I first noticed this limit on JJ's 12ax7 and 12au7 datasheets, where it's given simply as 150k. I found a Mullard 12ax7 datasheet where it's given as 20k(!) with the exception of a phase inverter directly driving the output tubes, where Mullard gives 120k. I can see there is an actual voltage differential between heater and cathode- each is at a voltage- but it's hard for me to understand how this resistance is between heater and cathode, especially when there is another figure also called Rh-k that figures in the hundreds of millions of ohms, having to do with the heater-cathode insulation.
I'm guessing they just mean the maximum cathode resistor to ground. If so, isn't the world full of cathode followers and DC-coupled stages with 12ax7's with much larger cathode resistors? When a 12ax7 might be passing a milliamp or less, how would you ever approach Vh-k max (180v to 200v given here and there) without a much larger resistor? What are real world concerns here? I'm tempted to use 200k cathode resistance to achieve something like 180v. ---sorry about the run on paragraph, this reply window won't let me indent! Thanks!
I'm guessing they just mean the maximum cathode resistor to ground. If so, isn't the world full of cathode followers and DC-coupled stages with 12ax7's with much larger cathode resistors? When a 12ax7 might be passing a milliamp or less, how would you ever approach Vh-k max (180v to 200v given here and there) without a much larger resistor? What are real world concerns here? I'm tempted to use 200k cathode resistance to achieve something like 180v. ---sorry about the run on paragraph, this reply window won't let me indent! Thanks!
HTML discards indents, unless tricks are used.
You are not paying for the paper. Separate with an extra line.
You do not want a heater totally floating. Normally you tie your grounded-cathode stages to zero or a small DC voltage, set your cathode followers with consideration for the breakdown voltage, and trust they won't float-away to some odd high voltage.
You are not paying for the paper. Separate with an extra line.
You do not want a heater totally floating. Normally you tie your grounded-cathode stages to zero or a small DC voltage, set your cathode followers with consideration for the breakdown voltage, and trust they won't float-away to some odd high voltage.
---> no, they don't !
---> this is not a property of the tube; it is a directive for you as the circuit designer to fit a resistance between cathode and heater no bigger than the value given here - which is 150kohms for JJ and 20kohms usually for vintage or NOS tubes
---> concern is to avoid build-up of any uncontrolled and maybe varying differential voltage between heater and cathode of indirectly heated tubes; heaters floating may cause hum injection from AC heaters into the cathode circuit
---> you are not supposed to ever reach that limit
---> you can use whatever cathode resistor you think you need need ... if your design requires the cathode to sit at 180v for whatever reason you should consider to elevate its heater supply to a similar voltage or even a little higher via a voltage divider with a combined resistance of less than or equal to these 150k or 20k or whatever the datasheet prescribes.
Still, it is completely unclear what criterion they used to arrive at 150 kohm or 20 kohm; the heater doesn't suddenly start floating at a specific resistance value, nor does it know whether the tube is used as a phase splitter.
Presumably some internal quality criterion can not be guaranteed to be met anymore when you exceed the specified value, but what exactly that criterion may be?
Whatever the cause is (an arguable point), you can find lots of 7199 tubes that came from the original Dyna ST70 amps that are now dead/not working properly (Unbalanced concertina in-phase output voltage versus out-of-phase output voltage).
The filament of a 7199 was allowed to float (and was in parallel with two EL34 filaments). But the cathode of the concertina was at about +110V, and went about +35V higher than that (+145V) to drive the EL34 grid to full signal. The only path to ground was from the 6.3V center tap through a ceramic capacitor to ground.
The negative feedback of the amplifier can only partially correct for the concertina Unbalance.
Float at your own risk.
The problem (and solution) has been described on this forum, and was written up in Issue #10 of "Sound Practices" magazine.
The filament of a 7199 was allowed to float (and was in parallel with two EL34 filaments). But the cathode of the concertina was at about +110V, and went about +35V higher than that (+145V) to drive the EL34 grid to full signal. The only path to ground was from the 6.3V center tap through a ceramic capacitor to ground.
The negative feedback of the amplifier can only partially correct for the concertina Unbalance.
Float at your own risk.
The problem (and solution) has been described on this forum, and was written up in Issue #10 of "Sound Practices" magazine.
No electrode inside the envelope should be allowed to float. That means some maximum resistance between electrodes. How they determine this is unclear, but I suspect that they are being cautious. The electrode which people always seem most tempted to float is the heater; some even 'ground' it via a capacitor! Hence they specify a maximum resistance from heater to cathode, but allow certain exceptions where they can assume the heater potential is still well-controlled.
Float, flout, flaunt...
I feel that if I violate this limit I should do so with a swagger. Thanks, and I'll look further in the archives- couldn't find anything earlier. It's quite unclear to me why this would be so much safer in Mullard's thinking for the phase inverter driving the power tubes- that would be the largest AC signal in the amp.
Thank you.
I feel that if I violate this limit I should do so with a swagger. Thanks, and I'll look further in the archives- couldn't find anything earlier. It's quite unclear to me why this would be so much safer in Mullard's thinking for the phase inverter driving the power tubes- that would be the largest AC signal in the amp.
Thank you.
It has nothing whatsover to do with AC signals. It is all about a floating electrode acquiring a sufficiently high DC voltage to disrupt normal valve operation in some way.
In a typical LTP phase splitter the cathode voltage is fairly well-defined whatever the value of the cathode resistor. The heater bias voltage will usually be well-defined (typically, in older designs, ground) so there is no problem. Hence an LTP or cathode follower can 'break' the guidelines.
In case someone thinks this is all rather too theoretical, there was a thread on here (IIRC) a few years ago where someone found strange results sometimes when using a twin triode with an intersection shield which he had not grounded. It turned out that sometimes the shield picked up a large voltage due to stray electrons and this affected operation.
In a typical LTP phase splitter the cathode voltage is fairly well-defined whatever the value of the cathode resistor. The heater bias voltage will usually be well-defined (typically, in older designs, ground) so there is no problem. Hence an LTP or cathode follower can 'break' the guidelines.
In case someone thinks this is all rather too theoretical, there was a thread on here (IIRC) a few years ago where someone found strange results sometimes when using a twin triode with an intersection shield which he had not grounded. It turned out that sometimes the shield picked up a large voltage due to stray electrons and this affected operation.
Basically all that is required is that everything within the envelope has a well-defined potential, because it is fed from some reference point by a suitably low resistance (typically 10's of K up to a few M, depending on the electrode). The datasheet requirement of a maximum Rh-k is somewhat misleading, which is why they have to list exceptions for cathode followers and LTP.
So far I haven't found any exceptions listed for cathode followers. If a triode stage is directly coupled to a previous stage, without a potential divider, would this put it in the exceptions category as well? It would have it's grid voltage defined by the previous stage's anode voltage, and it's cathode voltage defined by a cathode resistor, but I would think the grid voltage would have some additional stabilizing influence with an appropriate cathode resistance.
I think this thread is mixing two related but non-identical issues. One is floating heaters, which should be avoided because the floating potential may lead to unexpected effects on the electron stream (distortion). I have witnessed this happen with unconnected internal shields.
The other is the maximum recommended resistance between heater and cathode circuits. The purpose of this rating is partly to keep hum voltage below some arbitrary figure set by the manufacturer when AC heating is used and the cathode resistance is not bypassed. But more commonly it is a rating determined to swamp any heater-cathode leakage resistance which would otherwise affect tuned RF circuits and skew their performance. Like RDH4 says (p81) "The insulation resistance between heater and cathode should not be included in RF circuits where frequency stability is required or in AF circuits followed by a high-gain amplifier."
The other is the maximum recommended resistance between heater and cathode circuits. The purpose of this rating is partly to keep hum voltage below some arbitrary figure set by the manufacturer when AC heating is used and the cathode resistance is not bypassed. But more commonly it is a rating determined to swamp any heater-cathode leakage resistance which would otherwise affect tuned RF circuits and skew their performance. Like RDH4 says (p81) "The insulation resistance between heater and cathode should not be included in RF circuits where frequency stability is required or in AF circuits followed by a high-gain amplifier."
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I'm still not getting how this resistance is between the two circuits. In indirectly heated valves the cathode is separated from the heater by hundred of millions of ohms- although this is reduced in a hot cathode, which is what we're listening to, what Morgan Jones calls Rh-k(hot). Still, there's a resistance between ground and heater, maybe nearly nothing and more if they heaters are elevated, and the heater resistance itself. Then Rh-k(hot) is between the heater insulation and the heater. Then there's the cathode resistance.
Actually I guess I can see how Rh-k(hot) is internal to the heater and its insulation, and the cathode resistor is between the cathode and the heater insulation. But I can't see how the resistance between heater and ground is part of it, if Rh-k(hot) is not.
Unless Rh-k(hot) is parallel to one of the others? It's not between heater insulation and cathode, or between heater and ground...
If the cathode has a resistance to ground (R1), and you connect the heater to ground through another resistance (R2), then the (external) resistance between cathode and heater is R1+R2.
The heater-cathode insulation (inside the tube) is also in parallel with this external combination of resistors, yes. But we don't know what the insulation resistance is, so the datasheet is telling you to swamp it by using a 'relatively' small external resistance, so the insulation resistance become irrelevant to your circuit functioning.
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