Grid stopper resistors and screen resistors...clarification

[tubeswell]

Seems like there is some confusion between what occurs with dynamic changes in screen current during a signal cycle in an output stage, with setting overall screen idle voltage?


If overall screen voltage is decreased (i.e. throughout the signal cycle), there will (all other things being equal) be an overall reduction in tube current, and therefore an overall reduction in the amount of current transferred to the OT secondary, and therefore an overall reduction in output power. The amount of change in plate current for a given amount of change in grid voltage would also decrease, lowering the input sensitivity and gain. Surely?


I'm happy to be corrected in any misconception I may have, or to discuss the minutiae.

[HotBluePlates]

... If overall screen voltage is decreased (i.e. throughout the signal cycle), there will (all other things being equal) be an overall reduction in tube current ...

It is insufficient to cite general "If-Then" statements in this way for output tubes.  There has to be context provided, because "when does screen current increase" (in order for screen voltage to decrease)?  It's not "throughout the signal cycle"...

The attached graph for 6L6 G2 current shows for a given screen voltage, screen current doesn't radically increase until plate voltage is pulled down to the region that would be at/below the knee of the grid curves of the tube.

As a worst-case example, look at the G2=350v curve in the 1st attachment, and compare the screen current at 500v plate vs. 75v plate:  it has increased 20mA (from 22.5mA to 42.5mA).  If the typical 470Ω screen resistor is used for this tube, the screen voltage drop is only 470Ω * 0.02A = 9.4v!!

Now look at the 2nd attachment:
A Blue line is erected at 75v plate, to the G2=350v curve.  The Red line segment approximates 1/5 the distance downward to the G2=300v curve (for a ~10v reduction in screen voltage).  The 2 horizontal lines show the plate current for each condition, which drops only ~10-12mA.

This reinforces why manufacturers typically use small screen resistors:  because there is little impact to screen voltage, even when there are screen current peaks during peak positive grid-input, corresponding to peak plate current and a plate voltage minimum peak.  During the rest of the signal cycle, screen current stays relatively constant.

This also explains why I've said for years that "too big" screen resistors tend to choke back peak plate current and can yield a compressed sound when playing an amp full-tilt.

But my Comment in Reply #44 was in response to Rob's "catch-all statement" for power tubes taken from his website, and he cited the book Guitar Amplifier Overdrive as the source for his statements.  It was not until buying & reading the book that I found the book does not cover output tubes at all, and the source material was taken from a section on preamp pentodes.

Preamp pentodes use a different circuit configuration than output tubes, have screen resistors several-times bigger than the plate loads (where output tubes have screen resistors that are a fraction of the plate load), and the tubes themselves are evolved to have different characteristics (power tube mu under 15 is normal compared to a normal preamp pentode mu over 150).

Additionally, the book is about what happens when a grid input slams a tube stage way beyond its normal operating range (it was hard at first to take seriously the authors ramming a 20v peak input into a stage bias at -1v).

It was therefore inappropriate to extend the judgments summarized in that book to output tubes.

[jjasilli]

I have searched the internet & find little hard core info on the screen voltage, other than what Hotblue already posted re small bottle pentodes; EXCEPT for posts by tubeswell on this and other forums.  As Hotblue has already pointed-out, it's necessary to distinguish between preamp vs power pentodes.

Tubeswell: as a practical matter what actual circuit do you propose to lower screen voltage?  The Tube Cad Journal, among others, advises against voltage dividers.  KOC advises rather large 2.2K screen resistors for EL84's for overdrive tone.  The tone and "feel" didn't work for me, ruining clean and overdrive tone.  This corroborates Hotblue's comments above. 


Here's more rambling:  http://music-electronics-forum.com/t19103/


Of course there's UL but that's a different animal than mere screen voltage reduction, involving feedback & optimal winding taps.

[tubeswell]

Tubeswell: as a practical matter what actual circuit do you propose to lower screen voltage?  The Tube Cad Journal, among others, advises against voltage dividers.  KOC advises rather large 2.2K screen resistors for EL84's for overdrive tone.  The tone and "feel" didn't work for me, ruining clean and overdrive tone.  This corroborates Hotblue's comments above. 

For power tubes, you can run the screens off a separate winding, which is independently rectified and filtered, like an Ampeg SVT or this old circuit (attached) from a 1960s NZ tube amp. This keeps the screen supply a bit stiffer and improves output efficiency.

[jjasilli]

Many power tube specs call for screen voltage about 2/3 of plate voltage when the plates are run near max; otherwise they can be near equal.  Some power tube designs seem to always call for lower screen voltages, like 7027a and KT88.  It's not clear how this supports your theory.

BTW:  there are aficionado's of lower scree voltage.  Sometimes a zener is used to maintain a lower screen voltage.

[shooter]

like 7027a and KT88.

I tried fixing G2 on my breadboard with a KT88, and I didn't like the results, although I wasn't running the 88 near meltdown, but I think there are non equal comparisons  between big bottle PA and pentodes in the pre

[HotBluePlates]

Many power tube specs call for screen voltage about 2/3 of plate voltage when the plates are run near max; otherwise they can be near equal.  Some power tube designs seem to always call for lower screen voltages, like 7027a and KT88.  It's not clear how this supports your theory. ...

Tubeswell is 100% right that a screen voltage lower than plate voltage could be derived from a separate supply.

Where data sheets or practical amps have screens at a significantly-lower voltage than the plate, it is because a higher screen voltage is not necessary to generate the needed peak plate current (which happens when the plate voltage swings to a minimum, because plate current is at maximum and is dropping significant voltage across a load).

To reiterate,  my post on March 6 was a reply to Robrob's prior post, which took statements in a book summarizing preamp pentode behavior and extrapolated it to imply some similar behavior in output tubes, based on changing screen voltage.  Therein was the error.

(Didn't help that the book was under-whelming, and that the authors tried to claim pentodes behaved differently than triodes, while glossing over the fact they input 0.6v peak to a pentode biased at -1.3v, and compared it to a triode biased at -1v but slammed with a 20v peak input signal.  "The pentode doesn't suffer from grid-diode clipping..."  Well, duh, when you're not slamming the grid to melt-down like you did with the triode.)

[Lectroid]

> If only someone like Tesla had designed the guitar amplifier.
Yeah, I know. Tesla invented anti-gravity, the notes were taken to Area 51 to build the flying saucers which appeared "secretly" in the early 1950s, and are the forerunners of the flying saucers we all drive today.

 

[mresistor]

> If only someone like Tesla had designed the guitar amplifier.
Yeah, I know. Tesla invented anti-gravity, the notes were taken to Area 51 to build the flying saucers which appeared "secretly" in the early 1950s, and are the forerunners of the flying saucers we all drive today.

 

OK PRR  put the cognac down..   

[shooter]

it's been 5 years, pretty sure he's finished by now 
although if someone gave me a cognac, it'id still be sittin, un-touched.  never developed a taste for something that smelled like butane