Tuesday, November 29, 2011

Essential information regarding 6146 tubes by K9STH

The 6146 family of tubes has been used in many home-brew commercial and transmitters over the years. The following information explains important points to look out for when replacing these tubes or home-brewing your own 6146 based transmitter. Normally I don't just copy and paste an entire article but this information should be included in every 6146 based rig's instruction manual, in bold type, at the start. KF5CZO

6146/6146A/6146B/6146W 8298/8298A/6293 
*6883/*6883A/*6883B *8032/*8032A/*8552 
Tubes Explained 
(*12 volt filament)

The 6146 and the 6146A differ in the type of heater (filament) and can be "mixed" in use (i.e. one 6146 and one 6146A in a DX-100). There are no other differences. The heater in the 6146A was developed by RCA and is called a "dark heater". The type 8298 is the same tube as the 6146A.

The 6146B is the same tube as the 8298A. The 6146B is a different tube from the 6146/6146A/8298. Sometimes they will work in place of the 6146/6146A, but often they will NOT. This is due to the different bias requirements of the 6146B, different inter-electrode capacitances, among other things. Often it is impossible to either neutralize the final in a rig designed for the 6146/6146A or the neutralization does not "hold" for very long (often less than an hour).

The 6146B/8298A have a very bad habit of producing VHF and UHF parasitic oscillations which can cause all sorts of problems including TVI as well as the tube literally "burning" itself up. This is when they are used in circuits that were designed for the 6146/6146A.

Collins Radio had to modify production of the later S-Line units to allow the military to use 6146B type tubes. This required a redesign of the neutralization circuit which is in place in the later 32S3, 32S3A, KWM-2, and KWM-2A units. The earlier versions of these models must use the 6146/6146A tubes. The later version can use all three types.

There is a "pulse" tube that is a very heavy duty version of the 6146 and can be substituted without any problems. This is the 6293. A 6293 will outlast a "plain" 6146 by at least 5 times the life (over 10 times is not that unusual!). We would "kill" back in the late 1950s and early 1960s to get our hands on a pair of 6293 tubes for our DX-100s, etc.

The 12 volt equivalent of the 6146 is the 6883. The 12 volt equivalent of the 6146A is the 6883A/8032 and the 12 volt equivalent of the 6146B is the 6883B/8032A/8552. The same thing is true of these tubes, do not replace the 6883/6883A with the 6883B series of tubes.

Back in the late 1970s Motorola tried to replace all of their 6883A/8032 tube stock with 6883B/8032A/8552 type tubes. A very large number of Motrac units used the 8032 and only the very "latest" versions used the 8552 tubes. At the time I owned the Motorola reconditioned equipment center for the south-central US and we used, on average, over 100 8032 tubes a week. Just as soon as Motorola replaced the 8032 with the 8552, we started having virtually all of the Motrac units that were shipped to the customer arrive with the tube envelopes "shattered" by "normal" shipping. We had never had this happen before.

The construction of the Motrac is such that the tubes cannot be seen when the unit is assembled. What was happening is that in the 2 to 5 minutes that the radio was being tuned and final QC'd, that they were oscillating at UHF (parasitics) and the tubes were getting so hot that the glass envelope was destroyed!

We told Motorola what the problem was. However, they refused to believe us until they had well over 1000 warranty complaints from their service stations. It cost them quite a lot of money in warranty repair bills before they again started placing 8032 tubes in the boxes marked 8032!

I have quite a number of "boat anchor" rigs that use the 6146/6146A type of tubes including Collins 32S1, 32S3; Heath SB-110A, SB-401, DX-100, DX-35, Apache. I have owned rigs like the Knight T-150 and T-150A and others that were designed for the older tube. Frankly, all of them are much "happier" with the 6146/6146A instead of the 6146B.

Now, for the 6146W: Unfortunately, some of these are "ruggedized" 6146A tubes and the later ones are 6146B equivalents. The only way to tell is by the manufacture date on tubes by a particular manufacturer. Unfortunately, to my knowledge, there is no master list giving this information. Each manufacturer changed from the 6146A to 6146B construction in their 6146W at different times.

I haven't seen a notice from Heath about using the 6146B tube. However, several manufacturers, including Collins Radio, originally said that it was fine to use the newer tubes. But, after a very short period of time they found out different!

Thus, I would be very careful about using the 6146B and the 6146W tubes in place of the 6146/6146A. You might "get away" with it. However, you might also do some damage to your final amplifier section. If you decide to try the 6146W, then be sure and neutralize the final and check it after operating for an hour or two. If the neutralization remains OK for several days, then you should be "home free". But, if the neutralization changes, then you need to replace the 6146W tubes with the 6146/6146A types.

Glen, K9STH
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