[Heathkit] DX-60 Final Amplifier Tube Question

[Drew P.]

Glen Zook wrote:

> Both the DX-40 and the DX-35 heavily load the grid of the
> 6146 so as not to require neutralization. ?The 6146B/8298A
> and 6146W are more likely to work than in some of the other
> "boat anchor" transmitters. ?If you can't locate an earlier
> version of the 6146 family including the 6293, then I would
> say to "try it". ?If the final "takes off", then you know
> that your particular DX-40 doesn't like the 6146B/8298A or
> 6146W. ?Component tolerances seem to play an important role
> as to if the later tubes can be used or not. ?If the
> tolerances add "just right", there are no problems.
> 
> The last time I looked, which was a couple of months ago,
> several Internet sources had 6293 tubes for around what they
> were charging for 6146A/8298 tubes.

Does the 6293 cause the same problems as the 6146B sometimes does?
And, is the 7212 more like the 6146/6146A or is it more like the 6146B?

I have an SB-101 having 6146B installed (I didn't do it).  Even with careful adjustment of neutralization using an RF voltmeter on the output, the final amplifier is still sometimes unstable when attempting to load into loads not having a very low SWR.  The final will sometimes oscillate when attempting to load into a 100 watt light bulb.  The 6146B's are "Penta", and show a date code for 1993.  The workmanship is poor - one tube is 1/4" shorter than the other, on one the envelope leans quite noticeably to one side; on one, the electrode structure appears to be crooked and not square to the mica.  These reputedly were made for Penta by Shuguang.

I lately has the rig loaded into a non-reactive 50 ohm dummy load, showing a mere 100 mA of plate current.  I turned my back for a minute, and when I looked to the radio again, the plate current was 400 mA, the output was way up, and one final tube plate was glowing an ominous orange, the other was dark.  Before I could get the rig shut down, the incandescent tube flashed over and blew the fuse in the HP-23 power supply.  I guess it's time for new final tubes.  I like the idea of installing 6883/6883A.

Drew