While I have my amp on the bench looking at overload, the output stage bias point slowly shifts during clipping. I especially noticed this when changing the value of CRk.
I have heard and liked the effect of a grid choke. Is it the same to provide a low Z path to ground for low frequencies in the grid circuit as the cathode?
I have read of SY's using multiple LED's in the cathode of an output stage. Will this eliminate the problem (I estimate I will need 80 of them)? Maybe standard diodes would be more practical?
Will fixed bias for the output stage (and grounding the cathode) cure all these issues at once?
I am also concerned about driving my driver into class A2 when driving the output deep into cutoff. Maybe this shoud be a separate question?
I have heard and liked the effect of a grid choke. Is it the same to provide a low Z path to ground for low frequencies in the grid circuit as the cathode?
I have read of SY's using multiple LED's in the cathode of an output stage. Will this eliminate the problem (I estimate I will need 80 of them)? Maybe standard diodes would be more practical?
Will fixed bias for the output stage (and grounding the cathode) cure all these issues at once?
I am also concerned about driving my driver into class A2 when driving the output deep into cutoff. Maybe this shoud be a separate question?
You can't really separate the grid and cathode connections as the only impedance of interest is between the grid and cathode. A grid choke combined with fixed bias will keep the dc part of this impedance very low. Only an interstage transformer can match this but a decent grid choke will still provide a higher impedance at audio frequencies.
Sorry, no amount of leds will achieve anything similar 🙂
I'm thinking the impedance between the grid and cathode is Rk in series with Rg(or the impedance of the choke at low frequencies), all in parallel with the internal impedance between G and K, which is infinite during class A1 operation and finite during overload.analog_sa said:
Does the grid current need to find a path from the grid, through the grid and cathode resistances and back to the cathode, and that any resistance here will impede its dissipation?
I hate to contradict my good friend from SA, but I have an amp in front of me that disagrees with him. IMO, overload recovery is the single greatest factor in audible differences between otherwise good measuring amplifiers.
There are two forms of blocking overload- one is from the bypass capacitor in the cathode circuit, one is from the RC coupling in the grid circuit. They're both explained thoroughly in several articles by Norman Crowhurst in "Audio" magazine and in the book "Understanding Hifi Circuits", and again by Morgan Jones in "Valve Amplifiers".
The LED biasing scheme eliminates the first form totally. The second form is a bit trickier, but can be done in a brute-force manner by using cathode followers direct coupled to the output tubes' grids (this is the approach that Jones took for the Crystal Palace amplifier). If you use output tubes in pentode mode, you can do nearly as good a job by raising the value of the output tube grid stoppers to a high value (I use 47k in my LED-biased amp).
Proof is in the real amp- with overload, the bias doesn't shift, the idle current stays stable, and the recovery is faster than I can measure. The amp sounds like it's a lot more powerful than it is.
There are two forms of blocking overload- one is from the bypass capacitor in the cathode circuit, one is from the RC coupling in the grid circuit. They're both explained thoroughly in several articles by Norman Crowhurst in "Audio" magazine and in the book "Understanding Hifi Circuits", and again by Morgan Jones in "Valve Amplifiers".
The LED biasing scheme eliminates the first form totally. The second form is a bit trickier, but can be done in a brute-force manner by using cathode followers direct coupled to the output tubes' grids (this is the approach that Jones took for the Crystal Palace amplifier). If you use output tubes in pentode mode, you can do nearly as good a job by raising the value of the output tube grid stoppers to a high value (I use 47k in my LED-biased amp).
Proof is in the real amp- with overload, the bias doesn't shift, the idle current stays stable, and the recovery is faster than I can measure. The amp sounds like it's a lot more powerful than it is.
Thought I'd throw this one out, a way of providing negative grid clipping beyond the range of cutoff, to prevent blocking distortion:
http://www.18watt.com/storage/18-watter_buzz_info_311.pdf
http://www.18watt.com/storage/18-watter_buzz_info_311.pdf
SY said:
He,he. But i fail to see a contradiction. And I still maintain that a grid choke or interstage tr + fixed bias provides the best solution. And without involving sand noise sources and non-linearities.
Aha 😀
Simming a single ended CRk, it charges up when there is assymetrical clipping (with a DC component). There seems no ideal size, they all seem to have issues. It appears as if I will want either a CV bias of some description, or plain cathode bias.
I can see that the coupling capacitor could be charged by grid current. Perhaps it could be made small.
Aside from eliminating the cap - Is it true that a low DCR from ground to coupling capacitor (like the choke) shorts the grid current path negating it's effect?
-Wouldn't it be true that a low Zo conventional common cathode driver would inadvertently assist the charging effect?
I was once surprised to find that paralleled drivers actually made distortion worse, and I now suspect this issue. If this is in fact true, maybe cathode bias and a CCS would be a good choice for the driver (but maybe not).
BTW, Crowhurst seems knowledgeable, what I read of his was helpful. Is there a different article that would apply specifically to non-NFB SE?
http://www.milbert.com/articles/Crowhurst/AmpDefects/?milses
Simming a single ended CRk, it charges up when there is assymetrical clipping (with a DC component). There seems no ideal size, they all seem to have issues. It appears as if I will want either a CV bias of some description, or plain cathode bias.
I can see that the coupling capacitor could be charged by grid current. Perhaps it could be made small.
Aside from eliminating the cap - Is it true that a low DCR from ground to coupling capacitor (like the choke) shorts the grid current path negating it's effect?
-Wouldn't it be true that a low Zo conventional common cathode driver would inadvertently assist the charging effect?
I was once surprised to find that paralleled drivers actually made distortion worse, and I now suspect this issue. If this is in fact true, maybe cathode bias and a CCS would be a good choice for the driver (but maybe not).
BTW, Crowhurst seems knowledgeable, what I read of his was helpful. Is there a different article that would apply specifically to non-NFB SE?
http://www.milbert.com/articles/Crowhurst/AmpDefects/?milses
Putting things into practice. I shifted some EQ to the driver stage. 12nF working into 100k. I also biased the outputs with a string of zeners. I needed to bypass with a small film to short out the spikes.
Impedance calculates to 15 ohms. Overload seems nonexistant
Can't listen just now, too late to do overload testing.
Impedance calculates to 15 ohms. Overload seems nonexistant
Can't listen just now, too late to do overload testing.
Well, problem seems fixed, and I have all the headroom I had with plain cathode bias, plus I have the damping factor. The 1812 can't break it 🙂 .
I don't think it is very elegant, much tweaking to go, but thanks to all for helping me come to a solution.
I don't think it is very elegant, much tweaking to go, but thanks to all for helping me come to a solution.
I have on occasion used a s.s regulator type of topology to provide a fixed cathode voltage (mine about 44V), with the advantage over many zeners that you can adjust it and it can sink a rather large current. Regulation can be anything you want. The Zeq of mine is below 150 milli-ohm over the whole audio range. Topology can also include an overload sensor (to trigger an h.t. cut-out circuit) and other handy features.
Regards.
Regards.
I have a couple of mje340's. I might be tempted to wrap them around the zeners (ala simple Vreg). I hope they may at least shield the zeners from whatever makes them groan and oscillate, and reduce the impedance.
Careful here. Indm - they must be PNPs. The tube is the load, with + at anode and - at cathode, The transistor is between common and cathode, sinking the cathode current. Convenient to screw the collector directly onto the chassis without insulation.
My circuit was somewhat more involved, with a LTP (PNPs) sensing on one side and having a 6,2V zener on the other, then fed through a NPN to the power transistor (in my case TIP 42 - simply readily available; I needed to sink 1A). As said one can then sense over-current somewhere inside, and a compensating cap in the right place will extend the frequency response, as in giving a low Z over the audio range.
My circuit was somewhat more involved, with a LTP (PNPs) sensing on one side and having a 6,2V zener on the other, then fed through a NPN to the power transistor (in my case TIP 42 - simply readily available; I needed to sink 1A). As said one can then sense over-current somewhere inside, and a compensating cap in the right place will extend the frequency response, as in giving a low Z over the audio range.
One more take on blocking distortion - I'm going out on a limb here and suggesting that the current source in the cathode (suitably bypassed) as I used in the Baby Huey circuit here
http://www.diyaudio.com/forums/showthread.php?s=&threadid=72536
addresses both forms of blocking distortion - it eliminates that due to cathode bypass charging up with grid current and ingnores that cause by a grid voltage shift.
Am I deluding myself???
Cheers,
Ian
http://www.diyaudio.com/forums/showthread.php?s=&threadid=72536
addresses both forms of blocking distortion - it eliminates that due to cathode bypass charging up with grid current and ingnores that cause by a grid voltage shift.
Am I deluding myself???
Cheers,
Ian
I can't convince myself that this is immune to blocking. If, under overload, the bypass cap charges up to follow the CS voltage, when the CS voltage tries go the other way, it still has to discharge.
Or maybe I'm not seeing it right.
Or maybe I'm not seeing it right.
SY - you're right as usual.
If the bypass cap charges up as a result of overload then tube current will reduce by the same amount as the cap discharge current supplies part (or even all) of the constant CCS current.
I really need to think these things through a bit better before dashing of a post during my lunch hour.
Cheers,
Ian
If the bypass cap charges up as a result of overload then tube current will reduce by the same amount as the cap discharge current supplies part (or even all) of the constant CCS current.
I really need to think these things through a bit better before dashing of a post during my lunch hour.
Cheers,
Ian
Indm, can I use just a scope and signal generator to see my amplifier's overload recovery capability or lack of? If so, may I ask how?
Thanks for the heads up there Johan, I haven't thought through to the finer details yet. Your idea will take some digesting but it does appear more refined than what I have now. I haven't seen much like it on the www .
Arnoldc, with an output stage cathode resistor and bypass cap, I had one channel watching the cathode and one watching the dummy load. There was appreciable signal at the cathode despite the cap. I would wind the volume, and when the signal on the load revealed I was drawing grid current, the cathode trace would shoot off (with DC) and return shortly after (though I was AC coupling the channel come to think of it).
Arnoldc, with an output stage cathode resistor and bypass cap, I had one channel watching the cathode and one watching the dummy load. There was appreciable signal at the cathode despite the cap. I would wind the volume, and when the signal on the load revealed I was drawing grid current, the cathode trace would shoot off (with DC) and return shortly after (though I was AC coupling the channel come to think of it).
Hmmm, I'll try that but my amp right now shares the same cathode R. Hope it's close or similar in result.
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