30L-1 811A valve

30L-1 811A valve ... No.2 Amateur Radio 811A Service Tips for the 30L-1 811A Amplifier By: Dean Battishill, W5LAJ Silver City, NM The 30L-1 amplifier has been in use from the early '60s. Its users over the following thirty-plus years have included ...
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Times have changed, too: line voltages in general have crept upward, reaching nominally 125 volts, where the amplifiers where designed for 115 volts. In many areas, the nominal 1960 line-voltages were in the vicinity of 110 volts ac. All existing transformers reflect the 115-volt values, and that means, of course, that the 811A's (both in terms of filament and plate supply voltages) frequently are higher than "normal" with today's usual higher voltages. This will result in inordinately high resting current and quiescent plate dissipation. Frequently the resting current in the 30L-1 (unmodified) will exceed the maximum plate dissipation of the tubes. For example: At 1,990 volts on the plates, and without any modification, the resting current was 140 mA, developing 279 watts plate dissipation. According to the 811A data sheets, the maximum dissipation of four tubes is 260 watts ICAS. The following will describe the difficulties encountered in the operation of the 30L-1 as designed, but with current line voltages. It also will cover a number of technically sound solutions, which will be described in detail.

Most modern solid-state transceivers are capable of keying 12 Vdc at 1 ampere. The 30L-1 requires a sub-system capable of operating the T/R line at the negative 170-volt line (at about 75mA). It therefore requires an interposing relay, keyed by the transceiver T/R line. The contacts of the interposing relay then key the (-) 170-volt line of the antenna relay on the Collins. This is shown in one of the schematic diagrams (see fig. 3) as designed for interfacing with the Icom 751A. Other transceivers are similar. Current power amplifiers utilize antenna relay voltages which vary from 12 Vdc to 115 Vac. Make sure that their needs are met.

Whether the amplifier is your own or someone else's after a long lay-off, or during repair, this is "step one." Do these checks:

Today's typically higher plate voltage, as it may occur, can be tolerated easily by the tubes, but the extra plate dissipation can't be tolerated. In the amplifier described here a rather novel design, using several forward-biased diodes in the PTT line, was developed for additional bias to reduce the quiescent plate current to the proper value. The PTT line in the 30L-1 carries total grid current, and the forward voltage drop of the diodes produces about 0.6 volts per diode. On the amplifier described here, it requires seven diodes, or about 4.5 volts extra bias. The required extra bias really is determined by the operating plate voltage at any particular location. It is up to the owner to decide how many diodes to use. The criterion here is to develop 65 mA static plate current. There is an advantage in this design, because high-power Zener diodes are becoming scarce. The forward-biased diodes dissipate practically no power. There is an RCA Phono-jack on the back of the 30L-1, and it proved convenient to place the seven diodes in a short length of shrink-tubing, which are in turn soldered to the mating phono plug. It is shown as Fig. 1.

In Section IV of Service Tips for the 30L-1 811A amplifier (by Collins), there is important information regarding the filament fuse: "The filament circuit on the 30L-1 is fused by a length of number 30 wire in the center tap ground return of the filament winding on T1. The fuse is connected between the two outer lugs of a terminal strip located near R11 in the rear of the power supply compartment. Under some conditions the amplifier may appear to function normally, even though this fuse has blown; however, this causes hum to appear on the output signal. Check for shorts in the filament circuit."

The conduction angle of the 811A's is lowered as the bias is increased, and you will notice that a little more drive will be required, but the amplifier will be slightly more efficient.

The amplifier refurbished here required a set of new tubes. 811A's have been difficult to obtain up to now. The tubes used as replacements are Svetlanas, manufactured in Russia. An interesting sidelight is that four new Svetlana 811A's were more closely matched "right out of the box" than they would be if they were of other manufacture. They also have ceramic bases and ceramic plate-cap insulators, both of which can be seen in the photograph of the 811A on the first page. Svetlana engineers have made certain that the internal alignment and support of the electrodes in the 811A accommodate horizontal placement whenever the design requires it (as they are in the 30L-1), and they have proved to be excellent tubes in both construction and performance.

This amplifier has been modified so as to be "better than new." If the directions given in this paper are followed, yours should be, too.

**The information provided in this application note is intended for general design guidance only. The user assumes all responsibility for correct and safe usage of this information. Svetlana Electron Devices does not guarantee the usefulness or marketability of products based on this material.