• 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.

Free Lunch/Monkey/DRD inspired 2A3

So I made a prototype of the attached circuit. Must say it sounds pretty nice, but I get a hum (or is that a buzz?). It's a quiet hum. But if I listen real close (and backwards) it seems to sound like this "you don't know how to hook up a/c heaters." Weird, huh?

I’ve never heard a DHT before this and I don’t quite understand humbuckers, so any comments are very welcome.

I know a bit of the parts are over the top, but what can I be if not a wasteful American?

The 417A doesn’t fully drive the 2A3. Maybe a bit more out of the phono stage is in order. (Haven't listened to it with the CD yet.)

I’m going to try to figure out the hum, and then make a proper monoblock for stereo listening!

here's Johnny....


  • 2a3_monkey_drd.jpg
    29.6 KB · Views: 2,240
Add a hum pot to 2A3 filament

Hi Dan,

Surely it will hum. It is so strange that the schematic doesn't include a hum pot for killing hum at the 2A3's filament. A 50-ohm 2W potentiometer could be placed in parallel with the two 100R resistor. You may refer to the attached schematic.

By the way, the Hammond 266M2 may produce excessive voltage greater than 2.5Vrms for the 2A3 filament. Check the voltage!!! You may use a dropping resistor to reduce the voltage to 2.5V if necessary. My very simple calculation of the resistor would be:-

R = (Vout - 2.5V)/2.5

You would find a resistor that will give the voltage very close to 2.5Vrms across the filament.



  • simple45sc-tn1.jpeg
    14.2 KB · Views: 1,896
I wired up a 50R humbucker from RS (173-164). With the dial in the centre, the same buzz/hum. When I turned the knob right, I started to hear a hum. Turn it back and to the left, I started to hear a hum.

With the knob in the centre - no hum, just a buzz.

So now I guess I know what hum is. So maybe it is not the AC.

If I put a 100k from input to earth, buzz is gone, but so is the music. Well most of it. If I put a 1M, no effect. (Just guessing now)

But with my power supply, I just don't see it being the B+

Maybe swap the input valve........


Hi Dan,

Congratulations in fixing the buzz/hum problems in your 2A3 amp. It's time for you to enjoy the beautiful music coming out from your homebrew amp.

I really hope you enjoy it.

I have made the same mistake that the filament of the driver stage wasn't ground which cause big buzz noise. W

Anyway, it is nice having the hum pot to get the hum to minimum level.


Don't know if this is right, but it seemed to work for me.

I calculated the value of the cap for a 1hz cut off and for the total resistance that the cap sees. I (well, read out of morgan jones) took the total resistance as the value of the cathode resistors (50+390+4500) 4940ohms in parallel with the cathode resistance.

I understand the cathode resistance to be the ra of the valve summed with the anode resistor divided by amplification factor +1. But since I've got a transformer sitting on top of the valve, I figured that the impedance load was so big that it effectlively make the total resistance equal pretty much the value of the cathode resistor (I'm sure someone told me this, because I wouldn't have figured it out myself, and the cap value it gave me made sense).

so the cap value is:

1/(2 x pi x 1 x 4940) = 32uF

I konw I show 50uF on the circuit, but that will be changed to 33uF

I'm using a polyprop (solens) fo my cap

Good luck
DRD model

Have I correctly drawn the model of a drd output stage?
ra - internal tube resistance
Ka - amplification factor
us - grid voltage
rxfrm - output resistance (Ro x n^2)
Rk - cathode resistance
Cu - ultrapath cap

If I am correct, then your assumption is right, isn't it?


  • drd_model.jpg
    11.2 KB · Views: 989
monkey amp

I have built a very similar amp and like it very much:


I used lots of cheap poly-in-oil motor run caps, with one little solen to boost the voltage from the choke input filter.

As for hum - you might want to also put some grid stoppers on the input tube, as these high gain supertubes are good at amplifying RF and whatnot. I haven't had to go too crazy with ferrite beads and all that stuff - the grid stoppers seem to take care of it.