Question of power scaling.

[Alan0354]

I have question on 3 different ways of power scaling.

1) Using cathode bias.
2) Using fixed bias with grid voltage proportional to +B.
3) Using fixed bias with constant grid voltage.


1) With cathode bias using a common cathode resistor and bypass cap. This is Ian's measurement. Notice the bias current approximately proportionally to the +B. This mean at low +B like 150V, the bias current is only about 10mA vs the original setting of 24mA. So the amp is in deeper class AB when lowering the +B.

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

2) Fixed bias with grid voltage proportional to +B. eg. say +B=400V, grid voltage = -35V. But lowering to +B=150V, grid voltage = 13V. If you look at the data sheet the bias current shoot way high. This will potentially over heat the tube AND the MOSFET source follower in the power scaling circuit.



3) Fix bias with constant grid voltage. eg. +B=400V, grid=-35V. According to data graph, if I keep the grid constant at -35V and just lowering the +B, the bias current stay approximately the same ( slightly lower).


Seems like Fixed bias with constant grid voltage is the way to go. Please chime in.

Thanks

[jazbo8]

Not quite, when B+ is lowered, so does the screen voltage, i.e., the plate characteristic chart needs to be re-drawn, so (3) will result in crossover distortion. (2) is actually the right way to do it, and London Power sells just such a kit for fixed biased amps or you can make one yourself.

[Alan0354]

Not quite, when B+ is lowered, so does the screen voltage, i.e., the plate characteristic chart needs to be re-drawn, so (3) will result in crossover distortion. (2) is actually the right way to do it, and London Power sells just such a kit for fixed biased amps or you can make one yourself.

Ah, I forgot the screen voltage. thanks

No wonder when I did the power scaling in 78 using the variac and have separate supplies for filaments and preamp, it worked like a champ.
Alan

[Alan0354]

Anyone has the schematic of London Power power scaling? Seems like it has more components than my design. Do they have a patent on this? I did a quick patent search and to my surprise I can't find it.

[jazbo8]

I think Power Scaling is well covered in this thread, it's been awhile, perhaps you have forgotten.

[J M Fahey]

I forgot the screen voltage

[Chuck H]

Play nice Juan

[Alan0354]

Play nice Juan

It's a waste of time to even respond.

[Alan0354]

Dose London power have patent on the power scaling. How come I read they licensed out to other amp companies?

[J M Fahey]

Not on the power scaling principle or schematic, too late to patent a pot adjustable linear regulated power supply (what it actually is) ; I think that the license allows use of the Power Scaling *brand* .
As in you can´t freely use the Power Scale label on amp front panels or advertising.

[Alan0354]

Ha ha, who cares using the "Power scaling" label. I just care they don't have a patent so I can design on my own. I designed the first "power scaling" back in 78 using variac. I just put in separate 6.3V and 360V transformers to power the filaments and preamp, then variac on the original power transformer to control the power and PI tubes.

[J M Fahey]

I designed the first "power scaling" back in 78 using variac.

No comment .

I just put in separate 6.3V and 360V transformers to power the filaments and preamp

So you fed 509VDC to the preamp?
Wow !!!!!!!!

[Alan0354]

No comment .

So you fed 509VDC to the preamp?
Wow !!!!!!!!

What do you mean by no comment? This is not something that is so special. I did it and it's not a big deal. I just got my patent on noise cancellation for single coil guitar that ABSOLUTE DO NOT CHANGE THE SOUND OF THE GUITAR and THE GUITAR STILL WORK AND SOUND THE SAME EVEN IF THE BATTERY DIES, just no noise cancellation. That is a lot more involve in electronics than this. You think I am saying just for sour grape? I have other patent and articles published in scientific journal. This is not something that I would be proud of. Don't judge UNLESS YOU'VE BEEN THERE.

I don't remember the exact voltage. I did not even know electronics when I did that. I stack a bunch of 36V transformers and I don't remember the number. AGAIN, THIS IS CHILDS' PLAY.

I am trying very hard not to get into it with you. It's not worth it. Why don't we just quit this and you have your opinion and I have mine. You don't have to join in, this is my thread.

[Gingertube]

See the pdf I attached to this post:
Calculating cathode resistor for cathode biasing. - Page 2
Cheers,
Ian

[Alan0354]

See the pdf I attached to this post:
Calculating cathode resistor for cathode biasing. - Page 2
Cheers,
Ian

Thanks Ian

I was really studying your two attachments and I posted a few questions in the other thread. Please answer those if you have time.

It is very kind of you to help and I really learn quite a bit from those. I really appreciate help input.