Type 50 tube amplifier
Chandni Chowk, Delhi No. Company Video. Grant Road, Mumbai Shop No. Mumbai, Maharashtra.
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Retro Stereo 50
For many years the KT66 valve has been regarded by many as the hall-mark of a high-quality amplifier whether home made or commercially manufactured. With a total anode-plus-screen dissipation of 28 Watts, when operated with cathode bias, its power output, in push-pull pairs, ranges from the 12 Watts of the original Williamson amplifier , to 32 Watts when used in an ultra-linear output stage.
With this valve it is therefore possible to build amplifiers having higher power outputs suitable for public-address equipment and high-quality sound reproduction in general. Due to the lower anode impedance of the new valve, its higher power output is obtained without increasing the HT voltage requirements beyond the limits of normally available components.
For example if plugged into a KT66 ultra-linear output stage giving 32 Watts, a pair of the new KT88 valves will give 40 Watts with a corresponding increase in drive voltage. The maximum power obtainable from a pair of KT88s with cathode bias is slightly over 50 Watts with a HT line voltage of Volts. This article gives details of the design and construction of a 50 Watt power amplifier using KT88s. A new pre-amplifier suitable for use with this amplifier will be described later.
The two units have been designed to offer an improved performance and range of controls compared with previous designs, yet they include no complicated networks or unusual components, and are comparatively economical to construct. They will reproduce from radio tuner, any magnetic or crystal gramophone pickup, microphone, or direct from a magnetic tape replay head. A rotary switch selects the input circuit required and at the same time adjusts the sensitivity and frequency correction for tape or disc recordings.
The pre-amplifier is separate from the power amplifier to which it is connected by a flexible cable; its controls are similar in function to those on the GEC amplifier , but the operation of the treble slope and presence controls has been improved, and a rumble filter is incorporated.
The circuit of the power amplifier is shown in Fig. It contains a pair of KT88s connected in an ultra-linear output stage, a push-pull low-impedance double-triode driver stage using a B, and a high-gain B first stage incorporating phase splitting. Overall feedback of dB is used, and the input sensitivity is about 0. The Volt HT supply is provided by a U52 , and the electrolytic smoothing capacitors are protected by the use of a thermistor against excessive voltage during the warming-up period.
The ultra-linear connection for output tetrodes and pentodes has become popular during the past two years. As will be seen from Fig. Its advantages are that it gives a maximum power output at least equal to that obtained from the pentode connection, with distortion similar to or less than that for the triode connection, which gives less than half the power output.
For equal power output, the distortion from an ultra-linear output stage is about half that for a triode stage using the same valves. The ultra-linear connection also provides a low output impedance, roughly equal to the load, and a good damping factor is, therefore, easily obtain- able when feedback is applied.
A push-pull output transformer is required which has each half primary tapped 40 per cent turns ratio from the HT end. Leakage and inductance requirements are discussed later. The use of a push-pull pair of triodes for the driver stage was chosen so that the output stage would be symmetrically driven, and that no unbalanced operation would occur even at the onset of grid current in the output valves during overload.
The removal of the phase splitter to an earlier stage ensures that the time constants in the grid circuits of the output valves are the same. The B is used in this stage because it has a low anode impedance, about 10, Ohms. With this low value of driver impedance the phase shift due to the input capacity of the output stage is relegated to frequencies above 50 kHz, and this, combined with the symmetry of the circuit, greatly assists in ensuring freedom from HF instability when feedback is applied overall.
A high-gain first stage B is used to provide good balance in the phase-splitting circuit, and also adequate overall sensitivity after feedback is applied: the phase-splitter circuit used is one in which the input to the grid of the second or inverter triode is automatically balanced against its stage gain.
This circuit gives a push-pull output from the two anodes of the B, and as the amplifier is truly push-pull from this stage through to the output transformer little h. The push-pull output from the B stage is balanced to about 2 per cent, a high-gain stage being an advantage here. This balance is improved slightly by the use of an un-bypassed common bias resistor in the cathode circuit of the B driven stage. This degree of balance is very satisfactory for many purposes, and with close-tolerance cathode bias resistors the KT88 valves used in designing the prototype amplifier have given a consistently symmetrical output voltage waveform when driven up to full power output when the peaks just show flattening due to the onset of grid current.
However, it has been found on amplifiers of this type with unmatched output valves that minimum distortion is obtained when the push-pull drive is adjusted so that both output valves reach the onset of grid current simultaneously as the drive voltage is increased. Where facilities are available, and it is desired to make this adjustment, alteration of the balance of the push-pull drive is obtainable by relative adjustment of the two anode loads of the B, and accordingly a pre-set variable wire-wound potentiometer R 3 , is shown in Fig.
The waveform of the voltage across the secondary of the output transformer can be observed on a cathode-ray oscilloscope connected across a dummy load resistance, and R 3 , should be adjusted so that with a sinusoidal the input voltage of suitable value the output waveform shows similar flattening on both positive and negative peaks. Although judged visually, this adjustment can be made with more than sufficient accuracy, provided the input waveform is free of second harmonic distortion. To avoid phase effects a frequency is chosen between and 2,Hz.
When feedback is to be applied over an amplifier it is desirable that it is truly negative feedback over the whole frequency range that will be fed to the amplifier. At frequencies outside this range the feedback should be either negative or inoperative. If this is not so, the final frequency response of the amplifier will show peaks. Further increase of feedback, or in borderline cases certain types of input signal, will produce oscillation at these peak frequencies.
Even if oscillation does not occur the amplifier will ring at these frequencies; that is, when an input signal containing the peak frequencies is interrupted the output from the amplifier will not cease as abruptly as the input, the peak frequencies persisting, with a more gradually decaying amplitude. The peak frequencies usually occur at very low or very high frequencies, and are due to phase shifts in the inter-valve coupling circuits and in the output transformer itself.
The low-frequency peak occurs only when feedback is applied, and is due to the combined phase shift of the inter-valve coupling capacitors in conjunction with the associated grid leaks, together with the phase shift of the output transformers primary inductance in conjunction with the load and valve impedances. The peak in amplification commonly occurs well below 20 Hz and often results in low-frequency instability motor-boating when a pre-amplifier is connected to the same HT power supply.
The effect is reduced if the several phase shifts are arranged to occur at differing frequencies, for example in the circuit of Fig. Complete or nearly complete avoidance of a low-frequency peak can best be obtained by reducing the gain of the amplifier before feedback is applied at the frequency at which the peak is expected, without introducing additional phase shift at this frequency.
If a flat frequency response is required down to this frequency, then the reduction in gain should approximately equal the feedback to be applied. This is achieved by inserting a small coupling capacitor shunted by a high resistor, so that with the following grid leak the gain is reduced as the signal frequency is lowered until at the very low frequencies where a peak is expected the gain is reduced by a substantially resistive potential divider with very little phase shift.
For a 20 dB gain reduction the shunt resistance should be ten-times the grid leak. The capacitor should be sufficiently small to have an impedance at the very low frequencies equal to or higher than the shunt resistance. As the amplifier is push-pull throughout such a circuit has to be incorporated on each side, and on one side, in Fig. The valves chosen will give low-frequency stability with any output transformer capable of delivering the full power output down to 40 Hz. An advantage of the inclusion of this type of stabilizing circuit is that there is no tendency for the amplifier to motor-boat when the pre-amplifier is connected to the same HT line.
The smoothing used in the pre-amplifier supply is therefore economically chosen solely to give sufficient ripple reduction. At the high frequencies peaks may be detected in the response of most amplifiers when this is measured up to kHz or kHz before feedback is applied.
These peaks are due to resonances in the output transformer, the most important of which is the series resonance of the primary leakage inductance with the primary winding capacitance. This is commonly the cause of the first peak, i.
The response usually shows a general downward trend, and this is due to the total shunt capacities, including Miller effect, across each anode load in the amplifier. When feedback is applied the combined phase shifts due to shunt capacities and leakage inductance cause the peaks in the response to be exaggerated, and often rise above the mid-frequency level. With the output transformers used in designing the prototype amplifier the leakage inductances between the several windings were low, as described later, and the first high-frequency peak was detected about kHz.
Accordingly a stabilizing circuit, similar in principle to that used at the low frequencies, is incorporated. This consists of a shunt capacitor connected across the anode impedance of the first valve, with a series resistance to limit its shunting effect to about 20 dB and minimize phase shift at frequencies above 50 kHz.
In Fig. These values are sufficient to give stability when the amplifier is loaded capacitively, and to reduce ringing on a square wave input 10 kHz repetition rate to only about 10 per cent overshoot on a resistive load, and even less on an inductive load.
The use of capacitors for improving stability across any portion of the output transformer is not recommended in presence of the above stabilizing circuits, and was found merely to lower the resonant frequency, which was undesirable, and in some cases increased overshoot.
The use of such capacitors depends on individual transformer design, and is not suitable in this context. No reactances giving phase correction are included in the feedback network itself R 11 , and R 4 because the correct choice of reactance is critically dependent on the type of load and output transformer used.
For example overshoot, or actual instability, using a given transformer and dummy load resistance can be greatly reduced by shunting the feedback resistance R 11 , by a critically chosen value of capacitance, but this will be found to worsen stability on a reactive load such as a loudspeaker.
This behaviour is common to all feedback amplifiers, and the stabilizing circuits here incorporated within the amplifier itself give satisfactory results with a wide variety of loads, and with the several transformers used in testing the prototype.
Greater stability could be obtained with inferior output transformers by altering the capacitances in the stabilizing circuits so that the level frequency response of the amplifier, before feedback is applied, is further restricted.
The level frequency response at high and low frequencies will be restored when negative feedback is applied, but the amount of feedback difference in gain with and without feedback will be so reduced that the overall harmonic distortion at high and low frequencies will be considerably higher than at mid-frequencies.
In addition the valve preceding the stabilizing circuits handles a higher signal voltage at high and low frequencies, and extra distortion may occur here as well as the distortion inherent in using a poorer output transformer.
The stabilizing circuits shown in Fig. The components values have been found satisfactory for use with a typical minimum transformer, but are primarily intended for use with a transformer of the type described below. The reduction of feedback at 40 Hz and at 10 kHz amounts to some 6dB less than the dB feedback at mid-frequencies.
Although originally intended for operation with valves of lower power output, it gives a very good account of itself with the KT88 from 40 Hz to 20, Hz. Another transformer tried with excellent results was the Savage Type 4N1.
For its extra size and cost this would deliver the full power output down to a lower frequency than the WO The requirements for an ultra-linear transformer to be used with feedback are adequate primary inductance and low leakage inductances between primary and secondary as normally connected , between each half primary, and between each half primary anode-tapping and the associated screen tapping.
The primary winding capacitance must also be low enough to relegate the lowest high-frequency resonance to the region where a reasonable stabilizing circuit has reduced the gain of the amplifier without appreciable phase shift. Both the transformers mentioned gave measurements of all five leakage inductances less than 6 mH, and a high-frequency resonance in circuit operation above kHz.
The WO achieves this by the use of gain-oriented silicon iron, with moderate sectionalization of the windings, and the 4N1 achieves similar figures with a larger core of normal silicon iron by more sectionalization of the windings.
The accompanying photograph shows the underside of the power amplifier chassis. The prototype was constructed on a chassis measuring 14in x 9in x 3in. The mains transformer was of ordinary silicon iron, but the choke and output transformer were of grain-oriented silicon iron and were therefore comparatively small. A slightly larger chassis would be needed if larger transformers were used, but the same layout must be used, and the transformers positioned as in the top view of the amplifier.
Because of the high HT voltages it is advisable to mount the transformers tags down ; the elongated holes required can easily be cut with a valve hole-cutter and file. The heater wiring should be laid in first, with twisted twin wires laid along the bend of the chassis. The valve-holders are oriented to avoid the heater wires crossing the grid wiring. The second heater supply to the octal pre-amplifier socket connection should also be laid in.
Both supplies must have a centre-tap earthed to chassis, or an artificial centre-tap using two equal resistances, as shown. An earth point should be chosen next the first valve B, and a star tag bolted down with a serrated washer to ensure good contact. This will be the one earth point to which all grid, anode, and inter-valve coupling circuits must be connected by insulated wiring.
The signal input pin 8 on the octal socket should be wired as directly as possible to the grid of the first valve; the earth pin 1 on the octal connected to the star earth tag, and the grid leak connected. The cathode bypass capacitor with feedback resistance R 4 , in series should be connected between the cathode pin and the star earth tag using the smallest total loop area of wiring possible, and keeping the cathode circuit as close to the grid input lead as possible.
The cathode bypass capacitor of the second half of the B should be wired in an equally compact fashion. The grid of this valve is fed from the phase-splitting network connected between the two anodes, and this should be wired as compactly as possible consistent with good mechanical location of the components.
The tag-board is used for all the smaller components, but the larger coupling capacitors and the later cathode bypass capacitors are mounted by standard clips on the side of the chassis.
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There are a number of things to consider when looking to get your hands on a cheaper tube unit. The vocabulary is the same as the world of premium tube amps — with the preamp and power amp tubes, the total output wattage, the number of channels and the overall control you have with EQ, preamp gain and so on all affecting how the amp works and sounds. Cheaper tube amps tend to offer lower output wattages — but this is by no means a bad thing. More wallet-friendly amps are also more likely to feature just one channel, rather than separate clean and overdrive channels. A general rule of thumb is that tube amps need three times fewer watts to achieve a similar volume. So if you like how loud your watt solid-state amp is, then for similar dB results shoot for a watt tube amp. This means that a watt amplifier is twice as loud as a 1-watt amplifier, and a watt amplifier twice as loud as a watt amplifier. A modern iteration of the amps that started it all back in Chicago during the 50s, the Supro Blues King 8 includes an all-tube signal path 12AX7 and 12AU7 , vintage-style cabinetry and a custom eight-inch speaker.
Direct Heated Triodes
Question: What is a vacuum tube? Answer: A vacuum tube is an electronic device consisting of a minimum of four active elements: a heater filament , a cathode, a grid and a plate, all sealed in a vacuum glass enclosure to prevent parts from burning. Once heated, the cathode begins to emit electrons, which flow from the cathode which is negatively charged toward the plate which is positively charged. Question: What is a JAN tube?
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Often too much weight i. In fact, dare I say that in many cases — for example a reissue amplifier — a change to a higher-quality output transformer will have a greater impact on improving the overall tone than playing with different brands of tubes. Most commonly, there are two large transformers in an amp — The other besides the output transformer is called the power transformer — that is the one that is responsible for taking the wall current and transforming it into the DC power used by your amplifier. In fact, output transformers used to be thought of as simply items to be replaced when they failed. However, I can attest personally that in many instances a change in the output transformer to a quality unit as an upgrade can make a huge difference in tone — especially with those popular reissue Fender, Marshall, and Vox amps currently being made. The test-dummy amp used in the first case was a stock Marshall JMP 50 watt model Lead model — it had recently blown its output transformer after a tube had shorted a connection between two pins of the tube socket so it was the perfect candidate.
Single-Ended (SE) 50EH5 Tube Amplifier
Type 50 Tube Amp. I would like to build a type 50 tube amp. I am not the most technical but I have a friend who is retired and an very good tech, he has agreed to assist me with the build. I have a lot of gear here, but nothing I have built myself. My reference setup includes Shindo separates and I have a few choices for speakers most are high efficiency. I came across this circuit and was wondering everyone's thoughts? Anyway, thoughts on the above circuit? Anyone have any other the 50 tube SET circuits to consider?
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Most of our OPTs can be configured for different primary and secondary impedances. The use of C-cores with well-defined air-gap results in an almost constant inductance throughout the whole operating voltage range. Most of the transformers below are available with amorphous iron C-cores. From a purely engineering point of view, we cannot justify the fairly high price of our amorphous core OPTs. But we have actually received a lot of praise for their sound quality, such as this report. When considering testing an amorphous core transformer, please note that they are said to require much longer break in time than what is normally required for silicon-iron cores.
All work that requires the replacement of valves or biasing should be carried out by a qualified engineer. What do the different valves in my amplifier and combo do? Most of our units contain a mixture of valves responsible for preamp or the power amplifier. The number and type varies from one amplifier to another.
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