4-400A pentode as solid state or equivalent.

[Oneminde]

Hello.
I hope I am not asking a "stupid" question, but how does one replace a 4-400A pentode with a solid state circuit ? I am asking because I essentially have no idea.

[CJay]

You don't, not easily anyway.

I think it would be simpler to replace the entire amplifier than design and retrofit some sort of solid state replacement as that tube is capable of around 1KW output.

Investigate BLF188.

[Oneminde]

You don't, not easily anyway.

I think it would be simpler to replace the entire amplifier than design and retrofit some sort of solid state replacement as that tube is capable of around 1KW output.

Investigate BLF188.

Edit: Its a tetrode and not a pentode, my bad 

Anyway. CJay, thanks for your input, I'll take a look at  it. Perhaps the schematic will help - yes there is a tesla-coil in there

[dmills]

Self exciting screen modulated AM transmitter?

Nasty looking thing, I would stick to vacuum fets for that, maybe one of those Russian external anode jobbies, they will take the abuse far better then anything sand based.

A few spark gaps between the grids and cathodes might be a good idea as well (Also a 'glitch resistor'), just to reduce the risk arcing the output to G1 and destroying the tube.

Regards, Dan.

[Oneminde]

This is just part of the circuit and the final version had addressed all issues.

RF band: Short wave band, 7-20 Mhz, the screen modulated part I don't know. But it is used as "exciter" for a tesla coil. Does that help ?
 

[dmills]

Well you are applying the modulation to G2, so it is screen grid modulated.

I am not sure the circuit as given is really suitable for driving a classical loosely coupled quarter wave tesla coil, but it will poke out some high voltage RF.

This is not so much a tesla coil as a jammer IMHO.

Forget a sand based approach to this thing, a zero bias external anode tube is going to be more reliable in all ways if you do the design right.

[Oneminde]

You know does plasma tweeters available like Acapella and Lansche corona plasma tweeter, does are weak compared to this one.

[Oneminde]

I might not need to go solid state, Penta Labs are producing new 4-400CG tubes which is an improved version of the 4-400A.

[Cyberdragon]

Tubes are far more electrically durable than transistors. High power transisitors go bang at the slightest hint of trouble, and Tesla coils can be electrically abusive.  Plus...

[Oneminde]

You are correct. That is why I am so happy that Penta Labs produce them among many others.

[CJay]

Tubes are far more electrically durable than transistors. High power transisitors go bang at the slightest hint of trouble, and Tesla coils can be electrically abusive.  Plus...

In general I agree, valves/tubes are still around for high power RF for several reasons, one of which is durability and reliability but semiconductors are making inroads to the KW level hence why I suggested that if the OP *had* to go to silicon that they investigate the BLF188, there are some really scary impressive videos out there of people drawing arcs to ground,  open circuiting them etc. and the devices still work just fine afterwards.


https://www.nxp.com/video/sparky-vs.-extremely-rugged-ldmos-blf188xr-blf184xr-pre-ims-2013:RUGGED-LDMOS-BLF188XR-BLF184XR

[Oneminde]

@ CJay: The BLF188XR as far as I can tell seems like a real contender for a SS version for this project and I am very grateful for the suggestion and as you point out, it seems to survive open spark test aka plasma discharges

The picture is from the plasma project which the circuit is connected to, now THAT is a plasma speaker. Singing Tesla coils are cool but totally unusable for HQ audio applications.

[CJay]

@ CJay: The BLF188XR as far as I can tell seems like a real contender for a SS version for this project and I am very grateful for the suggestion and as you point out, it seems to survive open spark test aka plasma discharges

The picture is from the plasma project which the circuit is connected to, now THAT is a plasma speaker. Singing Tesla coils are cool but totally unusable for HQ audio applications.

As you've found a source for a replacement tube I'd suggest you stick with that, re-engineering it to use a BLF will take a significant amount of effort and expense for little advantage.

I would agree with DMills though, it sounds like it's going to cause a large amount of interference to local HF radio users, I hope it's going to have some attempt at shielding?

[dmills]

Seems to me that this application is exactly what the ISM band at 13.56MHz was intended for!

Rather them making the thing self oscillating, and thus prone to wonder all over the dial, use a 13.56MHz crystal and drive the grid with a couple of watts at that frequency.

It will be far better behaved.

73 Dan.

[Oneminde]

As you've found a source for a replacement tube I'd suggest you stick with that, re-engineering it to use a BLF will take a significant amount of effort and expense for little advantage. I would agree with DMills though, it sounds like it's going to cause a large amount of interference to local HF radio users, I hope it's going to have some attempt at shielding?

Plasma can generate 3 issues one might need to address: UV, RF/EM and NOx (nitrous oxides). All of them can and have been solved. Screening is important both from internal and external interference. Safety will always be a priority with projects like this, both for the living and inorganic like electronics.

I am glad new tubes are being made, not just because I first of all need to do a 100% replication but thermionic devices are awesome. A solid state version would solve some issues and address other. With it being completely SS, I can see several benefits and one is the size of the overall electronics package. Since the BLF is made to be extremely rugged, it might outlast a 4-400 tetrode.

[Oneminde]

Seems to me that this application is exactly what the ISM band at 13.56MHz was intended for!

Rather them making the thing self oscillating, and thus prone to wonder all over the dial, use a 13.56MHz crystal and drive the grid with a couple of watts at that frequency.

It will be far better behaved.

73 Dan.

Both ISM 13.56MHz and 27.283 MHz can be of interest and since you mentioned it, I will test them. But to be honest, it will be an overall observation of produced interference and issues.

[dmills]

The BLF188 will not be as robust as the tetrode, at least not without a hell of a lot of protection circuitry.

Those videos you see typically have the short near the amplifier so pretty close to zero reactive component, and have a power supply with serious current limiting. Also, the device starts off cold.

Four things kill ldmos, Overheating, Over voltage, Over current and Over drive, and it is the drive one that usually kills fastest, followed by drain over voltage.

I would commend the GU-34 to your attention, cheap as chips and fairly robust, or the GU43 for more woof. Note that indirectly heated tubes typically need the heaters to be on for a few minutes before applying HT. 

Regards, Dan.

[Oneminde]

Which of these Eimac's is close to the 4-400A/CG ??

http://www.cpii.com/product.cfm/9/22/76

[Oneminde]

Picture arrived today