I have not been able to find any information about using the the 12ax7 heaters as a bias resistor for the output tubes.
I need help to find a way to bias the op tubes.
Plate dissipation for all tubes comes to:
Diss/plate current
V5. 8.2 / 22.7
V4. 14.3 / 40.4
V10. 15.1 / 42.5
V9. 12.3 / 34.6
I would like to make some adjustments as i think the unbalanced tubes are causing some hum. I can't think of a way to bias this part of the circuit without messing up the rest.
Question 11.)
How do I bias a free dc cathode biased 6v6 output tube?
I need help to find a way to bias the op tubes.
Plate dissipation for all tubes comes to:
Diss/plate current
V5. 8.2 / 22.7
V4. 14.3 / 40.4
V10. 15.1 / 42.5
V9. 12.3 / 34.6
I would like to make some adjustments as i think the unbalanced tubes are causing some hum. I can't think of a way to bias this part of the circuit without messing up the rest.
Question 11.)
How do I bias a free dc cathode biased 6v6 output tube?
Can anyone help?
I can most likely find answers to some of my previous questions, but I need some good help with question 11 as I can't find much info.
Maybe someone can direct me to some good reading on the subject...
I can most likely find answers to some of my previous questions, but I need some good help with question 11 as I can't find much info.
Maybe someone can direct me to some good reading on the subject...
It's just arithmetic.
Your schematic has 24volts written on it (under the cathode of each 6v6).
Two 12.6v filaments in series give 25.2 volts - near enough.
Those filaments draw 150mA.
They can be replaced in the 6v6 cathode circuits by a cathode resistor and a bypass capacitor.
I suggest you separate each channel if you are doing this so you estimate 150/2=75mA per channel.
Ohms law says v=I*R, so R=V/I
The resistor you want from the 6v6 cathodes to ground on each channel is
25/.075 = 333 ohms.
Your schematic has 24volts written on it (under the cathode of each 6v6).
Two 12.6v filaments in series give 25.2 volts - near enough.
Those filaments draw 150mA.
They can be replaced in the 6v6 cathode circuits by a cathode resistor and a bypass capacitor.
I suggest you separate each channel if you are doing this so you estimate 150/2=75mA per channel.
Ohms law says v=I*R, so R=V/I
The resistor you want from the 6v6 cathodes to ground on each channel is
25/.075 = 333 ohms.
Oh yeah, pair up v4 and v10 in the same channel, and v5 and v9 in the other. Does this help the hum? Be prepared to replace v5.
Have fun.
JimG
Have fun.
JimG
If I remove the 12ax7s from the 6v6 cathode circuit and replace them with a bias resistor and cap I would lose the dc that is powering the 12ax7s. Then I would have to power the 12ax7 heaters by running another 6.3v ac line from the ip transformer. If I do this wouldn't there be too much current draw from the ip transformer secondary?
Ok... I have been studying a schematic of another amp(Harmon Karden A500) that uses the same cathode preamp dc heater trick(Free DC). This amp has a 100ohm pot between each push pull cathode.
http://shermanr.web.prw.net/A500 Version 2_02.jpg
Would this work for my Bell 3030?
The only difference between the A500 and the bell is the op tube types and a hum ballance pot.
Would I need to ground the grid 270k grid resistors through a connection with the bias pots as is shown on the A500 scheme?
http://shermanr.web.prw.net/A500 Version 2_02.jpg
Would this work for my Bell 3030?
The only difference between the A500 and the bell is the op tube types and a hum ballance pot.
Would I need to ground the grid 270k grid resistors through a connection with the bias pots as is shown on the A500 scheme?
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