Standby switching

Some different views.

The origin of the standby switch on guitar amps is obscure. The typical thing was to simply bung a 240 volt AC rated toggle switch in the 600 volt DC B+ line to the output stage, and everything seems to survive okay.

The main function of the standby switch has nothing to do with technology - it's to allow the amp to be silenced, say during breaks, without disturbing the control settings.

“But what about valve life? I've heard...”

...a lot of techno-waffle from guitarists as to why you should, or shouldn't, standby an amp. I'll take as my example this quote from an article on Harmony-Central.

“(You need) a 'standby' switch that lets the tubes heat up using the 6.3 voltage supply, but keeps the high B+ voltage off. After 2 or 3 minutes, then you can turn the standby switch to the 'on' position, which turns on the high voltage B+. Doing this will make your tubes last much, much longer. Tubes should warm up first before any high voltage hits them. Applying high voltage to cold tubes makes them wear out much faster.”
- Guitar Amp Evolution by Keenan.

Apart from making a very strong pitch about greatly extending valve life, he is quite specific about disconnecting the B+/HT. This seems to be a widely held belief, but how does it stack up in reality?

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Cathode standby

First let's look at the idea of disabling a stage by disconnecting the cathodes.

“A valve should not be rendered inoperative by disconnecting the cathode unless there is resistor not exceeding 250,000 ohms between heater and cathode.”
Pat Hawker, Radio and Television Reference Book, Newnes, 1960

This is to prevent a buildup of voltage on the disconnected cathode that would eventually break over the heater-cathode insulation. Anyone who ever thought of doing this apparently looked at the difficulties, then put a switch in the HT. It is a point to watch if you put current metering links in your cathode circuits - a small power resistor in each cathode is the safer method.

I've seen amps with fuses fitted in the cathodes and this doesn't seem like such a great idea either.

Mercury vapour rectifiers

Mercury vapour rectifiers are a well known special case which is worthwhile examining, if only to get out of the way.

“When mercury-vapour tubes are first placed in service, and each time after the mercury has been disturbed, such as by removal from the socket to the horizontal position, they should be run with filament voltage only for 30 minutes before applying high voltage. After that a delay of 30 seconds is recommended each time the filament is turned on.”
- Power Supplies, ARRL Handbook, 1963 ed.

In the 60's and 70's I visited many Radio Hams running rigs using mercury-vapour rectifiers and often got an explanation of the need for delay as the 60 second thermal delay in the power supply timed out.

To me Keenan's advice above seems to relate much more to using mercury vapour rectifiers than hard vacuume valves.

The big difference with mercury-vapour rectifiers is, of course, the mercury. When that is a hot vapour they are formidable, but cold they ain't worth a crumpet; can't carry current and can't withstand voltage. If tipped when cold the condensed mercury gets on the electrodes and has to be boiled off first, hence the 30 minute first run.

None of this applies to valve guitar amps because, the odd insane experiment aside, they were never used in guitar amps. Apart from having to “brew up” your amp every time you moved it, mercury-vapour rectifiers give off a lot of RF noise and typically have to be caged to keep it in.

Silicon Rectifiers

So what happens when you apply HT, say from a silicon rectifier, to cold valves?

Well, initially, nothing. As the cathodes come to temperature they start emitting and everything settles down. If any damage were going to occur it would be during the start of emission. But all we have here is emission-limited current rather than electrostatically, or “space charge”, -limited current, as it quickly becomes. The saturated emissivity of a cathode is generally more than ten times it's rated current and one can observe the abrupt rise of cathode current from zero as it comes up to temperature, taking only a second or two.

The RCA databook makes a long point of getting the heater voltage right because thoriated-tungsten cathodes (i.e. the ones we mostly use) are damaged by persistant running both under and over temperature, but nothing about a few seconds during warmup.

What can be a concern, particularly with retro-fit silicon rectifiers is not so much a bit more HT, but that it comes on very quickly before the valves have heated to load it. This means that all bypass caps on the HT line must be able to withstand peak HT for the warm-up period.

In such a retro-fit it would be wise to let the valves heat up before applying HT, but better to also retro-fit with higher voltage caps where needed, including coupling caps. With such a retro-fit a small value power resistor (50-200 ohms) between the rectifier and first filter capacitor is a good idea to limit the switch-on surge current.

This can also serve as a “sag” resistor to simulate a valve rectifier if that's what you want, but watch its power rating, the more you sag the hotter it will get. (e.g. see also Moody BA40)

Overbias

Another method for shutting down the output stage that didn't get used was the idea of increasing the output stage bias to choke it off (although Fender did use overbias to switch a tremolo oscillator).

“According to the valve data books this is if anything preferable to switching off the anode supply as prolonged operation with the heaters on without applying anode voltage gradually reduces the emissivity of the cathode.”
- “EL156 Audio Power Amplifier”, Gerhard Haas, Elektor, March 2005.

Certainly loss of emission from running a cathode for extended periods without HT was well known to Hams in the 60's and is mentioned by Pat Hawker and others.

Perhaps I should know Keenan like I know Sting, Liberace, Prince, Sade and other one-name identities. But I don't. Pat Hawker has a great deal to say about cathodes, and his overlooking a significant factor such as this seems unlikely to me. And my long-term understanding agrees with Gerhard.

Valve Rectifiers

Looking through various books trying to find something definitive I couldn't help noticing that vacuume rectifiers almost always have the AC applied to their anodes while they warm up. But I didn't notice that rectifiers wore out significantly more often than other power valves. In fact in a million mantle radios they actually seemed pretty robust. However the vacuume rectifier does apply power to the rest of the valves delayed, and very gently due to its own cathode warm up, and it does so looking into the discharged HT network.

In contrast there is nothing gradual about the average standby circuit - brutal is more like it. And it would worry us back then with vague talk of “cathode stripping”, but this is a dubious concern. (see addendum below)

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So we have some contradictory statements about how best to standby an amp, the health of the cathodes being the issue.

A look at a bit of valve theory may help to guide the way.

When electrons boil off the cathode they form a cloud around it which has a nett negative charge, naturally, and this is called space charge. This charge tends to repel electrons leaving later, back to the cathode.

When an electrostatic gradient is created by applying a positive voltage to another electrode, say the anode in a diode, the cloud is attracted away from the cathode then allowing other electrons to follow, and a nett current flows.

It is important to understand that in pentodes and beam tetrodes such as guitar amps use a lot, this initial accelerating field actually is created by the screen grid, grid 2. Because of the action of the supressor grid (pentode grid 3) or beam forming plates (beam-tetrode) the anode potential has only a little effect on the electrons until they are well on their way, accelerated for the trip by the screen grid. My own PM117 happens to use screen grid standby switching as an experiment, and if driven hard in standby will produce a watt or so of the most incredible shred.

The bias on the control grid, grid one, keeps this current in check by effectively repelling the space charge back toward the cathode, but even at currents as low as 1mA the space charge is concentrated away from the cathode surface.

With no current at all it is concentrated right at the cathode surface and all of the emission current will be returning to bombard it. It is not hard to imagine that prolonged bombardment like this might make the cathode less emissive since we know that ion bombardment certainly does.

Reducing the current in the output pair to a very low value by over-biassing them allows a small residual current to flow, silencing the amp while keeping the valve cathodes in a reasonably normal state. And making this ramp smoothly to 'de-plop' it seems easier than on the B+ where considerable current will flow.

Switching the HT means the network has to re-charge, and many amps produce a strong hum for the second or so this takes when switched back on from standby.

So in summary it seems that:

• valves are not damaged by HT when cold
• but heated with no cathode current they will slowly lose emission

Physical issues aside, by far the main determinant of valve life is how hard the cathode gets worked. If you are in the habit of “hammering the snot out of it” with your bias tweeked down, then get used to changing valves a lot. Vox AC30 owners know about this even without hammering it. Guitar amps frequently went off the data sheet and flogged the output valves, but you don't have to flog valves to get tone out of them.

If anything disproportionally shortens the life of a valve, it's running it off the data sheet outside the absolute ratings.

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Addendum: 7/11/05.

Some items have turned up which touch on these issues:

“A Deeper Look At The Phenomenon Of Cathode Stripping In Thermionic Valves” from a rec.audio.tubes article posted Dec.'96 by: J.H. van de Weijer

In summary; the idea is that the emissive coating in an indirectly-heated cathode is like a ceramic and that over time it gets many small cracks that form a granular surface. It is asserted that these cracks also hold residual charge.

If HT is applied before the cathode is heated sufficiently to allow this residual charge to leak away it is stripped off the cathode by the HT field and carries tiny chunks of cathode emissive material with it.

These fragments can attach to the nearby control grid, making it emissive.

Comment:

Certainly grid emission is a vacumme tube failure mode, but it wasn't all that common in the good old days, and then more common in equipment using pre-octal and octal types.

While his remarks above are directed specifically to low level stages the simple fact is that replacing a preamp bottle because it has gone grid emissive seems to be a very rare occurance in practice with miniature all-glass valves.

By far the most common reason for replacing a preamp valve is because it has gone microphonic, second because it has lost emission, and thirdly because it won't light up. Grid emission hardly rates a mention, even with bottles in service for thirty years or more.

What will cause cathode stripping is rough handling, and my guess would be that the lug to a single gig and back would cause much more cathode damage than any amount of powering on and off.

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“Kindness to Filaments - A Study of Different Filament Supply Regimes” by John Harper.
http://www.john-a-harper.com/FilamentHeating

John deals more with the processes when a heater or filament is warming up from cold and has experimental results from a 6SN7 (indirectly heated cathode) and 300B (directly heated filament).

He makes the interesting observation that the heat-up of the 6SN7 features a knee at around 1300° Kelvin where the temperature rise slows down, and that the directly heated 300B does not show this effect.

Certainly later valves such as the 12AX7 had what RCA called a “rapid warm-up feature”.

This causes a very bright flare at the base of the cathode for a second or two at switch-on. RCA don't detail this feature but it is likely to be the use of a short length of heater with a different resistance-temperature characteristic. This may explain the knee and why it isn't observed in an earlier directly heated valve.

Use of a constant-current supply reduces the peak thermal stress on the filament by about a factor of five...

Strictly speaking it extends the warmup time by roughly that amount, but drawing broad conclusions about stress in the heater is complicated by the fact that there are a fair number of different heater arrangements.

- In a 5Y3 directly-heated rectifier the heater consists of a flat strip that runs up the structure, over the top insulator and back to the base. In operation it can be seen that the top hoop rises a couple of millimetres above its cold position on the insulator.

- In a 6SN7 and similar octal valves the heater is likely to be a bifilar spiral running up the middle of the cathode tube. These generally show little or no signs of movement, the expansion perhaps taking place radially.

- In minature valves such as the 12AX7 we may have a heater that is folded to run up-and-down a few times. These often show some small lengthwise movement between cold and hot.

- In battery operated valves the 1.5 volt filament is a very fine single wire from top to bottom. It is fixed to its contact at the bottom but is held under outward tension by a small leaf spring at the top. The lifting of this spring during heating is quite obvious being 2-3mm in as many seconds. In fact this spring resting right up against the glass top dome was something you looked for as a standard symptom of a valve with a blown filament.

John makes some interesting points about the use of directly heated valves.

Of course there are people who believe that such regulation of the heater voltage somehow harms the sonic properties of the amplifier. This may well be true for a directly heated tube (due to interaction between the voltage drop caused by the signal flowing in the filament, and the regulator). It's pretty hard to believe for an indirectly heated tube.

With directly heated valves it is always important to keep in mind that your cathode is not equi-potential, that is the same voltage to the other electrodes, from end to end due to the heater voltage itself. This is a path of possible interaction between the heater circuit and the anode-to-cathode signal circuit that is not present in indirectly heated valves much more commonly used in guitar amplifiers.

John concludes;

The reduced thermal stress may increase tube life (although see below), but it may also reduce it. Unless the B+ supply has a delay, the period while the cathode is warming up will result in operation in saturated mode, where the plate current is equal to the emitted current and there is no space-charge of electrons surrounding the cathode. It is widely believed that this contributes to a phenomenon known as “cathode stripping”, where the oxide coating on the cathode is damaged by impact from positively-charged ions resulting from high-energy collisions between electrons and residual gas molecules.

Whether all this is of any practical value is open to debate. By the 1950s, lifetime was a predominant concern of tube designers and industrial users, and if switch-on surges would significantly reduce lifetime then this would be reflected in the literature of the time, which it is not. There could conceivably be some benefit with directly-heated filamentary tubes, whose filaments are more mechanically fragile. The high-end audio world is full of mystical beliefs, and very probably this is one of them.

I'm inclined to the view that like skin effect the dangers of cathode stripping are real enough is some situations, but that those situations do not apply to valves as they are used in guitar amps.

While it's quite true that when a valve goes gassy one of the things that happens is that the cathode gets poisoned, non-emissive, due to ion bombardment. But if a valve has gone gassy to this extent then a bit of cathode stripping is the least of your worries - the valve is “gassy” and stuffed anyway.

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This item is a post from a forum and I feel it summarises the whole situation quite well;

Source: http://www.guitarists.net/forum/view.php?forum=5&thread=50848

RE: New To Tube Amps...Standby?
From: Ricochet
Status: Preferred Member
Posts: 1,178
Join Date: 3/02/2003

It's really a myth about “cathode stripping” in small tubes like we use in our amps. That was something that happened in big transmitting tubes with much higher voltages on them.

But I still do leave the standby switch “off” until the tubes fully warm up, just in case it makes some miniscule difference in tube life, and also switch it “off” for a few seconds before turning the power off, to let the high voltage in the power supply's filter caps bleed down before cooling the cathodes. Unnecessary, but it makes me feel better.

I don't worry about amps that don't have standby switches, none of the consumer electronic equipment that used tubes in the old days had them. TVs, radios, etc.

The standby switch is mostly useful for silencing the amp when you want to be able to bring it back online quickly without having to wait on tube warmup.

Leaving tubes with the cathodes hot but no cathode current flowing (as with the standby switch off) for prolonged periods can damage the cathodes' emission capability, a condition called “sleeping sickness”.

I'm not worried about that with a guitar amp, either. I've often left mine idling that way for hours (and on one occasion, inadvertently for several days) with no evident harm. But the Radiotron Designer's Handbook, 4th Edition, has a lengthy discussion of these issues and recommends powering off the equipment in most cases if it's going to be unused for over 10 minutes or so.

Post Date: 8/1/2005 @ 10:02 pm

So I come back to where I started above; the primary use of a standby switch is to silence your amp without disturbing your volume control setting - a performance consideration rather than an electronic one.

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