SE build 6L6 redplating

[Tone Junkie]

I have built 2 of these already all at 365 B+ 470 ohm cathode resister these were 5K transformer ,thought I would try 7K tap used different power transformer B+ is 450 tube is red plating just a bit. Im not good with cathode bias amps. Do I raise or lower the resister to get were I want. Cant remember.
Thanks Bill

[DummyLoad]

one of three ways to get plate dissipation down. 1) increase the value of Rk and the simplest. 2) decrease just  the screen voltage: somewhat more complex. 3) reduce B+ overall: redesign or use B+ CT reduction methods - e.g., VVR, etc..


--pete

[DummyLoad]

put another 100 ohm 2W resistor in series with the 470 to test. problem is, as you reduce Ik, the B+ rises, so you end up using/needing even more resistance to overcome plate dissipation, and at some point you may get too close to or put the tube in cut-off region. if things come to that, then you'll need to reduce screen voltage and/or B+ overall.

you could also try a different brand/type of 6L6: some of the russian stuff is actually closer to 5881/6L6GB in ratings. NOS, svetlana/wingedC, MESA STR, and Grove tubes 6L6GC-GE are true variants of a 6L6GC.


out of curiosity, what is the DCV measurement from cathode to ground with the 470R cathode resistor installed? from screen (pin 4) to ground? from plate (pin 3) to ground? 


--pete

[shooter]

what is is the DCV measurement

+1
if you ignore the OT, consider the tube a "short" while conducting, 450VDC/470ohms puts you within a rounding error of 100mA for the tube.

[tubeswell]

365 B+ 470 ohm cathode resister these were 5K transformer ,thought I would try 7K tap used different power transformer B+ is 450 tube is red plating just a bit.

The formula for calculating the optimum reflected load for a centre-biased output stage running in Class A is:

Zout = Va/(Pa/Va)

where:
Zout = the reflected load
Pa = the maximum recommended plate dissipation of the tube
Va = the (idle) plate voltage

With a 6L6GC and 450 plate volts, the 'ideal' plate load works out to 6,750R. 7k is good

Vg2 can happily be 5V or so under the 450V mark if you are running close to an optimum plate load. Use decent-sized Rg2 for screen protection (3W 470R would be fine)

Ia = 30W/450V = 66mA  for Class A operation. Ig2 will be around 5-6mA bringing the total tube current to ~70mA.

Size the cathode resistor accordingly. If we extrapolate a Vg2 = 450 from an average transfer characteristics chart for a 6L6GC*, we'll see that when running the screens at around 450V, with 66mA plate current, the bias voltage wants to be about -39V. Therefore, working with our total tube current of 70mA (including screen current), 39/0.07 = 557R (5W resistor). If you want to run 470R (giving Vg1 = ~-33V), you can drop the screens to 400V. (This can be confirmed with a quick glance at the bottom chart on page 9 of the linked datasheet, if you superimpose a DC load line with the bias point situated on 450V at 66mA plate current)

(*e.g. see bottom chart of page 6 of 6L6GC.pdf)

YMMV

Of course, if you have a 6L6GB or a 5881 or an earlier 6L6 type, you need to work off a different tube datasheet.

[PRR]

> 2 of these already all at 365 B+
> used different power transformer B+ is 450

It was happy at 365V. Unhappy at 450V.

Get the B+ down. 500r-1K series with the OT feed gets the B+ down. Add a filter cap there.

[Tone Junkie]

Thanks guys I will get some measurements and change the resistor out. I had a 200 and 360 ohm I could put together. I have 100 uf cap with that .
I will print this one out and put on my wall o,knowledge. I forget things because I have sleep issues, having it handy were I can look real quick has served me well.
Thank You Bill

[Tone Junkie]

My voltage before the change with the 470 ohm R.was 425,pin 3 ,400 pin 4. 32v  across cathode cap and resister, after changing to 560 cathode resister, 460 pin 3 ,406 pin 4, and 35 across cathode cap and resistor. Tube is happy.
Thank you Bill

[shooter]

If I pushed the right buttons I have you Idling about 32W with those values

[tubeswell]

I get:

35V/560R = 62.5mA, minus ~5mA (Ig2) = 57.5mA



460-35 = 425V


425V x 57.5mA = 24.4W

[shooter]

  you win
I did 425/560 = 75.8mA, duh, what I get for multi taskin with my Gdaughter