Power & Rectifier Tube Notes

Capacitor Notes

Resistor & Transformer Notes

Altec, Acrosound & Dynaco

Eico, Grommes, HK & Heath

Pederson thru Stromberg

Notes About Small Signal Tube Substitutions Back to Dusty Files Online

There is a wide variety of substitutions that may be made depending on the circuit used.

There are three major varieties of circuits used with small tubes.

First, grounded cathode. This is by far the most typical. Tubes may be substituted with other tubes with a similar mu, aka amplification factor, with certain limitations. Adjustments to plate(anode) resistors and cathode may be necessary to maintain the maximum voltage output (usually abbreviated Eo) and gain required. See the resistance coupled amplifier charts included for typical examples.

Second, grounded plate, or cathode follower, and split-load phase inverters. These are similar for substitution purposes, since neither circuit has gain of more than unity (1).

Cathode followers are used as buffers or impedance converters where a low output impedance is needed but the the preceding stage needs a light load to amplify properly.

Split-load phase inverters split the signal between the plate and cathode, each side being 180 degrees out of phase. Generally, low mu triodes are always used, since these tend to give the lowest output impedence and split phase more evenly than higher mu types. Substitutions should be made on the basis of similar plate resistance and DC plate current. If tubes with a lower plate resistance are chosen, the resistors connected to the plate or plate/cathode can be made a lower value to take advantage of the lower output impedance available.

The third groups are the cascoded triodes and pentodes. These are grouped together because for malking gain calculations they act similarly. A cascoded triode is made by wiring two triode sections in series, then taking the signal off the plate (anode) of the top tube.

The gain of a cascode or pentode with no load is approximately the transconductance times the plate load, either

(plate load X micromhos) / 1,000,000 (if you're in North America)

or

(plate load X (ma per volt) ) / 1000 (if you are anywhere else)

thus a pentode/cascode with load = 200K and 1,000 umho (1 ma/v) has gain of approx 200.

(See resistance coupled amplifier charts for examples, or click here for a simple method of making gain calculation for triodes or pentodes) Thus substitutions should be made mostly based on the transconductace of the tubes, not mu factors. The mu factor on triodes will affect the biasing (value of cathode resistor for the same plate current), but not the gain of the circuit, all other things being equal. The disadvantages of the cascode/pentode arrangement are

(a) the condition of the tubes affects gain much more than in grounded- cathode (straight triode gain stage) circuits, thus the arrangement is not practical unless feedback is applied or closely matched tubes are used,

(b) output noise varies with the gain, and

(c) the output impedance is high relative to the other types of circuits and therefore the load presented at the following stage will affect gain, maximum output, and distortion much more as well.

Often pentodes/cascodes are directly coupled to the grid of the next stage (for example, Dynaco) to minimize distortion. Note that types designated as remote-cutoff or variable-slope should be avioded as these types are non-linear (they are intended for automatic gain control in RF and audio)...altho some people have has reasonably good results with certain substitutions, eg: 6ES8/ECC189 for 6DJ8/ECC88, 6922 or equivalent in low level circuits.

Power & Rectifier Tube Notes

Capacitor Notes

Resistor & Transformer Notes

Altec, Acrosound & Dynaco

Eico, Grommes, HK & Heath

Pederson thru Stromberg

Back to Dusty Files Online