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Old 13th October 2002, 10:04 AM   #1
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Default 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
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Old 13th October 2002, 11:01 AM   #2
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Default Re: how do I increase the bass in 12B4 pre-amp?

Quote:
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.

Quote:
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,
Remco
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Old 13th October 2002, 11:19 AM   #3
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Default 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.
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Old 13th October 2002, 11:24 AM   #4
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Default Re: Re: how do I increase the bass in 12B4 pre-amp?

Quote:
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!
where can I find the knowledge "how to bias tubes" etc?
this site?
or other good sites?

thanks.
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Old 15th October 2002, 07:46 AM   #5
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Default it works !

thanks ultranalog !!! after I changed the output resistor to 100K, the low frequency did come out more !


thanks for your help
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Old 15th October 2002, 08:34 AM   #6
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Default 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!

Quote:
where can I find the knowledge "how to bias tubes" etc?
What are the most helpful tube websites

Remco
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Old 15th October 2002, 09:52 AM   #7
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Default Re: Re: Re: Re: how do I increase the bass in 12B4 pre-amp?

how much should I increase to?
10uF in total ?

Quote:
you might want to try increasing the 4 uF, although it's a fine value as it is.
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Old 15th October 2002, 02:23 PM   #8
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You might also try increasing the cathode bypass capacitor substantially, perhaps to something like 1000uf. - Pat
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Old 16th October 2002, 07:54 AM   #9
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hi!
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 ?

thanks

Quote:
Originally posted by tubesguy
You might also try increasing the cathode bypass capacitor substantially, perhaps to something like 1000uf. - Pat
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Old 16th October 2002, 08:09 AM   #10
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Patient: "Doctor, it hurts when I smash my leg with a hammer!"

Doctor: "Well. DON'T!"






All capacitors have to be charged sometime. Imagine what that does.

Remco, who never turns on powerams before everything else
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