6. Diode limiter


A big problem for the settling node method, small settling signals are overrided by the 300mV clamping voltage from the Schottky diodes. Unfortunately there are not many diodes available with a lower voltage.

I thought much about many solutions, I didn't see any solution without switching.

Some offer RF diodes with a low VF voltage and low capacitances.

Tube diodes could be also a choice. let's try these old parts first. Sometimes it's a good idea start on the beginning than you'll understand the new days.



The Table is full of instruments.


In the past I build up flexible tube sockets for banana cables.
Not a good choice for higher frequencies but excellent for rapid prototyping.
Experiments starts with a double diode EAA91, a type with separate cathodes .



I tried many different tubes, diodes, pentodes and triodes in diode connection.
Here a PC900 VHF TV tuner triode under test. I even start to select them.



Also direct heated russian military GHz triodes were under test.


These are big russian military power GHz triodes.


Clamping Results

At first glance the PC900 and the EAA91 reaches very good clamping results.
All measurement made with the banana cable tube socket, they are a little noise.

Mesaured with a HP3325A generator and a 7A22 amplifier





Upper trace 1N5711 clamping voltage
Lower trace tube diode clamping voltage

PC900:

 
input 10Vpeak  - seems good                                                                                            input 1Vpeak  - seems good

 
input 100mVpeak  - bad, tube have too low resistance under low voltages                     input 10mVpeak - bad, tube have too low resistance under low voltages

 
input 3mVpeak - very bad, tube has too low resistance under low voltages.
Schottky clamps still correct.


EAA91:

 
input 10Vpeak - seems good                                                                                          input 1Vpeak -  seems good


 
input 100mVpeak  - tube clamps much                                                                           input 10mVpeak - high clamping

  
input 3mVpeak                                                                                      input 1mVpeak - tube clamps to 75µV, the schottky still clamps correct.

For the measurement with low voltages it was necessary switch the 7A22 for a lower frequency to 1kHz to reject the AC-line components.
For further comparison, the small russian triodes had the lowest resistance they clamped much (not shown).


Comparison list of different tubes driven with a ?V signal (I forgot), applying via 3k to two 1N5711 and 3k to the diodes:
(tt) 6072  300mVpp
(tt) ECC82 160mVpp
(tt) ECC83 150mVpp
(tt)  ECC84 150mVpp
(dd) EAA91 120mVpp
(tt) ECC86 120mVpp
(p) EF85 100mVpp
(tt) ECC81 90mVpp
(tt) ECC88 90mVpp
(tt) ECC85 70mVpp
(p) EF80 60mVpp
(p) EF800 55mVpp
(p) EF183 50mVpp
(p) D3A 50mVpp
(tt) ECC189 45mVpp
(p) EF184 30mVpp
(t) PCC900 28mVpp
(t) Russian GHz triode 19mVpp

(t=triode) (p=pentode) (d=diode) (tt, dd indicates double)

always best cases, some tubes were poor.


Problems with tubes:



Radiated free electrons from the glowing cathode cause an unwanted current, if not blocked by a neagitve grid voltage of approxiamtely -1V. Need a switch to avoid this inrush-current, grid switch.
  1. tube diode has a too low resistance under very low voltages.
  2. at zero anode voltage diode-tubes produce already a inrush-current. 
  3. very large offset problems caused by different inrush-currents. Clamping offset voltage can be reduced by selecting tubes and by choosing different heater voltages. A controled heater voltage could reduce the problem.
  4. Tube ageing, changed curve characteristics
  5. Heater supply

Unfortunately I don't see a change to work with tubes, but it was a nice experiment having some fun. I am still waiting for special diodes.


Where to buy? nowhere



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