670 signal amp/control amp woes

[DaveP]

No, that's wrong, dBm is related to 1mW in 600 ohms impedance.

I might as well prove this for the record.

square root of 0.001/600 = 0.001291 Amps

600 x 0.001291 = 0.77459 Volts, this is where the 0.775 VU comes from.  (0dBm)

You have 1.96Vrms, so you have to divide this by 0.77459 to get to dB.

1.96/0.77459= 2.53

Then you want 20xLOG 2.53 which equals 8.06dBm.

Well done! you are bang on spec ;D

DaveP

 

[Heikki]

I thought he was asking how to calculate voltage gain of the amp.

 

[skal1]

ok lads i' am still confused here , i think i might be ask in the wrong questions here.


so this is this calculation for  AC gain  Av=  ΔVoutput / ΔVinput,  what is this equation useful for ?

cheers davep/ Heikki
.

 

[DaveP]

If you want to know the gain in dB, you divide the output by the input as Heikki said.

If you want to know the output level, you follow my working.

The gain of the compressor without compression is the gain that is still present when the pot to the control amp is on zero so that no negative dc control voltage is getting to the signal amp tubes.

DaveP

 

[skal1]

The gain of the compressor without compression is the gain that is still present when the pot to the control amp is on zero so that no negative dc control voltage is getting to the signal amp tubes.

DaveP

yeah i get that .


Hey Davep  i think i should stick to RMS measurements  for the moment, it is much easier for me to understand.

So  Davep, is their a method or step by step procedure of testing  the signal amp , i want the amp i am working on to spec's out as close to  the original 670 signal amp as possible.

Thanks for all the help lads

regards

denis

 

[skal1]

so still working on the beast , but i have a few ? ,

whats the best why to adjust the voltage at the junction of the 12ax7 cathodes to get the 3.60v needed , should i put a pot in their, and use it to adjust voltage or should i shunt the 18k resistor with the pot and leave the pot in their, is this a good idea ?


cheers

Denis

 

[skal1]

ok have managed to complete 1 channel of the 670 , but the some reason , the screen voltage is pulling down when compressing a sine wave , is this normal ? , i thought it was meant to stay at 154v because it is fed by the regulated 240v supply.

Regards

Denis

 

[DaveP]

How much is it pulling down?

It's not fed directly from the regulated supply, so there may be a small drop across the divider.  A sine wave is a heavy load, its unlikely that music would be as much of a load.

DaveP

 

[skal1]

ok back on this again , managed to buy the replica output tx from sowter ,so  I'am doing a dual mono version without the latvert  switch i got a couple of ? about the pad .

1] without the switch ,is the secondary  of  t202 in series or parrell 

2] what is the load to the  primary of  t203 without the switch is this  series or parrell 


regards

Denis

 

[DaveP]

The secondary on T202 can be what you choose, just as long as it matches the primary of T203.

PRR told me that the pad between them was to give some isolation between the output and the input to the control amp.

The attached section of the circuit is how I connected T202/T203 and it works fine.

DaveP

 

[skal1]

cheers  DaveP, i did see that before,  but i have already bought the resistors for the padin the 670 schematic already, i guess if  your version works ,i will order from e ba  on monday..

DaveP  i need your expertise on something  can i email you..

regards

Skal

 

[DaveP]

DaveP  i need your expertise on something  can i email you

Sure
DaveP

 

[skal1]

Hi , still having problems with my screen voltage , fairchild schematic state 154v at the junction of  r315/r316 with the screen,s  loaded attached to psu , i am getting 136v at the junction of  r315/r316 on the power supply when load with the screens , with no  screens attached to power supply i get 186vdc , so thats a big drop , i think , it is due to the screen grid pulling to much current , am i right in my thinking or has it got something to do with the grid bias ?

i think the i will need to change out  r315 for a different  value  so it only drops 30vdc  instead of 50vdc

4 screens attached to psu  so stereo  version

No signal applied to input
Anybody care too comment

cheers

Skal

 

[skal1]

ok i have just purchased 2 new vu meter for the 670 fairchild , it has 4 diode on a circuit board on the back of the meter,
it all say that 1.233v is full deflection.

my first ? is ,do i need to remove the diode out of the vu meter

also how do i spec the new resistor for the meter / balance circuit..

regards

skal1

 

[xazrules]

Hi skal! How is sounding your comp? I’ve using mine since 6 months and I can’t live without it, but now I’d like to make a comparison between all the builders here in the forum, what do you think? So we can see if we made a good job replicating the original behaviour or find similarities between our builds. Or see which is the fastest! A dragster race

 

[xazrules]

My voltage at the plates are all over the place, but the most important thing is to have about -70vdc at the 6ba6 grids. I don’t think vu meters work here, I’ve used uA meter across a small plate resistor ala poor man 670 and edcor trafos.

 

[skal1]

my unit sounds great , it can mash and be smooth did not do the matrix thing , but i do want to put in the filter , i just did a check online about using a vu , you can ..

my set up

input 1:4 audomaintance

all other tx sowter

regards

denis

 

[skal1]

she's not pretty, but she works well , i'am up for comparison