Why exactly IS there an Rh-k limit? - UK Vintage Radio Repair and Restoration Discussion Forum

daviddeakin · 30th Aug 2018, 8:02 am

Can anyone shed light on what the heater-cathode resistance rating has to do with RF circuits? I can see why there would be an upper limit on Rhk in-circuit for reasons of hum, but when and how does this have anything to do with tuning stability? The reason I ask is this advice given in RDH4 (p81) says:
"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."

Similar advice is quoted in Philips SQ-tubes, Jan 1975:
"To minimise the influence of variation and spread of the leakage current between heater and cathode the resistance of the external heater to cathode circuit should not exceed 20kohm in R.F. circuits where frequency stability or preservation of wave form is required and in A.F. circuits with low signal level. However, when the D.C. value of Vkf is at least 3 times the RMS value of the heater voltage an external resistance between heater and cathode of max 220kohm can be used provided that the hum voltage which may then occur across the cathode resistor can be accepted for the application considered."

When do you ever have a high-Q tuned circuit in the cathode that might be affected by heater-cathode insulation resistance and hence need a realiable 20kohm damping resistance

Herald1360 · 30th Aug 2018, 9:41 am

I would simply say that any leakage resistance between signal and power circuits is undesirable. If itbvaried with time and temperature the problems caused could be worse still. At RF the effects would be ill defined if not unpredictable in terms of oscillator stability and AF hum and noise would be the major worry.

Chris55000 · 5th Sep 2018, 9:37 pm

Hi!

C.A. Quarrington has also made reference to cathode/heater capacitance and insulation resistance in several places in his "Radio and Television" four volume set, 1963 edition – this is why V.H.F. amplifiers/oscillators always have series chokes in their heater–supply leads.

They work by providing a high–impedance at operating frequency between the valve–heater and chassis, so the heater is at the same approx. R.F. potential as the cathode, therefore changes in heater–to–cathode parameters (insulation resistance, capacitance, etc.) have much less effect on stage–gain or frequency of oscillation as the case may be.

The requirement for a certain maximum heater–cathode resistance/impedance for most types of valves is rarely an issue in probably 99% of normal circuits, the only exceptions I can think of is the upper valve of cascode amplifiers and the series element in H.T. stabilising circuits.

Altho' the upper triode in cascode–amplifiers has it's cathode seemingly without a physical component returning it to heater–line potential (normally a TV series–heater chain), there is both the very high (but not infinite) insulating resistance of the alumina–oxide heater wire coating, plus a "hidden or phantom" resistance produced by the small number of electrons collected on the upper cascode grid, which will be returned to H.T. and earth via the bias potential divider, as well as a lower–impedance path produced by the ra of the lower cascode triode section, which has the same anode current flowing as the upper section!

The combined effect of these "hidden" resistances is that, altho' the circuit diagram shows no physical return from k to h, the valve is still operating effectively with a d.c. return produced by the normal electron emission process, allbeit indirectly!

The other type of circuit I mentioned, H.T. stabilisers, either have a direct potentiometer network from the cathode of the series stabiliser to chassis, or indirectly via the feedback amplifier valve and (usually) the neon–regulator tube used in most such circuits to maintain the cathode of the feedback amplifier at a constant reference potential – don't forget once a neon regulator has struck it is equivalent to a very low dynamic resistance!

Chris Williams

PS!

Sorry if this is a bit long–winded – I'll add drawings if anybody needs them!

PPS!

I forgot to mention the discharge valve of Puckles's classic hard–valve timebase used in many simple valved oscilloscopes – the discharge–valve's cathode has a dynamic return to it's heater via the conduction of the bottom constant–current pentode, with it's g2 usually used as the "timebase velocity" control!

PPS!

I suspect the reason for a Rh–k limit being quoted for certain types of valve could be to either prevent electrolytic effects or "tin–whisker" effects between heater & cathode from voltage–stress, but as I explained above, this requirement is rarely violated in practice, as a "dynamic" path is almost always present as an alternative to an actual physical component!

G8HQP Dave · 7th Sep 2018, 11:19 am

There has recently been a debate on this on DIYaudio forum.

My own view is that it is partly a matter of hum and partly a matter of avoiding any floating electrodes.

30th Aug 2018, 8:02 am	#1
daviddeakin Hexode Join Date: Apr 2007 Location: W Yorks, UK. Posts: 407	Why exactly IS there an Rh-k limit? Can anyone shed light on what the heater-cathode resistance rating has to do with RF circuits? I can see why there would be an upper limit on Rhk in-circuit for reasons of hum, but when and how does this have anything to do with tuning stability? The reason I ask is this advice given in RDH4 (p81) says: "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."* Similar advice is quoted in Philips SQ-tubes, Jan 1975: "To minimise the influence of variation and spread of the leakage current between heater and cathode the resistance of the external heater to cathode circuit should not exceed 20kohm in R.F. circuits where frequency stability or preservation of wave form is required and in A.F. circuits with low signal level. However, when the D.C. value of Vkf is at least 3 times the RMS value of the heater voltage an external resistance between heater and cathode of max 220kohm can be used provided that the hum voltage which may then occur across the cathode resistor can be accepted for the application considered." When do you ever have a high-Q tuned circuit in the cathode that might be affected by heater-cathode insulation resistance and hence need a realiable 20kohm damping resistance

30th Aug 2018, 9:41 am	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Why exactly IS there an Rh-k limit? I would simply say that any leakage resistance between signal and power circuits is undesirable. If itbvaried with time and temperature the problems caused could be worse still. At RF the effects would be ill defined if not unpredictable in terms of oscillator stability and AF hum and noise would be the major worry. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

5th Sep 2018, 9:37 pm	#3
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,868	Re: Why exactly IS there an Rh-k limit? Hi! C.A. Quarrington has also made reference to cathode/heater capacitance and insulation resistance in several places in his "Radio and Television" four volume set, 1963 edition – this is why V.H.F. amplifiers/oscillators always have series chokes in their heater–supply leads. They work by providing a high–impedance at operating frequency between the valve–heater and chassis, so the heater is at the same approx. R.F. potential as the cathode, therefore changes in heater–to–cathode parameters (insulation resistance, capacitance, etc.) have much less effect on stage–gain or frequency of oscillation as the case may be. The requirement for a certain maximum heater–cathode resistance/impedance for most types of valves is rarely an issue in probably 99% of normal circuits, the only exceptions I can think of is the upper valve of cascode amplifiers and the series element in H.T. stabilising circuits. Altho' the upper triode in cascode–amplifiers has it's cathode seemingly without a physical component returning it to heater–line potential (normally a TV series–heater chain), there is both the very high (but not infinite) insulating resistance of the alumina–oxide heater wire coating, plus a "hidden or phantom" resistance produced by the small number of electrons collected on the upper cascode grid, which will be returned to H.T. and earth via the bias potential divider, as well as a lower–impedance path produced by the ra of the lower cascode triode section, which has the same anode current flowing as the upper section! The combined effect of these "hidden" resistances is that, altho' the circuit diagram shows no physical return from k to h, the valve is still operating effectively with a d.c. return produced by the normal electron emission process, allbeit indirectly! The other type of circuit I mentioned, H.T. stabilisers, either have a direct potentiometer network from the cathode of the series stabiliser to chassis, or indirectly via the feedback amplifier valve and (usually) the neon–regulator tube used in most such circuits to maintain the cathode of the feedback amplifier at a constant reference potential – don't forget once a neon regulator has struck it is equivalent to a very low dynamic resistance! Chris Williams PS! Sorry if this is a bit long–winded – I'll add drawings if anybody needs them! PPS! I forgot to mention the discharge valve of Puckles's classic hard–valve timebase used in many simple valved oscilloscopes – the discharge–valve's cathode has a dynamic return to it's heater via the conduction of the bottom constant–current pentode, with it's g2 usually used as the "timebase velocity" control! PPS! I suspect the reason for a Rh–k limit being quoted for certain types of valve could be to either prevent electrolytic effects or "tin–whisker" effects between heater & cathode from voltage–stress, but as I explained above, this requirement is rarely violated in practice, as a "dynamic" path is almost always present as an alternative to an actual physical component! __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed! Last edited by Chris55000; 5th Sep 2018 at 9:59 pm.

7th Sep 2018, 11:19 am	#4
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Why exactly IS there an Rh-k limit? There has recently been a debate on this on DIYaudio forum. My own view is that it is partly a matter of hum and partly a matter of avoiding any floating electrodes.