670 signal amp/control amp woes

[DaveP]

Regarding your previous post on the power supply first.

The DC resistance of a normal choke is likely to be a 100~200 ohms so it will not drop 530V to 440V, check it with ohms law.

You have 4 x 100uF caps in series so the capacitance will only be  25uF, the original has 2x140uF in series so that will be 70uF.
You  have also missed out the balancing resistors, so it's not safe like that.  500V power supplies need care and respect!

You don't need as little as 4uF for the first cap either, 2x 100uF 350V caps with balancing resistors will be an improvement.

Regarding the EL34:  When you have done a good job on the power supply, there will be very little AC on the EL34, it is just there to regulate the DC voltage for the signal amp.  You might find that the choke radiates much more AC so be careful where you site that.  I have had problems with chokes like that so I used a resistor and as much capacitance as possible.

DaveP

 

[skal1]

ok Davep , i updated the power a long time ago , it is  more like the original psu now  , i can up load it  if want to take a  look.

regards


Denis

 

[skal1]

updated the power


Regards

Denis

 

[DaveP]

Denis, you will still need three equal balancing resistors across/ in parallel with, each capacitor.  Three 150K would be ideal, that way the voltage cannot exceed the voltage limit on each cap should they be unequal in any way.  This is standard guitar amp practice too.  Even with that configuration, you have still only got 40uF of actual capacitance compared to 70uF on the original.

DaveP

 

[skal1]

ok Davep ,

here is the revised version , i think this is 100 uf now , what you think ?

Regards

Denis

 

[DaveP]

That should do the trick.

My 660 drew 75mA, so the voltage drop with a choke of say 150 ohms would be 11V.

To get a final voltage of 440V, the input to the choke would need to be 451V.

If you are making a stereo 670 then you might draw 100mA and the drop would then be 15V.

Be very careful when playing with these voltages, don't mess with them when you are tired.

DaveP

 

[skal1]

cheers  Davep,

i  know ,it is well over that with 1 channel i had to put in a 1k ohms to drop it to 440 , so  i think i will start with about 500 ohms and go from their  and i place that in front of  the choke.

Regards

Denis

 

[skal1]

670 chassis complete

pic from front


Regards

Denis

 

[skal1]

670 chassis complete

pic from Rear


Regards

Denis

 

[DaveP]

Nice work, looking good.

DaveP

 

[skal1]

Hey, anyone  can tell what gauge of wire a chassis mount transformer should be connected with, can i just use the normal wire out of a IEC  plug.

cheers

Denis

 

[skal1]

i am working on the 670 bias  control circuit and i can not get the voltages  right ,-14v, -19v and  -6.2v , the dc input voltage after  filter cap is 36vdc  , is this due to the circuit not  been load ? ,  at what point's would i load the circuit to get the correct voltages.



Regards

Denis

 

[DaveP]

The bias control in the 670 is quite different from the 660, but you will not get the right voltages on it until it is fully connected to the signal amp and the control amp.  In the signal amp, part of the amp current to the balance pot goes to earth and part of it goes through the negative supply, which loads it up.

DaveP

 

[skal1]

cheers Davep...

DEnis

 

[skal1]

Hi  just an update on the chassis , all finished now , take a look

 

[skal1]

Hi  just an update on the chassis , all finished now , take a look

 

[skal1]

Ok its  been a long ting since  i have upated this thread , so here we go..

so i have completed 1 channel of the 670  all voltages seem to be ok ,  the only thing  which is  it giving me a  problem is the cathode balancing circuit  of the 6ba6's  , what i am finding is that  i can not drawn more 20mA of current  from each phase .

Could this be down to the vacuum tubes or  could it be my  output transformer i'am using a 10k:600  4:1 tx

any insight would help

regards

Denis

 

[DaveP]

I think we would need to see the voltages on the 6BA6's to check if they are turned on enough

DaveP

 

[skal1]

Hi Davep were would i check  at the cathode to gnd ?

regards

Denis

 

[DaveP]

Anode/plate voltages and cathode voltage all to ground, your grid voltage too, should be around -6V?

DaveP