screen grid capacitor in 6V6 push-pull output?

In the output section of the Sta-level schematic there is a capacitor (C7C) that goes between the 6V6 screen grids and ground. What is the purpose of this capacitor?
http://www.waltzingbear.com/Schematics/Gates/Sta_Level.htm

I have seen several 6V6 output stages, and often there's only a screen grid resistor placed between plate and screen grid to make Vs a bit lower than Vp.

I am working on a built Sta-level to see if there is a way to make the 6V6 stage clip later.


/Flundran

When the screen grid is not decoupled, it acts somewhat like a second anode. The overall behaviour of the valve becomes a tad more "triode-ish", i.e. the internal resistance is lower, which decreases the gain, the distortion character is different (more progressive and somewhat smoother), and the Miller effect is not as completely decreased.
In this particular case, the screens of both sides of the push-pull being connected together, I'm not sure that makes a lot of difference.
I believe PRR could nail it down.

Thank you for the reply, it cleared things up a bit. Today I tested lifting C7C and R42 (the parallell resistor) and it resulted in lower gain indeed. I tested a lower value of R24 (screen grid resistor) but this lowered the gain also.

My voltage readings around the 6V6's  matches the ones in the schematic. I got about 282V at the plates, and 12V at the cathodes. I have tried different loading on the output transformer secondary to improve things, but without any noticable difference.

Perhaps things are as good as they could be allready. But whenever there is a loud transient, the 6V6's is clipping before the compression kicks in, and I hear it as distortion on the transient. I guess I could try lowering the threshold, but then the ratio will be lower also... Would there be an idea to swap the power transformer for another one slighly bigger? I have a 275-0-275 transformer in it now.

I think it has nothing to do with the operating point; I would think it's just the way things are. Inevitably the compressor takes some time to react and it lets pass loud transients, which should be taken care of differently, by a very fast limiter or a clipper.

Flundran said:

In the output section of the Sta-level schematic there is a capacitor (C7C) that goes between the 6V6 screen grids and ground. What is the purpose of this capacitor?

Click to expand...

Look again at the network of R24, R42 and C7C    It's probably most familiar used as a 'virtual ground' in single supply circuits.

Often there's just a resistor from B+ to the screen, and the voltage drop across the resistor is caused by the current from screen to cathode.  So your screen voltage is dependant on screen current, and B+ voltage.
In this case, we have a voltage divider with a stabilising capacitor across it instead.  It provides a more stable screen voltage, that's less affected by these variations.

But in the end I also think Abbey Rd is right - sounds like you're trying to fix a feature  ;-)

> I believe PRR could nail it down.

Sore elbow.... maybe I can use the electric screwdriver?

> What is the purpose of this capacitor?

Reduce power buzz. You have rectifier, 20uFd, unknown choke, 20uFd, +300VDC. This point will be clean enough for plates but (maybe) not for screens. It is customary to take one more filter-stage between plate tap and screen tap.

> resulted in lower gain indeed.

Higher G2 voltage will require a higher G1 voltage swing. For -gain- you want G2 as low as will make your required Power. For Power you need G2 high enough to slam your load. Here they split the difference: good gain for effective NFB with ample power (+24dBm is less than 1/10th what 2x6V6 can make).

> resulted in lower gain indeed.

Then something is wrong. The R20 R6 network(s) constrain the gain of the 12AT7-6V6 stages with about 4:1 NFB margin. A 32V or 14% change of G2 voltage should not give audible gain change.

Since this stage is push-pull, the pentode-triode effects are nil, canceled from side to side. Removing C7C mostly raises the ripple-buzz. (It may or may not have good or bad effects on distortion, but this is very secondary.)

> make the 6V6 stage clip later.

This seems reckless. Two 6V6 will make Ten Watts, which can SMOKE any solid-state input behind it.

And I strongly suspect it is the 12AT7 clipping. The 6386 plates idle at 60V and go to 137V when limiting. The R35 C10 rise-time is faster than the C1 R14 coupling-network time constant. So 75V of that +77V jolt tries to pull 12AT7 grids positive. In fact the C1 R14 time constant now has the ~~1K forward grid resistance, oh and we should now account R11 and 6386 resistances.

Fixing the 12AT7 stage is trivial, but the fix will slam the 6V6es to "off", clipping there. While this too might be fixed (might be a full re-design), that just defers the problem to some later point down the chain.

It is NOT meant to be "slammed". It assumes a polite responsible professional operator. But accidents happen. And something always clips. This unit might feed a 25,000 Watt audio amp, the modulator for an AM radio broadcast transmitter. Clipping these things is illegal: it "splatters" hash into the air-space of other broadcasters. So illegal, that 25,000 Watt modulators are not built to stand heavy clipping. And when they fail it is exciting and costly.

With the Sta-Level, an "accident" gives <25mS of clipping in the 12AT7 (which is not harmed) and then it settles to non-clipping sorta-clean. As long as such accidents are rare, everything is fine. The 12AT7 clipping causes much less splatter than modulator-whacking, and the short duration of clipping/splatter may go un-noticed by other stations and audiences.

I'm not sure what you want a slow-attack limiter to do with a severe overdrive? It is designed to ramp-up over 20mS-30mS. Those transients are supposed to pass un-changed. Unless you work far below reference level, they MUST overload some later stage. If you need NO overload, you need to accept the over-control of 50uS (0.05mS) attack times and use a far beefier sidechain driver than a couple 0.1u caps and a hollow diode.

PRR said:

And I strongly suspect it is the 12AT7 clipping. The 6386 plates idle at 60V and go to 137V when limiting. The R35 C10 rise-time is faster than the C1 R14 coupling-network time constant. So 75V of that +77V jolt tries to pull 12AT7 grids positive. In fact the C1 R14 time constant now has the ~~1K forward grid resistance, oh and we should now account R11 and 6386 resistances.

Fixing the 12AT7 stage is trivial, but the fix will slam the 6V6es to "off", clipping there. While this too might be fixed (might be a full re-design), that just defers the problem to some later point down the chain.

Click to expand...

Interesting, I haven't thought about rise-time around the tube stages. I actually built my sta-level according to another Sta-level schematic (earlier revision I think) where the C and R are 0.1uF and 150kOhm around both the 12AT7 and the 6V6's. The risetime is a bit lower than R35 x C10, but still perhaps it's worth trying to lower these time-constants further? I noticed in the newer Sta-level schematic ( http://www.waltzingbear.com/Schematics/Gates/Sta_Level.htm ) that around the 6V6, C = 0.02uF and R=150kOhm. That yelds a really low time constant. I built my Sta-level with bigger C to avoid HP-filter for audio. But perhaps this was a mistake...


Thank you guys for your replies. It really cleares some things up to hang around here. 

Flundran said:

PRR said:

And I strongly suspect it is the 12AT7 clipping. The 6386 plates idle at 60V and go to 137V when limiting. The R35 C10 rise-time is faster than the C1 R14 coupling-network time constant. So 75V of that +77V jolt tries to pull 12AT7 grids positive. In fact the C1 R14 time constant now has the ~~1K forward grid resistance, oh and we should now account R11 and 6386 resistances.

Fixing the 12AT7 stage is trivial, but the fix will slam the 6V6es to "off", clipping there. While this too might be fixed (might be a full re-design), that just defers the problem to some later point down the chain.

Click to expand...

Interesting, I haven't thought about rise-time around the tube stages. I actually built my sta-level according to another Sta-level schematic (earlier revision I think) where the C and R are 0.1uF and 150kOhm around both the 12AT7 and the 6V6's. The risetime is a bit lower than R35 x C10, but still perhaps it's worth trying to lower these time-constants further? I noticed in the newer Sta-level schematic ( http://www.waltzingbear.com/Schematics/Gates/Sta_Level.htm ) that around the 6V6, C = 0.02uF and R=150kOhm. That yelds a really low time constant. I built my Sta-level with bigger C to avoid HP-filter for audio. But perhaps this was a mistake...


Thank you guys for your replies. It really cleares some things up to hang around here. 

Click to expand...

The changes around the 6V6's are not very significant. And it's not a matter of rise-time, in the commonly accepted sense. What PRR means is that the pulse that results from slamming the unit is governed by R35-C10, but what makes the 12AT7's clip is the result of amplifying it in the 6386's and high-passing it in C1-R14 & C2-R13. So this pulse is first low-passed then high-passed. The result is a band-pass. The mid-band attenuation of this band-pass can be increased by slowing down the attack (increasing R35 and/or C10) or increasing the high-pass frequency (decreasing C1/C2 and or R13/R14). This will reduce the thump, but also will make compressor even slower to catch attacks, or reduce the overall LF response of the unit.