Poor little 8CG7

[Diablo]

I've got a bunch of 8CG7 tubes, and trying to find out what they're good for. I see where they are identical to the 6CG7, except for 8.4 volt heater vs 6.3 volt. Who the heck makes a transformer with an 8.4 volt winding? Could I use the 8CG7 with 6.3 volts supplied to the heater - for the reverb driver in an Ampeg V4? Or will it sound bad or not work at all?

[Enzo]

Plug it in and try it.

The tube is a 6SN7 in a different form.

No one makes an 8.4v transformer. These tubes, both 6CG7 and 8CG7 have an 11 second heater warm up, and they were designed for series wired heater strings as used in TV sets of the era. That is the case with many of the odd voltage heater tubes. You ran multiple tubes with their heaters in series, the voltages simply add up. You get 120v worth, and you can power it right off the line, without a transformer. Providing the tube in two differnt voltages allows more options with your tube lineup.

[Diablo]

Thanks Enzo, very interesting. I'll give it a whirl in the V4 and see what it does. Now, you've got me thinking of how many tubes I need to add up to make 120V in series....

[Rob Mercure]

When you rectify and filter AC to DC you wind up going from the RMS AC voltage to the AC peak in DC - whatever AC RMS voltage you started with times 1.414..... Now once you load this the voltage will drop but I've run 8BQ5s off 6.3 volts rectified and filtered this way and with some trannies I get right on the voltage or even higher - worth trying. Udderwise you could use a 12.6 VAC tranny and dropping resistors or a DC regulator set up or, if you've as crazy as me, you can save odd transformers from monitors and receivers and such and with a good enough junk box can often find one with the voltage you need - or one that you can series a couple of tubes to meet - 2 tubes = 16V - etc., etc.,

Rob

[Diablo]

Thanks Rob for weighing in on this one. I stuck it in my V4, and it works just fine. Now I've got enough reverb tubes to last my for 200 years....

[6267]

Try 6CM7, 6FD7, some of the dissimilar ones might drop in or be easily adapted in circuit. Many of these are considered "dollar" tubes, and some might have better plate dissipation and resistance than the original 6cg7.

You can do this, and it'll work, but at the expense of tube life. The heater heats the cathode, which creates an "electron cloud" which is controlled by the grid, and electrons are "sucked" away by the anode's positive potential. If the cathode's not quite hot enough, the cloud will be small, and rapidly deplete. This can damage the cathode over the long term. That being said, if your heater volts are 7.1AC like many amps these days, it's not too far off from +/-10% for 8.5v.

Some very reputable manufacturers ran low-level preamp tubes a bit cold on the heaters because apparently that increases linearity to a point, say 11.2v instead of 12.6v on a 12ax7. A power stage would likely be subject to more cathode stripping because of the higher currents required from the cathode. I used 8BQ5s for a while in an EL84 amp, very durable looking Sylvanias. They lasted less than a year before their emission started dropping and they became noisier. Much lower power out right off the bat.