and use a resistor to convert to the voltage:

Regulated power supply for my SRRP tube stage constant current source

Q1 and R1 form a constant current source. As Q1 should be used DN3545 transistor. By using Q1, R1 and calculated R value, you could get whatever needed HV voltage. I chose a 250k pot to adjust the regulated HV. R2 slows down charging C1 and it takes few seconds (depending of C1 value) until your HV will reach a nominal value. I will use this circuit instead of the Zener diodes  to tweak my regulated power supply kit (via eBay).

The supplied 47uF value (not speaking about even higher values) high voltage capacitors are too high to whatever tube rectifier 6Z4 or 6X4. You can check their datasheets, which suggest only 10-12uF capacitor. The workaround could be to put high wattage resistor in series before the first HV capacitor. But you need to calculate its value.

Ok, we are done with HV.

What about the tube heater power supply?

The circuit is based on LT1085 regulator (via eBay). Let’s start that the heaters power supply can implemented to supply either 12.6V (for two heaters of the tubes connected in series) or 6.3V (for the heaters connected in parallel). Personally, I prefer the second one as some tubes use slightly different current and that  ends with supplying slightly different voltages. But let’s return back to LT1085.

Original circuit of this kit is quite bad quality but there is another alternative provided on this page:

Regulated power supply for my SRRP tube stage LT1085
Original circuit of this kit does not contain a pot for adjusting the heaters voltage and the resistors around are slightly different. But I chose to do these small modifications in order to get the heaters voltage adjusted too.

Few words about saving the kenotrons

There are plenty misleading circuits on the web with pretty high value capacitors following the kenetron. It does not matter which one you are going to use, simply do not overload it and use the values given in its datasheet. Or use inrush limiting resistor to save the kenotron 😉

The kit (via eBay) came with two Chinese kenotrons 6Z4. I already tried few other brands too: Tung-Sol 6X4WA (via eBay) and Sylvania 6X4 (via eBay). I liked more the sound of the last one 🙂 Another alternative is to use e.g. BYV960, which were given in Thorsten’s SRPP circuit. At the end of the day, it is all about the preferences.