Hello Im just starting to work with valves but have a question on the power supply side. The question is the amplifier I am building needs 300 VDC. As I understand voltage after bridge rectifiers across the smoothing capacitor increases. The transformer I have is 660 volts AC center tapped 120 milliamps.

The amp requires a single ended power supply. The multi meter shows 330 volts AC no load. but where I bought the transformer it says 290 volts ac and it could power the tubes Iam using. But the real question is will that voltage come down under load on the AC side and the DC side.

Im not sure if 330 ac is to much for the tubes. So what I was wondering if I needed something as low as 250 Volts AC. Then that would go up after rectifier and capacitor.

The bottom line is what ac voltage do I need to convert to dc 300 v. Is it the same or is the ac part lower ?

Also what would be the best way to wire the center taped output to get the most current out of the transformer because I only need one x 300 volts supply. The diodes I am using for rectification are 1n4007.

Im confused with this RMS and peak power. I know peak is higher and do the caps need to be at the peak voltage. The caps are rated at 450 volts The tubes Iam using are 6v6s and there will be 2 of those to make a stereo amp. There is also 2 x 6j5s which are driver tubes. A total of 4 valves will be used. The SE output transformer is 5000 ohms so Iam not sure if there is enough current. Would a larger capacitor and choke compensate the current demand at peak power?

sorry if this is long but any help would be much apreciated.


  • \$\begingroup\$ A transformers voltage is specified at its full load current, so your transformer is specified as 290V RMS with a load of 120mA. Unloaded its voltage will increase. When you measure 330V you are measuring this unloaded voltage. This is known as "transformer regulation". Your transformer appears to have a regulation of nearly 14% which sounds to be a little high but could be acceptable. \$\endgroup\$
    – Steve G
    Jan 3, 2017 at 7:57
  • \$\begingroup\$ Do you use 6J5s as phase inverters? Will you build only a "power amp" or a preamp stage as well? If you'll also build a preamp stage then what tubes are you using for preamp stage? Besides, if you'll build a preamp stage then you'll need at least one RC filter after rectification. R is important here, because it will also provide some voltage drop for supplying driver and preamp stage. If you answer my questions, I'll post a detailed explanation for your questions. Or, shortly, post your schematic. It's the best way ;) \$\endgroup\$ Jan 3, 2017 at 8:02
  • \$\begingroup\$ This is only a power amp and these are not used as phase inverters. here is the schematic electronicecircuits.com/electronic-circuits/… \$\endgroup\$
    – user134721
    Jan 3, 2017 at 8:17
  • \$\begingroup\$ You're starting from the wrong end. You need to decide your plate voltage, based on the chosen valve and its load lines and your desired operating point. Then choose your transformer. NB You should really use a tube rectifier, for startup reasons, but be aware there will be tens of volts dropped across it. \$\endgroup\$
    – user207421
    May 20, 2017 at 0:45
  • \$\begingroup\$ Ok thanks for the comment this is my first ampifier that i have succesfully built. \$\endgroup\$
    – user134721
    Jul 5, 2017 at 18:46

2 Answers 2


So you have a transformer with CT output then of course you can use only one side for rectification and leave another side open. Since it will show 290Vrms under full load and you don't have that much load, we can assume it will show 300Vac. As stated by RoyC, peak output voltage after rectification will be 1.4 times secondary RMS but we leave some margin (due to ripple) and let's say the output will be 300 x 1.35 = 405VDC under full load. But for 300VDC, you'll need 300/1.35=220Vac output. But you have 300Vac, so let me check if you can still use it.

For driver stage, since it has a triode and Class-A config, supply voltage directly affects on operating point (thus the harmonic content and distortion). So, if your circuit is designed for 300VDC then you'll need 300VDC. Note that ±20V is a fair margin for a Class A driver's supply. For power stage, it's not that important but 405VDC is dangerous for that stage.

Im not sure if 330 ac is to much for the tubes.

Let's check: According to 6J5 datasheet and the circuit given, quiescent current is 1.5mA (blue point) for 300VDC:

enter image description here

You can see that quiescent anode (plate) voltage is around 160VDC.

If you directly apply 405VDC to the driver stage without any modification, operating point will be like in the following (blue point):

enter image description here

As you can see, quiescent anode (plate) voltage will be around 210VDC. According to datasheet, maximum allowable anode (plate) voltage is 300VDC. Besides, voltage gain will be a bit different. And also, harmonic content for high (10Vpp) input signals will be quite different, but I think input won't be higher than 2Vpp in your application even if it's a guitar amp. So, you're still good.

But, I personally recommend this:

After rectification, put a RC filter with R = 22k/1W and C = 10uF/450V. This will provide a voltage drop for preamp stage and nice filtering for 100Hz ripples. You can also play with R value until you get enough performance.

  • \$\begingroup\$ ok so i need a transformer with 220 volts ac and it will work. Also what is the current draw from this circuit as i am building 2 circuits. \$\endgroup\$
    – user134721
    Jan 3, 2017 at 13:00
  • \$\begingroup\$ Is the driver stage just the same as a preamp basically. also will it accept a line input which is coming from a solid state turntable preamp. It will also go through a 3 band eq that im going to try and build. \$\endgroup\$
    – user134721
    Jan 3, 2017 at 13:01
  • \$\begingroup\$ also are the tube amps safe and wont send 300 volts out of a line in jack \$\endgroup\$
    – user134721
    Jan 3, 2017 at 13:05
  • \$\begingroup\$ 1) It's said that output power is 4.5W (per amp) in the project page. Let's assume %50 efficiency, so input power will be 10W under full load. And the current drawn from 220V secondary will be about 45mA. For stereo output, multiply this by 2. 2) Yes, the driver stage in this circuit is a simple Class A preamp. Input source (turntable, hi-fi amp, PC audio output etc.) is not important provided that having a low output impedance. Also, please don't apply higher than 5Vpp input signal. 3) You are welcome. 4) It totally depends on your "dexterity" :) \$\endgroup\$ Jan 3, 2017 at 13:20
  • \$\begingroup\$ ok so how loud would tube amp be at 4.5 watts because ive never actually heard one. \$\endgroup\$
    – user134721
    Jan 3, 2017 at 13:28

Your main sector is either 110V or 220V (or approx) RMS. The transformer multiplies that by a constant factor to turn that into 290V RMS. As your main sector may not be exactly 110 nor 220V but a bit (10% is ordinary) more, this difference is multiplied by the transformer and it outputs 330V RMS = 330 * sqrt(2) = 466V peak. Without load, no current is drawn from the transformer so diodes will redress the peak voltage and charge the first capacitor with Vpeak (466V).

With a charge, the transformer impedance will start to dissipate hence the voltage will be lower. As a rule of thumb, a loaded high voltage power supply delivers approx. RMS × 1.2 = 396V (400V) but the first capacitors must be able to withstand the full unloaded HT (466V). Choose at least a 500V rated capacitor.

PS: Yes tubes amps do love regulated power supplies.

  • \$\begingroup\$ PPS: 6V6 do not need such a high voltage. 300V DC (250V RMS transformer) is enough. \$\endgroup\$
    – greg
    Jan 3, 2017 at 9:59

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