Post Phase inverter MV on one tube only.

[chocopower]

Hi!

I asked this question in the "Disconnecting one output tube in a push pull amp" topic in the favorite area, but maybe the question was not well redacted.... so.

In a cathode biased amp, push pull, with 2 6V6 and looking for some attenuation, i grounded ONE of the power tube grids with great results. IMHO, a LOT better response that switching out the splited cathode caps.

Now, if i want it in a fixed bias amp, i suposse that bypassing one of the two 220k bias feed resistors will work.

Basically, is a post phase inverter MV aplied only to one tube. Any problem on sight?

thanks in advance.

[sluckey]

Look at reply #5...

http://www.el34world.com/Forum/index.php?topic=11422.0
 
For your purpose, only use one pot.

[kagliostro]

Try to read here, may be you can be interested

http://www.el34world.com/Forum/index.php?topic=11667.0

Kagliostro

[chocopower]

thanks for your replies.

I would like to use a switch, just for get it simple.. i attached a schemo.

Any correction?

[worth]

It looks like you're just breaking the grid connection , not grounding it ?

[Geezer]

It looks like you're just breaking the grid connection , not grounding it ?

I think this is what you want to do (?)

[darryl]

Chocopower's circuit retains the negative bias on V3, but Geezer's circuit leaves V3 without any bias when its grid is switched to earth.

[Geezer]

Chocopower's circuit retains the negative bias on V3, but Geezer's circuit leaves V3 without any bias when its grid is switched to earth.

Yeah........I'm so used to cathode bias (all my amps) that I forget about the bias voltage needed for fixed bias (DOH). 

[tubeswell]

thanks for your replies.

I would like to use a switch, just for get it simple.. i attached a schemo.

Any correction?

Maybe put the switch on the coupling cap side of the 220k, so that the grid of the output tube always has a connection to the -ve bias.

And also put the switch on the cathode side of the PI so that you still can use the PI stage somewhat as an inverting gain stage when in SE mode

[phsyconoodler]

What happens to an amp when you ground one grid on a push pull power section?Tonally that is.

  I know most push pull amps don't like having one power tube removed which is basically what you are doing,isn't it?

Just curious. Sluckey did mention the thread that shows the Lar-Mar master volume.
 Personally I can't think of any reason to cut off one power tube.

[worth]

It looks like you're just breaking the grid connection , not grounding it ?

Geezer.. What I meant was.. Cocopower said that he grounded the grids... his schematic did not show that.

[sluckey]

Don't forget that the bias supply is at AC signal ground due to the filter caps (same can be said of the B+ rail). Chocopower's schematic places one grid at AC signal ground which accomplishes the same thing as 'grounding' a grid in a cathode biased circuit.

[HotBluePlates]

Chocopower's first drawing was correct.

Stop and think guys... He is applying this to a fixed-bias amp. If he "grounds" a grid only, there is 0v on the grid, and tube current soars. He was right to simply swap the grid connection to the raw bias supply to prevent drive and keep idle current in check.

The only downside is this method is all-or-nothing. Sluckey recommended a simple volume control preceding the grid (you need a coupling cap between the pot and bias supply) which allows no drive, full drive or anything in between. Kevin O'conner calls this same technique a "Body" control.

To implement, lift the leg of the existing coupling cap that connects to the 220k bias feed resistor. Add a volume control to this point, but also add another coupling cap from the new volume's wiper to the 220k resistor. The new cap should be 10x the value of the existing cap to avoid rolling off low end.

[phsyconoodler]

Ok,now someone tell me what it does in terms of tone.This is interesting.

  I have used power 'dampening' where you remove the phase inverter bias cap and replace it with a 50k pot.It works pretty good.

[tubeswell]

My suggestion was to use a switch because chocopower said he wanted a switch to keep it simple and it doesn't require any more parts than what chocopower had in his original question.  If the switch is kept on the CC side of the 220k grid load resistor, he can keep the output tube grid permanently attached to the -ve supply, and should reduce switch pop.

[sluckey]

Lot's of ways to kill the drive to one 6V6. I'd probably just use a SPST across the 220K resistor. No chance of loosing bias that way. That would be the equivalent of "grounding" a grid in a cathode biased 6V6.

[HotBluePlates]

Ok,now someone tell me what it does in terms of tone.This is interesting.

Power output drops in a big way, in the same way that O'Connor's body control works.

You might ask how this is different from simply removing one tube. Since the idle current still flows in the undriven tube, OT saturation due to unbalanced d.c. is not an issue. Additionally, if you remove a tube, the remaining tube sees the plate-to-plate impedance divided by 4. With a tube still in the undriven socket and passing idle current, the driven tube will see the plate-to-plate impedance divided by 2.

The explanation for why the change from Rpp/4 to Rpp/2 is long, and if you'd like I can post a reference. But the same thing happens in class AB amps when one side is driven to cutoff.

[phsyconoodler]

there are lots of ways to lower output.Just wondering if this method has any better tone than all the rest? Seems like it wouldn't to me.

[HotBluePlates]

Just wondering if this method has any better tone than all the rest?

You'd have to try it and see if you like it.

Some folks love lobster; I hate seafood. So how could I recommend a good seafood restaurant to you?

[chocopower]

there are lots of ways to lower output.Just wondering if this method has any better tone than all the rest? Seems like it wouldn't to me.

To my taste sound a lot bether than any other i had used.
 It worth a try. I test in 5 seconds in a cathode biased amp (splitted cathode resistors) using a crocodile wire.