ruffrecords
Well-known member
Continuing the theme of low cost construction projects of classic kit I have been thinking about a poor man's tube compressor. The classic types use vari-mu tubes in complex push pull arrangements with lots of expensive iron or they use expensive light dependent resistors and plenty of iron too.
So I have been hunting around for an alternative tube based gain control element and came across this little gem in the Radiotron Designers Handbook (see pic). It uses a pentode with 100% NFB to create a variable resistor with the value 1/gm and hence by varying the grid bias you can vary gm and hence the resistance. I have done a quick simulation using an EF86 and it looks quite promising. Distortion is below 0.5% provided the signal is no greater than 1V rms. At first I thought this might be a bit low but then I had a look at the Neve 2254 which uses a diode bridge and that operates at a nominal -31dB. Allowing 26dB headroom and assuming the nominal 0dB reference is +4dBu means its maximum operating level is only -1dBu which is less than1V rms so we are in good company. I don't have much experience with pentodes but I think I might knock this up on the bench and give it a test. As it is unbalanced we don't need push pull circuits or much in the way of iron so an inexpensive tube compressor could be a possibility.
What do you think?
Cheers
ian
So I have been hunting around for an alternative tube based gain control element and came across this little gem in the Radiotron Designers Handbook (see pic). It uses a pentode with 100% NFB to create a variable resistor with the value 1/gm and hence by varying the grid bias you can vary gm and hence the resistance. I have done a quick simulation using an EF86 and it looks quite promising. Distortion is below 0.5% provided the signal is no greater than 1V rms. At first I thought this might be a bit low but then I had a look at the Neve 2254 which uses a diode bridge and that operates at a nominal -31dB. Allowing 26dB headroom and assuming the nominal 0dB reference is +4dBu means its maximum operating level is only -1dBu which is less than1V rms so we are in good company. I don't have much experience with pentodes but I think I might knock this up on the bench and give it a test. As it is unbalanced we don't need push pull circuits or much in the way of iron so an inexpensive tube compressor could be a possibility.
What do you think?
Cheers
ian