bottoming

[Andy Le Blanc]

I was going over an old electronics text book from the late 40's, when I found
the term "bottoming". This is a form of distortion, the bottom the wave form
is clipped, hence the name. "With tubes possessing a screen grid,... "
"flattening occurs whenever the instantaneous plate voltage becomes low enough
to cause a virtual cathode to form in the space between screen grid and plate".

I've not seen the term in other text, or on the web, so I did some digging.
Which led me to some very early and basic description of pentodes.

Ok, electrons are drawn from the space charge around the cathode by the
potential of the screen-grid and are accelerated to the suppressor grid, were
they slow, but because the plate is positive they continue on to the plate.
The suppressor, is at the same potential as the cathode, and acts as an
electro-static shield, between the screen grid and plate, keeping the potential
of the plate from influencing electrons in the space between the screen
and suppressor.

Bottoming happens when the "instantaneous plate voltage" drops to where
the potential around the suppressor acts like another cathode. "..the plate
current drops as a result of electrons turned back by the virtual cathode.
At the same time, the screen current increases because the screen
captures some of the returning electrons".

Bottoming is why you need screen-grid resistors. The electrons go to the
most positive potential, which when this happens, is the screen-grid.
Which cant handle the current. This is what happens when you push the
amp into distortion. This also helps to explain the compessive effects of changeing
the value of the screen grid resistor. By increasing the value of the
screen grid resistor you reduce the ability of the screen to attract from the
cathode as you reduce its potential between the control and screen.

[Jana]

sounds interesting. It explains why those early 50 watt marshalls with no screen grid resistor sounded the way they did.

Bottoming is also what my 49 Ford did when I drove it in a river.

[Structo]

So what affect does changing the screen resistors have in regards of tone.

You say the compression changes but does the tone change?

Or is it more of a feel type change?

In my D'Lite I originally had 1K screen resistors but later changed them to 470R. (Dogears advice)

At the same time I changed the grid resistors to 3K3 from 1K8.

[Andy Le Blanc]

This is what your modifying, when you start adding resistance to the screen
grid supply. By changing screen resistors you change when
the "virtual cathode" begins to affect the tone, the natural compression that
happens with pentodes when overdriven. You also change the the screen grids
potential field between the control grid and screen, between the screen
and suppressor, and the screens influence upon the cathode. I imagine that
it could also effect the input characteristics but I havent found any refference to that.
Ive seen a lot of discussion, in this forum, of how the tone changes when you
change screen grid resistors, but not a good reason why.

[skyboltone]

Thanks for this Andy. With a 470R screen resistor in a 6L6gc Dumble build, when the amp moves into full feedback and sustain the screens glow like a toaster heating element while the plates remain grey. This is the main reason I'd like to have a bias rite. I'd like to know just how much over on screen dissipation the tubes are running. With less resistance at some point the tube changes over to triode mode and with more resistance the amp should have more headroom and screen dissipation should drop; along with compression.

6L6GC has a screen dissipation maximum of 5 watts
6CA7/EL-34 can handle a max of 8 watts.

The plate dissipation ratings are opposite. The 6L6 is a 30 watt plate while the EL-34 is a 25 watt plate.

I would be very hesitant to run a pair of RCA NOS 6L6GC with 470R in the screen and a hundred ohms or so in the choke. I have not put the VTVM on the screens to find out the voltage they are running in full roar. I'll do that one of these fine Saturdays. KF figured 1470 ohms between plates and screens at 410VDC plate voltage. At the 460-475VDC plate voltage most 6L6 designs use, you just can't push the grids as hard as in an Express or you risk screen meltdown. In AM transmitter service the practice was to regulate screen voltage because you just couldn't afford dead air time. The tubes had to age in a linear way. Though it has been done, regulated screens are not popular in guitar amps. Too punchy or peaky I suppose. In AM radio studios the compression was added between the microphone and the modulator preamp. Like a distortion pedal only smoother as in the famed LA2-A.

I doubt very much that the plate curves were calculated with 470R screen resistors. Though the max ratings show most screens can take the voltage the "Typical" examples shown in the tube manuals often show over a 100 volts difference. But then those examples are not what we do. In any case, I like the sound of 1K to 1.2K screen resistors when used with a choked power supply. I suspect that Scott's preference for smaller resistors comes from his style of play as much as anything else. A jazz/fusion sound.

[Structo]

Thanks, very helpful!

[xk49w]

With less resistance at some point the tube changes over to triode mode and with more resistance the amp should have more headroom and screen dissipation should drop; along with compression.

I thought triode connection was like an ultralinear tap with the tap moved to the anode end. Pentode operation is with a fixed screen voltage (isn't it?), which would be the case with no screen resistance and a stiff supply.

Leslie amplifiers use no screen resistance and an OC3 to drop about 100V for the screens. They are very linear with 6550s (the overdrive sounds bad). Quite a nice overdrive when biased up for EL34s. This is with a Hammond B3 driving it. Always wanted to try it with guitar but never took the time.

[Andy Le Blanc]

Skyboltone mentioned transmitters, what pentode supressor grids are ment to prevent,
and what we use screen grid resistors to modify some what, when the tube
starts to have issues with large signal swings, is called "dynatron action".
good for transmitters but limiting for audio power.

[skyboltone]

Skyboltone mentioned transmitters, what pentode supressor grids are ment to prevent,
and what we use screen grid resistors to modify some what, when the tube
starts to have issues with large signal swings, is called "dynatron action".
good for transmitters but limiting for audio power.

I was actually talking about the modulator, or audio portion of the old AM transmitters. Gates for instance would drive the grids of a pair of 833 triodes with about 100 watts of audio power. The 833s would run at about 2000VDC and produce as much as 1KW of audio. This was in turn dumped (through an isolation transformer) on to the plates of another pair of 833s running at 2500VDC or so grounded grid. The second pair would be the transmitter pair, providing the carrier at the frequency assigned by the FCC. This audio would amplitude modulate the carrier which could be detected by the receiver which would also filter out the carrier frequency,

Anyway, the 100 watt driver stage was frequently made up of 4 6L6GCs with a pretty ordinary totem pole power supply(sometimes with a separate screen winding). We would all but eliminate the screen series resistance and put strings of O type regulator tubes into play (or later, high voltage darlington arrays) to regulate the screens at book value. Always much lower than the plates. The object was the opposit though of a guitar amp. We were attempting to keep the screen voltage from flopping around on program peaks. It gave the transmitted audio more punch and less out of band splash.

[skyboltone]

With less resistance at some point the tube changes over to triode mode and with more resistance the amp should have more headroom and screen dissipation should drop; along with compression.

I thought triode connection was like an ultralinear tap with the tap moved to the anode end. Pentode operation is with a fixed screen voltage (isn't it?), which would be the case with no screen resistance and a stiff supply.

Leslie amplifiers use no screen resistance and an OC3 to drop about 100V for the screens. They are very linear with 6550s (the overdrive sounds bad). Quite a nice overdrive when biased up for EL34s. This is with a Hammond B3 driving it. Always wanted to try it with guitar but never took the time.

Triode connection simply shunts the supressor grid and plates. Which is what you said. But the problem with fixed screen voltage is that it isn't fixed. If you eliminate series resistance entirely you are in Triode mode. With series resistance or a choke the more current flowing through the screen circuit the more voltage drop across that resistance. If you watch the screen voltage using sufficient isolation, (scope or VTVM) you'll see the voltage swing a very wide arc depending on how hard you drive the grids. And precisely as you note, when you regulate, or fix the screen voltage the tubes overdrive poorly. With more resistance between the plate supply and the screen supply (on the same tap) the whole maryann flops around depending on how hard you push the tube. The more the plate voltage sags the more the screens sag even more. My point is, that with a low series resistance on the screens, they will stay at a "too close" (or viewed another way, not close enough as in Triode mode) potential with the plates and begin to act as plates; dissipating more power than they are designed to dissipate. Operated within book values, vacuum tubes are designed to go for 8 to 10 thousand hours. Try that in your Marshall 1987.

[Andy Le Blanc]

Big fun, 833a?, class B?. Do the plates glow orange at max conditions like the sheet says?

[skyboltone]

Big fun, 833a?, class B?. Do the plates glow orange at max conditions like the sheet says?

Absolutely. And I don't know if the modulators were in Class B to be honest. When you think of it, a red plated tube is running at lots of idle current so I doubt it. And they are quick at color change too. Like an octopus. You can watch them change from bing cherry red to orange like a VU meter on voice or music peaks. The bias on the mod tubes was adjustable from the front panel and you would change bias for reduced power operation at night. The power supply was switched from one transfomer tap to a lower one, the bias set by an amp meter on the mods and then the load control on the transmitter pair was adjusted also by an amp meter. Don't quote me there. It's been a long time. At full power it was all about color.

The transmitter pair run flat out grounded grid class C. A little brighter than bing all the time and we ran them until they burned holes through the plates. Frequently we transfered modulator tubes to transmitter service and dropped a new pair of tubes in the mod chassis.

The whole transmitter was pretty foolproof except for the power supply. Constant problems with oil filled caps blowing up and high voltage cable failure. Full of PCBs they were. I was just a helper, not the site engineer. When the FCC changed to no full time engineer needed, (about the time solid state gear showed up) my engineer buddy would travel from station to station making repairs and doing preventative maintainance. I would go along for the fun.

[Ears]

Thanks for this Andy. With a 470R screen resistor in a 6L6gc Dumble build, when the amp moves into full feedback and sustain the screens glow like a toaster heating element while the plates remain grey. This is the main reason I'd like to have a bias rite. I'd like to know just how much over on screen dissipation the tubes are running.

Yes Dan, this is one of the great mysteries, I've been fishing a few times in the forum on how to generate screen current curves from general information presented in data sheets. Ocassionaly tube curve data sheets include screen current curves but unfortunately it is rarely included, and never for situations where screens are run outside typical operating potentials. We just get given a few max screen current figures for specific conditions.I guess it didn't occur to engineers of the period that there would ever be such an interest in conditions boardering on what they would term as component abuse. I think I'll end up approaching Kuehnel or someone similar for this information the information/proceedures I seek, if they even exist.

With less resistance at some point the tube changes over to triode mode and with more resistance the amp should have more headroom and screen dissipation should drop; along with compression.

6L6GC has a screen dissipation maximum of 5 watts
6CA7/EL-34 can handle a max of 8 watts.

The plate dissipation ratings are opposite. The 6L6 is a 30 watt plate while the EL-34 is a 25 watt plate.

Advance apologies if I have misinterpreted the gist of your posts but I don't think reducing resistance in screen cct toward zero accurately describes going from pentode to triode operation. More accurate is the issue of independance of the screen supply to instantaneous plate potentials. Said another way, and ignoring overall supply sag, plate and screen supplies that are independant of plate potential are in pentode mode, ranging through UL where screen tracks a % of instantaneous plate potential to triode where the electrodes are strapped.
But sag is all interelated too: general power supply sag affects both plate and screen supplies in all conditions but in addition we have screen sag due to the screen current fluctuation interacting with the source resistance of the screen supply (includes screen resistors). This effect decreases as independance between the electrodes also decreases (pentode -> triode). [edit: in effect you end up without a screen, so screen supply sag effects also disappear] Again, sorry if I'm just repeating what you already know.

I guess Andy has discovered Terman's descriptions

[Andy Le Blanc]

Austin V. Eastman. University of Washington, 1937, 1941.
Fredrick E. Terman is credited in the preface but not cited for that
information.
He is later cited from publications;

Proc. I.R.E. 1930, 1934, 1936, 1938, 1939
Electronics 1939
Communications 1939

always co-authored

and one from his book "Measurements in Radio Engineering" 1936

[/i]

[wscrane]

With a 470R screen resistor in a 6L6gc Dumble build, when the amp moves into full feedback and sustain the screens glow like a toaster heating element while the plates remain grey.

Might be a design oversight where you are cutting the Vg1=0 characteristic well below the knee. Could you pass along the B+ value and output transformer primary Z?