PlayJazzGuitar.com Forum

[randyc]

Apparently there is a long-standing argument regarding whether "standby switches" are required for vacuum tube amplifiers. Manufacturers are somewhat consistent but not always. Generally, the lower power amplifiers don't include them - Fender Princeton (12 watts) for example but higher power amplifiers (e.g. all Fenders from the Deluxe on up) universally use standby switches.

Here are the popular opinions for using the switches, based on internet research:

1. "The standby switch can be disabled for a break in playing, turning off the high voltage power supply but not the filaments. This extends tube life, as the filament doesn't have to be reheated often during a prolonged playing session. Example: a normal light bulb is rated at 2,000 hours life. That rating was measured by applying power to the light bulb once and then measuring the time elapsed until failure. In normal usage, one may get only 1,000 hours from that bulb due to the stresses of suddenly heating and cooling the filament each time that the bulb is switched on/off. The standby switch helps avoiding that initial tube filament shock every time you turn the amplifier on/off."

2. "Amplifier startup begins with the application of filament voltage to bring them up to the temperature needed to drive off electrons from the cathode. In most cases, it takes from 15 to 60 seconds to reach the proper operating temperature for the cathode. It is during this warm-up time that damage to the vacuum tubes occurs when the high voltage is applied and it peaks simultaneously. This is the time when the STANDBY switch should be OFF (open), preventing current flow and damage to the vacuum tubes.

How are the tubes being damaged? If voltage is applied to the tubes before the cathode is hot and is freely emitting electrons, the HIGH VOLTAGE on the plate is essentially tearing electrons from the cathode material (thoriated tungsten). Generally, this is called cathode stripping. When most of the electron emitting material is eventually depleted or stripped from the cathode, the tube is ruined and can no longer deliver enough electrons to operate as designed."

I've read counter arguments to "2" that state the process that this is intended to prevent only occurs on very high-voltage tubes (x-ray tubes and the like). I haven't seen any counterarguments to "1".

A recurrence of this argument recently, made me think more about standby switches and why they are still used. I don't "own" any of the conventional theories regarding applying high voltage to a tube that is not at operating temperature, my slight knowledge comes from reading old literature, like the RCA receiving tube manual. The scenario propounded in "1" above seems plausible, however.

Anyway, the problems of tube reliability, that usually justify the use of a standby switch, might include another potential failure mechanism in beam power tubes, occurring in the screen grid structures. Here are some quotes from a web source that I recently found, maybe the only site that is particularly focused on screen bias voltages and stress:

From http://www.webace.com.au/~electron/tubes/screens.htm


… a review of commercial circuits shows that for the whole of that 60 year period between 1940 and 2001, only a few basic types of tubes were used in all the audio amplifiers ever produced in the whole world.

The result is that:

1. there is very little literature about Screen Grids

2. there are are few examples of innovative design variants

3. audio amplifier design standards reflected the need for simple tubes that could be overloaded and abused by users

4. audio amateurs - ie hobbyists and project builders - have had to remain within a very rigid published design framework

5. published manufacturer's tube data invariably fails to provide information about the effect of Screen Grid voltage upon Plate Current

6. there is little published manufacturer's data available for non-popular tube types

7. there is little practical knowledge available to facilitate experimentation with non-popular tube types

8. a self-destructing commercial approach manifested that inhibited innovation in the tube based audio equipment industry, paving the way for their displacement by semi-conductors

This page attempts to quantify some of the major principles and possibilities regarding improving vacuum tube technologies in the area of Screen Grids.

RCA Receiving Tube Handbook RC-19 explains at Page 7:
"The Screen Grid is operated at a positive voltage and, therefore, attracts electrons from the cathode. However, because of the comparatively large space between wires of the Screen Grid, most of the electrons drawn to the Screen Grid pass through it. Hence the Screen Grid supplies an electrostatic force pulling electrons from the Cathode to the Plate. At the same time, the Screen Grid shields the electrons between Cathode and Screen Grid from the Plate so that the Plate exerts very little electrostatic force on electrons near the Cathode.

So long as the Plate voltage is higher than the Screen Grid voltage, Plate current in a Screen Grid tube depends to a great degree on the Screen Grid voltage and very little on the Plate voltage."

"Notwithstanding the above JETEC design specifications - determined from extensive practical and theoretical research, design type tested performance criteria and endorsed by leading manufacturers'- numerous examples of commercial Guitar amplifiers and Public Address (PA) amplifiers demonstrate typical design with a common Plate and Screen supply (as a cost saving measure) having B+ supply voltages well above the above specified maxima.

However this operating configuration does not promote either long tube life or high-fi standard performance - in fact some tube guitar amp designers deliberately configure the output stage to ensure desired distortion characteristics under sustained overload conditions. But it can also be a recipe for overheating, unreliability, short tube life, instability, parasitic oscillations and/or dynatron action in the output stage because the output tubes are running with the Plate Voltage less than the Screen Grid Voltage (because of DC voltage drop in the primary of the output transformer).

This is particularly true of low-cost output transformers having high DC resistance windings - not to mention low primary inductance and high leakage inductance which also facilitate parasitics."

It follows that the critical design element for a Tetrode, Pentode or Beam Power Tube is the Screen-Grid voltage, because this is the effective Anode voltage.

"As a rule of thumb, the screen grid supply voltage should NEVER be more than the manufacturer's rating. Higher applied Screen-Grid voltage is likely to cause self-oscillation, parasitic oscillation, dynatron action or thermal runaway - any of which can easily destroy a tube and associated components. MINIMAL Screen-Grid voltage will provide better performance including cleaner, crisper sound with less distortion."

"Tube Data handbooks typically recommend Screen Grid operating voltages at only half, or even less than half, the rated maximum for a given tube type, warning us of the great control the Screen Grid has in determining tube performance."

Radiotronics Magazine No. 80 of October 1937 says:
"The power dissipated in the Screen circuit is added to the power in the Plate to obtain the total B supply input power. With full signal input, the power delivered to the Plate circuit is the product of the full signal Plate supply voltage and the full-signal DC Plate current. The power dissipated by the Plate in heat is the difference between the power supplied to the Plate circuit and the power supplied to the load.

Screen dissipation increases with load resistance. In order to visualise this relation, assume that the sum of the Screen and Plate current is independent of Plate voltage for zero Control Grid bias, or for a negative value of it. A decrease in Plate voltage causes a certain decrease in Plate current; it is assumed that the Screen Current rises by an equal amount.

Hence, when the Screen Grid valve operates with a load which intersects the zero-bias characteristics below the knee, the Screen current rises to high values during low-Plate voltage excursions of the output voltage.

This action produces a rise in the DC value of Screen current with signal. Therefore, the Screen dissipation with full signal input may be several times the zero-signal value. To reduce Screen dissipation, the load should always be chosen so that it passes through the knee of the zero-bias characteristic.

Increasing the applied signal voltage to a value higher than that for which the load is designed also increases Screen dissipation. For this reason, it may be advisable to use a value of load which is slightly less than the optimum value. This precaution has another advantage, which is especially important at high audio frequencies.

The impedance of a loudspeaker increases with frequency. When the load is adjusted for the proper value at 400 Hz, the load is usually too high at 2000 Hz; thus a Screen dissipation limit may be exceeded at 2000 Hz even though operation is normal at 400 Hz. The use of a load which passes through the zero bias characteristic somewhat above the knee is desirable for these reasons."

Note: The conditions described above are very likely in lead guitar amplifiers where the signal is of a single frequency nature.

It is interesting to note also that although RCA state in Transmitting Tube Manual TT-4 at page 9: " Beam Power Tubes may also employ Suppressor Grids rather than space-charge effects to prevent the reversal of electron flow when the Plate swings negative with respect to the Screen Grid." - a study of tube specifications reveals that RF Beam Power Tubes always have a rated Screen Grid voltage substantially lower than the rated Plate voltage, thereby rendering the foregoing statement by RCA as somewhat theoretical for both Pentodes and Beam Power Tubes.

RCA Transmitting Tube Handbook TT-4 also states at p62:
"The danger of excessive screen-grid voltages is present principally when screen-grid voltage is obtained from the plate supply through a series dropping resistor. In this type of supply circuit, sufficient resistance is connected between the screen-grid and the plate supply to assure that the screen-grid voltage and dissipation at the values of screen-grid current, bias and driving voltage required for full output are within the maximum ratings for the tube. Any condition which reduces the current through the screen-grid dropping resistor to a very low value, therefore, may cause the screen-grid voltage to rise to an excessive value."

Voltage drop from DC Screen Current is a particular challenge with parallel-push-pull operation. Care is also needed with conventional Class AB or Class B operation of single paired tubes

OK, that's the end of the quoted passages. Please note that all of the underlining in these passages are mine. I wanted to stress those points that ermphasize screen grid potential problems most especially as they relate to guitar amplifiers.

Accepting that excessive screen voltage (a situation where the screen approaches or exceeds the plate potential) is undesirable, this suggests yet another good reason for including a standby switch in vacuum tube amplifiers. Bear with me for a moment while I consider the possible screen grid problem as it would relate to an amplifier not having (or not using) the standby switch:

We've learned that guitar amplifiers use beam power pentodes in the output section and that the screen grid of these tubes must always be operated at lower voltage than the plate. We've also learned that screen grids frequently CAN'T be operated at plate potential without catastrophic failure. Moving on …

The universal means of obtaining the bias voltage for the screen grid is by using a voltage dropping resistor from plate to screen grid. The correct screen bias voltage cannot be obtained without the voltage drop across this bias resistor.

This voltage drop can't occur unless current is flowing through the bias resistor. Current cannot flow through the bias resistor until it flows through the cathode - plate of the tube.

That can't happen until the filament/cathode are at operational temperatures.

Without current flow, the screen grid is at the plate potential.

AND without current flow through the output tubes, the power supply voltage is much higher than under normal loaded conditions (because guitar amplifiers have primitive, non-regulated power supplies).

As a practical example, consider an amplifier using EL-84 tubes as an example, where the plates are operated around 375 volts and the screen grids around 300 volts. At switch-on, there is no current flow through ANY of the tubes (we can assume that the rectifier is solid state).

There is no voltage drop across the secondary winding of the power transformer, so the voltage appearing at the plate AND the screen of the output tubes is near 420 volts while the maximum screen voltage specified for quality American-made EL-84 tubes is 300 volts.

Here is some information regarding amplifiers that I own or have access to:

Fender Champ, 6V6GT (1), 6W, 400V, 350V
Fender Princeton, 6V6GT (2), 12W, 420V, 415V

Fender Blues Jr, EL84 (2), 15W, 328V, 307V
Ampeg Reverberocket, 7868 (2), 18W, 360V, 350V
Epiphone 25*, EL84 (2), 22W, 400V, 396V
Fender Deluxe Rev*, 6V6GT, (2), 22W, 415V, 415V

Fender Bassman*, 6L6GC (2), 50W, 425V, 425V
Fender Showman*, 6L6GC (4), 85W, 450V, 443V
Fender Twin Rev*, 6L6GC (4), 85W, 460V, 458V

* standby switch

Of the two newest amplifiers, the Fender Blues Junior has reverse polarity diodes on both output plates so that the screen can NEVER be at a higher potential than the plate. The Epiphone Galaxie 25 has a standby switch.

So manufacturers are still clearly concerned about a potential reliability problem, which occurs during initial turn-on and warm-up. Of the listing of higher power amplifiers above, only one has no feature to prevent screen grid voltage from equaling or exceeding plate voltage, the Ampeg. (Neither the Champ nor the Princeton have this feature.)

Having said that, the Ampeg DOES oscillate (as was mentioned in the above quoted articles) at turn on with no signal present. (I turn the volume control "off", then turn the amplifier on and insert my instrument cable. In that sequence, the oscillation doesn't occur …)

I guess that I'll keep on using that standby switch. After all, three of my amplifiers are over fifty years old and still working fine. One of them has original tubes, although frankly I don't play it all that often.

[Jazzie]

Dude! That is way too long a post. No way am I going to read all that. Can someone give me the short version of what he just said?
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[randyc]

[Gorecki]

Jazzie, I highly suggest you DO NOT pursue a career in any form of engineering. It doesn't appear you have the palette for it.
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