Power & Rectifier Tube Notes

Small Signal Tube Notes

Capacitor Notes

Resistor & Transformer Notes

Altec, Acrosound & Dynaco

Eico, Grommes, HK & Heath

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Pederson W-50

This is another example of a Williamson amplifier using 12AU7 tubes. Of chief interest here is the simple implementation of a time delay (in the filament of the rectifier tube, which thus avoids putting high voltage DC across the relay contactors) and the damping factor control.

Not much experimentation has been done lately (by lately meaning in the past 30 years!) with damping factor controls, they may prove to be useful with today's more extended range speakers, or for coping with odd speaker loads. Note that the first two filter capacitors need to be rated 550V or more and that the 278-X transformer does not have a bias tap and a small 115V transformer may be used instead. See other examples in this volume of bias controls.

Pilot AA-902

This is a good example of a small, simple amplifier similar to the Eico HF-86, but using a 12AU7 tube. This can be a very cheap amp to build since the power transformer can usually be scavenged off of cheap used public address amplifiers that usually turn up in Salvation Army stores.

A couple of tricks to improve performance: try replacing the 100 ohm resistor coming directly off the 5Y3 tube with a small choke. Try replacing the 5.6K resistor (the one directly after the 100 ohm resistor) with a 10K resistor bypassed by a 47 volt 5 watt zener diode.

Note the second trick won't work if you unhook the screens of the 6V6-GT for an ultralinear or triode output tube connection. A 5AR4 tube may be used instead of the 5Y3-GT but expect 15 or 25 volts more off the rectifier tube. In that case you may wish to increase the value of the 6V6-GT cathode resistor. The 300 ohm cathode resistor bypassed by the 1000 ohm resistor may be replaced with a single 270 ohm 5 watt resistor. The 470K ohm resistors in the grid circuit of the 6V6-GT's are really a little too high for good practice, the amp will work just as well (and be a little more stable besides) with grid resistors of 100K to 330K ohms.

RCA OTL Amplifier

Power Transformer: None required but we strongly recommend using a large (250 to 500 VA) isolation transformer since one side of the chassis as designed is connected directly to the AC line which not only can be extremely dangerous under the wrong circumstances, but problems interfacing with other equipment, particulary modern solid state gear, can occur with equipment that is not properly line isolated. The isolation transformer solves both problems.

Note that many overseas users will have to use a 220V/240V to 120V isolation stepdown transformer to make this amplifier work, do not try connecting directly to 220/240V mains!

We also strongly recommend running the tube filaments off a transformer rather than off the line as is shown, this will eliminate the possiblity of bizarre failures occurring due to variations in filament to cathode insulation between different tubes. 6080, 6AS7-G, 6AS7-GA can be used in place of the 6082 tubes, the filament voltages are different, frankly the 6AS7-G is probably best as the large glass area of these tubes can dissipate the heat best. It may be possible to use one of the large 6C33 Russian dual triode in place of the three 6082 tubes, we have not tried it, the bias voltages on this unusual tube will certainly be different. The large industrial dual triodes 6528 and 6336 are possible substitutes as well. It may be useful to replace the 56K resistor in the cathode of the second section of the second 12AT7 with a 47K resistor and a 20K or 22K control to facilitate balancing the driver stage. This amplifier will work with 8 ohm speakers, but with reduced power output, probably about 10 watts.

Simple 10 watt Triode Amplifier

6BL7-GT tubes may be sustituted in this amplifier by changing the cathode resistor from 250 to 200 ohms. A single 6AS7-G may be used by changing to one resistor per cathode of 2500 ohms. 2A3's may be used with a single resistor of 600 ohms. For both 6AS7 and 2A3 it may be necessary to supply the driver stage with a more voltage (up to 150V more) the the output which may require an additional power transformer to supply the voltage.

The .02 coupling capacitors between the 6CG7 and the 6BX7's may be increased in value to .1 or .22 uF (400V minimum rating), the .05 capacitor connected to pin 7 of the 6CG7 may be increased to .22 uF.

Stromberg Carlson AP-80

Transformers Required: Power: Any combination of transformers/rectifiers that provide the required voltages, at least 300 ma for the 6550's. The 278-CX transformer used with silicon diodes and capacitor input will provide about 550 volts, the 717 transformer will provide about 600-625 volts using 5U4-G/GB tubes, between 650 to 700 volts using silicon diodes, with capacitor input. The 360 volts required for the screens may be obtained with a 261-E6 transformer used with a silicon bridge rectifier, capacitor input.

This is an example of an amplifier that may be built using Hammond 1650T or 1650W transformers.

Six 6CA7/EL34 tubes may be used in place of the four 6550 tubes listed. Plate current of these tubes should be held to no more than 50 ma per tube for 6550 and no more than 35 ma per tube with 6CA7/EL34. The 1650T and 1650W do not have extra feedback winding shown, if feedback is taken from the 8 ohm tap the value of the 150K feedback resistor should be made 68K ohms.

A good deal of caution should be exercised when testing or troubleshooting this amplifier, the voltages and currents involved are high enough to be potentially perilous to life, even if you don't get killed you can get knocked on the floor pretty easily, especially if large filter capacitors are used. 6550 tubes used at these voltages and in multiple pairs especially, can melt down or crack and shatter if unintentional oscillation sets in or bias voltage is wrong or lost. Frankly, we'd feel safer using the six-pack of EL-34's, which are inherently more stable. Chinese or Yugoslav tubes (other than the Ei 6CA7) are definitely NOT recommended for this project.

Click to see a general purpose tube regulated power supply for preamps & small power amps.

Power & Rectifier Tube Notes

Small Signal Tube Notes

Capacitor Notes

Resistor & Transformer Notes

Altec, Acrosound & Dynaco

Eico, Grommes, HK & Heath

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