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surfstu 6th May 2012 04:19 PM

12b4 bass rolloff problem
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Ever since I built this linestage I've noticed a distinct lack of bass. I am driving a leak stereo 20 that has input impedance of 1M, I am also trying a Rotel solid state that has input impendance of 33K.

Alot of the bottom end reappears when I drive the Leak, which leads me to belive I have a problem with the output impedance of the 12b4 linestage. I have heard this can dramtically affect the bass response. Although it is still not great and varies from song to song and at what volume I'm litening to the music at. There is almost no bass at low volumes.

I've done a calculation on the the final rc filter of the circuit and my coupling cap (3.3uf) should be big enough. I hope so too as these are huge ruissian PIO caps that were difficult to fix so I'm hoping they do not require changing.

I,m a noob and hoped that someone may take a glance at my schematic and notice an obvious problem....


(Not quite certain of the value of the anode choke and I have 149 Volts at the B+ due to the OD3 regulator)

artosalo 6th May 2012 04:29 PM

Better way than "ear engineering" is to measure the bass response.
You need an AC volt- or universal meter and your PC as a signal source.

DF96 6th May 2012 04:34 PM

I can't see anything obviously wrong, apart from a missing grid resistor, but this would not just affect bass so I assume the grid gets its bias from somewhere.

Yvesm 6th May 2012 05:46 PM


Originally Posted by surfstu (
. . .
(Not quite certain of the value of the anode choke . . .)

It's the first suspect !


surfstu 6th May 2012 07:21 PM

Thanks everyone.

I got the anode choke from i guy selling 12b4 kits, i just can,t remember the exact values. i do remember having the same troubles before using the choke.

If i adjust the value of the resistor across the output - will this affect the output impedance? how?

I,m concerned about its inability to drive the Rotel (33k input impedance)

Also, I,ve used Ohms Law at the cathode resistor to calculate 10.5V / 470R = 22mA

Does this mean that the tube is operating at 22mA? If so - I think this ok.

However, I,m unsure if I need to bias the grid voltage? or how to do it on this preamp and I still don,t think that this is the culprit. I feel that the fact that the line stage delivers very little bass especially at lower volumes must be the impedance? but I am a mere Noob


DF96 6th May 2012 07:34 PM

If your circuit is correct, with nothing missing, then you are relying on the source providing the DC bias for the valve grid. This is not a good idea but should not affect the bass any more than other frequencies.

If the anode choke was too low in value then this would be the same for both amps. The most likely cause for the symptoms you describe is that the output capacitor is much smaller in value than you think.

euro21 6th May 2012 08:40 PM

1 Attachment(s)
I think this is the correct schematic.

ROut about 1.7K, so bass roll-off frequency with extreme low loading (2K) about 30Hz (-3dB).

surfstu 6th May 2012 09:11 PM

Thanks very much euro21! I assume you put my schemtaic into software for that? really appreciated.

Can you see where I might make improvements? If it has rolled off -3dB at 30Hz - does this mean that it already started to lose volume at a higher freq than 30Hz?

I always thought that ROut was close to 600 Ohms. Now I know it's 1.7K, could I bennefit from decreasing this?

I am just not sure how to intrepret your findings - I just want more bass!! (ha)

@df96 - thanks for your input too. he output caps are definatley 3.3uf - they are huge green military ones from ebay and they say 3.3uf. Could this be wrong?

cheers Stuart

DF96 6th May 2012 09:13 PM

It is not unknown for components to be wrongly marked, even from reputable suppliers. From ebay, who knows? If possible, measure the output caps.

Eli Duttman 6th May 2012 09:52 PM

1 Attachment(s)
R4 is too small. It's only function is to discharge the O/P coupling capacitor. Refer to my version of a 12B4 preamp. It uses 1.5 MOhms in that position.

XC for 80 H. @ 20 Hz. is 10053 Ω. That can easily be too small working into the comparatively low SS power amp I/P impedance. Replace the choke with a good CCS. Then, even the nasty IHF 10 KOhm "standard" I/P impedance will not be a problem.

3.3 μF. is the absolute minimum value for the O/P coupler. IF the part is correctly labeled, replacement is unnecessary.

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