designing a 15W/30W power switching amp

[macwanj]

Hello folks,

I decided to build a tube amp head running with 2 (or 4) 6L6's PPP Class AB in the power stage to put out 15W (or 30W). I adapted the power amp circuit from the Mesa Boogie Lonestar design. However before I start building it, I wanted to ask the more experienced folks out here if the circuit makes sense. The two things that I'm unsure about is

1. The switch between tubes 6L6(1) and 6L6(2) is what controls the power output. When on, the connection to ground is made and the circuit is now working in parallel push pull to supply 30W output. When the switch is off the ground connection is broken and tubes 6L6(1) and 6L6(2) now work as a buffer stage. Once again, this is how I think this circuit works but am I wrong?

2. I'm still unsure about how to calculate the power output for a class AB PP amplifier. My OPT is a 4000:16/8/4 and so my loadline for 2 tubes (ie with the power switch off) is 1000R. My operating point lies at -22Vgk, Ia quiescent=40mA, B+=360V Iamax=180mA and therefore my maximum rms power is (180mA/sqrt2)^2*1000R (P=I^2*R) giving me 16.2W.

For all four tubes, my loadline is 500R, my Imax doubles to 360mA giving me Po=(360mA/sqrt2)^2*500R = 32.4W.

My maximum plate dissipation is about 15W so the tube should run well. Now is my reasoning sound? The thing thats bothering me is that I don't know how the screen voltage Vg2 comes into the picture. For now I have it running at about 355V. Secondly, does my power estimate take into account the effeciency of the PP topology or do I need to factor in 60% of 16.2W and 32.4W?

Please let me know what you think.

[macwanj]

This is the preamp schematic...

[loudthud]

My crude calculations estimate the power output at about 44W with a single pair of 6L6. Where did you get Imax=180mA? Looking at the curves in the RCA tube manual, with a screen voltage of ~350V, I max is about 300mA for a single tube. If the screen voltage is lowered to ~275V, Imax will be in the 200mA range. If the load impedance is doubled by connecting an 8 ohm load to the 4 ohm tap, power will be about half. The switch in the power amp schematic won't do much to reduce power output.

With 6V6 tubes, the picture changes. With a 4K load and all 4 tubes, power is about 40W and half that with two tubes and an 8K load.

There are some errors in the tone stack of the preamp schematic.

[macwanj]

Thanks for the reply loudthud. When I said Imax is 180mA, I obtained this figure by drawing a 1000R loadline (for two tubes when plate to plate impedance is 4000R). Since my B+=360V, I drew the load line connecting 0mA,360V(x axis) and 360/1000=360mA,0V(y axis) Then my Imax is where Vgk=0V and that was 180mA. Also sorry for the confusion because I used a 6L6GC.

Anyway, when I look at the data sheet for the 6L6GC it suggests using a 360V B+ and a 3800R plate to plate load impedance with Vgk=-22V to get 18W power for push pull class AB. The Imax (or maximum signal plate current) specified was 140mA while zero signal plate current or the quiescent current was specified to be 88mA. Isn't 80mA too hot to begin with. Where am I going wrong??

I'll also check the tone stack; must have made an error using ExpressPCB. Also why would the switch not work?

Once again, thanks for all your help!

[macwanj]

Heres the 6L6GC data sheet

[loudthud]

The operating conditions you have selected specify a 270V screen voltage. This severely limits the saturation voltage of the tube when it starts to clip (in class AB1). In the circuit you have drawn up there is considerably more screen voltage. Working backwards from 18W, 3800 ohms and 360V B+ the voltage across the tube at clipping is about 180V. Note that the operating conditions are almost the same as the class AB2 circuit that yields 47 watts. This tells us that we can get a lot more power out of the basic circuit if we can get more current through the tubes and with a screen voltage of 350V, we will. Look at the graph at the top of page 6. Note that each line is for a different screen voltage, not grid voltage. See how we can get a lot more current through the tube when the screen voltage goes from 250 to 350? Draw your load line on that graph and note how the tube will clip at something around 75V. That tranlates to a lot more than 18 watts.

The 88ma current is for both tubes. With an output transformer, the load line is drawn a little differently than with a resistive load. You would draw the line from 360V, 44ma to 0V, (360V/1000R)+44ma. The slope is the same but the current is moved up by the bias current.

The reason I said the switch wouldn't do much is because with the high screen voltage, a single pair of tubes will get most of the power that the B+ and load impedance will allow. If the screen voltage is backed down to 270V, then I think the circuit will do what you intended without having to change the speaker connection. Not a bad thing, but you are using 4 tubes to do it. You could accomplish the same thing with one pair of tubes and just switch the screen voltage form 360 to 270 volts (and change the bias voltage).

[macwanj]

You could accomplish the same thing with one pair of tubes and just switch the screen voltage form 360 to 270 volts (and change the bias voltage).

Now why didn't I think about that! Everything makes sense now. Thanks a lot loudthud.

[Old Tele man]

Now why didn't I think about that! Everything makes sense now. Thanks a lot loudthud.

...and, *if* you "scale" the BIAS voltage down from the "switched" (Vac) plate voltage (ahead of filter caps), you'll get a fairly accurate 'tracking/scaling' of the two different Vg/Vs-ratios should input line voltage change.

[macwanj]

Hello Old Tele man, thank you for your reply, but what do you mean by tracking and the Vg/Vs ratio? What are the implications of this in light of guitar tone? Plus what kind of switch can I use to switch Screen Voltage and Bias Voltage at the same time. I know how to modify the circuit to do this but am not too familiar with the kind of switch I need to do this. I do know that the switch will need to switch both the circuit junctions simultaneously. Some guidance about the switch model part on Mouser or Digikey would be most helpful indeed. Thanks once again you guys are so helpful! I also post on diyaudio.com but the guys there aren't very helpful since their focus is mainly on hi-fi amps.

[Old Tele man]

...to 'estimate' the control grid BIAS voltage at various Vs values (Vp basically doesn't matter, except when calculating idle wattage), use the equation:

Vg.q = 0.9*(3/2)*(Ip.q/gm) - (Vs/u1)

...for example, the RCA Tube Manual Class-AB1 55W 2 x 6L6GC amplifier bias is -37Vdc:

Vg.q = 0.9*(3/2)*(0.058A/0.006A/V) - (400/7.99) = -37.01 ~ -37Vdc

...so, when switching between Vs(hi) = 360Vdc and Vs(lo) = 270Vdc, the respective BIAS voltages would (assuming: Vp = 400W and 60% Ppd, so Ip.q = 45mA per tube) approximately be:

Vg.q(hi) = 1.35*(0.045/0.006) - (360Vs(hi)/7.99) = -34.93 ~ -35Vdc
Vg.q(lo) = 1.35*(0.045/0.006) - (270Vs(lo)/7.99) = -23.67 ~ -24Vdc

...thus, while the screen voltage "drop" ratio is 75% = 270V/360V, the control grid bias "drop" ratio is only 68% = 24V/35V...which is a Vg/Vs-change ratio of 90%...in other words, Vs and Vg are not directly proportional (and they're opposite polarities too!)...each bias voltage needs to be "scaled" to its respective Vs level, so that when/if line-voltage changes, both Vs and Vg will 'track' (as much as possible) together.

...of course, those numbers assumed same idle currents at both settings because Vp wasn't changed, that is, we just "lowered" Vs from 360Vdc down to 270Vdc.