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John Warren

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John Warren last won the day on October 4 2014

John Warren had the most liked content!

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About John Warren

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    "So much for the experts on this board"

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    Engineering-Audio, magnetics, materials for electronic and magnetic applications, engineering models and simulation, SPICE, MATLAB, FORTRAN, acoustics, complex algebra, physics of sound, microphones, vintage audio, loudspeaker design, amplifier design, McIntosh amplifiers, discrete semiconductor devices.....and movies including silents, foreign and indies.
  • My System
    12" Utah Tri-axial drivers mounted in LRE bass "reflex" enclosures.

    Sony Superscope FM only

    Lafayette Solid State Stereophonic Integrated Amp

    16 GA Lamp Wire

    Koss Pro 4AA

    Technics SL-QD33

    CD Player:
    NAD 325i (modified)

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  1. Primary impedance presented to the P-P plates using the Transcendar output transformers shown below. The OEM transformer is BLUE. Some differences but I'd be hard pressed to hear them using the 7591 output tubes.
  2. The Transcendars output transformers are excellent replacements. The bandwidth measurements are below. The OEM transformer is in BLUE. The Green and Dark Red traces are the two Transcendar units. Takeaway is the Transcendar replacements exhibits the same bandwidth. Load is non-inductive 8-Ohm. Analyzer is set at 194kHz sampling.
  3. Starting the second build of the LK72 amplifier. The replacement Heyboer PS transformer and Transcendar output transformers are being used in this build. The Heyboer transformer is a substantial transformer, a superior embodiment of the OEM hardware.
  4. Chassis cage attached on the Alpha. Next iteration of the amplifier (i.e. Beta) will have the Transcendar output transformers and the Heyboer power supply transformer. And of course, the performance will be measured and posted here.
  5. Presume you're referring to the blue, spiral-wrap bundle. Those are B+ leads and 380VDC screen voltage for the 7591s. The white lead is -45VDC for DC balance pots sourced from the on-board -45VDC supply. That supply is capable of driving numerous 12AX7 preamp cathodes, the bridge will provide about 2FLA if not limited by upstream fusing. The HV supply has both 190 and 255VDC rails to drive the LK-72A preamp section. A second iteration of the amplifier (which will be going to a customer) will have a few hardware and PC board tweaks including 600V barrier terminals, Transcendar 299C replacement output transformers (better bandwidth than both the Hammonds and the original Scott units) and a new Heyboer power supply transformer. The Heyboer transformer is a very well made unit (NOS valves to thank there). The original LK-72A/299C amplifier is, of course, not grounded to the Earth connection by the power line plug. So this needs to be considered. Center-tap drains are star-configured on all three boards, which is a painful routing exercise given the mix of high current AC, HV DC and low voltage, high current DC leads on each board. The stair "point of contact" from each power output board returns to the IEC ground prong via dedicated, 16 AWG leads soldered to the Earth prong tab located on the AC housing module (i.e. no isolation). The negative screw terminal lug on the blue, big-*** cap located on the HV board is tied directly to the center-tap which is also tied to the center-tap drains of the output boards. There's also a chassis ground on each board which drains small signal ground sheaths (only) to the chassis which is then tied to the ground prong through the AC input module which has, in addition to fusing, has an internal line blocking filter that eliminates powerline artifacts and protects the circuits from surges.
  6. So, this is what it looks like off the bench and running. I have a cage that covers the entire set of boards but it needs a few holes drilled in the top to allow the DC balance to be adjusted. I'll grommet the holes and install fiberglass shafts to allow adjust whilst operating. I posted a few more photos in the "show us your tube amp thread".
  7. Output module boards installed and running hard sinewave sweep at full power looking for trouble. While running there was a power failure, neighborhood wide. I thought for a second my little amp pulling a MASSIVE 1450mA brought the local grid down! What was nice, the power cycled back on a second or two after and the amp fuse tripped nicely (2FLA slow). I use an autoformer with a 3A fuse installed to feed the circuit.
  8. My clone of the Scott LK-72A amplifier section with the cage removed. Needs a nice wooden plinth. I have a pair of the Scott LK72A/299C output transformer replacements from Transcendar and will swap those in after spending time with the Hammonds. The Hammonds are a wee bit oversized for the power levels but I had them lying around so why not. Compensation needed some tweaking with the Hammonds. Also, the original design used silver mica in the FDBK return which I had a bit of a time with. For some reason, which I'm still noodling, MLCC provided much better stability based on the analyzer response. The amp is insanely quiet. Spectrum analyzer shows flat well out past 50kHz. If there's ever another Pilgrimage I might be tempted to drive this thing and a modified version that's almost complete down to Hope so we can all listen, compare, contrast.
  9. Boron Nitride plate used for load testing the HV supply. The loads get pretty hot!
  10. Also installed the HV supply board.
  11. Continuing the assembly, Installed the AC power entry module. Contains fuse and the switch glows red when on.
  12. A man that knows his waste treatment is always after my heart!
  13. Gents- No reason to apologize. The discussion is not unamusing, for sure. And, I've derailed plenty of threads too so there you have it!
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