Thread: Valve preamp.
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Old 20th Jun 2019, 7:07 am   #56
Electronpusher0
Nonode
 
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 2,288
Default Re: Valve preamp.

OK Andy,
I took a look at F. Nachbaur's pre-amp and read his write up on it.
I think one of the problems with the design, which he refers to, is that the differential amp front end has a poor common mode rejection due the fact that the common cathode load is not close enough to a constant current. He offsets this shortcoming to some extent by unballancing the Anode resistors.
Differential amps of this type are often called "long tailed pairs" because to get a close approximation to a constant current you really need a negative supply, say 120V and a larger cathode resistor, hence the long tail.
By only using a positive supply and returning the cathode resistor to 0V the design has to have the grids at about 27V which adds complexity with dc blocking capacitors.
This combined with direct coupling to the next stage means that the HT supply has to be so high, 430V!
With a negative supply the HT could come down to about 340V.

Another advantage is that the grids of the diff amp can now be at 0V which eliminates some components, the bottom of R102 can go to 0V, C102 goes as does R103 and R104 becomes a link.
There will be some changes in value eg R105 (the cathode resistor) increases to 120K.
There will also need to be a few mods to the dc feedback circuit because of the change in grid voltages, R 110 becomes 270K and goes to the -120V instead ov 0V.
The reduced HT means one of the neons is not needed, or repace the neons by a single 120V zener.

I will try and find time to model the version described above in LtSpice (I may even knock one up out of curiosity).

Peter
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