Exceeding V-heater-cathode

[SemperFi]

I've been using both 5687s and 6AS7s as rectifiers in low voltage stuff (up to 200volts DC out) for use in some preamps. But I'd like to use a 6AS7 in my 6W6 PSE amp and use a PT with 2x330V secondary. That will seriously exceed the V-heater-cathode rating (which 300v for the 6AS7).

How will I damage the tube? Are we talking rapid death murder kill, or just a slow decay in motivation over some years?

Of course I can use real rectifier tubes, but I'd have to go to the hastle of ordering them, spending money on them, and besides, I love the 6AS7!

[kevinkr]

Originally Posted by SemperFi

I've been using both 5687s and 6AS7s as rectifiers in low voltage stuff (up to 200volts DC out) for use in some preamps. But I'd like to use a 6AS7 in my 6W6 PSE amp and use a PT with 2x330V secondary. That will seriously exceed the V-heater-cathode rating (which 300v for the 6AS7).

How will I damage the tube? Are we talking rapid death murder kill, or just a slow decay in motivation over some years?

Of course I can use real rectifier tubes, but I'd have to go to the hastle of ordering them, spending money on them, and besides, I love the 6AS7!

Please use a real rectifier tube here.. The death in this case would probably be a quick one and for more reasons than you might think, one is that the heater/cathode insulation will be very overstressed and in this case might just short unless you elevate the filament supply. The bigger concern with the 6AS7 is with cathode stripping as the filaments warm up with 330VAC on the plates as well as potential arcing between the cathodes and plates. You would have to use a standby switch and I think the voltages will be excessive in operation for this type. Also most rectifiers are a good deal less lossy than the 6AS7 would be in this sort of use.

[Eli Duttman]

Kevin's post makes considerable sense.

If you are determined to go ahead, buy a separate, correct current rated, 6.3 VAC filament trafo, with a superior HIPOT characteristic. Then, tie the 2 cathodes and 2 grids to 1 end of the heater and take the B+ from that point. That setup emulates the construction of a 5AR4, which avoids the heater to cathode insulation breakdown issue.

[kevinkr]

Originally Posted by Eli Duttman

Kevin's post makes considerable sense.

If you are determined to go ahead, buy a separate, correct current rated, 6.3 VAC filament trafo, with a superior HIPOT characteristic. Then, tie the 2 cathodes and 2 grids to 1 end of the heater and take the B+ from that point. That setup emulates the construction of a 5AR4, which avoids the heater to cathode insulation breakdown issue.

This is what would be required to make it work, although I would probably connect the grids and plates together as was the practice before small tube rectifiers became available, but otherwise as ELI suggests. The recommended transformer which is necessary would probably cost more than you would save using this tube assuming that your new power transformer has a 5V rectifier filament winding. I would still expect relatively poor performance as a rectifier due to internal losses which would probably be much higher than for any typical rectifier tube.

If you like the shape look for old 5U4GB type rectifiers - they're cheap, rugged, and have the look you want to know better..

[tubelab.com]

How will I damage the tube? Are we talking rapid death murder kill

YES! I had a big bunch of tubes that got sold off, and some of the really nasty ones with severely corroded pins went into the dumpster. I experimented on a few tubes destined for the trash to see what will happen.

In the case of H-K overdose the first thing that will happen is a short between the heater and cathode. Depending on the external circuitry and how much current is available the next thing to happen varies from a blown heater to a volcano of sparks shooting out of the cathode sleeve. In a rectifier application a short will connect the B+ into your heater circuit possibly blowing other tubes and components.

[kevinkr]

Originally Posted by tubelab.com

YES! I had a big bunch of tubes that got sold off, and some of the really nasty ones with severely corroded pins went into the dumpster. I experimented on a few tubes destined for the trash to see what will happen.

In the case of H-K overdose the first thing that will happen is a short between the heater and cathode. Depending on the external circuitry and how much current is available the next thing to happen varies from a blown heater to a volcano of sparks shooting out of the cathode sleeve. In a rectifier application a short will connect the B+ into your heater circuit possibly blowing other tubes and components.

I can just imagine the excitement..

[SemperFi]

Thanks guy for informative answers. I've used triodes as rectifiers up to 115VAC without problems so far
I guess I will get my senses and bite the bullet and write up an order for a real rectifier. With as little time on hand I have these days the 6W6 amp wont be up and running for a while anyways.

Tubelab!!! U need to contact Discovery channel and help them blow things up That'd be a great show...hundred ways to blow up vintage electronics.

[tubelab.com]

Tubelab!!! U need to contact Discovery channel and help them blow things up

I could teach the Mythbusters guys a thing or two about microwave ovens!

I took some pictures of a cathode to plate arc in a serious overload situation. It starts in post #28 of this thread:

forced aircooling

[Eli Duttman]

Originally Posted by SemperFi

Thanks guy for informative answers. I've used triodes as rectifiers up to 115VAC without problems so far
I guess I will get my senses and bite the bullet and write up an order for a real rectifier. With as little time on hand I have these days the 6W6 amp wont be up and running for a while anyways.

Dude,

Acquiring a reasonably capable tube rectifier does not require that you spend heavily. The 100 mA. rated 7Z4 costs less than the requisite Locktal socket.