G said:
No, that's not the way to do it.
To make exact recommendations we would need to know your HT voltage and the current you are running through the 7189s.
Reference to the curves will enable you to select a suitable operating voltage for g2. The curves will also tell you how much current the screen will draw. Having decided on a screen operating voltage and knowing the current, you can then calculate the value of the screen supply resistors (i.e. one for each 7189). You must then by-pass this supply to ground; a typical capacitor would be about 3.3uF. Note that the behaviour of a pentode is almost entirely determined by the voltages at g1 and g2 - anode voltage has virtually no effect on the amount of current that the valve passes - which is why the curves are horizontal.
You will also have to adjust your bias accordingly.
Unfortunately, connecting as a pentode will a) raise the anode resistance of the 7189 from perhaps 2k to about 40k and b) will cause the valve to produce a considerable amount of odd-order harmonic distortion. The effect of (a) will be that the reflected impedance as seen at the speaker terminals will change dramatically. You may find gaps in the frequency response and probably rather strange bass - it is difficult to predict.
This is why pentode amplifiers are usually run with about 20dB of negative feedback; this largely cancels the distortion, improves bandwidth and lowers the output resistance of the amplifier so that it can drive loudspeakers properly.
Many people believe that feedback destroys the sound of an amplifier (this despite the popularity of "vintage" designs such as Leak, Radford and the Mullard 5-20, all of which used plenty of negative feedback).
Unfortunately for satisfactory operation it is necessary with pentodes. And this is one reason why triodes, or triode-strapped pentodes are so popular.
Best of luck
7N7
G,
When you convert to pentode mode the HT current will increase - will your power supply be able to handle the extra current?
Also, the optimum output impedence of the 7189 will change, and there will be a mismatch with the output transformer, although it would still probably work. However, because of the extra anode current drawn by a tube in pentode mode you need to check that the tranny can handle the extra current.
When you convert to pentode mode the HT current will increase - will your power supply be able to handle the extra current?
Also, the optimum output impedence of the 7189 will change, and there will be a mismatch with the output transformer, although it would still probably work. However, because of the extra anode current drawn by a tube in pentode mode you need to check that the tranny can handle the extra current.
G,
Having just seen the pictures of your amp on the other thread I don't think there would be any problem in their power handling capability for running the 7189s in pentode mode. Also, with a primary impedence of 4800 ohms they are probably a better match for pentode mode rather than triode mode.
If you are not using feedback then a small cap, about 0.001uf rated for 1000vdc across the primary of the output transformer will help reduce high frequency transients.
Having just seen the pictures of your amp on the other thread I don't think there would be any problem in their power handling capability for running the 7189s in pentode mode. Also, with a primary impedence of 4800 ohms they are probably a better match for pentode mode rather than triode mode.
If you are not using feedback then a small cap, about 0.001uf rated for 1000vdc across the primary of the output transformer will help reduce high frequency transients.
... and also...
In addition to all the good advice above, can I stress that in pentode mode the screen grid needs a low impedance dc source - relative to it's current requirement i.e. make sure that the screen voltage does not vary according to amplifier load. This is the quickest way of introducing psu modulation into a pentode - this is particularly true of SE designs.
Gary Pimm's schematic for his P-P DHP amp (see the thread with this title) shows a neat way of introducing voltage feedback to a pentode via a resistor between plate and control grid.
Finally I would use an anode stopper when running in pentode mode with a modern pentode as they will oscillate under certain conditions - a 150R carbon comp. is usually sufficent.
hope this helps
ciao
James
In addition to all the good advice above, can I stress that in pentode mode the screen grid needs a low impedance dc source - relative to it's current requirement i.e. make sure that the screen voltage does not vary according to amplifier load. This is the quickest way of introducing psu modulation into a pentode - this is particularly true of SE designs.
Gary Pimm's schematic for his P-P DHP amp (see the thread with this title) shows a neat way of introducing voltage feedback to a pentode via a resistor between plate and control grid.
Finally I would use an anode stopper when running in pentode mode with a modern pentode as they will oscillate under certain conditions - a 150R carbon comp. is usually sufficent.
hope this helps
ciao
James
Yes I agree with James, that a g2 supply should have a low output resistance.
My experimental "Instock" project, which was the subject of a thread about a month ago, uses pentodes (D3As) as cathode followers.
On these, g2 is supplied by a relarively simple emitter follower circuit. This actually consists of two resistors a transistor (MPSA92) and a 470nF capacitor. The two resistors form a potential divider which sets the base voltage of the transistor. The lower end of the potential divider is connected to the cathode of the pentode to g2 so that it stays in "step" with what's going on around it!
7N7
My experimental "Instock" project, which was the subject of a thread about a month ago, uses pentodes (D3As) as cathode followers.
On these, g2 is supplied by a relarively simple emitter follower circuit. This actually consists of two resistors a transistor (MPSA92) and a 470nF capacitor. The two resistors form a potential divider which sets the base voltage of the transistor. The lower end of the potential divider is connected to the cathode of the pentode to g2 so that it stays in "step" with what's going on around it!
7N7
G,
Do your output transformers have UL taps? (if they are Hammond they do). Connect your screen to those. You will get good sound plus more power.
As for how to connect the 7189 screens, all the spec sheets I have seen for them (EL84/6BQ5/7189) says 300 volts...anode voltage... or basically as you surmised, B+.
As for how 7189's sound in pentode, they sound very nice. I have heard an old SE stereo using them in a portable phono. Very nice. You won't be disappointed. At close to full power and clipping is where the distortion may eek.
AAMOF, I helped a young friend of mine who also built a 7189 based SE amp for a bi-amped system. We were both pleasantly surprised how good it sounded... after I helped tweak it for more output, similar to your problem.
BTW, NFB with pentodes is more for stability since pentodes have a very high amplification factor. Triodes can be as high as 100, where pentodes can be easily 10 times higher (this is because the transconductance is much higher... whatever that is
). Yes, this makes for more distortion, so both reasons are true. But they can be run without NFB as long as you don't use too much gain in your preamp/driver.
Hope this helps,
Gabe
Do your output transformers have UL taps? (if they are Hammond they do). Connect your screen to those. You will get good sound plus more power.
As for how to connect the 7189 screens, all the spec sheets I have seen for them (EL84/6BQ5/7189) says 300 volts...anode voltage... or basically as you surmised, B+.
As for how 7189's sound in pentode, they sound very nice. I have heard an old SE stereo using them in a portable phono. Very nice. You won't be disappointed. At close to full power and clipping is where the distortion may eek.
AAMOF, I helped a young friend of mine who also built a 7189 based SE amp for a bi-amped system. We were both pleasantly surprised how good it sounded... after I helped tweak it for more output, similar to your problem.
BTW, NFB with pentodes is more for stability since pentodes have a very high amplification factor. Triodes can be as high as 100, where pentodes can be easily 10 times higher (this is because the transconductance is much higher... whatever that is
). Yes, this makes for more distortion, so both reasons are true. But they can be run without NFB as long as you don't use too much gain in your preamp/driver.Hope this helps,
Gabe
Actually, it's the Rp, and thus effective mu, that makes a pentode belt out the gain. In the olden days, they used to spec mu on them, but for some reason it hasn't shown up in data sheets since the 30s or so..
The lack of internal NFB (vis-a-vis triodes) causes more gain and distortion, but that gain allows more NFB, which in fact gives less distortion for the same amount of gain. And because of the knee curve of the pentode, you can get far more power output than a triode.
Tim (listening to WJJO through Onkyo TX-2500 --> tube preamp --> Hept'AU7 (PP monoblock) and Revision 3 (6L6GC SE partly-integrated monoblock) )
In the olden days, they used to spec mu on them, but for some reason it hasn't shown up in data sheets since the 30s or so.. The lack of internal NFB (vis-a-vis triodes) causes more gain and distortion, but that gain allows more NFB, which in fact gives less distortion for the same amount of gain. And because of the knee curve of the pentode, you can get far more power output than a triode.
Tim (listening to WJJO through Onkyo TX-2500 --> tube preamp --> Hept'AU7 (PP monoblock) and Revision 3 (6L6GC SE partly-integrated monoblock)
)
Sch3mat1c,
Shall we break out the formulae?
I do not disagree the high plate resistance has a play. Since one formula for gain is Rp/Rk. However, it is the Gm in the case of pentodes that dictates it all. Hence why so many say a power triode is so "hard" to drive, where a pentode is so "easy" to drive.
Here is one point where I definitely admit I am probably off base, because most of my understanding of Gm with relation to gain is in solid state theory. So... if those terms are different for tube technology, then I bow to the more correct statement that gain in the pentode has to do with high plate resistance.
Of course, the 12AX7's plate resistance is between 62K and 80K, whereas the 7189/6BQ5's is 40K.
But, the 7189's Gm is 11,300µS, where the 12AX7's is about 1250 to 1600 µS.
Although, I am not comparing apples to apples because the 300B's plate resistance is 600 ohms... but its Gm and gain are very low. Mu is 3.8, while Gm is on the order of a few hundred.
As for damping factor... that's another more complicated story.
Gabe
Shall we break out the formulae?
I do not disagree the high plate resistance has a play. Since one formula for gain is Rp/Rk. However, it is the Gm in the case of pentodes that dictates it all. Hence why so many say a power triode is so "hard" to drive, where a pentode is so "easy" to drive.
Here is one point where I definitely admit I am probably off base, because most of my understanding of Gm with relation to gain is in solid state theory. So... if those terms are different for tube technology, then I bow to the more correct statement that gain in the pentode has to do with high plate resistance.
Of course, the 12AX7's plate resistance is between 62K and 80K, whereas the 7189/6BQ5's is 40K.
But, the 7189's Gm is 11,300µS, where the 12AX7's is about 1250 to 1600 µS.
Although, I am not comparing apples to apples because the 300B's plate resistance is 600 ohms... but its Gm and gain are very low. Mu is 3.8, while Gm is on the order of a few hundred.
As for damping factor... that's another more complicated story.
Gabe
However...
Yes quite so, but we do not expect to get power from a 12AX7 - only voltage.
The 7189 with its 40k or so is looking into 4.5k - hardly ideal for clean power transfer.
Pentodes inherently produce odd-order harmonics - it's a fact of life and goes with the territory.
The negative feedback is not for stability - in fact the reverse is the case: much work usually has to be done to make an amplifier with feedback stable i.e. to ensure that the feedback stays negative - as a basic example take a look at the step networks in a Williamson amplifier.
The feedback is there to lower the output resistance, thereby extending bandwidth and to reduce distortion.
Someone else suggested UL connection; I know people have done this and if they get an acceptable sound I would say that they are very lucky; a set of UL curves look rather like those of an early tetrode - again feedback is essential.
7N7
Gabevee said:
Yes quite so, but we do not expect to get power from a 12AX7 - only voltage.
The 7189 with its 40k or so is looking into 4.5k - hardly ideal for clean power transfer.
Pentodes inherently produce odd-order harmonics - it's a fact of life and goes with the territory.
The negative feedback is not for stability - in fact the reverse is the case: much work usually has to be done to make an amplifier with feedback stable i.e. to ensure that the feedback stays negative - as a basic example take a look at the step networks in a Williamson amplifier.
The feedback is there to lower the output resistance, thereby extending bandwidth and to reduce distortion.
Someone else suggested UL connection; I know people have done this and if they get an acceptable sound I would say that they are very lucky; a set of UL curves look rather like those of an early tetrode - again feedback is essential.
7N7
Gabevee said:
Not really.
Check out 2A3 for instance. 6000µmhos, yet takes 50Vp to drive! Check out 6F6, like 3000µmhos, yet only 20Vp!
Gm doesn't vary too much between pentodes and triodes, however triodes generally have lower, possibly because they didn't bother putting much effort into it.
(Pentodes are much more cost-effective after all.) Those where they did, however - acorns, nuvistors and other RF types where high Gm is important - show that triodes are clearly capable of it.Yep.
And quite simply, the NFB you apply outside the tube corrects this.Tim
G said:
Damping has absolutely nothing to do with a chosen operating point; usually it's for maximum power output. After all, you'd get miliwatts from a pentode if your load were the triode's standard 3*Rp figure. Apples to oranges - pentodes are little like triodes.
Check out the plate curves for them, draw a loadline or two, you'll see.
Tim
7N7,
Well, I guess that's another two textbooks I need to throw away.
That might apply with solid state op amps without internal compensation due to phase shift.
But tube amps? If there is such severe phase shift that NFB causes high frequency instability... it is a bad design to begin with, no disrespect to Williamson intended.
BTW, I have seen (and have on my hard drive) the original Williamson schematic... what stepped network??? Just a simple resistor to the cathode of the first pentode voltage amp tube.
Otherwise, as for voltage versus power amplification... that is why I said I was not comparing apples to apples. Then I compared it to the 300B. The 7189 can indeed and has been used as a straight voltage amplifier.
http://www.triodeel.com/achpwill.gif
http://www.triodeel.com/dynawil1.gif
http://www.triodeel.com/images/ehpa100.gif
http://www.triodeel.com/images/utc20w.gif
http://www.xs4all.nl/~ideas/amps/
(no, this is not my hard drive, but where I got them from)
Note the first three use a bypass capacitor for the compensation... though generally not necessary. I have used it once and found that my amp oscillated at 100kHz with it. So I don't use it. Also note the last one is of the original Williamson!
If five Williamson amp schematics with the simple basic NFB that virtually all tube amps use isn't convincing empirical evidence enough... I give up!
Gabe
Well, I guess that's another two textbooks I need to throw away.
That might apply with solid state op amps without internal compensation due to phase shift.
But tube amps? If there is such severe phase shift that NFB causes high frequency instability... it is a bad design to begin with, no disrespect to Williamson intended.
BTW, I have seen (and have on my hard drive) the original Williamson schematic... what stepped network??? Just a simple resistor to the cathode of the first pentode voltage amp tube.
Otherwise, as for voltage versus power amplification... that is why I said I was not comparing apples to apples. Then I compared it to the 300B. The 7189 can indeed and has been used as a straight voltage amplifier.
http://www.triodeel.com/achpwill.gif
http://www.triodeel.com/dynawil1.gif
http://www.triodeel.com/images/ehpa100.gif
http://www.triodeel.com/images/utc20w.gif
http://www.xs4all.nl/~ideas/amps/
(no, this is not my hard drive, but where I got them from)
Note the first three use a bypass capacitor for the compensation... though generally not necessary. I have used it once and found that my amp oscillated at 100kHz with it. So I don't use it. Also note the last one is of the original Williamson!
If five Williamson amp schematics with the simple basic NFB that virtually all tube amps use isn't convincing empirical evidence enough... I give up!
Gabe
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