SOFT START FOR POWER TUBES

By Jukka
Tolonen

Contents:

Saving
tubes and energy

Why
Tubes Fail

Reduced
filament power

In
Series for Soft-Start

Switch
Operation

Automatic
switching

Applications

Figures,
Tables

In recent years GA
has published several articles about soft-starting to extend tube life. These
articles covered DC-operated filaments, used mainly in preamplifiers. However,
power amplifiers with all those expensive tubes would benefit much more from the
added life span. Here are some guidelines on how to apply soft-starting and
reduce filament voltage in power amplifiers.

Saving tubes and energy

Some time ago I finished a built an all tube amplifier system consisting of a
modified version of Joe Curcio's
"Daniel" preamplifier (TAA
2/85, p. 7) and a 40 W Class A power amplifier based on the legendary Williamson
amplifier.

I needed a system in which I could turn on the power to all equipment with
onlyone switch, located on the preamp front panel along with other controls. In
principle this is not difficult to achieve with a bundle of extension cords and
a master power switch. With a tube-based power amplifier drawing over 300 W from
the power line, it's inconvenient to switch the system on in the morning and
then switch it off in the evening, which is exactly what I did with my previous
solid state amplifiers.

My tube preamp is on for an average about 40 hours per week and easily
accumulates 2000 hours per year. Fitted with a set of Russian 6922/6DJ8s
good for about 10,000 hours, the preamp will probably last for five years
without any tube changes. The power amplifier, on the other hand, might need new
power tubes once a year!

An audio operated power switch to promptly turn on or off a power amp without
extra stress on the tubes would be convenient. Switching on could be activated
by monitoring the input audio signal, while switching off could be delayed from
10 to 30 minutes after no input signal. Since the active use of the power amp is
probably less that half of the total on-time, I would more than double the
lifetime of the power amp tubes with this scheme.

Why Tubes Fail

My theory regarding the failure of tubes is as follows:

In low power tubes the dominant failure is a filament breakdown
in cold start. Repeated heating and cooling, plus the stress of the initial
cold current surge, cause Slow deterioration of the filament.. You can
sometimes see a bright initial light flash in the filaments of some miniature
tubes at startup, especially when those tubes are operated from AC. This clear
shows how large the initial current surge is. For maximum life, tubes should
be on for relatively long periods, and the cold current surge should be
eliminated.

In power tubes the filament must deliver such a high electron
flow that the filament also wears away, as an incandescent lamp. The
cold-current surge then eventually breaks the filament. Deformation of the
electrode structures due to overloading can cause premature failure in a power
tube. Optimal use requires minimizingthe active time with heavy anode current
and eliminating the cold current surge.

In all tubes, the full plate voltage should not be applied until
the filaments are warmed up to prevent cathode stripping.

Convincing? Unfortunately I haven't uncoverd any published information about
the effects of low filament voltage on tube life. My reference books (Eastman,
Fundamentals of Vacuum Tubes; Millman, Vacuum Tube and Semiconductor
Electronics; Valley & Wallman, Vacuum Tube Amplifiers) ignore
the topic completely. Therefore, my "theory" is based solely on my own reasoning
and some articles inGlass Audio Magazine.

The VTL Book (by David Manley, available through Old Colony)
contains one of the few fragments of information on tube life. The data sheet
for the GE's KT66 states that at full rated power the expected life time will be
about 2,000h, and at derated power 10,000h, provided that the tube is not
switched on more that 12 times per day! Trouble is that today's tubes are (among
other things) not what they used to be.

Reduced filament power

I experimented on my prototype amp to meet the challenge of creating a fast
switch-on for tubes. I attempted to determine the lowest filament voltage to
keep the tubes warm enough for immediate operation after restoring normal
filament and plate voltages (Table
1). The start-up time is the interval from switch-on to normal sound output.
Above 2.6V the plate supply voltage turn-on time primarily determines the delay.
If the filament voltage during the warm period is more than 2.6V, the amp will
operate (you can hear the sound) virtually immediately after the switch-on from
a low-level filament voltage to full supply voltages.

In Series for Soft-Start

The cold current surge is a serious problem in a power amps using multiple
output tubes. For example, the resistance of a cold 6L6-GC filament is roughly
3-4x that of a hot filament. A 6L6GC,
normally drawing 0.9A, has a surge of about 3A during a cold start. This becomes
even more hair-raising in my 12-tube amp (eight power tubes and four others,
with a normal heater current of 9.6 A), with the surge current exceeding 30A.

Figure
1 shows how to achieve lowered filament voltage when the plate supply is
switched off. When the power is first applied using the main power switch S1,
the relay K1 remains unenergized. The filaments of the stereo channels are
connected in series, along with an optional NTC resistor. Thus the filament
current will have a very smooth and easy rise with absolutely no cold current
surge.

In my amplifier the total cold filament current is initially about 1A, rising
to a steady 3A level in two minutes. The voltage drop over the NTC resistor
(Philips type 2322 644 90008 or Siemens type Q63036-S1509-M) was about 1.2V,
leaving some 2.6V over the filaments of both channels. Without the thermistor
the cold current peak is about 8A, dropping to 3.5A in 30 seconds, and the cold
current surge is no larger than the normal full-power heater current. Even if
you can't purchase a suitable thermistor, series connection of the stereo
channels will help reduce tube stress during switch-on.

In the standby mode my prototype amplifier consumes about 20W of line power,
which uses about 175 kWh energy per year, provided the standby power feature is
enabled full-time. At my local power company's current energy rates, the
additional costs is less than $20 per year.

Switch Operation

The reduced filament power lets you switch the power amp on or off as
necessary. To fully exploit this feature, I designed a control unit, which can
be fitted inside the amplifier (Figure
2). Alternatively, you can build the unit as a separate add-on in its own
case. See the parts lists in Table
2.

The operation is quite similar to "An Audio Activated Power Switch" in TAA
1/84, except a few details. This unit has an initial delay before power is
applied after a cold start. The input buffers feature very high input impedance
to minimize signal source loading. The power switching occurs with a relay,
since a multi-pole switching, such as K1 in Figure
1, is necessary.

After a cold start the unit functions as a delay timer, gently powering up
the tube filaments to a reduced operating voltage and then switching the amp on
to full power. This will take 136s at 60Hz line frequency or 164s at 50Hz
instead of the normal 10s (which is the price you pay for the soft start). After
the power-up it will then act as a full-power/standby-power switch, monitoring
the signal input. Switching from reduced standby power to full power is
virtually immediate.

During the initial power-up R1, C7 and IC3B generate a short reset pulse that
clears the counters IC2 and IC6. The flip-flop formed by gates IC5A and IC5D is
also reset at power-up. IC4F keeps the counter IC6 cleared as long as the
flip-flop stays reset. After a 136s delay, determined by counter IC2 output 13,
the flip-flop IC5A/ IC5D is finally set. This enables counter IC6 and switches
on the relay. Thus a warm-up delay of about two minutes is provided after a cold
start.

The dual operational amplifier (IC1) monitors the input signals to the power
amplifier. A noise-free input circuit is essential, since we must not inject any
noise to the input signal. The diodes D1-D4 protect the op amp inputs from
possible switching-on transients of a tube preamplifier. The capacitor C3 and
diode D7 form simple peak-to-peak detector.

On positive peaks capacitors C3 is charged through diode D7. On negative
peaks the entire peak-to-peak value (minus the diode voltage drop), referenced
to VCC, is available at the junction of diode D7 and resistor R16. As soon as
this voltage falls below the logic threshold of the Schmitt-trigger input
inverters IC3F/IC3E, the wired or CLEAR line is pulled low, clearing the counter
IC6.

Automatic switching

If the peak-to-peak level of the input signal is above 20 mV p/p, counter IC6
will be repeatedly cleared, thus keeping relay K1 on. With no input signal, IC6
counts output pulses from IC2, enabled with gate IC5C. The frequency is about 1
Hz (actually, 60/64Hz or 50/64Hz). After a time period, the jumper selected
output of IC6 will switch high. IC5C freezes the counter IC6 by blocking the
clock signal. Gate IC5B switches off the relay.

The amp is now switched to standby mode with reduced filament power and no
plate voltage, but the control unit is constantly monitoring the signal inputs.
When the signal level again exceeds 20 mV p/p, counter IC6 is cleared and the
amplifier is switched on to full filament power, now already warm and ready for
immediate operation.

You can change the timings by selecting other outputs of IC6 with the jumper
block J11 (Table
3). You can also connect an optional override switch in solder points J9 and
J10. If the override switch is on, the amp will stay on full power regardless of
the input signal level. The input signal sensitivity can be changed with
resistors R11..R14.

Table 3

IC6 pin

Delay (60 Hz)

Delay (50 Hz)

4

137 s

164 s

13

273 s

328 s

12

546 s

655 s

14

1092 s

1310 s

15

2185 s

2621 s

1

4369 s

5243 s

The power supply is very simple, so you can easily use any available
transformer secondary in the range of 6-9 V ac. Keep in mind, however, that the
ground of the power supply is connected to the input signal ground. Any noise or
hum on that ground will be superimposed to the audio signal. I suggest you use a
separate transformer for the control unit.

Applications

This concept is applicable to all tube power amplifiers. If you can't easily
connect in series the filaments of the stereo channels, you can add a suitable
series resistor, which is then by-passed in full-power mode with the relay
contact. In my amp, this resistor would have to be a hefty 0.5 (/25W unit. The
power consumed in the resistor is wasted, thus the power consumption in standby
mode would increase to about 45...50 watts.

If you don't want to use the automatic switching feature, just replace the
relay K1 in Fig. 1 with a 110 V coil unit and use a separate full-power switch
for manual control. I designed the printed circuit board layout small enough to
fit it inside an amplifier (Figure 3 and Figure 4). I needed to fit a few
resistors vertically. Don't forget to install the seven jumper wires.

I hope these guidelines help you extend the tube lifetimes and also make your
tube amplifiers more practical and user-friendly.

Figures, Tables

Figure
1: Lowered filament voltage schematic.

Figure
2: Schematic of the amplifier timer.

Figure
3: Printed circuit board (scale 2:1).

Figure
4: Stuffing guide (scale 2:1).

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Audio Article List

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ABOUT THE AUTHOR

Jukka Tolonen graduated with a M.Sc.E.E. degree in 1979 from the
Helsinki University of Technology in Finland. Since then he has been designing
medical instruments and industrial measurement and control systems for several
Finnish companies. Currently he works with a small consulting company,
specializing in ASIC (Application Specific Integrated Circuit) design and
demanding measurements systems. His audio hobby started at the age of 15, when
he built his first amplifier. He has been a tube addict since 1992.

Copyright ©1995 Jukka Tolonen

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