6K6GT push pull limits

[brewdude]

Hi all,


Anyone have any experience with using 6K6GT in a push pull PA?


I built an amp inspired by Fender’s 6G3 Deluxe.  It is a single channel with no tremolo.  It uses an Edcor PT w 270-0-270 secondary and a NOS 5Y3GT rectifier. 


I found that adjusting the negative bias effects the anode voltage and current as well as the screen voltage.


I’ve found that I can set the bias to be under 70% anode dissipation, but with the screen voltage higher than the listed maximum 285v.


Or, I can set the bias with the screen voltage below 285v but the anode dissipation is above the 70% of maximum generally considered to be the safe upper limit for fixed bias push pull operation. 


Which safe limit is priority?

[Willabe]

I’ve found that I can set the bias to be under 70% anode dissipation, but with the screen voltage higher than the listed maximum 285v.

How much higher? What is the screen dcv when you set the 6K6's at 70%?

Generally, you can over volt a tube some, but you can't over current them.

You might need a slightly higher value screen grid R to help the screen from drawing too much current.

[shooter]

my last amp I used 6K6 but it was cathode biased, not fixed.  once I set the current at the cathode, I never bothered checking the screen.

[brewdude]

From memory:

If I set anode current at 60% the screen voltage is more like 300v.


If I set screen voltage below 285v the anode current is above 70% (more like 75%).


It sounds like (in general) I’m better off pushing the screen voltage then pushing dissipation current(?).

[PRR]

> adjusting the negative bias affects ... the screen voltage.

Only if there is a significant screen resistor. Changing that is the more direct way to control screen voltage.

[brewdude]

I have a shared 1k/5W screen resistor feeding individual 1k/3W screen resistor placed at the tube socket—I don’t think more resistance is the way to go.  I upped the resistors to try to reduce voltage at the screen.


I am considering buying a 5R4G to see if it will drop a few more volts in hopes of eliminating my dilemma.


I’m open to suggestions.

[PRR]

Earlier/smaller Deluxes had as mush as 10k feeding screens and preamp. I've used 3k on a larger amp with happiness.

A 6K6 amp is obviously not a stadium blaster. Put some resistance in there.

[brewdude]

Thanks. 
I will see what I can come up with.

[brewdude]

I changed the 1k shared screen resistor to 3.3k


Now with bias set at about 70% max anode dissipation, the screens are at ~275v. 


I still want to order a 5R4 rectifier.

[bmccowan]

Nice neat work!(Brown Junior thread) Did you create a schematic?
5R4s are nice, but don't expect to drop much voltage. I think its about the same drop as NOS 5Y3s. But you are in a good spot with voltage now.

[pdf64]

Unless dissipation at idle is very high, for an AB amp, the electrode voltages aren’t really a concern.
Rather the the voltages with signal, especially full load, which are relevant.
Some valve info shows that very high electrode voltages are permissible provided that the current is very low / 0.
I think that’s what is meant by the Vao and Vg2o limits on p4 of https://tubedata.altanatubes.com.br/sheets/010/e/EL34.pdf

[brewdude]

Do you mean as long as the Vg2 drops to less then 285v when the current is being pulled down by the signal, all is good—as long as the dissipation is within spec?

[brewdude]

Nice neat work!(Brown Junior thread) Did you create a schematic?
5R4s are nice, but don't expect to drop much voltage. I think its about the same drop as NOS 5Y3s. But you are in a good spot with voltage now.

Thanks, I spent some extra time with DIYLC to work on the chassis layout.  It took me long time to clumsily learn to get the program to do what I wanted, but I think it paid off in the end.  And, this is the quietest amp I’ve built yet. 


The chart sluckey posted in another thread lead me to believe that the 5R4 might drop 3-7v.  So, I’m not expecting too much. 

[brewdude]

Regarding a schematic, I have a draft completed, but need to update it before posting. 

[shooter]

when the current is being pulled down by the signal

believe it should be; when Volts get pulled down by increased current

[pdf64]

Do you mean as long as the Vg2 drops to less then 285v when the current is being pulled down by the signal, all is good—as long as the dissipation is within spec?

Yes, as long as it doesn’t get crazy high, just applying voltage shouldn’t, per se, be a stress factor.

[tubeswell]

Do you mean as long as the Vg2 drops to less then 285v when the current is being pulled down by the signal, all is good—as long as the dissipation is within spec?

The change in B+ voltage under signal conditions in Push-Pull amps is not quite the same as the B+ in SE amps.


In PP Class A, when plate current in one tube decreases, plate current in the other tube increases (This relationship is more-linear for lower-amplitude signals). This 'kind of' evens out the peaks and troughs in B+ voltage at the reservoir cap. However, within each tube, screen current increases (slightly) as plate current increases, and amount of screen current and plate current increase keeps in proportion to the tube's published Ip:Ig2 ratio for the given bias point - and vice versa for the other tube.

But there's another thing going on under big signal conditions which triggers the Class B load line. In Class B, plate current in one tube shuts off for part of the signal cycle (but screen voltage - and screen current - in that tube 'stays the same'), while plate current in the other tube goes beyond (what would otherwise be) a 'sensible' increase. In this condition, the screen current in this other tube also shoots up. So the screen current scenario in Class AB, is that screen current sits at a certain level, but increases during every B-load line phase, and if you don't have a decent screen grid resistor to eat up some of this increase, then things can get ugly. The ugliness thing can happen as plate voltage swing bottoms, if it bottoms out to the left of the 'knee' of the grid curves. This is where plate voltage has gotten so low that the plate current suddenly stops increasing and decreases dramatically. In this condition, screen current increases just as dramatically, and this can spell death for the screen (and the tube). There are a number of ways of preventing this. You can either:

1) decrease the reflected load impedance (which rotates the load line so that it goes through or above the knee of the Vg = 0 grid curve, or
2) cool off the bias, so the load line is 'lowered' (which reduces the power output of the tubes and helps keep them within acceptable dissipation limits, or
3) lower the screen voltage, which lowers the grid curves, so that the knee of the Vg = 0 curve is at or below the load line (and this is why tube manufacturers recommend a maximum screen voltage on tube data sheets), or
4) a combination of one or more of the above three options.

All the above options decrease output power. There's no hard and fast rule. Its merely that as you increase screen voltage more and more, you risk getting into an undesirably high screen current situation. Tubes will take a certain amount of abuse, and some types with manage better than others as you ramp up the operating conditions.

[brewdude]

Thanks.  That’s a straight forward understandable explanation for something that is tricky for me to keep straight, forward or understood.