670 signal amp/control amp woes

[skal1]

yes the input xfmr for the control amp, i am using DMM to measure voltage at the xfmr

regards

skal

 

[abbey road d enfer]

yes the input xfmr for the control amp, i am using DMM to measure voltage at the xfmr

regards

skal

OK, so, apparently it lacks some gain but it shouldn't be a problem really.

 

[skal1]

@Dave p

how much is the feedback voltage on the cathode of the 12bh7 ,move from 2.6v when compressing

regards

skal

 

[DaveP]

Sorry, I don't know, I have yet to finished my compressor.

DaveP

 

[skal1]

yeah back on this thing again, had to get the sowter control amp transformers ,the  1's  i brought were wrong  so that why is never work as intend, another lesson learn't .just in the middle of hooking it all up again, using the 6973 tube  instead of  the el84  tube and circuit Davep came up with .

Ok the screen grid voltage is a bit off 168v instead of 154v  and the cathodes of the 12bh7 are sitting at 2.8v instead of 2.6v , could this be down to tube variation ?

i'am a bit confused about how to wire the null pot for the signal amp , i know it is  wired in parallel  , but are bout wipers  going in the same direction?

cheer

skal

 

[skal1]

ok sorted the cathode  null  pot  seems to be working  ok,  got voltage of 12..47vdc across  the 680 resistors coming off the  cathode pot.

i have a concern about the  tx i am  using for the signal amp  output , it is a 10k600 and it  seems ,it can not drive the  sowter 8345 control amp  input tx  when i attach the sowter 8345 to the signal amp output tx i loss have the output  signal , is this what you cll a mismatch  or is it something else .

cheers


skal

 

[DaveP]

600 to 600 is not a mis-match.

Have you got the fairchild pad in between?

DaveP

 

[skal1]

Hi  DaveP,

i  have just got 680 ohms across  secondary of signal amp  which then is attached to the primary of the  8345 which is  connected in series...


regards
skal

 

[DaveP]

The Fairchild also has 2 x 150 ohms in series with the 680 to make a pad.

But I think your problem is that you will need to have the primary of the Sowter  8345 in parallel not series.

This is because you are not using the double wound original output TX on the signal amp.  You just have a single 600 ohm output.

DaveP

 

[skal1]

ok,    i  just  attached the 2 x 150 ohms in series with the 680 to make a pad  like the Fairchild,  i get 2.5vac  on the  secondary of the  signal amp  before pad , and 1.5vac across the  680 resistor  thats fine i think ,i then attach to the primary of Sowter  8345  which  is wired in series  , yellow and violet tied together  red input (1)  orange input (2)  cool , now the secondary is also wired in series , this is then connected to the dual 100k  threshold  pot , reading across the secondary  of  Sowter  8345  with pot ccw is  44vac this a lot more than is needed for 2.445 volt input , so my >? is , how do i reduce this voltage without upsetting the  pad  and the loading to the signal amp .

regards

skal

 

[DaveP]

Increase the 150 resistors to values that suit your control amp.  This will give a lighter load to your signal amp which is a good thing.

The convention is low output resistance to high input resistance, say 600:10k  You don't have to make the total load 10k, just make it what gets you in the right ballpark.

DaveP

 

[skal1]

ok ,i have made the original pad like the fairchild , changed the signal amp output tx to a  CA-18-VTB2290 - Transformer: Audio Output (9600R:600R Gapped, Narrow Board Version) which is a split bobbin i believe , so  here are the  voltage through the pad.

signal amp output tx  secondary voltage = 2.6vac

after pad at  primary of    Sowter  8345  voltage= 1.4vac  each phase

secondary of  Sowter  8345  wired in parallel  , voltage across secondary = 28.8

all measurements done with threshold pot ccw short

So  from these measurements  it seem that the the control amp input tx has a gain 20  , correct me if i'am wrong

cheer

skal

 

[skal1]

hi  DaveP,

what  method did you use to create the  Fairchild control amp gain  structure  chart.

cheers

skal

 

[DaveP]

what  method did you use to create the  Fairchild control amp gain  structure  chart.

It was a long while ago, but I took the values from the published curves (input and output levels) and put them in an excel table with the amp gain levels etc.

DaveP

 

[skal1]

well i am still struggling with this project, i  feel like giving up :'( , but i will not, anyway enough of my  emotional state .

So i am  looking at the control amp again , i have strip it back to just the 12ax stage , and the dc threshold pot  , my ?  are


1)why  can i not get the 3.6Vdc at the cathode's of 12ax like on  the fairchild schematic without trimming the 18k cathode resistor

2) why is that  when you exchange the 12ax with another one the cathode voltage  changes from the 3.6Vdc

regards

skal

 

[DaveP]

I think you are talking about normal valve tolerances here.  The cut off point of 3.6V is due to a physical grid distance within the valve, but present production may not be exactly identical to 1950's production.

As long as you can trim it to cut-off, I wouldn't worry about it.

DaveP

 

[abbey road d enfer]

well i am still struggling with this project, i  feel like giving up :'( , but i will not, anyway enough of my  emotional state .

So i am  looking at the control amp again , i have strip it back to just the 12ax stage , and the dc threshold pot  , my ?  are


1)why  can i not get the 3.6Vdc at the cathode's of 12ax like on  the fairchild schematic without trimming the 18k cathode resistor

Maybe your B+ is not correct. hwo far are you from 3.6? You should know that voltages in most tube gear are +/-10%

2) why is that  when you exchange the 12ax with another one the cathode voltage  changes from the 3.6Vdc

Vacuum tubes 101b: dispersion of characteristics for tubes in the same batch is +/-10% at best, +/20% in different batches, +100/-50% from different manufacturers

 

[skal1]

abbey road d enfer, Davep


Thanks 

skal

 

[skal1]

Ok ,started work on the psu , front end is solid state  and then the  choke and Hv capacitor bank then tube regulator.

i do not know if i am using too much capacitance in the HV FILTER bank , could this effect, the way the choke interacts with the filter circuit .

any comments welcome / value changes.

cheers

skal

 

[skal1]

Hi lads,

just started work on chassis and i was wondering  will the  el34 radiate any noise into those to transformer either side of it .


regards

Denis