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-   -   Anyone with knowledge of the 12b4? (http://www.diyaudio.com/forums/tubes-valves/144020-anyone-knowledge-12b4.html)

surfstu 12th May 2009 09:43 PM

Anyone with knowledge of the 12b4?
 
2 Attachment(s)
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

hemgjord 12th May 2009 10:57 PM

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

surfstu 12th May 2009 11:03 PM

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

Gordy 13th May 2009 01:18 AM

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.

Eli Duttman 13th May 2009 02:32 AM

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.

surfstu 13th May 2009 04:23 PM

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

surfstu 13th May 2009 11:13 PM

2 Attachment(s)
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

mikvous 15th May 2009 05:51 AM

Stuart
Could you please give the specs of your power transformer?
Mike


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