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Old 12th May 2009, 09:43 PM   #1
surfstu is offline surfstu  United Kingdom
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Default Anyone with knowledge of the 12b4?

I am getting hardly any gain from my linestage.

B+ is 250V

V at pin 9 anode = 140
V before anode resistor = 230

V across cathode resistor = 10V

cathode resisor = 470

current at cathode = 10/470 = 21 mA

i'm using a 100K stepped attenuator

the sound is incredibly bassy and very low in volume

The guy who helped me build my psu says i should just use 2 ecc83 valves instead but i would really like to get this 12b4 working, i have little knowledge of tube electronics which is why I am calling on anyone who can help! I have a feeling that the 12b4 does not behave like a typical preamp valve and have attached the circuit as i have built it

thanks in advance

stuart
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Old 12th May 2009, 10:57 PM   #2
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Should work like a charm as drawn. Expected gain should be approx 5.
What are you driving and what is driving the amp?
Bassy sound indicates a hf roll off somewhere. That 220R grid stopper is really 220R and not 220k or something?

/Olof
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Old 12th May 2009, 11:03 PM   #3
surfstu is offline surfstu  United Kingdom
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i feel like a fool, 3 days i've spent on this, I just discovered that it does work like a dream like you say.... i had taken my output caps from the wrong side of the anode resistor, classic schoolboy error?

I have volume now...

but.... an even higher volume of continual hiss/hum... quite high in pitch....

and when i turn my psu off i get cristal clear music for a second or two before the line stage runs out of juice... thanks for your reply, any ideas on the noise?

cheers stuart
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Old 13th May 2009, 01:18 AM   #4
Gordy is offline Gordy  United Kingdom
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Sounds like you are using a poor supply. I recommend CRC and then regulation. Use a decent regulator, such as a TL783, for each channel. (Mouser part number 595-TL783CKCSE3, they deliver to the UK). Don't believe any of this audiophool folklore nonsense about unregulated supplies being 'more free, to let the music flow', because that is just misguided, sentiment-filled clap-trap perpetuated by the hard-of-thinking and the self-styled gurus. Ignore them and regulate.

Check you ground wiring too.
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Old 13th May 2009, 02:32 AM   #5
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Stu,

Look at the 12B4 data sheet. Your anode load resistor may be too small. The NET load impedance the triode is working into should be >= 3X RP. The net load is the parallel combination of the anode load resistor, the 470 KOhm part, and the I/P impedance of the down stream power amp.

The schematic implies DC heater power. If that supply is floating, try grounding it.

The 12B4 is a very nice tube. However, it has an unpleasant propensity to be microphonic. Place silicone rubber "O" rings on the envelope over the micas to hold microphonics down.
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Old 13th May 2009, 04:23 PM   #6
surfstu is offline surfstu  United Kingdom
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thanks eli, I'm actually posting my headaches in another thread called '12b4 linestage' started by coffeedj

i feel a little guitly posting in 2 threads, however it was your initial schematic that inspired me to make this, i would appreciate any help you can give me. The heaters are indeed DC, I have not referenced them to ground inside the linestage yet as i do not know how.

However this noise that i am hearing is more than just a little background hum, could you share also how you would route your signal around this linestage, I have avoided the same side as the heater supply. I have put together the two grounds of my 100K stepped attenuator and all input and output grounds connect to this and to the chassis.

cheers stuart
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Old 13th May 2009, 11:13 PM   #7
surfstu is offline surfstu  United Kingdom
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ok, things are getting better, i have extended the umbilical cord to remove the psu a couple of meters from the linestage. this helped a little. The sound was very nice but still with some interference.

I then grounded the LT- directly to ground. The interference still exists but now it does not increase in volume as i increase the gain, this is better as i can at least turn the volume up to reduce make the problem reduce to the background.

so now i have sound, the interference is like an untuned radio between stations.

I don't believe the sound is great either, the vocals do not seem sharp and clear but mushy.....

can anyone post a schematic similar to mine that they now works for sure?

would be much appreciated

stuart
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Old 15th May 2009, 05:51 AM   #8
mikvous is offline mikvous  Greece
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Stuart
Could you please give the specs of your power transformer?
Mike
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