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LAN_403

Conundrum

Sun Feb 03 2013, 08:38AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Hi all.

I noticed something interesting in that box of components obtained wayback, what appears to be several PL500's.
The problem is that these aren't really high power, apparently they are 4KV at something like 200mA max.
Is there any way to series/parallel them to increase this or is it a lost cause?

Thanks!
-A

Proud Mary

Sun Feb 03 2013, 01:46PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Beam output pentodes like PL500 were designed to be as much like an on/off switch as was possible with thermionic technology.

The key figure in the data sheet is the anode dissipation design centre limit of 12 W, and screen dissipation limit of 5 W.

With Va max 7 kV, and Ik max 250 mA, the on periods in the duty cycle must be made very short indeed if the valve is not to be destroyed.

Under what the data sheet calls the 'worst probable conditions' the valve will function with an anode dissipation of up to 17 W, and screen grid dissipation of 6 W, which will no doubt shorten its life.

There is no reason you cannot connect these beam power pentodes in parallel at the low frequencies generally employed in Tesla coils, but I would suggest that you made every effort to keep wiring as short as possible, and put anti-parasitic chokes in all the grid and anode connections immediately at the valve bases and top caps. It would be good practice to check the whole thing out with an absorption wavemeter to detect parasitic oscillation (commonly in the VHF), and ensure that harmonics and other spurious emissions are below statutory limits.

Ash Small

Sun Feb 03 2013, 03:27PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've a PL500 somewhere (possibly from a box you sent me, Andre, by the way, any news on the next one yet

) and it sounds from your post, PM, that I may be able to use it in a project I have underway for switching a magnetron based ion source, depending on duty cycle.

I'd be interested in those PL500's if you decide not to use them, Andre.

Marko

Sun Feb 03 2013, 03:59PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys

a while ago I used a pair of PL504's in a half bridge configuration much like mosfets to base drive a Tesla coil. Despite the tubes weren't at their best with emission I managed to get over 1kW throughput and no plate reddening at all; however I had to push the voltage upwards 5-6kV.

P6110001

I wrote an article about it on my site

Mr.Black

Sun Feb 03 2013, 04:48PM

Registered Member #3960 Joined: Mon Jun 20 2011, 06:27AM
Location: Wuhan Hubei China
Posts: 33

The PL504 really 1kw power output?
It sounds unbelievable!!!amazed

Proud Mary

Sun Feb 03 2013, 10:05PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

I've a PL500 somewhere (possibly from a box you sent me, Andre, by the way, any news on the next one yet

) and it sounds from your post, PM, that I may be able to use it in a project I have underway for switching a magnetron based ion source, depending on duty cycle.

I don't know the ins and outs of your project, Mr Ash, but I suspect the old PL500 might be underpowered. How about another line output valve, PL509, which can dissipate up to 30W

, or PL519 which can manage 45W?

Four of them in parallel would be a sight to see!

Ash Small

Mon Feb 04 2013, 12:09AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Proud Mary wrote ...

I don't know the ins and outs of your project, Mr Ash, but I suspect the old PL500 might be underpowered. How about another line output valve, PL509, which can dissipate up to 30W

, or PL519 which can manage 45W?

Four of them in parallel would be a sight to see!

I'll only be running it @~20W max, but I'm not sure about pulse width/duty cycle yet.

Proud Mary

Mon Feb 04 2013, 08:45AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The problem with TV valves is that most were designed for transformerless series heater operation, so we find PL500 and PL504 needing a 27V/300mA heater supply, and PL509 and PL519 needing 40V/300 mA. Clearly, this is a big turn off, but the plus side is that series heater valves tend to be cheaper. than comparable 6.3V heater types.

Dr. Dark Current

Tue Feb 05 2013, 09:59PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

These tubes have a really low internal resistance. If used in continuous wave mode, going over 600-900V on the plates doesn't have any benefit, up to around 1200V can be used when running in pulsed mode, more than 1500V DC can result in flashover, especially in older tubes.

I have found that these tubes easily take pulsed cathode currents of over 0.5A if the average current is kept below 0.25A, this means some 600W peak input per tube at 1.2kV. The plate efficency of class-C oscillators built with these tubes can be pretty high (probably over 80% in some cases), so the average input power can be several times the plate dissipation without overpowering the tubes.

As to the paralelling: Direct paralelling of tubes usually results in parasitic oscillations, put 220 ohm resistor in series with control grid of each tube to stop the oscillations.

NOTE: These data are based on short-term testing, the tubes may have decreased lifetime but in a Tesla coil it usually doesn't matter.

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