06-11-2016, 11:49 AM
(This post was last modified: 06-11-2016, 11:53 AM by Diabolical Artificer.)
No worries Mark, am still learning about transistors so all info is welcome. I find valves easier to understand than transistors, they use electrons only, non of these holes to complicate matters.
Anyhoo, rebuilt the amp yesterday and after a bit of fiddling, It works better. See pic of schematic. I'm going to stop with it here as I have a lot of stuff needs fixing, like solid state amps. I could improve it further by direct coupling and finding a better OPT, but to be honest I'm quite chuffed with it as is, it "sounds" quite good, not too bad for my second attempt at building one from scratch.
As you see on the schematic it says 1% ish distortion. this is because A) the Marconi isn't calibrated B) I'm not 100% certain I have the procedure right. I measured the frequency response which is pretty flat but not brilliant.
A few questions, we use RC filters to limit frequencies going into the amp, IE a high pass filter on the input etc, but where would we put the low pass filter to limit the amp to 15khz say? At the coupling stage between anode of ECC83/g1 of EL84? I'm also a bit unsure about how to work out the value of the cap that couples g2 to earth. In some schematics I've seen this cap left off. Also do we really need the cathode bypass cap on the ECC83 if we use NFN?
Lastly I'm a bit hazy on how to find the R and C's values on the NFB. I used a 20 k pot, put a 1k sine in the amp, connected the OP to the distortion meter and scope and turned it till it till the distortion went down but not too much that we lost amplitude. I used a 390puff cap as that is what Mullard used in the 3-3. AFAIK though you put a 10k sq wave in and adjust C so you get flat horizontals.
Cheers, Andy.
Anyhoo, rebuilt the amp yesterday and after a bit of fiddling, It works better. See pic of schematic. I'm going to stop with it here as I have a lot of stuff needs fixing, like solid state amps. I could improve it further by direct coupling and finding a better OPT, but to be honest I'm quite chuffed with it as is, it "sounds" quite good, not too bad for my second attempt at building one from scratch.
As you see on the schematic it says 1% ish distortion. this is because A) the Marconi isn't calibrated B) I'm not 100% certain I have the procedure right. I measured the frequency response which is pretty flat but not brilliant.
A few questions, we use RC filters to limit frequencies going into the amp, IE a high pass filter on the input etc, but where would we put the low pass filter to limit the amp to 15khz say? At the coupling stage between anode of ECC83/g1 of EL84? I'm also a bit unsure about how to work out the value of the cap that couples g2 to earth. In some schematics I've seen this cap left off. Also do we really need the cathode bypass cap on the ECC83 if we use NFN?
Lastly I'm a bit hazy on how to find the R and C's values on the NFB. I used a 20 k pot, put a 1k sine in the amp, connected the OP to the distortion meter and scope and turned it till it till the distortion went down but not too much that we lost amplitude. I used a 390puff cap as that is what Mullard used in the 3-3. AFAIK though you put a 10k sq wave in and adjust C so you get flat horizontals.
Cheers, Andy.
