That spec was with a grid voltage of 0 volts. At initial startup the grids would be almost at supply. One would think the current would be higher and the plate resistance lower.
The 5600 plate resistance number you mentioned was with a grid voltage of -1volt in a class A amp configuration.

True, but the device has a maximum positive grid bias of zero; and from the Ra vs. Vg graph, it doesn't seem that it gets much lower than 5.6k.

 

from the Ra vs. Vg graph, it doesn't seem that it gets much lower than 5.6k.

I'm not sure about that. It appears to me that the curve is trending down as the grid voltage exceeds 0 volts.
Is it going to damage the grid if the bias is +30 volts?

 

It depends on how much current flows through the grid-cathode diode. As the manufacturer didn't intend it to be used with a positive grid voltage, they haven't put that information on the datasheet!

 

Hello after a couple of months!

I finally got all the parts I needed, and got around to putting everything together. But, it doesn't work...

These are the circuit and the tube (6FH5):





This is what I made (not that you can tell much just by looking at this picture, but I thought I'd post it anyway):




To power the tubes I use an AC26V transformer, and I fully rectify the output toa DC45V (approx.). The DC6.3V I get from the two coin cell batteries. This is something temporary, when I get this to work, I'll use a transformer.

I double checked all the connections, and it's the same as the schematic diagram. I double checked the voltages, and everything checks out. However, the circuit doesn't do what it's supposed to do. Both LEDs are permanently ON and dim (the one on the right is slightly dimmer), and that's it...

Is there anything that is very obvious, and I'm doing wrong?

I thought of these possibilities:
- One tube or both tubes don't work. If that's the case, how can I check?
- Pin 7 (cathode) should be connected. I didn't connect it because in all the schematics I've seen, this pin is not connected. Pin 1 is also a cathode and it is connected. Pin 6 (internal shield) is also not connected.
- The coin cell batteries are not good enough for the heaters (again, these are temporary).

Since I'm not an expert on tubes, I wanted to confirm before connecting pin 7 and ruining the tubes.

Any other ideas?

Thanks!

6FH5 Datasheet

 

- The coin cell batteries are not good enough for the heaters (again, these are temporary).

No chance, they need 400mA between them!
Are they glowing?

 

No chance, they need 400mA between them!
Are they glowing?

Yes, very dimmly, but they don't go on and off. I'll try a 6V transformer right now.

What about pin 7 (the other cathode)? Should I connect it?

 

Why are you still using coin cells when you were told that they would not work?
Get a proper 6VAC or 12VAC transformer.

 

Yes, very dimmly, but they don't go on and off. I'll try a 6V transformer right now.

What about pin 7 (the other cathode)? Should I connect it?

If pin-7 is connected to pin-1 then you don't need to connect it.

 

Why are you still using coin cells when you were told that they would not work?
Get a proper 6VAC or 12VAC transformer.

Because I'm a dumbass.
I thought the heaters needed DC. I have a DC6V transformer somewhere.

 

Alright. Both cathodes on both tubes are connected to negative, and I'm using a DC5V/2A for the heaters (that's all I found), and a AC26V/300mA for the rest. When rectified it becomes DC45V. Is that enough amps? The next transformer I have is DC12V at 1500mA. Both LEDs are dimmly glowing, but they won't do anything.

 

and I'm using a DC5V/2A for the heaters

You need to get 6.3V ± 10% capable of supplying at least 400mA for the heaters. Using 5V will seriously degrade the function of the tubes and I am not surprised the circuit doesn't work.

 

Alright. Both cathodes on both tubes are connected to negative, and I'm using a DC5V/2A for the heaters (that's all I found), and a AC26V/300mA for the rest. When rectified it becomes DC45V. Is that enough amps? The next transformer I have is DC12V at 1500mA. Both LEDs are dimmly glowing, but they won't do anything.

Put both heaters in series and use the 12V 1500mA

 

Put both heaters in series and use the 12V 1500mA

I just did that. Still nothing.

 

It works now. I found out the problem. I was interpreting the pinout as the bottom of the tube, not the top. I checked every pin with my tester in continuity mode and found out pins 4 and 5 where connected instead of 3 and 4 (the heaters).

However, as I'm writing this, the circuit stopped working. The LEDs are permanently on again.

 

However, as I'm writing this, one of the tubes stopped working. It's no longer glowing, and the LEDs are permanently ON.

How do you have the heaters connected?

 

How do you have the heaters connected?

DC12V/1500mA in series.

I turned everything off, let it rest for a few minutes, and turned it back on. It works for now, but it starts with both LEDs turning on and off at the same frequency, then one of the two LEDs slowly begins to stay on longer than the other, and the difference keeps increasing.

 

then one of the two LEDs slowly begins to stay on longer than the other, and the difference keeps increasing.

Make sure both capacitors are connected the right way round (positive side to anode).

 

DC12V/1500mA in series.
then one of the two LEDs slowly begins to stay on longer than the other, and the difference keeps increasing.

Never mind, I removed the capacitor at the output of my rectifier, and it works now.

Now it looks messy because of the mess of cables, and being on a breadboard. I'll put it on a PCB and post pictures.

Thank you everybody for your help!

 

Schematics are the language of electronics.
Could you draw a schematic of what you did?

Even if you made it work today, what will happen 6 months or 6 years down the road? Would you still remember how you did it?

 

Even if you made it work today, what will happen 6 months or 6 years down the road? Would you still remember how you did it?

Yes, because I posted the schematic in this thread already: here, here, and here.

I thought I would post a picture as a courtesy to everybody who helped, as opposed to just disappearing because I already got what I wanted from this thread. Also, besides seeing the schematics, I like to see pictures of the actual thing to get ideas about how people mounted something, what materials they used, whether they used a box for electronics or they recycled something they had at home, etc. I wasn't aware that posting a picture was an issue.