Slow Start

Not everything in life needs fast start! Some things in life are meant to have a slow start, like the tube filament! The inrush current surge is one of tube killers! 

This solution mentioned here are for DC filament only where one is using the very common 3-pin regulator, LM317 and the likes, to power the tube filament. 

LM317HV regulator

 

Picture above shows a normal / fast start LM317 regulator. As the reference voltage is available almost instantly, the output is programmed to the preset (R1/R2) value swiftly.  

LM117 with slow start

In this slow start configuration above, the inclusion of an extra transistor (2N2905), R3 and C1 makes it sure that the output of the circuit turns ON gradually after an input supply is applied. The output switch on ramp-up will depend on the value of R3 and C1. Increasing the values will produce higher time delays and vice versa. 

Almost all small transistors will work in this circuit. BC327 (pnp) was tried for LM317 and BC337 (npn) for LM337. For LM337, use npn transistor and turn all the electrolytic, instead of pnp transistor for LM317. 

What if I bought readily made modules without slow-start? Shown below is a standard module sold by some vendors online. It has to be modified slightly to have the slow-start function. 

R3 of 50k and pnp + npn transistors need to be added. D7, D8, C5 and C6 pins connecting to ADJ pins have to be disconnected and re-connect to gate pin of transistor. R3 should be added as per the original slow-start schematic. 

It’s not difficult to modify but the ROI is very high.

LM317 regulator without slow start

LM317 regulator modified with slow start

J&K Audio Design
30/12/2013

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Tube Amplifier Repair

Tube Amplifier Repair

Ken's 降龍十八掌 - after learning this kungfu, you should be able to tackle most tube amplifier problems.

Tube amplifier repair is not difficult if one knows how to relate symptoms to the problematic area. This requires experience and know-how plus some luck too. Here are some of the simple diagnoses towards tube amplifier repair.

Hum

• Improper grounding – ground loop
• Poor filament transformer grounding
• Poor filament termination
• Plate and filament winding short-circuit
• B+ voltage filter capacitor – insufficient or capacitor malfunction
• Bias supply filtering – insufficient or capacitor malfunction
• AC filament – voltage too high for AC operation, imbalanced AC feed, DC filament filtering – insufficient or capacitor malfunction

Distortion

• Push-pull topology – imbalance operation on driver or power stage (one tube is aged or damaged), causing imbalance gain = distortion
• Push-pull topology – output transformer winding damaged (open or short-circuit)
• Push-pull topology with pentode – grid resistor damaged
• Coupling capacitor damaged / leak / change in value / short circuit
• Bias runaway
• Input stage headroom is lower than input signal level
• Input signal level causing grid current (power stage not designed for A2 grid current operation)
• Insufficient drive strength or aged driver tube
• Plate load resistor, cathode resistor, and grid resistor value resistor change (damage) causing operating point to shift
• B+ voltage runaway – damaged choke  / filter / regulator / transformer

Tube plate becomes red / exceeds dissipation

• Overloading – output tap mismatch with speaker impedance, short circuit, output transformer primary short circuit, or improper operating point design
• Bias runaway – bias supply damaged, cathode bias capacitor or resistor damaged, improper bias setting, tube socket damage (open circuit)
• B+ too high – power supply damaged, high voltage transformer short circuit, bleeder resistor damage, choke short circuit, load current too low causing B+ to rise.
• Pentode grid voltage too high – grid resistor damage causing excess grid voltage
• High frequency oscillations – improper circuit design, component quality, lack of grid resistors
• Push-pull topology – tube aged, causing imbalance and over-plate dissipation, by either push or pull tube, or one of the paralleled tubes.
• Push-pull topology – partial output transformer winding short-circuit
• Excessive input signal, causing overload
• Improper amplifier design

 Parasitic oscillations

• Feedback circuit – component damage or improper feedback design
• Plate or grid resistor damage, or improper material of grid stopper resistor used (inductance in grid stopper resistor will cause oscillations)
• Excessive B+ voltage – working out of normal operating range

Power on or operation fuse blow

• Check all solder joints to all components
• Check components polarity
• Check if output transformer is short circuit – primary itself, or primary to secondary, or secondary itself
• Tube load short circuit or overloaded
• Change to slow blow fuse – overcome turn on surge

Lower output power, output soften

• Low filament voltage
• Wrong tubes operating point
• Tube aged – check plate current versus bias
• Check tube operating point
• Check tube bias
• Check driver tube drive strength / headroom
• Output transformer short circuit
• Check coupling components – capacitors / interstage transformer if damaged
• If parallel tube, check if any tube is weaken / damaged or not properly connected
• Check cathode bypass capacitors / resistors

That covers most of the issues a tube amplifier will have. If you have more tips, you’re very welcome to add it at the comments section. We will put it in accordingly.

Note: multi-meter, oscilloscope, and other measurement equipment might be needed for the above tests.

J&K Audio Design

29/12/2013

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Output Transformer Design

Output Transformer Design

Would you like to know how to design output transformer in step by step?

1^st, calculate the inductance required.  For higher primary impedance, larger inductance is needed and therefore larger core and / or more turns are needed. This also affects the high frequency response.

               L = Z / (2 * pi * f)

2^nd, calculate the primary voltage and current. If the power (Watt) and impedance are know, then you can use Ohm’s law to calculate the voltage and current needed.

E = sqrt (Z x W)

I = E / Z

3^rd, calculate the secondary voltage and current. Secondary voltage can be calculated from impedance ratio.

               E2 = sqrt ((E1² x Z2) / Z1)

               E2 / E1 = I1 / I2

   I2 = (I1 x E1) / E2

4^th, determine the core size and material. Since the core size will be dependent of the materials used and manufacturer electrical properties, we will not go into details. For small low level unit, material can be 50% or 80% nickel. For others, perhaps 4% silicon steel is needed. One can refer to the manufacturer datasheet that is usually specified for 50-60Hz operation, and de-rate it for 20Hz audio usage. The manufacturer datasheet should provide formula for inductance calculation. Follow that and choose the right core size to meet the inductance requirement. 

5^th, determine primary / secondary wire size. This can also be found from manufacturer datasheet of the wires used. 

6^th – calculate if the core size can fit the wires needed, for both primary and secondary turns included. Primary wire should take up 50% and vice versa. 

7^th – determine the high frequency response. High frequency response is controlled by the impedance and the leakage inductance. The leakage inductance is proportional to the square of the turns. Therefore, the higher the number of turns, the lower the high frequencies limit. The number of turns is already set to get the needed primary inductance for low frequency response. So, it’s a balance between low and high due to the law of physics. 

Leakage inductance can be reduced by interleaved primary and secondary windings. Interleave as in split the windings and wind primary first, the followed by secondary, and so forth. Some examples of interleaving could be 1:2, 3:4, and etc. 

The high frequency limit can be calculated with respect to the leakage inductance and the normalized impedance, as follow:

Zn = (N1/N2)² x Z2 + Z1

F2 = Zn / (2 * pi * L-leakage)

Do note that slopping winding or improper winding implementation will increase the leakage inductance. The windings must be directly above each other. Commonality is called for in every case if the wires cannot fill out the core, like equal spacing, spiraling and etc. No cross over windings is allowed. Good and patient workmanship is required to obtain excellent results. 

That sums up the output transformer design introduction. There are many more steps involved and we shall continue again in future articles.

J&K Audio Design

29/12/2013


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Frequency response measurement

Frequency response measurement

The frequency response can be measured with the circuit below:

Measure Transformer Frequency Response

• Rs = impedance of primary
• RL = impedance of load
• E1 = source signal
• E2 = output signal

E1 voltage must be held constant for all frequencies for frequency response measurement. The output voltage E2 is reference at 1kHz usually, and the rest of the voltage present at other frequencies are referred to this reference value for deviation calculation.

The voltage level of all frequencies (intended range) should lie within +/- 3dB or even +/- 1dB, or to the extreme of +/- 0.5dB or +/- 0.1dB. The -3dB frequency means that it will have a voltage level of 70.7% of the reference frequency. 

The lower limit of the frequency range (bass) is determined by the primary inductance, as mentioned previously. The upper limit of the frequency range (treble) is determined by the reactance of the leakage inductance. 

Please refer to the past articles of the primary inductance versus lower frequency calculation and the measurement of the leakage inductance. 

The impedance of both the primary and secondary windings can be calculated once the voltage level are known too. 

Z1 / Z2 = (E1 / E2) squared

There you have it, basic but useful transformer frequency response measurement. 

J&K Audio Design

29/12/2013

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* Finished amplifiers, DACs, audio gadgets, upgrades and repairs - this is not our core business and we do it out of passion. We do not have fixed models, fixed price and we customize for each individuals. The sky is limit of creativity.  

* Our product lines are always improving and increasing. If you do not see what you want, contact us!  

* Please email for volume discounts, distributor and OEM pricing.