Power & Rectifier Tube Notes

Small Signal Tube Notes

Capacitor Notes

Resistor & Transformer Notes

Altec, Acrosound & Dynaco

Pederson thru Stromberg

Construction Notes

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Eico HF-20

This is a version of the Williamson amplifier using 12AU7/ECC82 tubes, which may be preferable to those who feel than the large octal type 6SN7's sound too dark. 12BH7 (or 6CG7/6FQ7 or 6GU7 but changing the filament connections of the socket) may be used in lieu of the 12AU7 for either or both stages if you want an intermediate effect between the 12AU7's and 6SN7's. A number of substituions may be made for the 6L6-GB and 5U4-GB tubes.

Eico HF-22 and HF35

This is another variation on the Mullard 5-20 amplifier, but with low enough output impedence in the driver circuit to accomodate fixed bias should you wish to implement it. With fixed bias the EL34 tubes may use 1650N output transformer, 6550 tubes may be used with 1650N or 1650K output. Note that phase splitter tubes with higher gain than a 6SN7 should not be used with both fixed bias and either 6550 or 6L6 output tubes, since a higher value of grid circuit resistor (usually 220K to 270K ohms) would be required with higher gain tubes (example, 12AT7 or 12AX7), in order to swing enough output voltage. EL34 (as well as some other "true pentode" tubes) can accomodate a grid circuit resistor up to 270K ohms with fixed bias. Otherwise, most of the same rules apply as with the Dyna Modified Mullard circuit. A 5AR4 tube may be used in place of 5U4-GB provide the filter capacitors are rated for the additional voltage. The preamp power takeoff socket and connections may be omitted.

Eico HF-86

Transformers Required: Power- Any combination of transformer and rectifier that supplies the requisite voltages with at least 100 ma current. We suggest Hammond 272-BX transformer with 5Y3-GT,5AR4 or 6CA4 tube or 270-EX with silicon diodes.

This is a very simple, straightforward design with a driver circuit very similar to that used in the Quicksilver 60 watt monoblock amplifiers. This driver tube arrangement may be used with many other output tubes (6550,EL34, etc) by using a higher supply voltage and reducing the value of the 82K phase splitter resistors, usually to 47K or 22K. The value of this pair of resistors is quite flexible so long as they are matched well, we have seen production examples of this circuit with values as low as 15K and as high as 100K ohms. Note that if tubes other than 6BQ5/EL84 are used the 165 ohm cathode resistor must be changed in value. Typical examples are shown elsewhere in this volume. If 6V6-GT tubes are used we suggest a 250 ohm resistor, and reducing screen voltage to between 250 to 300 volts. Adapting to fixed bias is possible as well, see Eico HF22/HF35 above for further details. A ultralinear screen connection may be used instead of the pentode connection shown. In most cases you will want to use a larger value of coupling capacitor than the .025 uF shown, we suggest a .1 uF with the 330K grid resistor shown, .22 if you use a lower value of grid resistor. Note that the feedback resistor in this amplifier is connected to the 32 ohm tap. Most output transformers do not have 32 ohm taps. If connecting to the 16 ohm tap, make the resistor 22K ohms and the bypass cap 100 pF, if connecting to 8 ohms 15K bypassed by 150 pF.

Eico HF-87

This is another variation on the Mullard 5-20 amplifier, but with a triode front end in lieu of the usual pentode. You should review notes about the other Mullard-variation amplifiers (Eico HF-22, etc) prior to attempting to construct this type, most of the same rules apply, except the notations on the EF86 tubes which are not used here. For full bass response the .1 uF coupling capacitors should be replaced with .22 uF. The 25 uF and 50 uF cathode bypass capacitors can be replaced with larger values, (we suggest replacing with 47 uF and 100 uF respectively) and paralleled with polypropylene capacitors, we suggest .22 uF (50 volts minimum rating).

Grommes 215-BA

This is neat adaptation of the standard Williamson circuit with a push-pull feedback loop and a cathode follower driver circuit with a very low output impedance. The driver circuit has enough "punch" to drive tubes that need a wide voltage swing (i.e. 2A3, 300B, or 211A), and low enough output impedance to drive multiple pairs of beam tetrodes (such as 6L6 or 6550). By increasing the supply voltage for the second 6SN7 (one idea would be to use a choke in place of the 10K dropping resistor shown) and/or using multiple 6SN7's in parallel in place of the single 6SN7 cathode follower shown, even greater voltage swing and/or lower output impdence could be achieved. Different bias voltages can be had by changing the values of the 10K and 20K resistors connected between the anodes of the 6X5GT and ground. Variable bias can be had by installing a control (10K or 22K would work) between these two resistors. More negative bias can be generated by reducing the value of the 68K resistor hooked to the 6X5GT cathode. Several other types of rectifier tube (6CA4, 6X4, 6V4) or a silicon diode could be used in place of the 6X5GT. The values of the 10 uF and 20 uF filter capacitors could be increased considerably. The .1 uF capacitors between the third 6SN7 and the 807's could be increased to .22 or .47 uF. Note that the grounded tap of the output transformer secondary is the 8 ohm tap. This amplifier uses a certian amount of positive as well as negative feedback in order to increase damping factor. A standard feedback arrangement can be had by connecting the zero (common) lead to ground instead of the 8 ohm tap, then omitting the 3.3K resistor between the common and the first 6SN7, and omitting or jumpering out one of the 1K resistors between the cathode and ground of the first section of the first 6SN7.

Grommes 260-A

Transformers Required: Power, 274-BX. Output, 1650K (screen taps not used). Choke, 157R.

This is another variation on the Mullard long-tail pair theme, this one with a cascoded triode front end in lieu of the usual EF86. A single 5AR4 tube should be used instead of the dual 5U4-GB tubes shown (unless you have a power transformer with a 5 volt, 6 amp secondary!) The 274-BX transformer specified does not have a bias tap. Any small transformer supplying between 105 to 125 volts AC may be used in place of the tap. The bias-regulator circuit is very simple and effective and may be adapted to other amplifiers. It is possible to substitute a 0C3 gas VR tube bypassed by a 47K 1 watt resistor in place of the 6L6-GB tube shown, you may want to try it both ways. The damping factor control ( a dual 1 ohm 5 watt part which is hard to find) and the adjacent .5 ohm and 2200 ohm resistors may be omitted, in fact later models did omit these parts. The output stage may be wired as triode or ultralinear (screen-tap) connection, while the 1650K transformer will work we suggest using at 1650N instead. More bias voltage (45-55 V) will be required in those cases, in a few cases you may have to reduce the value of the 22K resistor attached to the 0B2 tube to get enough negative voltage to keep the output tube current within spec (70 ma per tube for 6550, 50 ma per tube for 6CA7/EL34).

Harmon Kardon Citation V

This is an example of an amplifier that may be constructed with the Hammond 1650H output transformer. 6CA7/EL34 may be used with the 1650N transformer, 6550A with 1650K transformer when wired as shown. The 12BY7 tube may be replaced with any tube with similar electrical characteristics, for example 7054/8077, 6AC7, etc., or a different front end may be grafted on from one of the other amplifiers in this volume. The main advantage of the 12BY7 or other pentodes of its genre is a considerably wider open loop gain bandwidth than can be had with typical audio pentodes. The 1 ohm resistors in series with the 12BY7 filament reduce the filament voltage a little so the tube runs cooler, thus reducing hum and noise pickup. A 5 to 5.5 volt DC supply could be used here, less the resistors.

Heath W3-AM

This is the "textbook example" Williamson amplifier with an ultralinear output. 6CA7/EL34 tubes may be used by changing the cathode resistor from 360 to 250 ohms, and changing the feedback resistor/capacitor to 8.2K bypassed by 220 pF (instead of 4.7K/ 350 pF). The 100 ohm balance pot and the adjacent 100 ohm resistors may be eliminated, use a 390 ohm cathode resistor (for 6L6 types) or 300 ohm resistor (EL34), and use matched pair of tubes. 200K (anything from 180 to 220K is fine) 2W resistors should be used instead of the 100K 1 watt resistors bypassing the 20 uF 350V capacitors off the rectifier tube. It would be safer to use filter capacitors with a rating of 500V instead of the 20 uF 450V units shown. Of course these may be made a much larger value if you wish and have space to accomodate it.

Heath W6-A

This is a version of the Williamson with a cathode follower driver that is directly coupled to the grids of the output tubes. This has the effect of extending "headroom" by allowing the grids to be driven somewhat into the positive region on extreme signal peaks. The net effects are that the output tubes clip at the the plate (rather than as usual when the control grids begin to draw current) and more power may be extracted from the amplifier without exceeding plate voltage or plate (anode) dissipation ratings. Thus with the same plate voltage there is more power put out. This driver circuit has low enough output impedence to drive single pairs or multiple pairs of either pentodes, tetrodes (wired as tetrode, triode or ultralinear), or triodes, by adjusting the negative bias to suit the tubes in question. The DC grid circuit resistance of the output tubes is low enough (10K ohms) to suit just about any triode or beam tetrode (such as 300B, 2A3, 6550 or 6L6-GC) that needs low DC grid circuit resistance to be operated in fixed bias. 300B, by the way, will use the same output transformer-about 4000 ohms- as a 6550, and can in push-pull fixed bias put out 40 watts per pair with this arrangement..maybe more if the driver circuit can put out enough voltage. Four 300B or 6550 tubes wired as triodes using the 1650T output transformer should be give 75-80 watts or more, with enogh plate voltage. This amplifier has more gain than the usual Williamson due to the 12AX7 in the driver circuit. This tube may be replaced with a lower gain tube if desired, see the other Williamson amps for examples of 12AU7,6SN7, etc. This reduces the open loop voltage gain by about half, thus the feedback resistor may be cut in half to about 5K bypassed by 100 pF. If triode or triode-connected output tubes are used you may wish to dispense with the feedback altogether. The damping factor control may be done away with by eliminating the 300 ohm/10 ohm ganged control and the low value resistors connected to the speaker socket, then using a 470 ohm resistor in the cathode of the first section of the 12AU7. The .005 capacitors used between the 12AU7 and 12AX7 tubes may be made larger, however it may be necessary to increase the value of the 10K feedback resistor if motorboating occurs. This effect is highly dependent on the output transformer and you may wish to use a variable control in place of the resistor in order to determine the optimum value. We suggest replacing the 20 uF 350V capacitor connected to the plates of the 12BH7 tubes with a larger value and bypassing with a .47 400V polypropylene so the 12BH7's do not get the idea that they are plate loaded at some very high or very low frequency. The surgistor shown is normally only necessary with the voltage doubler shown, if a voltage doubler is not used it may be safely omitted.

Power & Rectifier Tube Notes

Small Signal Tube Notes

Capacitor Notes

Resistor & Transformer Notes

Altec, Acrosound & Dynaco

Pederson thru Stromberg

Back to Dusty Files Online