Some time ago I finished a built an all tube amplifier system = consisting of a=20 modified version of Joe = Curcio's=20 "Daniel" preamplifier (TAA=20 2/85, p. 7) and a 40 W Class A power amplifier based on the legendary = Williamson=20 amplifier.
I needed a system in which I could turn on the power to all equipment = with=20 onlyone switch, located on the preamp front panel along with other = controls. In=20 principle this is not difficult to achieve with a bundle of extension = cords and=20 a master power switch. With a tube-based power amplifier drawing over = 300 W from=20 the power line, it's inconvenient to switch the system on in the morning = and=20 then switch it off in the evening, which is exactly what I did with my = previous=20 solid state amplifiers.
My tube preamp is on for an average about 40 hours per week and = easily=20 accumulates 2000 hours per year. Fitted with a set of Russian 6922/6DJ8s=20 good for about 10,000 hours, the preamp will probably last for five = years=20 without any tube changes. The power amplifier, on the other hand, might = need new=20 power tubes once a year!
An audio operated power switch to promptly turn on or off a power amp = without=20 extra stress on the tubes would be convenient. Switching on could be = activated=20 by monitoring the input audio signal, while switching off could be = delayed from=20 10 to 30 minutes after no input signal. Since the active use of the = power amp is=20 probably less that half of the total on-time, I would more than double = the=20 lifetime of the power amp tubes with this scheme.
My theory regarding the failure of tubes is as follows:
Convincing? Unfortunately I haven't uncoverd any published = information about=20 the effects of low filament voltage on tube life. My reference books = (Eastman,=20 Fundamentals of Vacuum Tubes; Millman, Vacuum Tube and Semiconductor = Electronics; Valley & Wallman, Vacuum Tube Amplifiers) = ignore=20 the topic completely. Therefore, my "theory" is based solely on my own = reasoning=20 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=20 for the GE's KT66 states that at full rated power the expected life time = will be=20 about 2,000h, and at derated power 10,000h, provided that the tube is = not=20 switched on more that 12 times per day! Trouble is that today's tubes = are (among=20 other things) not what they used to be.
I experimented on my prototype amp to meet the challenge of creating = a fast=20 switch-on for tubes. I attempted to determine the lowest filament = voltage to=20 keep the tubes warm enough for immediate operation after restoring = normal=20 filament and plate voltages (Table=20 1). The start-up time is the interval from switch-on to normal sound = output.=20 Above 2.6V the plate supply voltage turn-on time primarily determines = the delay.=20 If the filament voltage during the warm period is more than 2.6V, the = amp will=20 operate (you can hear the sound) virtually immediately after the = switch-on from=20 a low-level filament voltage to full supply voltages.
The cold current surge is a serious problem in a power amps using = multiple=20 output tubes. For example, the resistance of a cold 6L6-GC filament is = roughly=20 3-4x that of a hot filament. A 6L6GC<= /A>,=20 normally drawing 0.9A, has a surge of about 3A during a cold start. This = becomes=20 even more hair-raising in my 12-tube amp (eight power tubes and four = others,=20 with a normal heater current of 9.6 A), with the surge current exceeding = 30A.=20
In my amplifier the total cold filament current is initially about = 1A, rising=20 to a steady 3A level in two minutes. The voltage drop over the NTC = resistor=20 (Philips type 2322 644 90008 or Siemens type Q63036-S1509-M) was about = 1.2V,=20 leaving some 2.6V over the filaments of both channels. Without the = thermistor=20 the cold current peak is about 8A, dropping to 3.5A in 30 seconds, and = the cold=20 current surge is no larger than the normal full-power heater current. = Even if=20 you can't purchase a suitable thermistor, series connection of the = stereo=20 channels will help reduce tube stress during switch-on.
In the standby mode my prototype amplifier consumes about 20W of line = power,=20 which uses about 175 kWh energy per year, provided the standby power = feature is=20 enabled full-time. At my local power company's current energy rates, = the=20 additional costs is less than $20 per year.
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=20 be fitted inside the amplifier (= Figure=20 2). Alternatively, you can build the unit as a separate add-on in = its own=20 case. See the parts lists in Table=20 2.
The operation is quite similar to "An Audio Activated Power Switch" = in TAA=20 1/84, except a few details. This unit has an initial delay before power = is=20 applied after a cold start. The input buffers feature very high input = impedance=20 to minimize signal source loading. The power switching occurs with a = relay,=20 since a multi-pole switching, such as K1 in = Figure=20 1, is necessary.
After a cold start the unit functions as a delay timer, gently = powering up=20 the tube filaments to a reduced operating voltage and then switching the = amp on=20 to full power. This will take 136s at 60Hz line frequency or 164s at = 50Hz=20 instead of the normal 10s (which is the price you pay for the soft = start). After=20 the power-up it will then act as a full-power/standby-power switch, = monitoring=20 the signal input. Switching from reduced standby power to full power is=20 virtually immediate.
During the initial power-up R1, C7 and IC3B generate a short reset = pulse that=20 clears the counters IC2 and IC6. The flip-flop formed by gates IC5A and = IC5D is=20 also reset at power-up. IC4F keeps the counter IC6 cleared as long as = the=20 flip-flop stays reset. After a 136s delay, determined by counter IC2 = output 13,=20 the flip-flop IC5A/ IC5D is finally set. This enables counter IC6 and = switches=20 on the relay. Thus a warm-up delay of about two minutes is provided = after a cold=20 start.
The dual operational amplifier (IC1) monitors the input signals to = the power=20 amplifier. A noise-free input circuit is essential, since we must not = inject any=20 noise to the input signal. The diodes D1-D4 protect the op amp inputs = from=20 possible switching-on transients of a tube preamplifier. The capacitor = C3 and=20 diode D7 form simple peak-to-peak detector.
On positive peaks capacitors C3 is charged through diode D7. On = negative=20 peaks the entire peak-to-peak value (minus the diode voltage drop), = referenced=20 to VCC, is available at the junction of diode D7 and resistor R16. As = soon as=20 this voltage falls below the logic threshold of the Schmitt-trigger = input=20 inverters IC3F/IC3E, the wired or CLEAR line is pulled low, clearing the = counter=20 IC6.
If the peak-to-peak level of the input signal is above 20 mV p/p, = counter IC6=20 will be repeatedly cleared, thus keeping relay K1 on. With no input = signal, IC6=20 counts output pulses from IC2, enabled with gate IC5C. The frequency is = about 1=20 Hz (actually, 60/64Hz or 50/64Hz). After a time period, the jumper = selected=20 output of IC6 will switch high. IC5C freezes the counter IC6 by blocking = the=20 clock signal. Gate IC5B switches off the relay.
The amp is now switched to standby mode with reduced filament power = and no=20 plate voltage, but the control unit is constantly monitoring the signal = inputs.=20 When the signal level again exceeds 20 mV p/p, counter IC6 is cleared = and the=20 amplifier is switched on to full filament power, now already warm and = ready for=20 immediate operation.
You can change the timings by selecting other outputs of IC6 with the = jumper=20 block J11 (Table=20 3). You can also connect an optional override switch in solder = points J9 and=20 J10. If the override switch is on, the amp will stay on full power = regardless of=20 the input signal level. The input signal sensitivity can be changed with = resistors R11..R14.
| 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=20 transformer secondary in the range of 6-9 V ac. Keep in mind, however, = that the=20 ground of the power supply is connected to the input signal ground. Any = noise or=20 hum on that ground will be superimposed to the audio signal. I suggest = you use a=20 separate transformer for the control unit.
This concept is applicable to all tube power amplifiers. If you can't = easily=20 connect in series the filaments of the stereo channels, you can add a = suitable=20 series resistor, which is then by-passed in full-power mode with the = relay=20 contact. In my amp, this resistor would have to be a hefty 0.5 (/25W = unit. The=20 power consumed in the resistor is wasted, thus the power consumption in = standby=20 mode would increase to about 45...50 watts.
If you don't want to use the automatic switching feature, just = replace the=20 relay K1 in Fig. 1 with a 110 V coil unit and use a separate full-power = switch=20 for manual control. I designed the printed circuit board layout small = enough to=20 fit it inside an amplifier (Figure 3 and Figure 4). I needed to fit a = few=20 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=20 tube amplifiers more practical and user-friendly.
Back to = Glass=20 Audio Article List
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