#1 Grid stopper resistors
Posted: Sun Feb 28, 2021 6:54 pm
What do we think about grid stopper resistors?
When building the 300B's, I remember considering whether to include them or not, as they were not specified in the Lynn Olson schematic or the Reichert circuit on which it was based. I do remember that I decided to build without them in the end, as the *SN7 has a reputation for being well behaved and the following stages had quite low gain. No RF tubes in this design.
However there seems to be a mix of opinion - some say you should include them regardless, some say you should only include if the circuit is unstable without them. I've seen no sign of any oscillation issues - nothing showed up on the scope tests but I may not have seen anything occurring in the Mhz range as I'm unsure how far up the Analog Discovery goes. Also no strange behaviour concurrent with oscillation. The leads from the 12SN7 to the 6V6 are very short - well they are the capacitor lead-outs. The wires to the 300B's are a little longer - between 4 and 5 inches I think. I've tried to keep signal paths as short as possible.
I suppose the question is, does this circuit raise any red-flags in terms of oscillation risk, and is there anything I should be looking for that may point to oscillation that I haven't detected? The output transformers remain cool in operation, and the amps themselves have been working fine the last couple of weeks or so - we use them for several hours every evening. So I'm thinking - leave well alone, but would appreciate opinions, dissenting or not.
Circuit here.
When building the 300B's, I remember considering whether to include them or not, as they were not specified in the Lynn Olson schematic or the Reichert circuit on which it was based. I do remember that I decided to build without them in the end, as the *SN7 has a reputation for being well behaved and the following stages had quite low gain. No RF tubes in this design.
However there seems to be a mix of opinion - some say you should include them regardless, some say you should only include if the circuit is unstable without them. I've seen no sign of any oscillation issues - nothing showed up on the scope tests but I may not have seen anything occurring in the Mhz range as I'm unsure how far up the Analog Discovery goes. Also no strange behaviour concurrent with oscillation. The leads from the 12SN7 to the 6V6 are very short - well they are the capacitor lead-outs. The wires to the 300B's are a little longer - between 4 and 5 inches I think. I've tried to keep signal paths as short as possible.
I suppose the question is, does this circuit raise any red-flags in terms of oscillation risk, and is there anything I should be looking for that may point to oscillation that I haven't detected? The output transformers remain cool in operation, and the amps themselves have been working fine the last couple of weeks or so - we use them for several hours every evening. So I'm thinking - leave well alone, but would appreciate opinions, dissenting or not.
Circuit here.