• 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.

how do I increase the bass in 12B4 pre-amp?

hi everyone:

I built a pre-amp based on 12b4 with "Grounded Cathode Amplifier" circuit.

using CADDOCK 5K for B+ intake. (220VDC when loaded).
4uF REL Cap for signal output following the CADDOCK 5K Resistor.
MILLS 470Khoms to ground Cathode. (with 22uF Philips in parallel to slightly increase the output gain).
15Kohms HOLCO after output Cap (4uF REL) to ground.

ALPS 100K volume pot, then go into a 1Kohms to 12B4's grid.

however, I don't find the bass is powerful when compared to other solid pre-amp. the bass is kinda sloppy ...

how do I increase the bass respond?
please help help
MILLS 470Khoms to ground Cathode.
Really kiloohms?

Then the tube would be drawing microamps at best. Given your high plate voltage, I bet I'm not far off. Plate voltage will drop when you start drawing current.

Reduce the cathode resistor to 470 ohms. Maybe 560 if your PSU drops too far down. Cathode should be around 17 to 20 Volts for 30 mA.

15Kohms HOLCO after output
Increase to 100k. It will increase bandwidth.

Nice to see you using quality components, but the circuit is flawed. It seems you copied it from somewhere.

May I suggest you read up on how to bias tubes. These things are too dangerous to fiddle with without proper knowledge... really!

Circuit before components!

Be safe,
Re: Re: how do I increase the bass in 12B4 pre-amp?

woops @@
it's typo !!! sorry. you are right~!
actually they are MILLS 470ohms instead of 470K in my pre-amp.
then it's not a problem here.

so you are saying that increaing the Resistance followed the output Cap? (to 100K potentially ?)

ok, I'll give it a try !! thanks a lot.

by the way, I did copy the circut from someone else, who used "Tube Cad" helped me to calculate all component values I need.
although the outout resistance was 10K in the original circuit... (I used 15K... but seems I'll have to try 100K now )

I just guess it's called "output resistance", right?

yup yup! I'll be working really really hard on understanding audio circuit and electronic characterastic etc :)

I really love to play around with them.
tho my next plan is to replace my OP power-amp to DIY Aleph 2.

wish me luck !
and I'll let you know how the effect goes after adjusting the resistance you suggested :)

thank you very very much.:angel:
Re: Re: Re: how do I increase the bass in 12B4 pre-amp?

Good to see it helps! Apparently the input impedance of your poweramps is pretty low. For even better bass response, you might want to try increasing the 4 uF, although it's a fine value as it is.

Good luck with the design and happy listening!

where can I find the knowledge "how to bias tubes" etc?

What are the most helpful tube websites

thanks ~
I increased the value to around 330uF...
and now is perfect ! :)

BUT... there still has uncomfortable sound coming out whenever I turn on the tube pre-amp prior to the power amp !!!

and it's loud !!!

does anyone has idea how to get rid of it ?


tubesguy said:
You might also try increasing the cathode bypass capacitor substantially, perhaps to something like 1000uf. - Pat