1H4 at 9V plate potential, open grid versus pull up bias, microphone booster preamplifier experiments

The schematic used is the Pleiades V6 adapted for 1H4 use.






(A fuse should always be used in series with a voltage supply for safety).


A 1.2V battery was used for the filament of the 1H4. A 9V battery for plate supply.


Cc is 22nF. The input transformer an Altec 4722.


The MD441 sounds bass heavy at less than 10in. Much more natural sound is above 12in but acoustics and noise considerations make a conclusion inaccurate. Hiss from the booster amplifier may become quite apparent too.


Signal path, setup:


Voice at 12in - Sennheiser MD441 U3 - Pleiades V6 with 1H4 - Sony TC-D5 Pro - Sennheiser HD580


A phase change adaptor is also used for corect phase between mic and headphones. Positive pressure on mic producing positive pressure at headphones.


The grid at space potential sounded more big and gave less noise. Anode current is 10μA.


(Further reducing the heater voltage by a 10Ω resistor was also tried. The sound was very nice and had a little less bass and perhaps more definition).


Using direct 1.3V at the filament and various pull up grid resistors (from grid to anode) of the order of Megohms and increasing the anode current up to about 100μA gave progressively worst noise results. And more bass heavy sound. (Later addition, the reason seems to be that when 9V is on the plate the grid is at 50mV space potemtial, so it is already pulled up. See end of post).


An obvious thing to try is say 3V plate supply and pull up bias until anode current becomes 10μA.


Also trying the EF183 or Nuvistor 7865 at the above schematic without pull up bias, ie grid at space potential, and higher plate voltage than 4V to listen if the booster amplifier will become even more low noise or not.



The grid of the 1H4 becomes 50mV positive when 9V are on the plate. See next post:
http://euroelectron.blogspot.com/2018/08/the-grid-space-potential-increases-when.html