• 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.

I´ve built a 3A5 preamp

3a5 are very microphonic in general. It can be hard enough to distinguish hum from grounding, with hum from filament noise, with hum from mechanical vibration coupling to the valves.

If you have a transformer on the unit that vibrates for example, you will hear it amplified. One way to test that is to touch the tubes and it usually damps or changes the noise. Be careful with the voltage to the heaters - if you go past 1.6V you will burn out the heaters very quickly. Try them with a D-cell and see if the noise is gone, and you know then if its the heater supplies.

Another thing I found out is that if you are using a transformer with multiple windings - eg 150V for the B+ and then 2 x 12V for the heaters, you can get hum in the heaters. Use separate transformers and no hum. You could temporarily drop the black pcb from the chassis and just leave it sitting on its wires and see does that change it.


Paid Member
2004-09-17 12:16 pm
If the noise not hum type, try to temporarily RFI shielding tube with grounded aluminium folie.
I have in my collection ancient DHT tubes which tend to working as AM radio and cellular telephone detector. :-(

p.s. flicker noise is annoying phenomenon of old tubes. Sometimes without any sign the well working tube starts sizzling.

p.s. 2: are you "AC grounded" filament FT (pin 4) with 0.1uF capacitor, or it's a "cathode"?
Last edited:
Can I check:

have you paralleled both triodes in each valve? So the anode of the gyrator is going to pins 2 and 6?
Pins 1 and 7 are joined together, then to the 10r resistor, and the other side of this resistor is connected to ground?
The positive of the Coleman regulator is connected to pin 4
The negative of the Coleman regulator is connected to ground
Like this..... note there are no input caps marked in this schematic, and this is the filament boas from Ales website.


  • IMG_20220123_223011-01.jpeg
    40.8 KB · Views: 50