6BM8 plate voltage

[pullshocks]

I was looking at the RCA tube manual...it says the pentode max plate voltage is 500.  Has anybody used them that high?

[DummyLoad]

haven't seen 6BM8 with 500V on the plates, however, bob danielak built a SE amp using the pentode section DC coupled in a cathode follower config. operating in triode mode with 400V on the plate.

http://www.tube-town.net/info/sed-technotes/pic/no33fig1.gif

he seems to be fond of that envelope, another design of his using the 6BM8 choke loaded driving a 3CX300 RF triodes for finals.

http://www.reocities.com/bobdanielak/pp3cx300.html

[PRR]

In the RCA manual?

What exactly does it say?

Philips ECL82 (same tube) datasheet says Max Vao is 550V but max Va is 300V. This means 550V when no current is flowing, at switch-on. It better come down to 300V by the time it heats up.

Also: why? Power output is often limited by dissipation, not voltage. If you get a good amount of power with 10K load at 250V, then to get the same output and plate-heat at 500V you would have to load with 40K. That is not a common OT value, and makes frequency response hard to get.

> cathode follower config. operating in triode mode with 400V on the plate.

AND 187V at the cathode. 213V across the tube.

[DummyLoad]

AND 187V at the cathode. 213V across the tube.

given Va max rating of 300V - i assume that's given reference to ground - regardless of Vak - is that assumption wrong?

btw, that got me thinking... what happens when the tube swings + with his 80V or so peak wouldn't Vk exceed the Vhk spec of 100V significantly? at 187V Vk he's already well over the Vhk rating.

never mind... looking at the PS schema - he lifted the heaters 80V. still doesn't add up though. lifting the heaters 80V - 187V  we still have 107V Vhk diff. would Vhk + Vpk with signal still not exceed the 100V rating? or i'm i still clueless? 

 

[pullshocks]

Here is the reprint.  Actually it says 600v!

Why?  I want to build with the Little Wing type power amp and a deluxe reverb preamp.  I have a 350-0-350 PT and would rather not mess around with zeners to drop voltage.

http://i214.photobucket.com/albums/cc253/vision83/6BM8.jpg

[Geezer]

Notice the grid #2 (screen) voltage is 300v max.......you DO NOT want to exceed that #!

If you operate the plates @ higher voltage, you are going to have a huge Rk value in order to keep the Pdiss within spec.

G

[HotBluePlates]

I was looking at the RCA tube manual...it says the pentode max plate voltage is 500.

Here is the reprint.  Actually it says 600v!

How are you gonna lose 200-300v between the plates and the screens? More than 300v on the screens and you'll kill that tube in a flash.

I have a 350-0-350 PT and would rather not mess around with zeners to drop voltage.

If you want to use the 6BM8, you'll need either a different power transfromer, or you'll need a second transfromer and power supply for the screens and preamp. 350 * 1.414 = ~500v. You won't be able to drop 200v between the plate and screens in any way that won't zap the power of this amp to nothingness. You also can't bridge rectify the secondary and take half the voltage from the middle of a series stack of caps because you'll still wind up with 500v, along with a 700v * 1.414 = ~1kV output that you can't use.

If that PT is rated for any kind of current (and I suspect it is being 350-0-350), it's for a bigger, badder output tube run in cool class AB and making fair output power. It seems like the wrong iron for this job.

[pullshocks]

Yeah it is from a Hammond organ 6v6 amp.   I guess I'll stick to the 6V6s, though I really don't need that kind of power, and a separate PI tube takes up more space.

[PRR]

> Here is the reprint.  Actually it says 600v!

"Plate Supply Voltage"!!

"Plate Voltage" is 300V max.

The Plate-Cathode Voltage in normal running MUST be less than 330 Measured!

(The "300V" number is Design Center, includes some leeway for your production tolerances.)

> i assume that's given reference to ground

How can the tube know where "ground" is? What it "feels" is the pull from cathode to plate. That tells how hard those electrons will hit. Like a sled-run: a given class of sledder may survive a 300 foot hill easily, may die on a 600 foot hill. Does not matter if the "bottom of the hill" is really in Denver 5,000 feet above the sea.

Tube ratings are "usually" given relative to Cathode. Where you put ground is your problem.

> he lifted the heaters 80V. .... exceed the 100V rating?

I did not see that detail. Yes you should worry about it. Yes, you can sometimes cheat the heater-cathode rating, at least long enough to try and publish a schematic.

> I have a 350-0-350 PT

Sometimes good design forces tough choices. If you are really wedded to that PT, you may use a large resistor to drop voltage. This is WHY the tube has a 550V rating. Your PT will give 490V while tube is cold. For good load, you want to arrive near 250V, and current is pretty steady 35mA-45mA. A 6K (10W!) resistor will get your 490V down toward 300V.

(In fact the Hammond may have been taking a LOT of drop in series losses, possibly even field-coils.)

[jojokeo]

Notice the grid #2 (screen) voltage is 300v max.......you DO NOT want to exceed that #!

If you operate the plates @ higher voltage, you are going to have a huge Rk value in order to keep the Pdiss within spec.

G

I've been running my 6BM8 amp w/ 365v's on the plates but used large 22k screen resistors to get them down to 300V and my cathode resistor took 670ohms for proper 7 watt dissipation on NOS Valvo, Amperex, and RCAs w/out any problems.

*I would like to see more afordable & lower voltage trannies available than all of the higher ones seemingly everywhere.