Estimating power output

[Tubes4e4]

Hi tubelab,

The spec for a 6V6 is 12 watts for the plate and 2 watts for the screen grid this implies that you could do 13 or 14 watts in triode mode.

I would be careful with such implications; just summing up max. Pg2 and max. Pa for total max. Pd in triode mode often doesn´t work out to what arithmetics suggest.

In tetrode/BPT/pentode mode, situations can occur that impose much more Pd on the screen grid - and for that (native) operation mode the max. Pg2 is given. These situations hardly apply when just tying g2 to anode, having it swinging along: The average Pd impose on g2 in triode mode usually is much lower than in tetrode/BPT/pentode mode, since the anode will never swing below g2 potential.

The only safe assumption is, not to exceed the max plate Pd stated for tetrode/BPT/pentode (!) mode when triode-strapping such a tube, or else you very likely will just overload the plate Pd capabilities, since the screen grid doesn´t take its full (rated) Pd share - its Pd capabilities simply are not exhausted.

Ignoring this is especially harmful when triode-strapping bigger TV deflection tubes, which usually have massive Pg2 ratings - suiting circuits, where f.e the anode is at 90V, while the screen grid is at >250V.

Regards,

Tom

 

[Tubelab_com]

I would be careful with such implications; just summing up max. Pg2 and max. Pa for total max. Pd in triode mode often doesn´t work out to what arithmetics suggest.

Yes, I know this and have made several measurements that prove that this is not the case. In a tube with aligned grids, the screen can draw very little power in triode mode. I have measured screen dissipation as low as 1.2 watts in a 6L6GC while the plate was cranking at almost 30 watts. Many current production tubes that should have aligned grids (6L6GC) vary considerably in the alignment quality. Eight identical Shuguang 6L6GC's drew from 2.6 mA to 9 mA of screen current at 450 volts. The grid bias was adjusted to put the plate dissipation at 30 watts in each case. Only the tube was changed in each test.

This shows that you can't simply add the two dissipations together. Considering all of this, I still think that you can hit most any 6V6GT with 13 or 14 watts. I would be gentler on 6V6GA, 6V6 metal, or any rare or vintage tube. Fender blasted many 6V6's with up to 350 volts at about 15 watts. They held up reasonably well.

Any time I set up a tube near (or above as in the 6L6 case) the maximum ratings, I measure the screen current and examine the tube in a dark room. I have found that glowing grid wires will kill a tube quicker than a slightly glowing plate.

Many sweep tubes have a very low screen voltage rating compared to the plate. This often limits their use in triode mode. Often the screen grid limit can be exceeded in triode mode by 200 volts or more (6AV5's), but on some tubes (6CD6's) the tube will go into a runaway condition if the screen limit is mildly (50 volts) exceeded. Often different brands of the same tube will behave very differently in this regard. Careful testing is needed when trying this.

Dsavitsk decided that he will use a 6W6. I have several of these, but have not tried them yet. Next time I have the 6V6 amp hooked up I will stuff a couple of them in and see what happens. I can lower the B+ by removing the input filter cap. I have read that the 6BQ7 is too nonlinear to use as the driver in a SE amp, but I have read the same thing about the 12AT7, and I don't see this with a CCS load. I have used the 6BQ7 in LTP splitters, but not SE, yet.