6V6 line preamp

[DimZ]

yes Mike,
i am going to try ss, 274b and the russian 5u4g5s3c .. and maybe some others too (waiting for a telefunken rgn1064) ...

but progress is a little slow ( two kids and a full time job) .....
hopefully by christmas time this linestage will be singing .......

 

[mgreene]

Sweet! Thanks I am so slow it will probably Feb before I get mine built anyway. I will be looking forward to your results...

 

[DimZ]

by Feb I hope I will be done with the 26 linestage , got almost all parts ....

 

[mgreene]

Dont get me started about how many projects I have staring me in the face

I just finished rebuilding my main speakers with a new woofer and crossovers... been working on them in various guises for about 2 years...

Of course I am getting old too

 

[DimZ]

same story here ....
done with current speakers last year .... still I have something in mind for the future

 

[DimZ]

fired it up .......

58ma stable, B+ at start was 335V soon to drop to 240V ....

dc offset was strange , both channels werent stable, one measured between 20-22mV and the other 76-80mV ... changing all the time ...

output cap leaking ? oscillation ?

at least no boom kaboom , no smokes ...


output cap similar to this one only 2uf (http://www.ebay.com/itm/MBGP-2-Pape...526&pid=100011&prg=1005&rk=1&sd=250906640986&)

 

[Salas]

How you got the sense and force wires hooked up? Can you describe or sketch the power and ground nodes they meet?

 

[DimZ]

forgot to mention also, 840 heatsink after 5 minutes cold as ice, 2540 sink hot .....

 

[Salas]

Maybe it draws much. Try boost the CCS at say 70mA to see if it holds before other investigations.

 

[DimZ]

How you got the sense and force wires hooked up? Can you describe or sketch the power and ground nodes they meet?

they are tied together along with cathode resistor ground / pot ground and ac mains earth ...

 

[DimZ]

Maybe it draws much. Try boost the CCS at say 70mA to see if it holds before other investigations.

give me 5 min to report back ....

 

[Salas]

Its OK. How are the + pair hooked?

 

[DimZ]

the + pair tied together then splits to 2*10k parallel anode resistor per tube ....

 

[Salas]

That's OK too. Was it 335V until the heaters really booted up off their Isource reg?

 

[DimZ]

yes, then droped to 240V ..

trimed for 70mA but by mistake I turned r11 anti-clockwise ...

shall I put it at 340V ?

840 sink cold (not ice ...) ...

 

[Salas]

340 is OK.

 

[Salas]

How high a reg input DC you got to work from?

 

[DimZ]

input dc is 360V ...

with R4 at 70mA trimed R11 ... only reaches 300V ...
dc offset still varies ...
I think i smelled something like smoke (not sure) ... turned off power ...

 

[Salas]

There is strong biasing. It may be up to 30-35mA per tube though and not be abnormal. The fact that you can go up for B+ as you allow more CCS shows there is current demand in the output for one thing.

 

[Salas]

Keep a second DMM on one cathode resistor and if you got a third one on the other so to estimate bias current by voltage/680R.