Resistance to change right in accordance with the specifications. For the type of lamp. On the left collector - gun cathode established positive potential. Potentials on the cathodes will be different, but equal currents.
Actually the 6AS7 is one double triode where you have to discard a lot of valves to find two well matched units!
I will side with Artosalo. Once it is balanced it will stay balanced for a looong time. No need to check every month. Having some DC handling capability in a PP OT is not difficult and will completely eliminate the issue. Once you will notice some imbalance it is time to get new valves....😀
There can be a dc offset even if the idle currents are exactly the same - to my mind that form of offset is worse, as it happens when music is being reproduced rather than when it is not (and hence is of no consequence).
If you have a small gap to handle some DC unbalance then you can get rid of it. Except uncut cores (typically toroids for power supplies) all the others always have a minimum gap. Especially EI's. Then you can make it bigger if you want more and still get much higher inductance than an equivalent SE OT.
You can also balance the output stage at medium power using a low frequency sine wave (say, 40Hz) rather than the idle DC current.
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The arrangement advocated here means that within reason any 6AS7 will simply plug in an go without issue. No matching necessary unless they are seriously unbalanced.
All I can report of the actual experience of running these this way is that they give about 4000-5000 hours of service before emissions falls to below the point where they can maintain the DC balance - at this point they start making a high pitched whining noise which is most probably the CCS (in my case) starting to oscillate. the solution is to pull and replace - no catastrophic failures yet and no 6AS7 or 6080 so out of spec it hasn't worked first time. All I can report is that I am very happy and I have been doing it this way for 5 years on multiple different amps.
Shoog
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Thus far we have only qualitative advice. A general statement does not define the real effect. I can also not offer any measurements, but would dare to say that the d.c. balance factor in OPTs are somewhat overrated. Any measurements? If the inductance is say halved at some low frequency, what will be the effect?
Regarding the reliability/mainatinance of valve bias settings, I have over decades not experienced re-adjustment more often that perhaps once/annum. But that of course depends on tube quality, particularly using old/new tubes simultaneously. I would also stick my old neck out and say that an imbalance of up to 10%, depending somewhat on the circuit, is most unlikely to cause audible effects.
But perhaps those fortunate enough to possess a proper spectrum analyser or even a t.h.d. meter, can do a practical test and enlighten us (it should not be difficult to either get dissimilar tubes or generate such an imbalance).
Regarding the reliability/mainatinance of valve bias settings, I have over decades not experienced re-adjustment more often that perhaps once/annum. But that of course depends on tube quality, particularly using old/new tubes simultaneously. I would also stick my old neck out and say that an imbalance of up to 10%, depending somewhat on the circuit, is most unlikely to cause audible effects.
But perhaps those fortunate enough to possess a proper spectrum analyser or even a t.h.d. meter, can do a practical test and enlighten us (it should not be difficult to either get dissimilar tubes or generate such an imbalance).
Yes, I know. My comment was in general as I don't like that sort of "brake" into the cathode. With a proper transformer you don't need it.
It's not just the inductance which decreases but also the fact that the transformer might not be able to handle the specified power. There are too many variables to give numbers. Anyway if you look at the Japanese OT's specs you often find how much inductance will decrease with a given max recommended unbalance, that typically is 3-5 mA. They don't say anything about the power it can handle but in most cases, typically C cores, it will be about the same with no DC unbalance up to the max recommended unbalance, having left some headroom before saturation. With Lundahl's it's even easier, you choose the gap!😉
It is wrong to assume that there wont be a performance penalty of having standing DC in a PP transformer even if it can tolerate a small amount.
Shoog
Shoog
If the transformer has a gap it will behave more linearly actually! The price you pay is lower inductance but as it doesn't need to withstand the full DC current it will be still high enough to ensure low distortion down to low frequency. For sure down to 25-30Hz in most cases. You can have it down to 20Hz if you wish.
Shoog,
Er .... yes; nobody has assumed that; that is what the discussion is about. I think though the question is what practical sort/degree of penalty? I agree with the argument, advantage of small air gap etc., but it all depends on which combination of parameters one would be willing to accept for a certain design. Firstly the penalty is at the low frequency end only; would one want the complicity/extra-cost of an air gap/larger-core/whatever for an extra few Hz, or extra few watt? Also as said some of the consequences are rather difficult to estimate in design; it may be more practical to measure at the end (and hopefully it will be acceptable because it cannot be remedied cheaply!).
My recent experience was with a 100W tube amplifier, response 'calculated' down to 8 Hz, 100W at 20 Hz. Low level response (-3dB) turned out to be down to 6 Hz, maximum available power at 20 Hz: 80W. Big deal? Fortunately not in this case. 'Recalibrate' my design procedure for next time .... This does not have to do with current imbalance, but a similar situation.
Example for a 6AS7 PP. I pick up the Lundahl LL1627 which is 2.3K/8R for lowest power loss. Let's ignore the DC resistance for simplicity. The SE versions have a gap which will give 0.9T DC induction at the specified DC current and it will handle 13W @ 30Hz with the remaining 0.7T. More than enough for 6AS7 PP.
The standard version is gapped for 90 mA and has 18H. In this case I really don't need a gap for 90 mA. Let's exaggerate and go for 30 mA gap. The inductance will be about 55H. Is this too low? A simple calculation of the effective impedance at 20Hz for 8R load will tell that the primary impedance will be about 2.2K! No problem at all.....
The standard version is gapped for 90 mA and has 18H. In this case I really don't need a gap for 90 mA. Let's exaggerate and go for 30 mA gap. The inductance will be about 55H. Is this too low? A simple calculation of the effective impedance at 20Hz for 8R load will tell that the primary impedance will be about 2.2K! No problem at all.....
If you are not adverse to some solid state assisting the tube circuits then use a bias servo to eleiminate any DC imbalance and go fixed bias.
For pure class A a simple circuit can be used but you might as well use one of the circuits which will cope with Class AB as well.
I think the one here, by Rob, is as good as I've seen -
Motional feedback intro
The Norman Koren "Tina" scheme which runs one tube as a master and slaves the other to it is also good.
For purchase off the shelf the Menno/Guido scheme is good:
Tube bias control
Cheers,
Ian
For pure class A a simple circuit can be used but you might as well use one of the circuits which will cope with Class AB as well.
I think the one here, by Rob, is as good as I've seen -
Motional feedback intro
The Norman Koren "Tina" scheme which runs one tube as a master and slaves the other to it is also good.
For purchase off the shelf the Menno/Guido scheme is good:
Tube bias control
Cheers,
Ian
Thank you all for the replies. I emailed Edcor, and they told me that for this transformer (mentioned in my post), 5-10mA of DC shouldn't be a problem. This seems to make sense to me as well -- at full output, one tube is almost shut off while the other is passing over 100 mA! I think I'll keep it simple and use one shared resistor for both tubes, and use 10Ohm resistors to measure current and sort of match the tubes. I mean, the small-signal tubes won't be matched for gain anyway, so the phase splitter won't be perfect.
Mostly, I'm looking forward to exploiting the fact that pure Class A push-pull covers a multitude of sins. I'll update when I actually build the thing (may be a while).
Mostly, I'm looking forward to exploiting the fact that pure Class A push-pull covers a multitude of sins. I'll update when I actually build the thing (may be a while).
There is no DC-offset i.e. idle current difference if the difference is adjusted to zero.
If you mean that an instantaneous difference of signal current or a transient in the music can produce dc-inbalance,
please show me how that takes place.
I do not really get your idea. Could you try to enlighten me with a bit more detailed example ?
If the PP tubes have exactly the same idle dc current, but different conductance performance, then there will be an effective dc offset over a waveform cycle.
With respect to BH curve locus, one side of the excursion will move waveform peaks in to worsening linearity region before the other side - which is the same distortion related mechanism that is being discussed when there is a dc idle unbalance but otherwise perfectly matched valves.
With respect to BH curve locus, one side of the excursion will move waveform peaks in to worsening linearity region before the other side - which is the same distortion related mechanism that is being discussed when there is a dc idle unbalance but otherwise perfectly matched valves.
That is not a DC-offset. It is AC-unbalance and can be adjusted to zero if the design allows AC-balance adjustment (which it should).
It should be obvious that well designed PP-output stage should have DC- and AC-balance adjustments.
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