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John Warren

More Tube Pain

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8 hours ago, mark1101 said:

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.

 

Your amp was the inspiration Mark!  

 

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On 2/8/2021 at 7:33 PM, mark1101 said:

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

 

Check out the brain on Fragnito:)

 

How on earth did you know that from looking at a photo and a tube? You never cease to amaze. I'm going to hold my opinion until I see what kind of knobs it comes with.

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8 hours ago, dwilawyer said:

Check out the brain on Fragnito:)

 

How on earth did you know that from looking at a photo and a tube? You never cease to amaze. I'm going to hold my opinion until I see what kind of knobs it comes with.

 

I have had various HH Scott amps over the years.  Love those old amps with Klipsch.  I know their layouts pretty well and have gone through the schematics time and again.  The 6GH8 tube which is marked on John's output PC board is the phase splitter tube on an LK-72.  But he does not have the preamp section with the 12AX7 driver tube and so it threw me at first.  Wasn't exactly sure what I was looking at initially.

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12 hours ago, John Warren said:

 

Your amp was the inspiration Mark!  

 

 

Well you have outdone yourself on this one.  Seriously nice job.  👍

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Thank you.

 

The original amp is a two prong whilst this thing is a three-prong so there's now a "chassis" ground and "0V reference" (i.e. the center-tap).    

 

On the early McIntosh solid state amps the small signals were handled using single conductor instrument cable with the shield drain lead connected to the chassis at both ends of the conductor, the lead break-out was connected to the shield braid using a small crimp clamp and then the entire end of the conductor insulated with shrink.  I've followed that approach here, the shield drain is connected directly to the amp chassis. 

 

1189935309_build2_21.thumb.jpg.780773d3e5a3fc95fc04951d444558e1.jpg

 

On the RCA input side of the amp, the ground sleeve is actually a 0V reference and is handled differently, it's connected directly to the PS transformer center-tap.

That's what's going on here, the green leads are 0V references, the black whips are chassis ground connected.

 

The point in all of this is to make the amp as quiet.  My take is it worked.

 

1541951093_build2_26.thumb.jpg.662cb16602a45cb76ffbfcdbc16397ed.jpg

 

 

 

 

 

 

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On 2/9/2021 at 10:38 AM, mark1101 said:

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

He just sent back my rebuilt McIntosh MC250s.  Amazing.  JW knows his stuff.

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On 2/13/2021 at 6:53 AM, jwc said:

He just sent back my rebuilt McIntosh MC250s.  Amazing.  JW knows his stuff.

 

Thanks JC, I'm pleased to hear they're working well.

 

When I was done with JC's 250 amps, they both measured well, low distortion and ruler flat bandwidth.  After a couple hours of running hard and loud, both were cool.  I was pleased when they went out the door.  When JC hooked them up to his rig, he's initial impression was they were hard on his ears.  Apparently a wee-bit of fiddling with the setup got them to behave.

 

I'll post a few photos of the work I did on JCs amps in the solid state forum.

 

 

 

 

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Getting back to the Scott clones, here are the two amplifiers, the one on the left is the first iteration (i.e. alpha) using the first set of PC boards I designed for the project.  It uses the Hammond output transformers.  The second iteration (i.e. the beta) is to the right.  That guy uses nice wire to board connectors a bit better trace layouts especially with regard to handling traces that drain back to the c/t.  It also uses the Heyboer Scott 299C replacement PS transformer and the Transcendar 299C replacement output transformers.  From the photo below you can see these are not small.1057036122_build2_41.thumb.jpg.eda5d08f63806967076f46f4ff167146.jpg

 

407018126_build2_42.thumb.jpg.0137e0873aafe336cfcb84366599ce6b.jpg

 

Here's the Beta with cage installed.  It went out FedEx yesterday so let's see how well it survives the trip to Chicago!  The chassis plate is 1/4"thick 6000 series Aluminum and all the fasteners are good quality.

 

2077455864_build2_40.thumb.jpg.37440680fd12015f3499501b601b7944.jpg

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Some performance data comparing the two designs.  Here, the analyzer was calibrated and left alone, the amps then measured sequentially.   Same tube types and manufacturers (JnJ) but the alpha tubes have more time on them so not entirely an apples 2 apples comparison.  DC balance was checked and adjusted before the test.   

 

1kHz %THD.  Both channels in the beta are similar in performance.

908064367_THD_alpha_beta.thumb.jpg.e33a9ee1addac65889c4959f75e6d3c5.jpg

FFT of the alpha unit, both channels (CHA is top):

 

1kHz-FFT_alpha.thumb.jpg.711bbfa93fc1f88b9dad6a23ec1741ef.jpg

FFT of the beta (CHA is top):

1kHz-FFT_beta.thumb.jpg.a092aad596fd88d890b177e772f40b7a.jpg

So some differences with beta being a bit better but not dramatic differences.  Both sound great and are fun to use.

 

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Here's the beta board set.  The boards are designed using a package that links the schematic to the layout.  So you first layout the schematic and all the painful details associated with the size of the leads, component body, wattage, pin identification and then commence with the layout.

 

Dual traces (top and redundant bottom trace) are dark shadowed in the photo.

589947077_build2_boardset.thumb.jpg.958592412426bb87a30455f040a99568.jpg   

 

Here's closeups of each board

2070205878_build2_HV_board.thumb.jpg.19d898f51dfc210ef413886665ee4ca7.jpg

 

1253621559_build2_CHA_board.thumb.jpg.08d44073dc9bf846e0a88015c9e8685c.jpg

 

339554516_build2_CHB_board.thumb.jpg.87aab47eb8721cf011d910357ebd5490.jpg

 

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So someone in Chicago wound up with the Beta?

 

Since there is no preamp at this point is it designed to operate with a DAC or other player directly, or do you need a preamp?

 

Are you going to build that preamp separate and make it able to be used with other amps besides those Betas?

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1 hour ago, mark1101 said:

So someone in Chicago wound up with the Beta?

 

Yes, he contacted me a few weeks ago asking if he could purchase it so I sent it away.  I've serviced a couple of his tube amps in the past. I'm curious to get his take, he has many vintage tube amps including some rare pieces. 

 

I've have a second Beta built and parts for a third.  PC boards and the machined chassis plate have a small price break at three pieces.  

 

Quote

 

Since there is no preamp at this point is it designed to operate with a DAC or other player directly, or do you need a preamp?

 

The amp will produce full power into an 8Ohm load  (~12VRMS) and <0.5%THD with an input signal of ~1.6VRMS which allows for DVDs, CDs, tuners are other "line level" outputs including pro hardware.  With sensitive speakers the output would be painfully high with a direct plugin of CD player and listener a few feet away.

 

It will require a preamp for a phono inputs. 

 

There's a separate level adjust for each channel and a stereo balance potentiometer.   There's also a line level center-channel output RCA jack with level adjust too.  I always wanted a center-channel in my setup and now I've have one.

 

Quote

 

Are you going to build that preamp separate and make it able to be used with other amps besides those Betas?

 

I have the preamp board on order but have yet to consider how it will be packaged.  I'm less excited about tube preamps.  I've designed solid state preamp boards with greater than 100dB distortion free output. 

 

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2 minutes ago, John Warren said:

 

I have the preamp board on order but have yet to consider how it will be packaged.  I'm less excited about tube preamps.  I've designed solid state preamp boards with greater than 100dB distortion free output. 

 

 

This is an important comment for me.  I have multiple two channel systems and every one uses a tube preamp.  Some use solid state amps and a couple use tube amps.  I found I achieved the type of tube sound I am looking for from the preamps much more so than the amps.  That's just me.

 

Not that I am not interested in those awesome looking mono-blocks...........But I was going to say to you that I would be interested in your preamp once it's built.  To be honest much more interested in the preamp given it has the parts that elevate the sound of that old circuit.  😃

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9 hours ago, mark1101 said:

To be honest much more interested in the preamp given it has the parts that elevate the sound of that old circuit.  

Whoa.  This thread.  Really.  There will be a preamp only version?

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I'll be giving a pair of Lundahl output tramsformers a go in this amplifier.  I'm curious to measure how a C-core transformer measures against the conventional units.  The LL1620/PP has the correct primary impedance (6k Ohms), wattage (25W) and max plate current (150mA) into an 8 Ohm secondary load.  There's a nice powder coated metal enclosure that can be purchased too.  It's almost an exact replacement for the OEM version but, theoretically, a better version.

 

It will require a third set of mounting holes be added to the chassis plate.  

 

The advantage of a C-core transformer is the magnetic field is well contained and lower stray flux, no sharp corners in the iron circuit.  The coupling between the primary and secondary should be better.

 

But can it be heard? and can it be measured in %THD?  .  

lundahl_ll1620_pp1-600x600_0.jpg

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On 2/15/2021 at 8:58 AM, mark1101 said:

 

This is an important comment for me.  I have multiple two channel systems and every one uses a tube preamp.  Some use solid state amps and a couple use tube amps.  I found I achieved the type of tube sound I am looking for from the preamps much more so than the amps.  That's just me.

 

Not that I am not interested in those awesome looking mono-blocks...........But I was going to say to you that I would be interested in your preamp once it's built.  To be honest much more interested in the preamp given it has the parts that elevate the sound of that old circuit.  😃

 

A tube front end is the ticket for me as well

tc

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Spent Friday last listening to the Transcendar and Hammond output transformer versions of the amplifier.  I have a well broken in set of Sylvania 7591s that I was using, swapping them from one to the other.  Speakers are JBL L200s.  I also revised the feedback on the Hammond to provide a bit smoother response above 50kHz. 

 

Here's the distortion curves for both channels with the Transcendar outputs and Sylvania tubes, virtually identical.  There's a leveling off as the power approaches 8W RMS or so then there's a significant rise past 15W RMS.  The plates can dissipate 19W RMS.   

CHA_CHB_1kHz_THD_Transcendar.thumb.jpg.9d8a6124f4a2e80db01598d252bfda72.jpg

Same tubes with the Hammond outputs.  The Hammond is rated for 60W, a much larger core than the Transcendar which is a 22W transformer. 

 

CHA_CHB_1kHz_THD_Hammond.thumb.jpg.8de626daee2d11c50629b04aea42bc9b.jpg

And here's the amps

 

build2_49.thumb.jpg.f6af384476a10a2619512d9ac7003f39.jpg

 

 

 

How did the sound?

 

Hard to discern a significant difference between the two but I did like the Hammond version better.  The L200 isn't that good in the mid range and the bass is "tubby" but, that said, both amps sounded good to my 62 y/o ears.  I was plugging a 24-bit CD player directly into the RCAs, no preamp.

 

The distortion curves were taken after my listening so I wasn't entirely biased based on measurements.

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On 2/19/2021 at 5:43 AM, John Warren said:

I'll be giving a pair of Lundahl output transformers a go in this amplifier.  I'm curious to measure how a C-core transformer measures against the conventional units.  The LL1620/PP has the correct primary impedance (6k Ohms), wattage (25W) and max plate current (150mA) into an 8 Ohm secondary load.  There's a nice powder coated metal enclosure that can be purchased too.  It's almost an exact replacement for the OEM version but, theoretically, a better version.

 

Took some time but the Lundahl units have arrived.   They're 25W RMS, 6K primary 4/8/16 Ohm secondary, C-core transformers.

 

 Lundhal_1.thumb.jpg.db0c9c0a52d1e9c05c25a05449e33eab.jpg

 

Lundhal_4.thumb.jpg.7ef7cc7fe997beb55b250adf77321746.jpg

 

 Here's the primary side of the output transformer, eight solder pins.

 

Lundhal_Pri.thumb.jpg.18919951fec0f998aa363748750c9a8f.jpg

 

And here's the secondary side, sixteen tinned wires.  The enclosures are optional, total cost with enclosures is $625 for a pair.

 

Lundhal_Sec.thumb.jpg.93ed5ac47d528aca6fd56d2569169d65.jpg

 

 

The secondary is a bit of a pain to wire.  To measure the primary impedance, I temporarily soldered it for an 8 ohm load per the schematic below.

Lundhal_Sec_schematic.thumb.jpg.f0dda55559762cd3a65616408ba1d9d4.jpg

GREEN is the primary impedance of the Lundahl with non-inductive, 8 Ohm load and, for comparison, the Hammond 1650PA PURPLE and the Scott OEM output transformer, GOLD.

The Lundahl is a bit flatter than the other two.  Output tubes are 7591s in push-pull arrangement. 

 

Lundhal_HammondPA_OEM.thumb.jpg.49c2375462838978c9b4fc11e643feab.jpg

 

 

 

 

 

 

 

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