Calculating cathode resistor for cathode biasing.

[Gingertube]

This may be a bit of a "hang over" from my tube HiFi work but I think it is worth paying attention too. It is imporatant to keep that noise put of heater supplies and in particular out of bias supplies.
I have found that 1N5408 have really bad switching noise, 1N4007 not as bad but still bas enough.
Cheers,
Ian

[Alan0354]

This may be a bit of a "hang over" from my tube HiFi work but I think it is worth paying attention too. It is imporatant to keep that noise put of heater supplies and in particular out of bias supplies.
I have found that 1N5408 have really bad switching noise, 1N4007 not as bad but still bas enough.
Cheers,
Ian

Thanks, I have R55 and R52 ( 500 and 150) is series before the cap. I am hoping this will eliminate the switching noise. Maybe I should add a 50 ohm in series of the main rectifier. I try not to use 1N4007 because it's only 1A diode with proportionally lower surge current rating.

So far, I used 1N5408 in my prototype amp, It is quiet as a mouse. I don't have any input series resistor at the guitar input to lower the noise, the white noise(hissing) of the first stage still dominates the noise. I hear no hum or buzz even when I crank the cascade gain up.


BTW, did you write the book in the pdf file you posted? You circuit looks the same as in the pdf. I really like the inverting amp using the two transistors that form a closed loop opamp type circuit to get the proportional grid bias. I am going to try that out. It does not require all the protection diodes that I have to put in with the opamp design. I don't have to worry about the power supply for the opamp also. It might not be as accurate as the opamp circuit and it's not ground reference, but I think it's good enough.

[Alan0354]

Alan,

I've posted this a few other places but repeat it here .

Powerscaled 6V6 ( a quad) with fixed/cathode bias switching on each pair and and triode/pentode switching on each pair.

For your possible interest here are my power amp set to work notes:

Set to Work of the Power Amp.

Recall that I am using Hammond 370FX Power Tranny, 4 x 6V6G (ST Shape) Output Tubes and a 50W Marshall Output Transformer(Raa = 3K4).

Measured B+ unloaded (no tubes) => +378 Volts
Measured Raw Bias (no tubes) => -77 Volts

Set Power Scale Control to 10
Set Compression (SAG) Control to 0
Set Fixed Bias
Set pentode Mode

A quick calc for 70% of 12 Watts Anode dissipation gives 22mA per tube.

Fit the 4 x 6V6G Output Tubes and selected 12AX7 PI tube (selected for matched triode sections). Actually the best I had was a CV4004 so used that.

Set the 4 bias contrls for 20mA per tube as a "start point".
Remeasure (loaded) B+, now at +349 Volts
Redo the calc above and get 24mA per tube.
Allow to warm up for 10 minutes and then set bias pots for 24mA per tube.

Checks:
B+ = 345 Volts
Bias Current/Voltages
24.0mA @ -27.1V
24.0mA @ -26.9V
24.0mA @ -26.6V
24.0mA @ -26.7V

Switched to Triode Mode - each tube idle current drops about 0.3 to 0.4 mA
Switch back to Pentode Mode

Set Power Scale Control for B+ = 250V
Checks:
B+ = 250V
Bias Current/Voltages
16.5mA @ -19.5V
16.9mA @ -19.3V
16.6mA @ -19.1V
16.6mA @ -19.1V

Set Power Scale Control for B+ =150V
Checks
B+ = 150V
Bias Current and Voltages
9.8mA @ -11.3V
10.4mA @ -11.2V
9.7mA @ -11.1V
10.0mA @ -11.1V

Set Power Scale Control back to 10 => B+ back to +345 Volts

Wind Compression Control from 0 to 10 => B+ drops to +105V
This looks a bit "drastic" but actually sounded OK.

Set Compression back to 0 and switch to Cathode Bias (each 6V6 pair has a common, unbypassed 470R 5W).

Checks:
B+ = 343V
Bias Currents / Voltages
Pair#1 Cathode Volts = +23.9V
25.9mA , 25.3mA tube currents
Pair #2 Cathode Volts = + 23.7V
25.2mA, 25.4mA tube currents

Cheers,
Ian

Just build the circuit in the attachment and check the current. I set to 25mA at highest voltage, as I scale down the +B, the current DOES NOT remain constant. Result is very similar to what you posted here. I don't think using fixed bias and adjustable with +B is any better off than yours.

[Gingertube]

Alan,
The book extract .pdf file is from Kevin O'Connor TUT ("The Ultimate Tone") Series of books. That particular one is TUT5. The design is from what Kevin calls the "Standard" Amp, in that it incorporates most of his "standard" circuit elements and arrangements.
Tube Guitar & Hifi Amp Kits, Tube Amp Books - London Power
Cheers,
Ian