 |
 |
| A pair of 6JS6C Colour TV valves | The 'heavyweight' 6146W valve |
OBSERVATIONS FROM SCOTLAND 30_June 2008 GM1SXX
Restoring valve radios.
Many of the 'hybrid' radios of the 1970's used TV sweep valves in the PA stages. In particular, the 6JS6C was used in many Yaesu radios along with a 12BY7A driver. These valves are long obsolete, having been intended originally as the 'sweep' or line timebase amplifiers in early hybrid technology colour TV sets. With the advent of solid-state colour TV's and therefore no primary market to serve, the manufacturers 'pulled the plug' on the production of these valves thereby forcing the radio manufacturers into using PA transistors instead. The remaining stock of valve (tubes... or 'toobs' in the US) left lying on shelves in TV repair shops and in stock are the only ones you are ever likely to see. As a consequence, these valves are no longer cheap or plentiful and bidding on the remaining stocks is 'competitive' to say the least.
|
|
|
| A pair of 6JS6C Colour TV valves | The 'heavyweight' 6146W valve |
The 6JS6C, 6KD6, 6LF6 and other large all-glass TV valves were never intended for use in radio transmitters. Their characteristics are not ideal ... they were designed for LF Amplification duty ... and their construction is 'lightweight' compared to small purpose made transmitting valves such as the 6146 and it's relatives. The 6146 is an octal valve and so doesn't physically fit the valve holders used in the Yaesu amateur radios, but the radios *can* be modified to accept the 6146B valve. These valves are more robust and more available than the TV sweep valves, so for the restorer wishing to get a 70's hybrid transceiver back on the air, they are worth considering. The 6146B is still manufactured in small quantities by the Chinese and sold under various brands. Golden Dragon is one such brand while the actual valves are made by the Shuguang factory in China. I should mention that not all 6146's are born equal. There is the 6146, 6146B and 6146W The 6146B seems to be the only one still in (limited) production and makes a suitable alternative to the 6JS6C with a few circuit changes and a change of sockets. If you are not a confident constructor please have someone else do the work for you! The circuit changes required and not too difficult but be sure to use capacitors of suitable voltage rating. The 6JS6C has different inter-electrode capacitances to the 6146B so some circuit changes need to be made.
I first read of replacing the rare and expensive 6JS6C's with 6146B's in an article in Practical Wireless. It's not rocket science but you do have to be methodical and document which connections go where and to adhere to good RF practice. If you can, take digital photos during the work to serve as a reference point. I've since discovered a posting on a BB of some material from QST magazine. It's in two parts and shows what's involved in the process. The pages are in PDF (Acrobat) format.
I haven't modified any of my own radios (yet!), but when I do, I'll be converting to the 6146B.
 |
| 12BY7A Driver |
While I'm rambling on about PA valves, I should mention something interesting I've noticed about the 12BY7A's used in these radios as a driver. These never seem to last very long and a typical life for one of these valves is around 5 years of service. By contrast, when used as TV distribution amplifiers, they last a very *long* time indeed .... well over 10 years of 24*7 operation was usual!
I'm sure I now know why they are so 'unreliable' in amateur service. Their quoted absolute anode rating is 250V DC. I've never come across one in amateur service that had anything like that little voltage on it. My FT102 has well over 300V on the driver's anode. My FT200 is not very different. These valves were designed for use in TV distribution amplifiers where reliability was a major consideration, so it seems that Yaesu were asking far more of these valves than is reasonable. This could be why they don't last very long. I've also noticed that every 12BY7A I've tried, and that's a fair number, glows faintly blue inside when drive is applied. Not like the harsh dark blue glow around the support structure and inside the glass that indicates a soft or gassy valve but a pale blue glow inside the valve's structure. You can only see this in a darkened room. It's only visible when the valve is in conduction. My best guess is that it's caused by the ionisation of stray gas molecules inside the valve. It's not possible to make a perfect vacuum, so there will always be *some* residual gas molecules inside. You can have a good 12BY7A that tests perfectly on a valve tester yet it will glow internally. Weird! I'd be willing to bet that if the anode voltage was reduced to 250V or less, the glow would disappear and these valves would last a lot longer. I'm sure that you'd get less drive also. After all, these devices were intended for TV use and not as a transmitter PA driver.
The datasheets for both the 6146B and 6JS6C valve types are available here in PDF format.
73 Al.
GM1SXX