Exceeding Maximum Plate Voltage at Startup

Hi everyone,

I've searched around on this topic both here and on the web in general, but haven't found any specific answers yet. Maybe its a stupid question! Here it is anyway:

Is it acceptable to exceed a valve's specified maximum plate to cathode voltage for a short time (i.e. at startup, before the cathode is hot and current is flowing)?

I'm currently playing around with a few valve circuits and have a PSU which, unloaded, sits at 400vDC. The valve I'm interested in is the 5842 which has max plate to cathode voltage of 200v. I can, of course, set the resistance in the power supply's CRC filter such that the plate voltage will always sit under 200v when the valve is operating normally. My concern is about the risk of internal arcing for those few seconds where no current flows and the plate sees 400v. I guess the obvious answer is to use a more appropriate power supply, but I get the feeling this scenario will come up quite often while throwing test circuits together with a generic PSU...

Thanks for any comments on this!

Matthew

If you take a look at the maximum ratings in the datasheet, you will see VA0. This is exactly what you need, the anode voltage during start up. And as you can see it's 400V, so everything is fine. Tube are actually pretty forgiving to big overstresses for short times.


Volker

http://tubedata.milbert.com/sheets/009/5/5842.pdf
This datasheet says 400v max VAo (short peak, no current, I guess)...
But what seems odd to me is the 100% voltages difference between loaded and unloaded PSU.
Isn't your power transformer a little weak for this circuit ?

EDIT: forgot the anode resistor...

Laurent.

Volker, thanks very much for that, I wasn't aware of that parameter before. Awesome!

Laurent: indeed the anode resistor is dropping some voltage, as is the 10k 10W resistor in the CRC filter! (I actually only want 200v B+ at the top of the anode resistor - the PSU is overkill.)

It is normal for tube circuits to get the full unloaded (or roughly near) voltage during start up - that's why you see 600V coupling caps.

I would say the plate dissipation is the parameter you least want to exceed and you can't really do that with no current flowing in the tube ( those first few seconds of start up ).

As for the brief overvoltage period being a problem,  I've built circuits with 6C5s(250V plate max)  and exceeded the  max during start up with no apparent problems.  On the other extreme - I regularly read of guitar amp builders flogging the daylights out of EL-84s - running well above the max voltage(but below dissipation) under nominal use.  Again - the plate dissipation is what is likely going to melt the tube.

This is a good question and since the 5842 is a sub for the WE 418($$) so I'm not going to say do it worry free though I suspect it will be ok.

One thing you can do whenever in doubt is add a stand-by switch to your PS.  This will prevent the momentary voltage soar by allowing the cathodes to heat up before the HT is thrown , and is also a little easier on the caps as well.

Adding a choke will drop ~50V off your final B+ should that be needed to solve design issues.  

lassoharp said:

On the other extreme - I regularly read of guitar amp builders flogging the daylights out of EL-84s - running well above the max voltage(but below dissipation) under nominal use.  Again - the plate dissipation is what is likely going to melt the tube.

Click to expand...

I guess what happens is cathode-stripping; the overfed anode is just trying to suck electrons and rips the cathode. PRR has probably a more educated comment.

> short peak, no current, I guess

No.... essentially ZERO current for long time.

The warm-up issue, when current is rising toward normal level while raw supply voltage has nt yet sagged, is a gray-area.

> 6C5... EL-84s - running well above the max voltage

Those are fairly "large" tubes with large internal spacings.

5842 is super-carefully wound with very small electrode spacings.

It might take 400V forever, if dissipation is well under the limit. But the 5842 works best at high current and modest voltage. When bias is adjusted for high current at 200V, you have HIGH current at 400V sagging to 300V.

At $4, I might just risk it. At $100, I would not try. At $20.... I dunno. I put 400V on $18 "300V" EL84, but that's not a 2:1 over-voltage, I need big volts for big power, and I have reason to suspect that modern gitar-market EL84 mostly take 400V.

> I guess the obvious answer is to use a more appropriate power supply

That would be wise. There isn't much reason to have more than 250V DC on hand until you get to POWER stages (which can be smoke-tested at 250V before pushing to 300V-400V....).

Thanks for all the input, this is very useful stuff!

If I've understood correctly, the Vao parameter tells us how big a voltage we can apply without a risk of arcing, but the bigger issue is actually the plate dissipation in the grey area that PRR described? (This is where my theoretical knowledge gets a bit shaky - I'm guessing the likelihood of plate to cathode arcing is always the same whether we have no plate current flowing or, say, 20mA?)

To take my initial question further, how is this issue dealt with in designs which integrate pre and power amp stages? I've seen plenty big guitar amps fired up by hitting the mains and standby switches simultaneously and none of them have ever died as a result. Is the large cathode bias resistor typical of those preamp designs saving the day by keeping the plate dissipation well under limit, irrelevant of how high a voltage the plate sees?

Thanks again for the great comments!

Matthew

MatthewF said:

Thanks for all the input, this is very useful stuff!

If I've understood correctly, the Vao parameter tells us how big a voltage we can apply without a risk of arcing, but the bigger issue is actually the plate dissipation in the grey area that PRR described? (This is where my theoretical knowledge gets a bit shaky - I'm guessing the likelihood of plate to cathode arcing is always the same whether we have no plate current flowing or, say, 20mA?)

To take my initial question further, how is this issue dealt with in designs which integrate pre and power amp stages? I've seen plenty big guitar amps fired up by hitting the mains and standby switches simultaneously and none of them have ever died as a result. Is the large cathode bias resistor typical of those preamp designs saving the day by keeping the plate dissipation well under limit, irrelevant of how high a voltage the plate sees?

Thanks again for the great comments!

Matthew

Click to expand...

Max plate dissipation is what occurs when the current has reached nominal operating level - well after initial start up and after the gray area that PRR mentioned.  It's the voltage that momentarily soars - while current is ramping up.  The final operating current is dictated mainly by the cathode resistor.  So, the danger of exceeding dissipation is encountered once the tube current has reached normal levels - not during the start up phase.

The cathode resistors in the preamp stages are chosen for giving the best operating point for the tube with regards to distortion, voltage gain and bandwidth.  They are usually ran at a few mA and dissipation is seldom an issue here.

With the big guitar amps being powered up sans stand by - same as above.  The worrisome issue would be the voltage ratings on the caps.