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John Warren

More Tube Pain

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You guys sure make my morning egg sandwich and coke enjoyable.... (not a coffee person)

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15 minutes ago, Edgar said:

And with that we've taken this thread off the rails

Guilty as charged.  My apologies John.

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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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Continuing the assembly,

 

Installed the AC power entry module.  Contains fuse and the switch glows red when on.  

 

chassis_9.jpg

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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.

 

chassis_18.jpg

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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".  

chassis_15.jpg

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Bundled wires!  AAAAAAAAARGH.  NO!

 

Pay me no attention John, looks absolutely fantastic.

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On 9/9/2020 at 9:56 AM, CECAA850 said:

Bundled wires!  AAAAAAAAARGH.  NO!

 

Pay me no attention John, looks absolutely fantastic.

 

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. 

 

chassis_19.thumb.jpg.5ee6c6d0be018bafca4a78b5063f0059.jpg

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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. 

 

chassis_20.jpg

chassis_23.jpg

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I'm reasonably handy with tools of various sorts, but I look at some of the build threads here on the Forum (like this one) and I am humbled.

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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. 

 

 

build#2_1.jpg

build#2_2.jpg

build#2_3.jpg

build#2_4.jpg

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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.

transcendar_output_transformers_BW.jpg

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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.

 

transcendar_output_transformers.jpg

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For the second iteration of the amplifier revisions were made to improve build quality and simplify the assembly.  I've become a big fan of German Z&F ferrules for wire termination so those will be used for terminal block connections.   The crimping tool is a bit pricey at $240USD but lower cost ones can be found that do the same thing.  The higher cost units have interchangeable dies and are certified to crimp per a particular industry standard (DIN, CE, UL, etc).    

 

1128131033_build2_8.thumb.jpg.79c3c9dc4b051b85ae1cdabc5f6accfb.jpg

 

235203268_build2_9.thumb.jpg.d40fb61f95da65ca9370433f656cb462.jpg

 

The first two amplifiers were built using 400VDC Phoenix terminals and hook-up wire which doesn't cut it for voltages that exceed 460VDC at 125VAC input.  The revision will use 600VDC terminal blocks and Silicone insulated hook up for all +400VDC wiring.   

 

The wire to board connections will allow removal of the board without having to de-solder and the boards were revised to accept AMP 250 Fastons (both flags and verticals) and 7778 screw connect wtb terminal lugs for filaments.  And if they're good enough for the McIntosh retro MC275.....

 

Simple 2-layer boards made is USA!

 

729904468_build2_7.thumb.jpg.8ee490c22d3d3dfd81b9e5064425c9e9.jpg.    


The remaining hardware is due in this week.

 

1000054116_build2_5.thumb.jpg.a52c610734aadb497178aab43bebc137.jpg

 

 

     

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Continuing on with the second iteration of the amplifier.

 

The high voltage board terminals are now 600V Phoenix type.  A dedicated terminal block ties center-tap leads from the CHA and CHB output modules back to common junction which then ties to chassis ground and the IEC grounding lug located on the filtered AC intro module.  AC filament power now comes from Keystone terminal lugs.  The large cap is a screw terminal type that allows for easy field replacement.     

build#2_10.jpg

build#2_11.jpg

build#2_16.jpg

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