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300B IT 1:1 845, A2 possible?
I think this topic discussed few times, but I still couldn't achieve A2 on 845, wondering how Cary could claim 50W output with single 845.
With CF it is easy to drive 845 into A2 which I have tried ,but I still want IT to achieve A5 for another A2 portion
I recently use 300B 1:1 IT running 30 mA 330V plate to Cathode, but failed to pusg to A2
Follow Ups:
The curves of 845 haven't the portion in positive grid and it is the fact
So I can't understand why you push in A2 where the grd current is not linear and the drive circut is more complicated and the THD will increas rapidly
For a tube with an high linearity ( as 300B) the use of A" ia a not sense.
My opinion
Walter
The problem lies with the chosen 300B operating point. At the 30mA standing current it's too low for the required swing. I will assume a 5K primary impedance for the IT in which case you only have about 120V one-way swing on the negative going cycle before the tube goes into cut-off. The typical 845 OP requires around 180V one-way swing at the grid for A1 operation.If you're limited to the 30mA current for the 300B by the IT, it will be very difficult to impossible to push the 845 into A2 operation. By my (very rough) calculations you will need to set the driver tube closer to 400V Va-k at a plate current of 60mA. IOW you will need a very good interstage to pull it off.
If you're not averse to silicon, the easiest and cheapest way to accomplish A2 operation is something like Tubelab's power drive circuit. Can't speak from first hand experience on the sound quality that this circuit imparts, but from a technical point of view it will accomplish what you're after.
Edits: 12/28/21 12/28/21
-!
The Mind has No Firewall~ U.S. Army War College.
Would be interested to see a schematic if you have a link.
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Buy Chinese. Bury freedom.
This I have intentionally never released out of respect ... George has been gone a long time and this schemo is only a snapshot in time as I have seen many of George's iterations and have seen @ least 4 different designs. The man never stood still. The one shown is my favorite for sound. Notice the size of the coupling cap?
The Mind has No Firewall~ U.S. Army War College.
I thought you were discussing A2 operation? Can't drive grid current into the 2A3 through a coupling cap.
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Buy Chinese. Bury freedom.
Do I have to draw you a diagram? C'mon the grid IS being driven positive... Scrutinize closely. The driver to grid impedance is about 1.5k ohms and the driver produces about 55 volts before saturation sets in, more than enough to provide class A2 service. The heater supply technique should not be ignored.
The Mind has No Firewall~ U.S. Army War College.
Edits: 12/29/21
Why don't you try reading it before you post? It clearly says "This driver will drive 2A3 into A2 with distortion..." If all you want is volume, stick your head in a tailpipe.
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Buy Chinese. Bury freedom.
The distortion involved PALES in comparison to anyone listening with cones and domes.
A marked fact completely ignored by many too many audio fools.
The Mind has No Firewall~ U.S. Army War College.
Wrong. That amplifier will produce tons of blocking distortion at higher signal levels. The HD and IM will far surpass anything produced by a good speaker system. In no way does that even qualify as an A2 amplifier.
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Buy Chinese. Bury freedom.
You have indeed surprised me.
You are not near as knowledgeable as I thought. I made mention for you to scrutinize the schematic. As Paul Joppa made mention years ago, "George was one smart cookie "
"contempt prior to investigation will leave one in everlasting ignorance"... Herbert Spencer.
The Mind has No Firewall~ U.S. Army War College.
I'm anxiously awaiting your explanation...
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Buy Chinese. Bury freedom.
it samples off the 6sn7 heater to counteract distortion?
You were already given important clues and then you embarrass yourself by touting blocking distortion. No., your arrogance will not be rewarded with being spoon fed. Earn the answer yourself. The answers are there In the circuit . I suppose you also think one must have a capacitor for a Class A p/p Amp on the cathode resistor as well. That was also a hint, I must be getting soft.
The Mind has No Firewall~ U.S. Army War College.
End of story.
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Buy Chinese. Bury freedom.
Hi Sideliner,
> > The typical 845 OP requires around 180V one-way swing at the grid for A1 operation
My plate volt on 845 is 910V, running 85mA, so 845 grid is -125V fixed bais
One thing I am so sure, if my drive swing more than 125V peak, say from 126V to 160V does it mean these 126 to 160 already in A2 region?
Yes, any swing above the bias point at -126V will put the tube in A2 mode as the grid will start drawing current. Your current driver tube/IT setup barely gets you there. You will need to up both the plate voltage and current through the 300B for the needed higher swing to accomplish your goal.
Hi Sideliner,
I assume 360V p-p (180V peak) swing should bring me to expected output power. I use 10K output tranny
180V x 4.77 (845 mu x 90%) / 36 (10K:8) x0.707=16.9V rms
16.9V square / 8 ohm = 35 watt
But my 845 plate volt is only 910V, meaning my A1 is only up to 125V peak at grid, meaning 18watt A1, to reach total 35 watt the grid need positive voltage of (180-125=55V)
However, the deeper in A2 the more substantial decrease of grid impedance
Yes, the standing current need increment. I simulate the load line with 50mA and ASSUME tge grid drop to 5K ohm in the attached diagram
While I haven't checked your calculations, 35 Watts at A2 seems to be in the ballpark. At the chosen OP for the 845 you will need to swing the grid to appox -360V. This can be seen on the lower right corner of the attached image where the 10K loadline approaches zero current (cut-off) and also crosses the -360V gridline in that vicinity.The required swing from your bias point set at -125V, will be 360V-125V=255V one way. To accomplish this with the 300B while allowing for some margin (so that the driver tube's grid does not start drawing current itself), you will need to set its OP to Va-k=360V and Ip=50mA.
At the chosen Va-k=330V the 300B will only be able to swing 240V one way before it enters A2. The amp will have already entered into a high distortion regime, so why compound the situation. 360V/50mA is the minimum I'd set the 300B at, if lower distortion and a smidgen of headroom from the driver is the goal.
Edits: 12/30/21
Hi Sideliner,
I thought the gradient of the load line (your blue colored line) depends on the loading impedance of the 845 grid, if interstage is to be used
I am not sure wasn't it supposed to be nearly horizontal when secondary loading during A1, only starting in A2, the load line begin to inclined?
To complicate matters even more, the load line is really an ellipse. I used the graphical approach to better illustrate the thought process, but it is an approximation - albeit a decent one for design purposes.Also keep in mind, as Dave Slagle mentioned in another post in this thread, grid impedance in A2 could drop down to around 1.5K Ohm in which case the load line would be even steeper. Moral of the story is that you need to design into the driver ample headroom in terms of both plate voltage and current to keep the tube out of cut-off on the positive swing and from drawing grid current on the negative swing.
Edits: 12/31/21
> > Dave Slagle mentioned in another post in this thread, grid impedance in A2 could drop down to around 1.5K Ohm
If the 845 grid drop to 1.5K, there no way to use 1:1 IT with 300B, because the output impedance is nearly the the same as loading impedance
A2 operating circumstance may vary the 845 grid differently
I am guessing the 845 grid starting to enter to A2 would be around 20K, but if we push further positive on grid, it may drop continuously until the extreme case like 1.5K. The intention is to know, when using 300B as plate load driver with 1:1 IT, to estimate how much positive Volt this IT design could produce on 845 grid
Typo
One thing I am NOT so sure, if my drive swing more than 125V peak, say from 126V to 160V does it mean these 126 to 160 already in A2 region?
My ITC is 1:1 20Henry primary, How to tell what impedance it is?
Transformers don't have a characteristic impedance. The impedance you see attributed to IT and OPT windings is the loading that the manufacturer considers appropriate. That would be the operating impedance.
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Hi Triode Kingdom,
I am not sure for the following
The interstage may need to consider the secondary loading when the power tube grid goes into A2
When the swing within A1, the load line is almost horizontal but starting from > zero grid voltage, such as 845, starting from 20K (I guess) all the way to extreme 2K grid impedance
The transformer should be used at its specified operating impedance with the amplifier in A1. Note that some transformers may benefit from a resistive load across the output winding to dampen resonance and reflect a fixed impedance back to the source. Anyway, the maximum recommended operating impedance is a function of several things, including the inductance of the windings. A certain minimum inductance is necessary in order to provide the desired low frequency response at a specified operating impedance. When the amplifier enters A2, the operating impedance will decrease. This generally means less inductance is required, so that's no longer an issue. However, now the transformer is being tasked to deliver power, not just voltage. So, other factors, such as core size, create additional limitations.
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Buy Chinese. Bury freedom.
Edits: 12/29/21
surely the dc resistance of the transformer takes the A2 portion off the predicted (predicted as if dc resistance didnt exist) characteristic curves.Step down transformer or direct couple.
Aside from that, far be it from me to burst youre bubble, but if you download Stephies excell spreadsheet on the 801a, as you push it into A2 the 3rd harmonic is greater than 2nd and thats probably a greater effect on sound to the detriment than tube rolling and choosing the worst tube.
i.e.
[url=https://postimg.cc/fV1xpQ2m][img]https://i.postimg.cc/q7vjtk2b/3817-EDC6-E5-B7-4-FAE-A487-7-CE3-CA5-CAF5-F.png[/img][/url][url=https://postimg.cc/XZFw01mh][img]https://i.postimg.cc/TPNQgZ9R/72104159-059-B-45-BB-86-E2-FA8-AFB33-A844.png[/img][/url]
[url=https://postimg.cc/jwbyqQrj][img]https://i.postimg.cc/RF3dRXsw/AD428941-9348-45-BB-A450-EAB050-FBF86-C.png[/img][/url]
[url=https://postimg.cc/QFFQfdbj][img]https://i.postimg.cc/kgcTXVRR/C57-F25-D0-893-A-490-A-BCC2-4-C656-CEE98-A7.png[/img][/url]
Edits: 12/27/21 12/28/21
Im not up to speed with posting images here. In this distortion chart, if you can see it, guess which power level is in A2!
I think Im allowed one photo a week. So Ill have to explain rather than show pictures that tell the story best. If you operate 801a in A2 the distortion against power output is as it is here up to 6.4 watts. The minute you pass over to A2 distortion skyrockets. This is before you add to these figures the effect dc resistance in coil has on grid current.
Edits: 12/28/21 12/28/21
"The minute you pass over to A2 distortion skyrockets."
I'm not familiar with the 801A, but my 211-based SETs don't respond like that. The transition from A1 to A2 is very smooth, just a steadily increasing level of distortion (mostly even-order) without any unusual characteristics. Regarding the OP's question, it sounds as though the DCR of the IT might be too high.
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Buy Chinese. Bury freedom.
A 300B has a rP in excess of 1kΩ @ 30ma and then both the primary and secondary DCR needs to be added to those numbers which will take the drive impedance up even higher. I think there is a general ROT that the input impedance of an A2 grid drops like a rock and could very well be equal to the 1500Ω output Z of the driver which would net a 6dB loss in the positive going direction at the crossover point to A2.
Not to sidetrack too much but Douglas (pak protector) has used the drastic grid A2 grid impedance change to advocate for cathode drive where the load impedance even though is low, stays constant. The other interesting thought around this came from Bob "Darling" Danialak where he showed a 0 bias 833 amp at NY Noize. He made the argument that while the A1 vs. A2 grid impedance is radically different... Placing the crossover point between them at 0V keeps things symmetrical and any distortion caused by the crossover happens at all output levels. (He did drive it with a 20Ω source Z) To this day that amp has been the only A2 amp that has really impressed me with its sonics.
dave
Hi Paul,
Thx for response, some tubes like 805 requiring positive grid bias, normally 805 SE amp need NFB to sound normal, but my intention is drive 845 without NFB
Your distortion photo concern me and I do not expect A2 power would be such sky-high distortion
I heard many cases that 845 with beefy driver can push 845 to A2 also maintaining low distortion, something like extra 15watt A2 on top of A1 30watt,total around 45watt, the A2 extra 15watt is supposed to handle instant peak passage
With CF direct couple, I can get A2
But with IT, as stated above, not able to drive 845 into A2, in fact for the webs I did search do not show successful A2 operation, so I am wondering how Cary 805C could use one 845 to put out 50watt RMS(Cary 805C use 300B IT to 845)
I'll have to dig out some old posts but I looked at the 805c circuit which is nothing more than a IT coupled driven by a 300b.
I'm doing something similar with a GM70 and easily getting A2 out of it as well. But, it does have some distortion (on paper). I think the Cary is the same.
I know A2 doesn't always sound bad, but the 801a looks bad. I have an 801a amp that I kept to A1 because of those findings, You get a very nice 6.4 watts easily, I am using interstage coupling.Our UK gathering Nick Gorham and I did a collaberation, he built the driver I brought the power supply and output stage 833a's
Nicks circuit is an adaptation to provide more current, so he put 6BX7 as driver. Its a level shifting circuit , bias adjustment on input valve so whole cirquit must float at input, we used a cap, could use input transformer, with voltage potential difference allowed.
the level shifting circuit is known as ACF-2. Its really mint sounding, don't dismiss it because its not modern and doesnt use niche valves. It sounds good and drives A2 grid current .
https://www.desmith.net/NMdS/Data/955_Augmented_Cathode_Followers.pdf
Edits: 12/28/21
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