Power & Rectifier Tube Notes

Small Signal Tube Notes

Capacitor Notes

Resistor & Transformer Notes

Eico, Grommes, HK & Heath

Pederson thru Stromberg

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Acrosound High-Power Williamson Amplifier

The 25K 5 watt bias control called for in this design may be difficult to find. It's quite alright to substitute the bias supply circuit from another amplifier in this volume or rearrange things (provided adequate bias voltage is available) in order to eliminate this part. The 47K matched resistors in the grid circuit of the output tubes may be made a higher value (up to 100K for 6550,KT88 or large bulb 6CA7, up to 220K for "thin bulb" EL34) so long as the 470K matched resistors are increased in value proportionately, e.g., if the grid resistors are made 100K, the 470K resistors should be changed to 1 meg.

Acrosound KT-88 Self-Bias Amplifier

The 200K 2 watt resistor shown should actually be a 4 or 5 watt. The capactiors in the power supply may be made up by hooking lower voltage capacitors in series, the bypassing each with a 200K or 220K 2 watt resistor. A total voltage rating of 600V (instead of the 1KV and 700V listed) should be sufficient. Oil or oil/polypropylene capacitors with a voltage rating of 375 volts AC or better will fill the bill nicely, too. There is a filter capacitor missing between the 100K 2 watt resistor and the two 220K 12AX7 plate resistors. A capacitor between 22 and 100 uF, 500V DC minimum rating, should be installed at that point.

Acrosound Push-Pull Parallel Amplifier

6L6-GC or Russian 5881 tubes should be used in lieu of the KT66 tubes which are generally unavailable. A single 5AR4 tube may be used in place of the 2 5V4 tubes shown. The filter capacitors may be made up in the manner described above. This amplfier may be built using 6V6-GT or 6BQ5 tubes by substituting a lower voltage power transformer. In the case of 6BQ5 tubes the cathode resistor should be made 75 ohms, instead of 150 ohms.

Acrosound 6L6 Tetrode-connected Amplifier

A 5AR4 or 5V4-G/GA tube may be substituted for the 5U4-G tube shown, provided filter capacitor ratings are increased to 500 volts. Silicon diodes should be substuted for the selenium ones specified, or regulated screen voltage may be supplied by using a gas VR tube as shown on the Altec A-333-A following.

Acrosound Triode Amplifier

2A3 tubes may be used instead of the 6B4 tubes shown by using a separate 2.5 volt filament transformer (Hammond part # 166-Q2).

Altec A-333-A

Transformers Required: Power, 282-X. Output, 1618, 1649, or 1650F. Choke, 193-D. Note that this amplifier uses a choke-input power supply! This is a good example of a choke-input supply that may be adapted to other amplifiers. A capacitor-input supply that provides an equivalent voltage may be used, for example using 273-BX transformer. A 5AR4 rectifier tube may be used with the circuit shown, change the power transfomer to 279-X. We would suggest bypassing the 0A3 VR tube with a 47K, 1 or 2 watt resistor.

Dynaco "Classic" Circuit

Note that the 278X transformer does not have a bias tap. Bias can be supplied by using a separate transformer or the capacitive-divider arrangement shown on the Dynaco Modified Williamson amplifier. If 6AN8 or 6AN8A tubes are used we have found the latter day versions work better with a 470 ohm cathode resistor in lieu of the 680 ohm shown. Note that any tube or combination thereof which is roughly equivalent to a 6AN8 may be used, for example the 6SJ7 and 6J5 combination used on the Altec A-333-A. A cascoded dual-triode in lieu of a pentode (example, Grommes 260A) can be used as well. Depending on tube(s) chosen it may be necessary to adjust the value of the 680 ohm and/or 270K resistor in order to maintain the pentode plate (anode) voltage between 80 and 130 volts. Too low or too high of a voltage at that point will not allow the phase splitter to swing adequate voltage.

Dynaco Low Power Amp

A choke with a minimum current rating of 100 ma or more may be substituted for the 100 ohm, 10 watt resistor shown. A 5AR4 may be substituted for the 5Y3-GT tube provided that the filter capacitors have a minimum rating of 400V.

Dynaco Modified Mullard Circuit

A 5AR4 tube may be used in lieu of a 5U4-GB if either (A) a lower voltage power transformer (such as 274-BX) is used, or if amplifier is converted to fixed bias. This amplifier may be converted to fixed bias by using any convienient arrangement to supply a negative bias voltage (see other examples herein), eliminating the 250 ohm cathode resistor and 100 uF bypass cap, and using 1650N output transformer. In the arrangement as shown the EL34 tubes are pushing their limits in terms of plate dissipation. (Some cheaper makes of EL34, especially Yugoslav and Chinese, will probably glow a dull red and not last too long under these circumstances) You can make the tubes run cooler by increasing the size of the 250 ohm resistor to 300 or 350 ohms. The original version of this circuit used a 12AX7 in place of the 6CG7. This may be desirable if additional gain is required. The 62K resistors must be changed to 100K and the 22K cathode resistor to 33K in that case. A number of other lower gain tubes (12AT7, 6AQ8, 5965, 12AY7, etc.) may be used if the gain of the 12AX7 is excessive. The 12AX7 will not split phase as evenly as a 6CG7 and a balance control (say 22K or thereabouts) may be installed between the 100K resistors to adjust for this effect. The gain may be "dialed in" as well by adjusting values of the EF86 plate and cathode resistors. If gain is changed the value of the 18K feedback resistor must be changed as well in (roughly) inverse proportion to the gain change, you may wish to use a control instead of a fixed resistor for flexibility if you wish to experiment a lot. 6550 tubes may be used in place of the EL34's, but only if the amplifier is cathode biased, under fixed bias condition the value of the 240K grid resistors is too high for a 6550 or other beam-tetrode tubes.

Dynaco Modified Williamson

This amplifier is very similar to the Acrosound High-Power Williamson amplifier (not surprising, since both can be attributed to David Hafler). For the most part the same rules apply. This amp has a neat and cheap to implement capacitive-divider bias voltage supply which may be adapted to many other amplifiers. Note that making the .05 capacitor larger will make the bias voltage more negative. 5AR4 tubes may be substituted for the 5V4-G tubes, but if a 278X transformer is used the filter capacitors must be increased in voltage to 550 volts or more. Of course a value larger that 10 uF may be used, up to 50 uF directly connected to the rectifier tube, up to 470 uF to replace the one between the choke and the output transformer.

Power & Rectifier Tube Notes

Small Signal Tube Notes

Capacitor Notes

Resistor & Transformer Notes

Eico, Grommes, HK & Heath

Pederson thru Stromberg

Back to Dusty Files Online