I am quite a novice, but I do understand the nature of graphically drawing a load line for a triode. I have yet to find a good explanation and illustrations of pentode load lines.
Any points to good basic explanation?
Roger
Any points to good basic explanation?
Roger
One of the big differences is the effect of screen voltage on the curves. Quite often, at least for power tubes, the data sheet will include an graph of the 0V grid lines for different values of screen voltage. I like to draw a load line on this graph to help decide what a good screen voltage will be for my project. Otherwise it's similar to doing a triode but you try to shoot for an intersection with 0V grid so it doesn't hit in the knee area where it would be higher distortion at full power.
The online load line calculator can be instructive to see what happens at different screen voltages. It will give you a prediction of the THD and 2nd and 3rd harmonic content at different operating conditions.
https://www.vtadiy.com/loadline-calculators/loadline-calculator/
John
The online load line calculator can be instructive to see what happens at different screen voltages. It will give you a prediction of the THD and 2nd and 3rd harmonic content at different operating conditions.
https://www.vtadiy.com/loadline-calculators/loadline-calculator/
John
The load line impedance as it intersects the triode plate curves . . .
Is exactly the same as the load line impedance as it intersects the pentode plate curves.
The only difference is the name (triode/pentode), and the shape of the triode/pentode plate curves.
Different models of triodes plate curves have different spacing between them mu (u), and different plate impedances (plate curve slopes)
Pentode plate curves has different spacing between grid voltages, and different plate impedance (higher than most triodes).
A pentode has different plate curves versus the voltage to the screen. Just think of that as similar to multiple models of triodes, except that you get multiple results with only one tube, as the screen voltage is varied.
The concept of a pentode load line, and a beam power load line are the same.
Have Fun!
I only use Triode Wired Beam Power Tubes, Triode Wired Pentodes, and Triodes. But then that is a whole number of different threads than this one.
Is exactly the same as the load line impedance as it intersects the pentode plate curves.
The only difference is the name (triode/pentode), and the shape of the triode/pentode plate curves.
Different models of triodes plate curves have different spacing between them mu (u), and different plate impedances (plate curve slopes)
Pentode plate curves has different spacing between grid voltages, and different plate impedance (higher than most triodes).
A pentode has different plate curves versus the voltage to the screen. Just think of that as similar to multiple models of triodes, except that you get multiple results with only one tube, as the screen voltage is varied.
The concept of a pentode load line, and a beam power load line are the same.
Have Fun!
I only use Triode Wired Beam Power Tubes, Triode Wired Pentodes, and Triodes. But then that is a whole number of different threads than this one.
Last edited:
See - http://www.valvewizard.co.uk/pentode.html whilst aimed at guitar amp design the basic premise is the same.
Andy.
Andy.
Thanks all. I have visited those sites and have a handle on it for now. Perhaps more specific questions down he road.
Roger
Roger
The fun (or rather tedious version of it) comes when selecting a signal pentode for low distortion and low noise, alas when the optimum screen grid voltage is roughly 1/4 of the anode. Hardly any manufacturers give graphs of distortion versus load and screen grid voltages because most tubes were used in the TV front ends and very few for audio. The only one I can recollect is the 7199 by RCA. For a given anode load and voltage, depending on makes, and I have been through boxes of 7643-ECF80-E80CF-6BL8 and so on, and finding the optimum screen resistor varies from 500K-1M. If only there were a simpler way predicting the screen grid for distortion performance. Strangely the gold pin Valvo/Philips had the worst variations in performance and distortion and the Westinghouse 7643 was by far the most repeatable in every aspect. There are many concertina phase splitter designs which typically us a pentode direct coupled with fairly low anode voltage, and perusing through many circuit diagrams, some can be optimized for very low distortion when they are far from it.
Yours
Bench Baron
benchbaron,
My comments about triode, pentode, and beam power, load lines, and their similarity was just a first order analysis.
But I alluded to the issue of distortion (between the lines), when I mentioned the spacing between plate curves at different grid voltages, and the different slopes of the plate curves.
Slopes: straight, curved, or kinky.
Mu (u), plate curves evenly spaced, or non linearly spaced.
Understanding a load line is one thing.
Understanding distortion is another.
Understanding a pentode or beam power tube variations versus quiescent voltages and currents (including the screen voltage, something a triode does not have [fortunately]); and how those plate curves intersect with pure resistive load impedance is complex.
I often calculated by hand the 2nd harmonic distortion, right off the load lines.
Find the plate voltages at these 3 conditions: quiescent grid voltage (bias); 0V grid; and grid at 2 x bias voltage.
Starting with a load line, and quiescent operating condition . . .
Suppose at 40V grid bias, the plate voltage is 300V.
Suppose at 0V grid bias, the plate voltage is 150V.
Suppose at 80V grid bias, the plate voltage is only 420V.
With a linear grid signal of -40V to +80V, the plate voltage is Not linear . . . 150V swing in one direction, and 120V in the other direction. The 2nd order non-linearity total is 30V.
Or, divide 150/120, and consider that level of distortion.
Easy to see, easy to calculate, right off the plate curves as they intersect with the load line.
And, then there is the issue that many load lines are not straight:
Example 1: An output transformer's primary inductance, capacitive reactances, and leakage inductance . . . even when the load is a pure resistor.
The "load line", is not a straight load line at all frequencies, the load lines are often elliptical.
Example 2: A "perfect output transformer" (which is impossible), is loaded with a real world loudspeaker . . . a loudspeaker that has C, L, RLC, R, and combinations of those at varying levels . . . the R has a straight load line; whereas C, L, RLC all have Elliptical and/or even Circular load "lines".
(Those reactive impedances of the loudspeaker reflect through the real, or perfect, output transformer all the way back to the output tubes . . .
The resultant "load lines" are not always very pretty.
One thing that was pointed out to me many decades ago . . .
A Class A single ended amplifier can have more distortion when the load line is elliptical;
A Class A push pull amplifier usually deals fairly well, when the load line is elliptical.
But the real key to all of this is, it often sounds better than what all the factors might tell you to expect and to worry about.
Relax, sit back, and . . . "Enjoy the Music" - stolen from a hi fi reviewer.
Have Fun!
Note: A pentode that is DC coupled to a triode concertina . . . often, the filament voltage needs to be elevated, whenever the concertina quiescent cathode voltage + the cathode voltage peak swing is to large versus the filament voltage (according to the tube specification).
Many Dyna Stereo 70 have destroyed many 7199 tubes. Two resistors, and a bypass cap can easily fix that (Sound Practices Issue 10)
Thanks Joe Roberts!
My comments about triode, pentode, and beam power, load lines, and their similarity was just a first order analysis.
But I alluded to the issue of distortion (between the lines), when I mentioned the spacing between plate curves at different grid voltages, and the different slopes of the plate curves.
Slopes: straight, curved, or kinky.
Mu (u), plate curves evenly spaced, or non linearly spaced.
Understanding a load line is one thing.
Understanding distortion is another.
Understanding a pentode or beam power tube variations versus quiescent voltages and currents (including the screen voltage, something a triode does not have [fortunately]); and how those plate curves intersect with pure resistive load impedance is complex.
I often calculated by hand the 2nd harmonic distortion, right off the load lines.
Find the plate voltages at these 3 conditions: quiescent grid voltage (bias); 0V grid; and grid at 2 x bias voltage.
Starting with a load line, and quiescent operating condition . . .
Suppose at 40V grid bias, the plate voltage is 300V.
Suppose at 0V grid bias, the plate voltage is 150V.
Suppose at 80V grid bias, the plate voltage is only 420V.
With a linear grid signal of -40V to +80V, the plate voltage is Not linear . . . 150V swing in one direction, and 120V in the other direction. The 2nd order non-linearity total is 30V.
Or, divide 150/120, and consider that level of distortion.
Easy to see, easy to calculate, right off the plate curves as they intersect with the load line.
And, then there is the issue that many load lines are not straight:
Example 1: An output transformer's primary inductance, capacitive reactances, and leakage inductance . . . even when the load is a pure resistor.
The "load line", is not a straight load line at all frequencies, the load lines are often elliptical.
Example 2: A "perfect output transformer" (which is impossible), is loaded with a real world loudspeaker . . . a loudspeaker that has C, L, RLC, R, and combinations of those at varying levels . . . the R has a straight load line; whereas C, L, RLC all have Elliptical and/or even Circular load "lines".
(Those reactive impedances of the loudspeaker reflect through the real, or perfect, output transformer all the way back to the output tubes . . .
The resultant "load lines" are not always very pretty.
One thing that was pointed out to me many decades ago . . .
A Class A single ended amplifier can have more distortion when the load line is elliptical;
A Class A push pull amplifier usually deals fairly well, when the load line is elliptical.
But the real key to all of this is, it often sounds better than what all the factors might tell you to expect and to worry about.
Relax, sit back, and . . . "Enjoy the Music" - stolen from a hi fi reviewer.
Have Fun!
Note: A pentode that is DC coupled to a triode concertina . . . often, the filament voltage needs to be elevated, whenever the concertina quiescent cathode voltage + the cathode voltage peak swing is to large versus the filament voltage (according to the tube specification).
Many Dyna Stereo 70 have destroyed many 7199 tubes. Two resistors, and a bypass cap can easily fix that (Sound Practices Issue 10)
Thanks Joe Roberts!
Last edited:
There's not a lot to explain, the basic process is identical to a triode, if you're talking about a preamp stage (rather than a power output pentode).
The only thing to remember is you must use the curves that correspond to the actual screen voltage, which sometimes means you have to sketch your own curves.
A little more wind in the sails.......The 7199 RCA application note is the only example, esp fig 4 of distortion vs. screen grid and anode current. Generally for signal pentodes originally destined for the TV market, the curves are typical to one can expect with some variations. Out of 250 tubes of various makes, the 7643 Westinghouse version is the most consistent distortion performer for a given screen resistance followed by the Philips 6BL8. I am using the pentode with lower current than often used; input stage anode (150K) 220V B+ directly coupled to the concertina with 350V B+ (anode/cathode both 18 Kohms # cathode 560+100ohms) implies quite low pentode anode and screen grid voltages. The concertina anode B+ must be higher to avoid early clipping. The stage signal swing should provide 25+25V RMS at around 0.5% distortion, dropping to remarkably low levels with lower swings. An excellent start and headroom for an amp. The screen grid must be decoupled to the cathode and not to ground. A drawback is that 150K as a pentode anode load will produce slightly more noise than 100K, but since noise is already -70dB down, I´m not grumbling.
The rule regarding signal pentode screen voltages is often generalized as roughly 1/4 or 1/5 of anode voltages.
Unfortunately, other than using distortion analyser and vari psu, I see no other easy way other than the load line info 6A3s has submitted.
As for the elevated heater voltage, the cathode of the concertina is already sitting at around 100V so it is imperative the heater rail is raised or sep. supplied and smoothed. Don´t overdo it as then the pentode heater to cathode section becomes leakage vulnerable ! There are solutions i.e running the concertina cathode negatively as a compromise but that only complicates.
I´m using such a configuration in my 200W amp rebuild. With 600V B+ I am concentrating staying alive and using 100:1 probes.
Bench Baron
The rule regarding signal pentode screen voltages is often generalized as roughly 1/4 or 1/5 of anode voltages.
Unfortunately, other than using distortion analyser and vari psu, I see no other easy way other than the load line info 6A3s has submitted.
As for the elevated heater voltage, the cathode of the concertina is already sitting at around 100V so it is imperative the heater rail is raised or sep. supplied and smoothed. Don´t overdo it as then the pentode heater to cathode section becomes leakage vulnerable ! There are solutions i.e running the concertina cathode negatively as a compromise but that only complicates.
I´m using such a configuration in my 200W amp rebuild. With 600V B+ I am concentrating staying alive and using 100:1 probes.
Bench Baron