[Ronja] Vacuum tube in Ronja preamplifier

Gullik Webjorn gullik.webjorn at flysta.net
Fri Oct 12 13:26:01 BST 2007

I worked a lot with mobile radio when I was 13-20 years old.
Most installed base VHF and UHF radios had a dual triode cascode
as first RF input stage and were beeing replaced with solid state x-ceivers.

Noise factor was not so good in these designs, so a common "fix"
we did to "beef up" old sets was to replace the dual triode with
a dual gate mosfet. The lower noise figure gave new life to old
radios. Often this could be done "airwire" by soldering the fet
to the tube socket, and changing bias and DC. Sensitivity could
go from 1 uV @ 20 dB SINAD to 0.2 uV , more than doubling range.

To me this indicates there were other sources of noise, a guess
thermal from the 800 deg cathode electron cloud.

In these cases, the Cin of the tube was of course resonated with the
input L/C circuit, so the capacitance did not disturb, whereas in Ronja
it will create a pole with the PD resistor.

Are you trying to get more speed or lower noise figure?



Someone suggested that vacuum tubes have very low noise and suggested
PCC88/ECC88. It has
1.4pF input capacitance which is better than the best transistor (BF908,
2.4pF) and 12.5mS
forward transfer admittance. BF908 has 36-43-50 mS but is impossible to get.
BF988 which is
#1 has 21-24- mS and BF960 which is also a common replacement has 13mS.

Now if the dominant noise in the transistor is not generated by the forward
admittance, but rather by the real part of the input admittance, then the
vacuum tube has
a great chance to be actually better than a transistor.

Does anyone have PCC/ECC88 at home or knows where to buy it easily?
with vacuum tubes?  What do you think about it? Isn't there some other noise
source in vacuum tubes that isn't present in transistors that would kill the

It would be definitely great apart from increased resistance to lightning we
could start
claiming that digital music transmitted through Ronja has a better, warmer,
fuzzier, more
natural sound.


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