John Warren Posted March 8, 2020 Share Posted March 8, 2020 Completed the power modules and HV supply for the LK-72A clone amp. Lots going on here but so far, after 4 hours of swept sine cycling from 0.1 to about 40W/ch, nothing has burned, smoked, caught fire, changed color or melted. Running the OEM output transformer against a Hammond 1650P. I developed the PC boards from the Scott schematic. This is the first iteration of the amplifier. I will redesign the entire amp after this one is completed. 2 1 Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 8, 2020 Author Share Posted March 8, 2020 Some %THD data at 1k (bottom) and 10kHz. The 1650P seems to be a wee bit better at both frequencies. Needed to tweak the FDBK loop off the OT secondary to avoid a nasty oscillation using the 1650P. Load is 16Ohm wire wound resistors. All's good with the world. Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 8, 2020 Author Share Posted March 8, 2020 1650P and the Scott TR-11-2. The 1650P is 6600Ohm P-P with 16/8/4 secondaries. Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 8, 2020 Author Share Posted March 8, 2020 Here was the problem. There was a NASTY oscillation in the neighborhood of 56kHz that needed taming. This is the FFT with the medicine applied, about 4dB bump and stable at all output levels. The oscillation was well over 12dB using the OEM FDBK loop. The blue and red plots are the OEM OT. Two output levels are shown. Quote Link to comment Share on other sites More sharing options...
Edgar Posted March 8, 2020 Share Posted March 8, 2020 8 minutes ago, John Warren said: Completed the power modules and HV supply for the LK-72A clone amp. Lots going on here but so far, after 4 hours of swept sine cycling from 0.1 to about 40W/ch, nothing has burned, smoked, caught fire, changed color or melted. Running the OEM output transformer against a Hammond 1650P. John, is that amp board a North Reading Engineering product? I have my father's LK-72 sitting in the basement, gathering dust. I remember it as one of the sweetest sounding amps I've ever heard. I been contemplating a multichannel (more than two) 7591 basic amp ... Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 8, 2020 Author Share Posted March 8, 2020 Hi Edgar, Yes, they're my "creations" but not so sure I'd call them products. As you can see, they're really not for the faint of heart! jw FFT of the two OTs, two different output levels. The OEM OTs are a wee-bit less "wiggly" but I can improve the response of the 1650P with a little tweaking! Quote Link to comment Share on other sites More sharing options...
Edgar Posted March 8, 2020 Share Posted March 8, 2020 4 minutes ago, John Warren said: Yes, they're my "creations" but not so sure I'd call them products. As you can see, they're really not for the faint of heart! They're far more than I have now, which consists of a March, 1961 issue of Electronics World with a build article for a 7591 stereo amp. Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 8, 2020 Author Share Posted March 8, 2020 B+ comes in at 428VDC The two modules draw about 1560mA AC at 117VAC The -45VDC (on-board) for 12AX7 (x4) preamp modules and DC bal pots comes in at a little under -48VDC. For this test, the 12AX7 filaments are simulated using a 340Ohm power resistor that gets HOT AS HELL. I used a muffin fan to cool everything because I was starting to get warm hanging over this setup with probes. Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 8, 2020 Author Share Posted March 8, 2020 12 minutes ago, Edgar said: They're far more than I have now, which consists of a March, 1961 issue of Electronics World with a build article for a 7591 stereo amp. I've reproduced the circuit faithfully, same component call-out IDs, level adjust for each channel, channel balance and even a center channel out. Beefed up the HV power supply and the -45VDC supply can supply about 2A of current. In an earlier test of the HV supply, I tortured it by step loading it up until either a trace lifted/melted or I blew-out the fuse. It was able to provide close to 8AMPS at 425VDC and didn't burn a trace but it did pop the fuse and smoked the rectifier tube. That's over 3kW of power! Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 8, 2020 Author Share Posted March 8, 2020 Now at 15kHz the OEM transformer does a better job, that's because I'm attenuating the 1650P at the higher frequencies to calm the beast at 56kHz. Load is 16Ohm wire wound resistors. 15kHz testing is hard on tubes, they sing loudly as the high power levels are reached. That means mechanical stress and potential for damage. Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 8, 2020 Author Share Posted March 8, 2020 The bottom plot is the FDBK sent to cathode of the 6GHB Pentode section. It's global FDBK (top plot) and sourced from the top of the OT secondary. It shows how the filter was modified to tame the beast. This is where circuit simulation comes in, makes life a lot easier. Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 8, 2020 Author Share Posted March 8, 2020 32 minutes ago, Edgar said: They're far more than I have now, which consists of a March, 1961 issue of Electronics World with a build article for a 7591 stereo amp. If the OTs are good and the supply transformer works you could have a "project" on your hands! FWIW, I am sourcing a chassis and preamp boards for this effort. Also, Heyboer makes a killer replacement power transformer for the LK-72A and 299C. Quote Link to comment Share on other sites More sharing options...
Edgar Posted March 8, 2020 Share Posted March 8, 2020 41 minutes ago, John Warren said: If the OTs are good and the supply transformer works you could have a "project" on your hands! FWIW, I am sourcing a chassis and preamp boards for this effort. Also, Heyboer makes a killer replacement power transformer for the LK-72A and 299C. The amp worked 100% last time it was used -- probably 40 years ago. But I really don't need a 2-channel integrated nearly as much as I need a 4-channel (or more) basic amp, because I like biamping and surround. That's why seeing the modules in your posts caught my eye. Meanwhile I'm going to look into those Heyboer transformers. Thanks. Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 8, 2020 Author Share Posted March 8, 2020 Here's the Preamp section. Manages to squeeze two channels onto the board. Has Bass/Treble adjust! Uses 12AX7. 255 VDC taps off of the HV supply board and -45VDC for cathodes sourced from on-board supply located on CHB output module. Quote Link to comment Share on other sites More sharing options...
OO1 Posted March 8, 2020 Share Posted March 8, 2020 - Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 8, 2020 Author Share Posted March 8, 2020 Absolutely, they're reliable and hold firmly. 250V hold-off capability between connections. Love them! Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 15, 2020 Author Share Posted March 15, 2020 Spent today getting the output modules stable with the Hammond 1650Ps output transformers. Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 15, 2020 Author Share Posted March 15, 2020 CHA and B distortion with the Hammond 1650P output transformers at 1kHz (GRN, RED) and 15kHz (YEL, PURPLE). 15kHz testing is very hard on the tubes. Load is 16Ohm resistor on each channel. For a tube amp, the responses are quite good! Quote Link to comment Share on other sites More sharing options...
OO1 Posted March 15, 2020 Share Posted March 15, 2020 - Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 17, 2020 Author Share Posted March 17, 2020 I haven't given that question any thought. Not clear yet if I want to offer the boards for builders, it's a bit complex. I also want to get the pre-amp sections build and tested but the Coronavirus shut my PC board supplier down on the west coast. Power output should be in the 30W RMS/ch. The 1650Ps are 60W transformers. 1 Quote Link to comment Share on other sites More sharing options...
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