Do not expect it : measure it 🙂, it's best, because there are surprises sometimes 😕... Here what I do, just in case if the procedure seems obscure, but it's quite simple to do, in fact :
Starting from the basic formula U=E-Ri, using an audio generator, a DMM, and a variable load,
1 - measure E amp at unloaded out, and at moderate output power (say a few watts maximum, to be safe).
2 - without touching the settings, apply now a load R at the output, and adjust it to obtain U=E/2
3 - Power off, disconnect and measure the resulting value of R : this is the internal resistance of your amp shown at its output.
Then consider Znom, the nominal impedance of your speaker, say 4ohms or 8ohms, or any other value for which you want to determine the damping factor.
4 - now calculate Znom/R, and you have the damping factor provided by your amp to the speaker.
I measured several R and damping factors values of my amps, but sorry : I do not have them on hand while writing this. If some of you are interested, I can certainly retrieve and post them...😉
T
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tubelectron,
You are correct! Good Point!
Always measure damping factor; I always do.
But many readers of Tubes/Valves posts do not even have a DMM or sine wave signal source.
For those who are designing an amplifier who do not know how to predict a reasonably close estimate of the damping factor,
then they should ask if someone on Tubes/Valves threads can help them estimate the damping factor . . .
(of course a complete and accurate schematic is needed with voltages, resistances, tube types, etc.).
Most can not hit the bulls eye with an arrow, if they are blindfolded.
Hint for Newbies doing their first simple tube amplifier:
Predicting damping factor of an amplifier that is open loop is much quicker and easier than if it has Schade or Global negative feedback.
You should calculate the open loop DF first; so when feedback is applied, the damping factor has to be re-calculated.
You are correct! Good Point!
Always measure damping factor; I always do.
But many readers of Tubes/Valves posts do not even have a DMM or sine wave signal source.
For those who are designing an amplifier who do not know how to predict a reasonably close estimate of the damping factor,
then they should ask if someone on Tubes/Valves threads can help them estimate the damping factor . . .
(of course a complete and accurate schematic is needed with voltages, resistances, tube types, etc.).
Most can not hit the bulls eye with an arrow, if they are blindfolded.
Hint for Newbies doing their first simple tube amplifier:
Predicting damping factor of an amplifier that is open loop is much quicker and easier than if it has Schade or Global negative feedback.
You should calculate the open loop DF first; so when feedback is applied, the damping factor has to be re-calculated.
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Miniwatt,
Thanks for your post!
The triode wired curves of EL84 Pentode and 6V6 Beam Power?
Single ended, push pull, or both?
(curves overlaid in push pull have certain cancellations).
And, if triode wired 6V6 Beam Power is more linear than triode wired Pentode;
Then how about a triode wired 6BQ5 Beam Power, are its curves more linear than the triode wired EL84 Pentode?
Carefull: Some tubes are dual marked on the glass, EL84/6BQ5, or 6BQ5/EL84. You have to look at the tube construction in order to determine/know if it is a Pentode or a Beam Power tube.
Uh Oh! . . . Now you make me think of triode wired EL34 Pentode versus triode wired KT77 Beam Power.
Just food for thought, and for other's comments too.
Happy designing, building, and listening!
Thanks for your post!
The triode wired curves of EL84 Pentode and 6V6 Beam Power?
Single ended, push pull, or both?
(curves overlaid in push pull have certain cancellations).
And, if triode wired 6V6 Beam Power is more linear than triode wired Pentode;
Then how about a triode wired 6BQ5 Beam Power, are its curves more linear than the triode wired EL84 Pentode?
Carefull: Some tubes are dual marked on the glass, EL84/6BQ5, or 6BQ5/EL84. You have to look at the tube construction in order to determine/know if it is a Pentode or a Beam Power tube.
Uh Oh! . . . Now you make me think of triode wired EL34 Pentode versus triode wired KT77 Beam Power.
Just food for thought, and for other's comments too.
Happy designing, building, and listening!
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I think the trioded 6V6 has a special reputation for greatness (albeit at an inconveniently low power level) and it might not be safe to extrapolate to all beam tubes.
My triodewired el 84 Se sounds wonderful. I have built about ten different tubeamps the last 30 years, seven of them Single ended, and 5 of them el 84. https://www.diyaudio.com/community/threads/from-triode-wired-to-pentode-wired.389229/ This one sounds great with Hammond 260E, and Hammond 125BSE.
Strange : all the EL84/6BQ5 that I ever saw were all pentode tubes... I mean : always with a suppressor grid, no beam deflector plates à la 807, 6L6 or 6V6. I also never read that some EL84/6BQ5 could have been built as beam tetrode. 🤔
OK. I have some samples of different eras : I'll take a more closed look to it, though ! 😉
T
I meant SE EL84 in triode mode.
I have no experience with trioded 6V6, I just looked at the datasheet.
nicoch58,
You said: "yep nice thd profile"
What did you mean?
THD is the Sum of all harmonic distortion (odd and even harmonic distortion).
Did you mean the Total Distortion,
Or the Rates of harmonic distortion fall off from low order to high order . . .
Like typical SE harmonic distortion:, 2nd dominant, 3rd 15dB lower than 2nd, and few other harmonics of any significance?
6V6 in single ended?
6V6 in push pull?
Both?
Thanks!
You said: "yep nice thd profile"
What did you mean?
THD is the Sum of all harmonic distortion (odd and even harmonic distortion).
Did you mean the Total Distortion,
Or the Rates of harmonic distortion fall off from low order to high order . . .
Like typical SE harmonic distortion:, 2nd dominant, 3rd 15dB lower than 2nd, and few other harmonics of any significance?
6V6 in single ended?
6V6 in push pull?
Both?
Thanks!
tubelectron,
There were old patents that for some decades required manufacturers to use Either Pentode, Or Beam Power construction for their tubes.
Nobody wanted to pay royalties, and perhaps some/no company would agree to license such arrangement.
I believe that even after the patents ran out, those companies were set up to manufacture Either suppressor grids, Or beam formers.
The setups built them in different sizes for different models (6BQ5 EL84; KT77 EL34), but the procedure was known and fixed. To change the engineering and process was expensive for a manufacturer who had never built the other tube type.
The 6BQ5 originally started out as a beam power tube.
The EL84 originally started out as a pentode.
Those tubes were competing with each other for sales; the specifications were so close, just like two peas in a pod.
Long after the patents ran out, marketing/engineering/sales came up with a brilliant idea: Dual Label the tube; the tube wholesalers and the retail stores loved it!
No need to manufacture 2 tube types, no need to stock 2 tube types.
Just as I understand this true/untrue/part-true story.
To each his own belief!
There were old patents that for some decades required manufacturers to use Either Pentode, Or Beam Power construction for their tubes.
Nobody wanted to pay royalties, and perhaps some/no company would agree to license such arrangement.
I believe that even after the patents ran out, those companies were set up to manufacture Either suppressor grids, Or beam formers.
The setups built them in different sizes for different models (6BQ5 EL84; KT77 EL34), but the procedure was known and fixed. To change the engineering and process was expensive for a manufacturer who had never built the other tube type.
The 6BQ5 originally started out as a beam power tube.
The EL84 originally started out as a pentode.
Those tubes were competing with each other for sales; the specifications were so close, just like two peas in a pod.
Long after the patents ran out, marketing/engineering/sales came up with a brilliant idea: Dual Label the tube; the tube wholesalers and the retail stores loved it!
No need to manufacture 2 tube types, no need to stock 2 tube types.
Just as I understand this true/untrue/part-true story.
To each his own belief!
Without the negative feedback, the speaker and amplifier are interacting with each other based on the input impedance of the speaker and the fact that without NFB, the amplifier's output impedance is high. Voltage divider effect comes into play. The speaker is pushing the amplifier around in a non-euphonic way based on the speaker's impedance characteristic which is likely uneven over the audio frequency range. FB stiffens the amplifier's output so that the speaker cannot have as much influence on its sound. Amp says, "Speaker you're coming with me, and you will behave yourself.". Speaker says, "Y-y-y-yessir.".
nicoch58,
I hope you find that link, and post it.
And I hope it does not require me to Register/Join some web site just to read it.
When it comes to the "sound of a particular tube", I tend to take it with a grain of salt . . .
"All generalizations have exceptions".
I hope you find that link, and post it.
And I hope it does not require me to Register/Join some web site just to read it.
When it comes to the "sound of a particular tube", I tend to take it with a grain of salt . . .
"All generalizations have exceptions".
Unfortunately, EL84 (and 6BQ5) is missing from that list, because that list was of commonly available octal output tubes.
EL84 triode curves:
6V6GT triode curves:
Note the differing scales. The EL84 has grid lines spaced 2V apart, the 6V6 5V apart.
I think they both look pretty good. It's obvious that the EL84 is easier to drive, because of its higher mu (almost 20). If the driver stage doesn't have to swing as many volts into the output stage, it runs cleaner, and that could make the whole amp sound cleaner. Everything's relative.
Is that this one?
http://www.pmillett.com/pentodes.htm
EL84 triode curves:
6V6GT triode curves:
Note the differing scales. The EL84 has grid lines spaced 2V apart, the 6V6 5V apart.
I think they both look pretty good. It's obvious that the EL84 is easier to drive, because of its higher mu (almost 20). If the driver stage doesn't have to swing as many volts into the output stage, it runs cleaner, and that could make the whole amp sound cleaner. Everything's relative.
Is that this one?
http://www.pmillett.com/pentodes.htm
A thought that is possibly outside scope of original post, re curves above, I have had good success with driving a trioded 6V6 into grid current (class A2) with Tubelab's "Powerdrive" topology. Can get 3 watts out of a 6V6 in triode at 250V 45mA 3000Ω load. Yes that load is a bit low, but works fine.
To the original question about sound with no feedback, a triode-wired tube will have better damping, and I note the use of Klipsch speakers, they will likely have very peaky looking impedance vs frequency, which will exaggerate the lesser damping from disconnecting feedback, especially in pentode mode. I have a pair of Klipsch Heresy speaks myself, they can sound a bit hideous on amps with poor damping.
To the original question about sound with no feedback, a triode-wired tube will have better damping, and I note the use of Klipsch speakers, they will likely have very peaky looking impedance vs frequency, which will exaggerate the lesser damping from disconnecting feedback, especially in pentode mode. I have a pair of Klipsch Heresy speaks myself, they can sound a bit hideous on amps with poor damping.
Thanks for all of those links!
But . . .
I think I am suffering from Information Overload (all of those many tables of tube / performance data).
One of the most interesting tables is the Western Electric 300B chart that gives performance with plate to filament voltages from 200V to 450V.
The plate loads are many, the plate currents are many, and the bias is according to the plate to filament voltage and plate current.
Looking at those tables, the 2nd harmonic is between -20dBc to -31dBc, and the 3rd harmonic is between -29dBc and -45dBc.
A harmonic of -26dBc is 5% harmonic distortion.
The output power is rated for each of those parameters.
I never allowed more than 5% second harmonic distortion when I rated the power of single ended amplifiers that I designed and built.
I do not want to listen to that much distortion, but you are free to give my power amplifiers a higher Watt rating than I did.
A moderate amount of 2nd harmonic distortion is OK for me.
-29dBc is 3.55% 3rd harmonic distortion
I do not remember rating the power output of any push pull amplifier that I designed that had 3rd harmonic distortion of -29dBc (3.55%).
If it had that much 3rd harmonic distortion to get the power output I wanted, I would tear it down, and design a different amplifier.
Such a large amount of 3rd harmonic distortion is Not OK for me.
I am talking about amplifiers that I designed and built.
They did not use global negative feedback, and often did not have local negative feedback.
Negative feedback that I used was Ultra Linear or Triode Wired pentode/beam power tubes;
and for some push pull / balanced amplifiers, I used cathode to cathode feedback (common self bias resistor [*] without a bypass cap).
I know, that [*] breaks one of my fundamental rules of using individual self bias resistors, but in the case that the output tubes are super well matched, a single common self bias resistor without a bypass cap will do.
Have Fun Designing, Building, Modifying, and Listening!
But . . .
I think I am suffering from Information Overload (all of those many tables of tube / performance data).
One of the most interesting tables is the Western Electric 300B chart that gives performance with plate to filament voltages from 200V to 450V.
The plate loads are many, the plate currents are many, and the bias is according to the plate to filament voltage and plate current.
Looking at those tables, the 2nd harmonic is between -20dBc to -31dBc, and the 3rd harmonic is between -29dBc and -45dBc.
A harmonic of -26dBc is 5% harmonic distortion.
The output power is rated for each of those parameters.
I never allowed more than 5% second harmonic distortion when I rated the power of single ended amplifiers that I designed and built.
I do not want to listen to that much distortion, but you are free to give my power amplifiers a higher Watt rating than I did.
A moderate amount of 2nd harmonic distortion is OK for me.
-29dBc is 3.55% 3rd harmonic distortion
I do not remember rating the power output of any push pull amplifier that I designed that had 3rd harmonic distortion of -29dBc (3.55%).
If it had that much 3rd harmonic distortion to get the power output I wanted, I would tear it down, and design a different amplifier.
Such a large amount of 3rd harmonic distortion is Not OK for me.
I am talking about amplifiers that I designed and built.
They did not use global negative feedback, and often did not have local negative feedback.
Negative feedback that I used was Ultra Linear or Triode Wired pentode/beam power tubes;
and for some push pull / balanced amplifiers, I used cathode to cathode feedback (common self bias resistor [*] without a bypass cap).
I know, that [*] breaks one of my fundamental rules of using individual self bias resistors, but in the case that the output tubes are super well matched, a single common self bias resistor without a bypass cap will do.
Have Fun Designing, Building, Modifying, and Listening!
radiotron,
Interesting!
A Triode Wired 6V6 in Class A2
What is the distortion at 3 Watts of your 6V6 amplifier with 250V, 45mA, and 3000 Ohm load?
2nd harmonic ____ % ?
3rd harmonic ____ % ?
Is the bias about -13V?
Thanks!
Interesting!
A Triode Wired 6V6 in Class A2
What is the distortion at 3 Watts of your 6V6 amplifier with 250V, 45mA, and 3000 Ohm load?
2nd harmonic ____ % ?
3rd harmonic ____ % ?
Is the bias about -13V?
Thanks!
@6A3sUMMER, I don't have distortion numbers, have simply set up by eye with a nice clean sine wave and scope, and measured. Far as I know, slight visible softening of the peaks equates to 2-5% distortion. Exact bias voltage subject to tube variation, -13V is a good approximation.
I use a stabilised (not regulated) HT supply that has some adjustment, can dial in plate voltage after drop across output transformer, making it easier to work from sheet of plate curves.
PS, being SE, the distortion will be mostly 2H, and since distortion in a triode is proportional to output, at levels under a watt on sensitive speakers, it does sound very clean.
I use a stabilised (not regulated) HT supply that has some adjustment, can dial in plate voltage after drop across output transformer, making it easier to work from sheet of plate curves.
PS, being SE, the distortion will be mostly 2H, and since distortion in a triode is proportional to output, at levels under a watt on sensitive speakers, it does sound very clean.