670 signal amp/control amp woes

[skal1]

OK  ere we go

24OV @ CT of output TX

238V @ plate  of output TX

10V @  top of 680r node cathode

7V @  top  of 680r node to cathode


---------------------------------------------
-6.2 V @ CT of input tx to 6ba6 grids

- 6.2 V @ 100R  balancing pot

-------------------------------------------

Regards

Denis

 

[DaveP]

Denis,

It looks like you have current through the tubes, but not through the -ve supply,

I have attached my calculation and the next post will be CJ's which agrees with mine.

DaveP

 

[DaveP]

Re-posting CJ's calculation.

DaveP

 

[skal1]

yeah i have seen those before ,  i do have current  through the -ve supply its about 140mA  i just can not get that +14 v at the  top of 680r node cathode, could it be the pot is wired in correctily?

Hi dave should i be seeing  the 14v  without any tube in.

cheers

Denis

 

[DaveP]

No, the voltages in the schematic are with tubes in.

The junction with the pot should be -2.5V

The dual gang pot has to be reverse wired on one side, check that happens, it's easy to make a mistake with that.

Then, try varying the bias voltage to see if that varies the current up and down.

DaveP

 

[skal1]

ok took another look at the balancing pot, and i rewired it like you said it works  ,it balances the current in the plate  thats good but like a said the current in the plate is  still low about 17mA per phase , so i thinking i should change the 680 ohm cathode resistors for a lower value , does this make sense?

regards

Denis

 

[DaveP]

What type of tubes are you using, how many and what make?

DaveP

 

[skal1]

@ Davep brimar 6ba6  8  of  4 per phase

cheers

Denis

 

[DaveP]

OK,
The bias of 6.2V is designed to set the tubes on the straighter portion of the gm/-Vg curve, but it sounds like your tubes have less gm to start with and might benefit from a lower bias.  I would try reducing from 6.2V until you get the current you require.

I would leave the 680 R's alone at this stage.

DaveP

 

[skal1]

Davep you might be right , i up the voltage to the plate around 290v and the current in both halves of the primary is 25mA.

OK,  I would try reducing from 6.2V until you get the current you require.

I would leave the 680 R's alone at this stage.


DaveP

Is this  -6.2v on the grid? or the -6.2 v on the cathode.

cheers

Denis

 

[DaveP]

The grid voltage is set from a pot from the -14V supply, I would see what happens as you reduce it to 5.5V etc.

You should have around 12-14v on the cathodes (top of 680r's), so reduce the grid voltage until you get the correct voltage and current.

Is the heater voltage correct for these tubes?  Sometimes voltage can drop at the end of long runs, this might be why they are not drawing enough current.

DaveP

 

[skal1]

OK  these are the results of my testing

grid bais set too -1v

cathode negative bias set too -6.2

12.86 top of 680r  left

10.55  top of 680r right

22mA  in each phase of  transformer


Cheers Davep

regards

Denis

cheers Davep

 

[skal1]

oh yes sorted the low primary current out it was due to a doggy tube socket in the GR signal amp , all voltages on Gr amp as  schem now ...



i have a few other problems in the side chain amp ,low  grid voltage  it's about -10v should be -12  this is a easy to fix that..

Now  i am off to see the wizard  ;D

regards

Denis

 

[skal1]

Just a front pic 1 channel 

 

[CJ]

nice!  lots of good info here for future builders,

 

[skal1]

Ok lads i am kind of stuck  on the 3db  pad  on the secondary of the signal amp,  i have know idea ,what rms    voltages i should be getting ,before the pad and after the pad and  also when loaded with the control amplifier input  tx.

Signal amplifier output TX  VTB 2290  4:1 10k:600 and control ampifier  input TX is Sowter 8345e 17+17:1+1 600ohms/170k


this is the 670 compressor  pad i am using


any help would be appreciated ..

Regards

Denis

 

[DaveP]

It's the output voltage of the compressor divided down, say about +8dbm 1.95Vrms output

Pad is 680/980 = 0.69

So input to control amp is 1.95 x 0.69 =1.34Vrms

This is the ball park figure for 8dB output.

If you don't have the dual inputs then whatever connection you make you will need to get a similar input level or else the control amp will come on too quickly or too late.

DaveP

 

[skal1]

Ok i finished the pad for the output to the control amp  i get 1.96vrms at the out of the signal amp and 1.36vrms after the pad to the control amp ;D.

so my next issue is the gain of the signal amp ,  the 670 manual states 7db (no limiting) what does this mean?
it also state  +4  or +8 vu(+27dbm clipping point) ?

how would i calculate the db  of  the amp  , with (no limiting)

regards

Denis

 

[Heikki]

the 670 manual states 7db (no limiting) what does this mean?

The amplifier has gain of 7dB when there's no gain reduction.

it also state  +4  or +8 vu(+27dbm clipping point) ?

Nominal level and clipping point. Probably should say +4 or +8dBm

how would i calculate the db  of  the amp  , with (no limiting)

20*log(Vout/Vin)

 

[skal1]

Hi thanks Heikki,

just did that calculation  so i get 20xlog10( vout/vin)        20xlog 10 (1.96 vout/  vin 1.10 )=4.707db

is this right?

regards

Denis