Cathode resistor value..Zenith phonograph to 5D3 Deluxe

[Blindog]

Thanks! That's what I thought, just wanted to be sure.

Mark

[Andy Le Blanc]

very glad that wayne responded....... get the power side straight.....
the tubes you want .....biased correctly for the B+...... then start looking for
better tone.... its a floating paraphase... there are three things that need to be
addressed to get good band width and beter over all performance out of the circuit

1..... feed back.... the floating arrangement privides a differential signal
and a source of feedback WITHOUT global feedback.... one simple way to get
a little more is to use a common chathode resistor for both halves of the inverter...

2..... DC balance...... make sure that both plates have an equal voltage
this makes a big differnce

3.....AC balance.......notice that one side of the paraphase arrangement in
your amp has a 270k resistor while the other side has a 220k resistor.....
make one a pot. so ac or signal can be made equal for a more ideal push-pull operation......

both the DC and AC balance need to be readjusted every time
you swap the inverter tube..... but this circuit once its optimized can be very good.....
I gig with one that is 6l6 based that make about 45 watts love the breakup
once your power side is nailed down and the inverter is balanced..... youll need to reconsider the chathode values for
the pre and the inv. .... to get the gain where you want it.... and look to
maybe reduce the value of the first coupling cap to smooth out the break up
keep in mind that this circuit is never really clean in tone it colors the tone

[Blindog]

Thanks so much for this great info Andy. I'm really learning a lot from
this project. I'm a little confused on the measuring the millamps across the 1 ohm. Am I just measuring the voltage across the 1ohm resistor? The meter has to be in series to measure current, correct?

Mark[/quote]

[Andy Le Blanc]

just measure volts........ then move the decimal three places to the left to
convert to amps..... useing the meter in its "current" setting... you need to
make the meter part of the circuit... which you cant do easily if you need to
make many reading around the circuit...... so you used a 1 ohm resistor to
take the measurment across and make the math easy...... as mv.\ma. ...

[Blindog]

OK, I've rewired the power supply and the PI just like the 5D3 schematic in my earlier post. Only exceptions are .1uf couplers instead of .05uf 'cause thats all I had, 22uf bypass caps, and I used 22uf filter caps. I put a new 220 ohm cathode bias resistor in series with a 1ohm to ground like this:

Pin 8-----/\/\/\/\/---/\/\/\/\---ground
220r 1r

Here are the new voltages

B+1 371v
B+2 305v
B+3 265v

6J5
pin 3 57v

12AX7
pin1 174v
pin2 175v

6V6
pins 4 304v
pins 3 365v

I have 17v across the 220r and .078v across the 1 ohm

How does this look?

[Blindog]

double

[Andy Le Blanc]

OK...... so thats 365 X .078 = 28.47........divide by 2 and each tube is idleing
at 14.24 watts which is right on for that type of of circuit.....
remember that its mv/ma measured across the 1 ohm and it becomes amps
when converted.......the voltages look better too....

[Blindog]

I still want to be sure on the dissipation measurement. The .078v is the actual reading across the 1 ohm.(no decimal point moving) So using the formula and I move the decimal point 3 places to the left, that would make it .000078?! I must be doing something wrong, or misunderstood the formula.

Also, what do you think of the low plate voltage on the 6J5? Would it be better to run the stage closer to 100v? I'm not sure what effect that would make in this circuit.

[Andy Le Blanc]

check the range on your meter..... mine automatically ranges.... the figure
as given makes sense....... .078v is 78 ma...... milli.......

see if you cant find a chart for that tube type..... it might be good to go....
a chart like the resisitance coupled amplifier charts in the back of the RCA
tube manuals.... I think I saw a sylvania manual in PDF on line that has it....

[Blindog]

Yeah my meter is autoranging as well. I agree, it looks right...and sounds pretty good too. Did you maybe mean to say to move the decimal 3 places to the right instead of left? That works out if that's the case. I think i'm going to tweak the 6J5 next and see what kind of trouble I can make.
Thanks for all the input Andy!

Mark