Valve question
 

Having grown up in the era of transistors, I'm not very familiar with the practicalities of thermionic valves!

Can someone please tell me which, if any, of the following conditions is deprecated, and if so, what duration is considered OK/ not OK.

1. HT applied, heater cold
2. Heater on, no HT

3. Heater on standby/low power, HT on
4. Heater & HT both low.

JOhn

Re: Valve question
 

1 is OK. (Dodgy condition is HT on, lukewarm heater).

2 is not brilliant, but lots of equipment did it. Can lead to cathode interface resistance. If it's going to happen, it'll take tens to hundreds of hours.

3 Avoid this (unless you can keep a strong negative grid bias to cut off the valve).

4 OK. (If HT is there, make it very, very low). A cool heater will vastly increase the time the problem in (2) takes to appear. You really don't want to try to pull any significant prolonged current with a cool heater - cathode bombardment damage can arise.

Re: Valve question
 

Many thanks: most helpful.

John

Re: Valve question
 

In my youth I did once apply HT to heaters that were still warming up from cold by virtue of having strapped a silicon diode across the rectifier valve of a non-working radio. This produced crackling and miniature pyrotechnics displays in the valves, but it did actually fix the set, which had previously been silent. After a couple of futher brief cold applications of HT, and removal of the diode, it worked fine. An old hand told me that the valves had probably gone low emission and applying the full HT to cold valves had stripped the outer layer from the cathodes, revealing fresh emissive matter underneath. There was little interest in old bakelite radios then, and if I hadn't got it going, it would have been scrapped. Not to be recommended for valves that are working ok!

Re: Valve question
 

In certain types of electronic organ using separate oscillators with HT keying, the oscillator valves run with full heater and no HT until required to speak. Certain notes are so seldom played that for every thousand heater hours, anode current might flow in the relevant oscillators for all of a minute or two.

When first checking over my 4-rank Miller, which arrived in unplayable condition, most of the valves that run continuously with full anode current were found to be completely life-expired with woefully low emission. This was true regardless of whether they were original or replacements. In contrast, many of the oscillators are still running on their original Brimar 12AU7s, registering 50-75% emission on test; I have not yet investigated how much of the measured reduction is due to conventional wear on the cathode surface and how much to interface resistance or other deterioration mechanisms. More conspicuous was the low insulation resistance; hundreds of kilohms being typical, in some cases only tens of kilohms. I would like to study the valves further to discover whether this is related to the unusual operating conditions.

Re: Valve question
 

Most of the early battery set circuits I've seen just switch off the valve filaments and leave HT and GB connected.

John

Re: Valve question
 

When 625 lines was starting and TV sets had uhf tuners in addition to the VHF tuner people who had television sets often did not watch BBC2 or it was not available in their area. They continued to watch BBC 1 and ITV on VHF 405 lines,
When they did switch to 625 they often got no signal as the PC88 and PC86 valves in the uhf tuner had failed due having been running whenever the set was in use with their heaters on but with no HT. A nice quick service call.

John

Re: Valve question
 

Juke box amplifiers usually leave the heaters on and only apply HT when a record is selected. The HT is supplied via a directly heated rectifier so it comes up quickly so by the time the mechanism is ready to play the record the HT is up to its working voltage.

Some amplifiers run the heaters at around half the normal voltage with no HT applied. When a record is selected a winding on the HT transformer applies the other half of the heater volts (assuming the winding is phased correctly) and the HT via the directly heated rectifier.

With the expected usage of a juke box I doubt if the valves ever suffered from cathode poisoning!

Keith

Re: Valve question
 

The Mullard valve tester leaves the heaters on when a valve is not being tested and therefore there is no HT so no surprise the EL37 is often very low emissions.

Re: Valve question
 

Some mono TVs had FM radio. Although the crt heater was off in radio mode, the other
TV valves were run with low HT e.g. 10V to prevent cathode poisoning.

Some early colour TV portables had a quick start option where the crt heater was
permanently run warm at 2-3 volts when the set was off. This was not a good way to
extend tube life.

Re: Valve question
 

I thought that applying HT to a valve without having the heater up to temperature was a recipe for cathode stripping and shouldn't be done. Most, if not all of the valve guitar amplifiers using a thermionic rectifier that I have seen have a standby switch that prevents HT getting to the valves. This switch has two functions; it prevents cathode-stripping on turn-on and saves a bit of power between uses of the amp. If a thermionic rectifier is used, the slow warm-up of the rectifier prevents HT being applied to the valves before their heaters have warmed sufficiently to emit electrons.
Colin.
PS. It's a rare occasion when I can offer advice to John_BS rather than asking him for some!

Re: Valve question
 

Yes. Amplifying what I was on about in post #2:

When valve cathodes are up to temperature, there is a decent cloud of electrons surrounding the cathode. If not up to temperature, there ain't.

With anode current flowing, any residual gas in the envelope can be ionised by the fast-moving electrons knocking electrons out of the gas atoms which then are left with a positive charge. These ions are then attracted to the negative electrodes within the valve, notably the cathode.

When there is a healthy space charge, as the ions approach the cathode, they are neutralised by the space charge and attraction-to the cathode then stops. But if there is only a small, or no, space charge, they will not be neutralised and they will crash into the cathode surface causing damage.

The degree of damage obviously depends on how good the vacuum is, which will vary between samples (perfect vacuum means no gas to ionise) and the anode current (no current means no ions created). Also, to an extent, the anode (or other electrode) voltage - high voltages mean faster-moving electrons with better ionising potential.

The brief period during warm-up and cool down, if HT is left on, is generally short enough to not cause a problem, unless you do hundreds of times a day.

So, if you want to run heaters at standby powers, have zero (or very low) HT. The good news is that the effects which give rise to interface resistance with hot cathode and no current flowing, will happen very much more slowly at lower temperatures thus no need to worry!

Re: Valve question
 

The issue of cathode stripping is only applicable to transmitting valves operating in the kilovolt range, it is not relevant in receiving valves operating at a few hundred volts.

Standby switches on guitar amps came about when Fender included one on the Fender Bassman amp, there are a number of theories as to why this was done but they certainly do nothing to extend valve life and there are good arguments that they actually do the opposite. Of course when Marshall copied the Fender Bassman for their JTM45 amp this meant two of the major guitar amp manufacturers were using standby switches and they became common on many guitar amps despite the fact that they are completely unnecessary.

Some good information on the subject here http://www.valvewizard.co.uk/standby.html

Re: Valve question
 

When in standby, the early Grundig remote control CTV sets used a series resistor in the heater circuit, but a negative voltage (-180 I seem to recall) was connected to the first anodes. This prevented the cathode poisoning as described above.
With the main switch off, the TV was completely off, but I seem to recall that the Saba of the same period left the set in standby at all times. Only the mains supply switch on the wall actually switched it completely off.
Les.

Re: Valve question
 

Some of us unscrewed the screening cans and applied a tap-tap-tap with the butt-end of a 4BA nut-spinner. Restored the emission (long enough to beat a retreat, anyway).

Re: Valve question
 

The comment in #13 is noted. I can only say that the radio I mentioned in #4 was an ordinary domestic AC/DC table receiver (I forget which model: probably early post-war, not a DAC 90 but about the same size) wIth a tapped dropper resistor for mains adjustment. The section for 240V was open circuit and the radio must have been used for some time on 240V mains on the 200V tap, possibly accounting for its valves' low emissions. The very audible sparkler-like flashes were accompanied by audible crackling sounds. The volume, initially silent, got progressively louder with each application, and I stopped when no further increase was heard, about 5 or 6 applications of a few seconds each from memory. I had fitted a power resistor to replace the failed dropper section before doing anything else.

Re: Valve question
 

Last Thursday I found the fault causing a frame collapse on a 405 line TV, a grid-anode short on one half of a 6SN7 frame blocking oscillator.

The "short" was in fact of the order of a few hundred ohms, and was "tapable".

Pulling the valve out while it exhibited a low resistance and applying 200v between pins 4&5 appears to have cured the fault, hopefully permanently, and given the audiophool prices being asked for some 6SN7s I'm rather relieved.

In this particular blocking oscillator, the anode only has 25V when working normally, and I am convinced this fault would not have occurred in a more conventional circuit.

I really wasn't expecting a G-A short given the wide gap compared to cathode to grid, perhaps it was some debris in the wires near the pinch rather than the electrodes?

Re: Valve question
 

Can I refer you to "Valve Amplifiers" by Morgan Jones, page 354? He specifically mentions cathode damage when HT is applied to cold signal valves in audio amplifiers. Morgan Jones is an ex-BBC engineer and this book is considered to be the most complete guide to valve amplifier design. Admittedly, he doesn't condemn nor praise the standby switches used on valve guitar amplifiers.
You choose your information source as you see fit.

Colin.

Re: Valve question
 

And if you continue to read he goes on to discuss cathode poisoning which can occur when the cathode is heated to its normal working temperature with no HT voltage applied (which is exactly what happens in a guitar amp with a standby switch). He points out that this is not advisable and should be avoided.

You can indeed choose your information source as you see fit, equally important is how you interpret that information.

An internet search on the subject of cathode stripping will show that vast majority of informed opinion is that it is not a problem in receiving valves. Additionally I am not aware that any valve manufacturer has ever recommended that their receiving valves need to be pre heated in any way, they were (and still are) manufactured on the basis that they will likely be used in equipment where the HT and LT voltage are applied simultaneously from cold. There are a great many 70 and 80 year old valve radios still operating on their original valves that are surely testament to the fact that this is not a problem!

I have however seen many guitar amps with poorly implemented standby switches that have caused problems, switching high voltage DC is not trivial and the majority of the switches used for this in guitar amps are neither suitable nor rated for such applications. They typically use normal mains rated switches which are not rated for high voltage DC and over time arcing in the switch will cause failure. I have also seen amps with valve rectifiers (some from well-known manufacturers) where the standby switch has been placed between the rectifier and the reservoir capacitor, this is most definitely a bad idea as the current surge to charge the reservoir capacitor when it is switched on can cause arcing and failure of the rectifier valve.

The main reason guitarists like having a standby switch is that it is a convenient way to mute the amp during breaks without having to wait for it to warm up when you return. A much simpler and better way to do this is to fit a mute switch that grounds the signal path, no high voltage switching, no problems with cathode poisoning and as far as the end user is concerned it does exactly the same job. At best standby switches are unnecessary and in all probability they actually do more harm than good.

Re: Valve question
 

This discussion reminds me of a saying, which applies to me as much as anyone else:
"Opinions are like backsides, everyone has one, but no-one really wants to hear from anyone else's".

Colin.