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John Warren

More Tube Pain

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CHB output module is next up.  I didn't get much further than getting the Fastons soldered in.  Completed the -45VDC bias and preamp filament supply.  

 

The 299C and LK-72A both use a separate bias winding off the power supply transformer to provide AC voltage to the supply. That's nice.

 

build#2_13.jpg

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Thanks.  

 

Took about 4 hours to build what's shown below including testing the HV outputs.  There are five HV rails.  To get the module good and hot, ran them into resistive loads for about an hour at 125VAC input.  Draws about 1100mA.  B+ at 125VAC is about 460VDC.  Uses 5AR4 rectifier, which I have many. 

build#2_17.jpg

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Really enjoy following this build. So refreshing to see work of this caliber being done by someone who obviously knows what he's doing. Thanks for sharing this.

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@John Warren, if you ever decide to sell these as modular kits, by all means please let the Forum know. Speaking only for myself, I have no need for preamp functions (tone controls, input selector, derived center channel, etc.), but would love to be able to put together 4 or 5 or 6 channels of basic 7591 amplification. 

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On 11/15/2020 at 10:10 AM, John Warren said:

 The crimping tool is a bit pricey at $240USD

John , do you have a link to that Crimping tool - tx

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Thank you all for the comments, I'm glad a few of you find it of interest. 

 

The first iteration of this thing, posted earlier in the thread, was to determine if the basic layout of the components produced a low noise, hum free amp that could operate reliably for 100s of hours of operation.  Here, a few revisions were baked in but the overall layout works well so I didn't change too many things with this go.   

 

Completed the CHA and CHB boards.  Next step is wiring it up.1296740350_build2_18.thumb.jpg.e96d189b9ce5d5074b4698bae9b775d5.jpg

 

There are three returns back to the HV supply card (green leads below), each lead sinks return currents from CHA, CHB and bias supply star grounding traces back to the center-tap. The HV supply utilizes it's own star grounding as well.  Every component with a return back to center-tap has its own return to the center-tap which took some time to layout.

 

2086238400_build2_25.thumb.jpg.6c4362a42e7e647bd87335056f9f04ac.jpg  

 

I'm a fan of the AMP Faston connectors, they make w-t-b connections simple to enable.  Purist would prefer solder but I'm not into making life miserable for fly-shit.

On these units, I'm using the Scott replacement output transformers from Transcendar so I should be able to use the same feedback circuits as the factory originals.  That was not the case with the earlier build using the Hammonds.  A tweak might still be necessary.

 

113219323_build2_19.thumb.jpg.126fcfeea8ed8a64f8f7706bbda5d7fa.jpg

 

On 12/27/2020 at 8:53 PM, RandyH000 said:

John , do you have a link to that Crimping tool - tx

 

I use a couple for ferrules depending on the connection.  Here I used the this one ($120USD) made by Zoller + Frolich (Crimpit F 6 L).   

 

  1420845127_build2_23.thumb.jpg.4b0c4cfe03ab26e5610cbd2aabe23562.jpg

 

 

201920202_build2_24.thumb.jpg.d17fc24dd469c0a7a1d8665aa007be22.jpg

 

 

 

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I actually had a few hours today to finish the second build.  Some small changes to the PC boards to make it a bit easier to build.  1197565620_build2_33.thumb.jpg.0de5c140c4df193f212206814579b5ea.jpg

 

This amp sounds different than the first build shown a few pages back.  The output transformers are the Transcendar 299C replacements for the OEM transformers.  These present a bit better load to the 7591s than does the Hammond (6000 Ohm v. 6600 Ohm, respectively).  I also have a very low dollar 5AR4 in the power supply section, that's nice and bright when on. 

 

1662103842_build2_34.thumb.jpg.e16337cc51450bb6d5eea21d45f407a5.jpg

 

 

Bottom side below.  The small signal leads are instrument cable, the shield drains (black leads) source to the chassis ground.  The approach of attaching the drain using a seperate lead is similar to the McIntosh approach used in 60s and 70s.  

 

The green leads to the center-tap.  What's very cool about this amp is that it has a center channel small signal out which I kept in the clone.  The four pots are CHA level, center channel out level, CHB level and Stereo balance.  I thought about putting all of this on a PC board but boards in small quantities boards are pricey.  The ones used on this build are sourced from Sunstone Circuits, top quality supplier.

 

 141936461_build2_26.thumb.jpg.4c22d2199604a70d0f3a3547569ccd7d.jpg

 

I left the leads long on the PS transformer.  If this was a production unit they'd be trimmed.  The AC introduction module has a filter in it to eliminate line glitches and other crap.  

I'll post some of the measurements taken on this amp next day or so.

 

1670995807_build2_27.thumb.jpg.a27306e92edff9059c6ed76a39296c06.jpg

 

 

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Being true to the design original has it's problems.  There are two 100uuF Silver Mica caps in the OEM units and one is in the feedback circuit sourced from the output transformer secondary.  The amp will oscillate using the Mica cap in the feedback circuit and cause the output tubs to red plate.  A 100uuF MLCC however completely defeats the oscillations.  It's the little yellow one in the photo below.   An explanation may be in how the caps are constructed.

 

1313577598_build2_37.thumb.jpg.ea24998a0949fe12c2def596ee984fcc.jpg

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CHA and CHB bandwidth with the Transcendar LK72/299C output transformers.  Some improvements over the Hammond units.  -3dB is about 40kHz for each, not bad for a 60 year old design.  The plots look similar to the OEM responses too.  Input signal is 200mV cross-correlated MLS.   Level pots open wide.

CHA_CHB_BW.thumb.jpg.bde0a790591791eed1706e8d9027daa6.jpg

%THD at 1000Hz below for each channel.  The uptick in the distortion curves is associated with oscillation pulses that occur in the output when the tubes approach the design limits. 

1252743801_CHA_CHB_1kHz_THD.thumb.jpg.d1081e0c40afcbdda8e635b46b91ac29.jpg

 

 

 

 

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I actually just found this thread.  I remember you saying something about maybe doing this.  But I Didn't know you had.  Very impressive.  My LK-72 is still kicking.  Never been better.

 

So how do you split the phase?  You just have a single line stage input?

 

Edit:

Never mind about the phase.  I see the preamp board is not shown (built) and clearly see the 6GH8.  This whole thing is very cool.  I bet it sounds good too.

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Great project.  Way above my head.

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John I did not know what a LK72 was until I googled and now know it is the Scott LK-72. https://www.hifiengine.com/manual_library/hh-scott/lk-72.shtml  At the beginning of this thread there is no mention of what you were creating so I just glanced at this thread and moved on thinking it was some boards you bought on Ebay. Now knowing what you are doing has peaked my interest and I am glad to see members here doing such things. Are you using any distortion analyzer software like Arta or either a distortion analyzer to see what kind of harmonics this build has? I have an old distortion analyzer but I have found software like Arta to be as good as the one I have. Being a PP amplifier I would expect to see less 2nd and more 3rd but one never knows until tested. Great instructive build thread, or should I say design thread. 

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About a year ago I sent John my HH Scott LK-72 to replace the power transformer and make a few repairs and he not only did that..............he did all this.

 

He also rebuilt 2 McIntosh 2100s for me prior, which are both still excellent.

 

He sure can design and turn boards, chassis, and whatever else darn fast.

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21 hours ago, henry4841 said:

John I did not know what a LK72 was until I googled and now know it is the Scott LK-72. https://www.hifiengine.com/manual_library/hh-scott/lk-72.shtml  At the beginning of this thread there is no mention of what you were creating so I just glanced at this thread and moved on thinking it was some boards you bought on Ebay. Now knowing what you are doing has peaked my interest and I am glad to see members here doing such things. Are you using any distortion analyzer software like Arta or either a distortion analyzer to see what kind of harmonics this build has? I have an old distortion analyzer but I have found software like Arta to be as good as the one I have. Being a PP amplifier I would expect to see less 2nd and more 3rd but one never knows until tested. Great instructive build thread, or should I say design thread. 

 

I'm pleased you find something of interest here!

 

I'm using a CLIO 12 24-bit analyzer @192kHz sampling for distortion analysis.  It's a high precision FFT analyzer with a wide range of input signals for analysis.    I also have Rohde & Schwarz UPV, Krohn-Hite and Keithley analyzer.  I can certainly provide FFT spectrum but find %THD to be a pretty good indicator of amplifier capability (as plotted above).

 

And yes, it's a HH Scott LK-72A/299C copy and you won't find this on Ebay.

 

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8 hours ago, mark1101 said:

Where is the preamp board?

 

It's a stand alone "basic" amplifier with CHA and CHB input level adjusts, a Center channel out level adjust and stereo balance pot.  

 

I actually did design a preamp board.  I can duplicate the entire amplifier, including the tone controls.  I designed the preamp board and ordered all the parts for the build, just need to order the board (the entire LK-72A preamp is on a single board!).

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