John Warren Posted November 20, 2021 Share Posted November 20, 2021 Here's a model the Scott LK72 (minus the phono pre), thought some here would be interested. The tube models are both my own and a few found on the web. I'll first post a few screenshots of the schematic and the run a few sims to show what's possible. Some advantages of running a simulation before committing to hardware: Determine dissipation power of components, find hot spots and size accordingly View current waveforms Peak-to-peak voltages Bandwidth, instability analysis and compensation Output transformer loading Predicted behavior with reactive speaker loads Overall model: Power supply transformer: Power supply: Channel A: Channel B: 2 Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 20, 2021 Author Share Posted November 20, 2021 I didn't bother modeling the AC entry module EMI/RFI filter I installed on the unit I built. Transformer coupling coefficients are set at 1.00 but are generally lower. Transformer resistances are measured immediately after shutdown while still hot. Capacitors are modeled with series and parallel parasitic elements (Rser, Lser, Rpar, Cpar, RLshunt) Filaments are modelled as pure resistances Wall power is perfect, 60Hz sinusoidal although noise and HF artifacts can be added. Rectifier tube 5AR4 has parameters in the tube sim that simulate rail sag (not easy but did it!). Speaker load is 8 ohm non-inductive resistor but reactive loads can be simulated easily. Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 20, 2021 Author Share Posted November 20, 2021 Plots show rectifier tube turn on current thru D1 side of 5AR4 (black) and voltage at Pin 4 (red). Blue is power dissipated by R48 which, at steady state at about 5 seconds, is ~12WRMS. Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 20, 2021 Author Share Posted November 20, 2021 DC rails Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 20, 2021 Author Share Posted November 20, 2021 425VDC center tap out sim v. measured. Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted November 20, 2021 Share Posted November 20, 2021 I think I just found a new friend!!! I even have the same Tektronix 475 analog scope LTSpice is a powerful tool in the arsenal for amp builders but many home brew builders choose not to learn how to use it. I tend to use it more for my own designs that way I can tweak stages for optimal performance etc.. A long time ago I did make a folder of vintage amps that I built in LTSpice, I think I still have the folder somewhere in my backup hard drive. You listed stability and compensation but I have found that the real world amp will be much different with layout parasitic variables (mainly stray capacitance at the plates of gain stages) and getting a precise output transformer model in LTSpice isn't easy either so I wait until the amp has been built to do my feedback compensation networks. I use my scope in X-Y mode with the input and output signals and use Lissajou graph to find the amplifiers breakpoints. Basically a 45° angle has a very specific pattern on the scope in the form of an elongated circle, I sweep the frequency until the exact elongated circle dimension is reached and then write down the frequency. This will be the phase shift difference between input and output in 45° increments which I then use to make a bode plot. The bode plot is needed to calculate the lead/lag compensation network values. Once they are determined I install them into the amp and re-test with square wave testing. I have found this to be the quickest and most accurate way to do this. Sometimes if it doesn't give the results wanted you may have to tweak the values up or down a little to see if they improve but if you take the time to do the breakpoints accurately it should be good to go the first time around. Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 20, 2021 Author Share Posted November 20, 2021 wrong amp Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 20, 2021 Author Share Posted November 20, 2021 4 hours ago, captainbeefheart said: I think I just found a new friend!!! I even have the same Tektronix 475 analog scope I purchased the scope when I was 29 years old from the on-site Tektronix FS engineer while I was working at Raytheon Microwave Power Tube Laboratory (Waltham, MA). That was in 1987!. Also, it's a 465 but from the photo could be a 475. Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted November 21, 2021 Share Posted November 21, 2021 1 hour ago, John Warren said: On the compensation side, the simulator gives a sense where the to start. For example, here's AC analysis of CHA output (S21 in S-parameters = power output relative to power input) as the feedback compensation cap is stepped from 10, 100 and 200pF. In the hardware 200pF works. Below 100pF causes red plate. Plots below are at a couple of output transformer coupling coefficient, .98 (top) and .99. When I start testing for the breakpoints there are no compensation caps in. The cap you are changing is the lead network as it causes it to break upward and lead which is usually set at the second breakpoint (-90°) after you set the lag compensation. The lag compensation breaks downward and is most likely C10 and C17 in your schematic (100pF at plate of input pentode). This frequency is chosen by the very first breakpoint, -135° when testing with no compensation. Most people only mention or fiddle with the lead compensation cap in parallel with the feedback resistor but the lag network is just as important if not more so. Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 21, 2021 Author Share Posted November 21, 2021 The plots I posted were for another circuit, not the Scott. My apologies! Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted November 21, 2021 Share Posted November 21, 2021 1 hour ago, John Warren said: I purchased the scope when I was 29 years old from the on-site Tektronix FS engineer while I was working at Raytheon Microwave Power Tube Laboratory (Waltham, MA). That was in 1987!. Also, it's a 465 but from the photo could be a 475. Glad to see someone else enjoying the quality of the Tektronix scopes to this day. I have nice digital scopes also but for some reason I use the 475 the most on the work bench. 7 minutes ago, John Warren said: The plots I posted were for another circuit, not the Scott. My apologies! no worries Quote Link to comment Share on other sites More sharing options...
DirtyErnie Posted November 22, 2021 Share Posted November 22, 2021 Sidebar, Could either of you two post/pm some good Spice tutorials? Thanks! Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 26, 2021 Author Share Posted November 26, 2021 The plot below is a simulation of the Scott clone 299C bandwidth and phase to 100kHz. The schematic is shown above but a few of the values shown there have been revised to better approximate the amplifier configuration tested. The Hammond 1650PA, 6600 Ohm input impedance output transformer is modeled based on the specifications provided by Hammond (input impedance, DCR each winding, primary inductance and leakage). Below is measured response of the amplifier. Reasonable prediction of the result. 1 Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 27, 2021 Author Share Posted November 27, 2021 Plot at left show current flow in bottom half of output tube primary (blue trace, I(L13)) and black trace top half I(L8). Summation is red trace (I(L13) + I(L8). Same current plots as above but with addition of grid and anode voltages for each pentod3 (U2 is plotted top, U3 bottom) Blue plot is current trace on secondary running thru 8 Ohm resistor (1.2A RMS), black is current in primary (89mA RMS). Secondary current is ~13.5x wrt to primary. 1 Quote Link to comment Share on other sites More sharing options...
John Warren Posted January 1, 2022 Author Share Posted January 1, 2022 Let's break out a 12AX7 preamp board from the LK72 schematic. I managed to squeeze both channels including bass and treble potentiometers on a 2-layer, 6.6x3.18" board (the max size allowable before incurring a much higher fabrication cost). Requires two inputs: 255VDC (anode) and -45VDC for filament heaters. Simulated bandwidth using LTSPICE. Input will be adjusted using external audio taper potentiometer. Quote Link to comment Share on other sites More sharing options...
mark1101 Posted January 6, 2022 Share Posted January 6, 2022 Nice. I like the amp but I really like the miniaturized preamp. Quote Link to comment Share on other sites More sharing options...
John Warren Posted January 11, 2022 Author Share Posted January 11, 2022 Yes. I've wanted to get this going for some time. The boards are due in on the 19th from the supplier. Takes a morning to assemble then test! Quote Link to comment Share on other sites More sharing options...
mark1101 Posted January 11, 2022 Share Posted January 11, 2022 I know you are busy, but this is good stuff if you can find time to share. So what other controls were you able to squeeze in if any? Balance, loudness, stereo/mono/reverse stereo, CH A, CH. B ? Any of these original Scott controls? Is the circuit the Scott LK-72 preamp circuit or modified? Quote Link to comment Share on other sites More sharing options...
John Warren Posted January 12, 2022 Author Share Posted January 12, 2022 15 hours ago, mark1101 said: Is the circuit the Scott LK-72 preamp circuit or modified? It's bone stock with added traces to probe the behavior. There will likely be a few components that differ from the OEM configuration to accommodate for any instabilities that might occur. Those are flushed out on at test. The board contains two independent channels with bass and treble pots. A log taper level adjust will be on the chassis. No additional mode selectors will be considered but the board allow for then to be added. Also, I used the same component IDs on the board as is on the OEM schematic so you can follow along at home! Quote Link to comment Share on other sites More sharing options...
John Warren Posted January 15, 2022 Author Share Posted January 15, 2022 Boards came in a bit earlier than expected. Checking first that the pc board hardware can be installed. Bass and treble pots. I'll start a separate thread on the preamp. Board mounted RCA terminals as inputs. 1 Quote Link to comment Share on other sites More sharing options...
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