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Jeffrey D. Medwin

KT88 Direct-Coupled Design

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I have been designing and building tube amps ( for my own use ) for 40 years.  Decided to do a thread, where I show people the next new design.  This will be a  KT88  SE  DIRECTLY - COUPLED  two-stage amplifier, with a L.S.E.S. Power Supply ( Low Stored Energy Power Supply ).   Its all designed, and will be DIY building it over the next couple of months.  Since this section of the Forum deals with TUBES,  I thought some might enjoy seeing this build process, in photos.

 

I start all amp builds with a custom fabricated 14 gauge steel chassis, and prefer the sound of steel over aluminum.   The chassis dimensions are Golden Ratio, or a 1.62 to 1 ratio, length to width.

 

I carefully position major component placements, as well-thought-out lay-out in a SE amp is one of THE most important areas needing one's total attention and optimization.  Here is the start :

 

P1010118 Explained.jpg

P1010124 explained.jpg

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19 minutes ago, Jeffrey D. Medwin said:

since I prefer the sound of steel over aluminum.

Well, this is an excellent start.

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Another day, ( 3-22-19)  very minor progress, the Cardas CCGR Speaker Binding Posts positioned in place.  These posts, sourced from Michael Percy, are of high quality, some say undetectable - as far as sonic loss .

 

Starting off RIGHT :

 

Notice where the RCA INPUT jacks are to be purposely placed, and the resulting ultra SHORT wire lengths achieved

in the chassis spacing - shown below,............. ( input to output ). 

 

Most amps we see mis-place their RCA jack entry,  away from an optimal position.  Such poor attention to detail,

is akin to " starting off - on the wrong foot ".

 

In addition to having a short wire length, ( under two inches, and I use double Cardas 19 AWG polished silver

wire, RCA jack to the Input tube's control grid ) there is also - in the very front of the amp - the lowest level of stray / unwanted fields, by simply being positioned furthest-away from all of the 120 VAC wiring, power transformers, chokes, output lead wiring fields, etc. ). 

 

The Input signal IS the lowest signal level that the amp "sees", and it must be thoughtfully treated, prior to its amplification.

 

 

P1010015, explained 4.jpg

 

Cardas CCGR Speaker Binding Posts.JPG

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Hi Jeffrey!

 

I also love building and working on tube amps.  So direct-coupled two-stage input to output?  Interesting!  Direct coupling has often sounded better than more common approaches, and it's always nice to be able to do away with a series cap if possible (or in your case, desirable).  So, single-ended, with input jacks of front and away from heavy AC currents for reduced power supply rejection, short lead length, etc.  In the style of Leak and early Dynaco.  There have been wars waged here and elsewhere over the benefits of the phono jack position on the chassis, but logic and common sense would show it to be much better up front, IMO, even if somewhat less convenient at the back.  Of course there are some who would say, " Well, I put mine to the back, even right next to the mains input, without any problems at all."  So, each to his or her own.  This have the benefit of the signal input within very close reach (and inch or so!) of the grid of the first stage, and that's definitely better than having to route along the side, with shielded cable, from the back of the chassis up to the front, and on the way picking up stray capacitance trying to hitch a ride.  So, great!  Do you have a schematic to share, if you don't mind?  Are you using a tube rectifier, solid state?

 

Thanks for posting this here! 

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On 3/23/2019 at 1:22 PM, erik2A3 said:

Hi Jeffrey!

 

I also love building and working on tube amps.  So direct-coupled two-stage input to output?  Interesting!  Direct coupling has often sounded better than more common approaches, and it's always nice to be able to do away with a series cap if possible (or in your case, desirable).  So, single-ended, with input jacks of front and away from heavy AC currents for reduced power supply rejection, short lead length, etc.  In the style of Leak and early Dynaco.  There have been wars waged here and elsewhere over the benefits of the phono jack position on the chassis, but logic and common sense would show it to be much better up front, IMO, even if somewhat less convenient at the back.  Of course there are some who would say, " Well, I put mine to the back, even right next to the mains input, without any problems at all."  So, each to his or her own.  This have the benefit of the signal input within very close reach (and inch or so!) of the grid of the first stage, and that's definitely better than having to route along the side, with shielded cable, from the back of the chassis up to the front, and on the way picking up stray capacitance trying to hitch a ride.  So, great!  Do you have a schematic to share, if you don't mind?  Are you using a tube rectifier, solid state?

 

Thanks for posting this here! 

 

 

Thanks for such a thoughtful reply Erik.  In the first post's second picture, those are two 5U4GBs in the far-right rear-most-corner of the amp - distanced away from all else.

 

'Will provide you with a more detailed layout photo Erik, posted below. 

 

"Will also supply a " Master Schematic ", which you can see below, .....a simple D.C. circuit.

 

This Master Schematic  can NOT BE A DIY BUILDER's  SCHEMATIC, because my up-coming implementation contains proprietary design details, not originally my " intellectual property ", that I have promised not to put in the public domain. This Master Schematic I post, however, will give you an accurate view of the design, and someone could adapt it to their own operating points. 

 

I use CONSERVATIVE operating points in all I execute, so that it lasts longer, and typically,  it sounds better for much longer, less thermally stressed !!

 

 

P1010013 rev 8.jpg

 

 

SNIP Master KT88.JPG

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Sure, it was great to see you posting here!  Ok, I guess I didn't see the 2nd picture....but got it!  Same rectifier I use for my 300B amp, and direct-coupled 6SN7 input and driver.  And also like you, a portion of the design is borrowed, with permission, and I don't post the schematic.

 

I'm always much more curious about circuit design than cosmetics, so tend to ask more about that.  What speakers will you use with it when done?

 

 

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Oh! I seemed to get to the part of this thread that completely missed the other pictures.  So, ok!  I've got now. ;) 

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On 3/23/2019 at 3:24 PM, erik2A3 said:

Sure, it was great to see you posting here!  Ok, I guess I didn't see the 2nd picture....but got it!  Same rectifier I use for my 300B amp, and direct-coupled 6SN7 input and driver.  And also like you, a portion of the design is borrowed, with permission, and I don't post the schematic.

 

I'm always much more curious about circuit design than cosmetics, so tend to ask more about that.  What speakers will you use with it when done?

 

 

 Erik,

 

Take a look and see !!  I no longer use Consumer Speakers.   ' Been using Pro Speakers.  Under my name-tag in each post, left hand side is " See My System:  "  use URL referenced to view photos of the speakers : simple,  two-way, time aligned, both drivers are DIRECT FRONT - LOADED horns.... 8 AWG equivalent wired.

 

I no longer can stand to listen to a three stage tube amp Erik,  ' been spoiled rotten.......

 

 

 

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VOTT - cool!  I search on those every day, knowing full well I'd never be allowed to bring them into the house! :)  Trapezoid Klipsch La Scalas were accepted as good industrial design as a contrast to softer and more comfortable looking furnishings.  I sold those and bought a pair of really nice birch designer La Scalas, which made. my wife happy.  So, no 3-stage amps, and no 3-way loudspeakers!  Great looking crossover, and look at the size of those monster L-pads!  Good for 1k watts+ I bet!  The only thing I'm worried about is that I don't see any non-polar electrolytic in there. ;)

 

I have to say, Jeffrey, even though your really like the big VOTTs, the La Scala, and Klipschorn are also fully horn loaded, though there are many who re-use the LS bass bin with a wide-band horn on top in a two-way configuration.

 

I could swear I've seen your initials on the lower right of other schematics I've seen online -- I've studied many hundreds.  Have you posted them elsewhere?  Nicely drawn, by the way.

 

 

 

 

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On 3/23/2019 at 7:08 PM, erik2A3 said:

VOTT - cool!  I search on those every day, knowing full well I'd never be allowed to bring them into the house! :)  Trapezoid Klipsch La Scalas were accepted as good industrial design as a contrast to softer and more comfortable looking furnishings.  I sold those and bought a pair of really nice birch designer La Scalas, which made. my wife happy.  So, no 3-stage amps, and no 3-way loudspeakers!  Great looking crossover, and look at the size of those monster L-pads!  Good for 1k watts+ I bet!  The only thing I'm worried about is that I don't see any non-polar electrolytic in there. ;)

 

I have to say, Jeffrey, even though your really like the big VOTTs, the La Scala, and Klipschorn are also fully horn loaded, though there are many who re-use the LS bass bin with a wide-band horn on top in a two-way configuration.

 

I could swear I've seen your initials on the lower right of other schematics I've seen online -- I've studied many hundreds.  Have you posted them elsewhere?  Nicely drawn, by the way.

 

 

 

 

 

 

Hi Erik, 

 

In my decades of  transition from Consumer Speakers to Pro Speakers ( Movie Theatre ) I owned / used a pair of La Scalas,  stock and highly modded, and many of my friends run Corner horns.  Eric, there is a difference between Consumer speakers and Professional speakers.

 

I started with 1930's - 40's RCA Pro stuff MI-1428B field coils, and double 15 inch Ubangie Bass Horns, and now have gone to ALTEC , drivers like the 515B woofer and 802D compression driver, both very capable. I adore the present A7-8s, as they have been lightly modded. 

 

The front horn loaded 515B 15 inch driver, ( emphasis of " front " - from 135 hZ up ), is really COOL to hear.  I simply prefer the 15 inch to be front horn loaded .

 

Did you see anything unusual in the Master Schematic ??

 

Those L Pads sounded terrible, BTW, and I ditched them in Q1-2019. 

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Jeffrey,

 

regarding your our question about the schematic, I was actually wondering if there were a couple of mistakes (I don't at all mean to disrespectful, so please don't take offense).  Everything looked normal.....and then I thought, huh!?  And then I considered the possibility that it was intentional.  So! To answer your question, yes I did.

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On 3/24/2019 at 12:04 AM, erik2A3 said:

Jeffrey,

 

regarding your our question about the schematic, I was actually wondering if there were a couple of mistakes (I don't at all mean to disrespectful, so please don't take offense).  Everything looked normal.....and then I thought, huh!?  And then I considered the possibility that it was intentional.  So! To answer your question, yes I did.

 

What was different on Schematic ??  What were you thinking was a mistake ?  Might be very important Erik. Please let me know.

 

Thanks !!

 

 

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Jeffrey,

 

No, it's good.  I was really tired when I looked at it last night, but it had to do with the rectifier filament wiring.  Makes total sense now.

 

Thermistor on the mains input is a good idea.  That's become common practice for me too.

 

No worries!  It was my misreading, not your drawing, which is very clean and accurate.  Direct coupling is excellent.  In fact, I made reference to it in another recent post I made regarding a designer's choice to do away with inter-stage coupling caps.  I would be interested in giving your design a try!

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On 3/24/2019 at 8:49 AM, erik2A3 said:

Jeffrey,

 

No, it's good.  I was really tired when I looked at it last night, but it had to do with the rectifier filament wiring.  Makes total sense now.

 

Thermistor on the mains input is a good idea.  That's become common practice for me too.

 

No worries!  It was my misreading, not your drawing, which is very clean and accurate.  Direct coupling is excellent.  In fact, I made reference to it in another recent post I made regarding a designer's choice to do away with inter-stage coupling caps.  I would be interested in giving your design a try!

 

 

Erik, 

 

I have written a White Paper, over the last several years, on Direct Coupling of tube amps for Audio.  If you PM me, and send me your email address, I would be glad to send it to you, as long as you do NOT publish any parts of it, since it is NOT all my design work, and I need to protect the original designer, who is a commercial amp builder / manufacturer. 

 

You are at a stage in audio where you will like this a lot !! 

 

Sunday afternoon, back to drilling holes and tapping 6/32nds threads into this 14 gauge steel chassis.  Have decided where to place Under-The-Deck parts, and need to provide threaded holes, placed precisely, for below-deck component mounting.  Ugh, but it needs to be done ...and done carefully / well.

 

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I appreciate that kind offerJeffrey, and I most certainly would never even think about sharing it.  Other designers have shared their hard work with me in the past in confidence, and it's something for which I have much respect.  And I applaud your explaining that you are in turn being influenced by others rather than taking full credit.

 

I would VERY much like to read your paper!

 

Thanks for offering!

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3-25-19 PROGRESS Photo :

 

For several days now. am still hand drilling , chassis punching, and tapping holes in the chassis, to mount various components. 

 

There are 24 Larger holes, and 64 smaller ones - tapped 6-32 nds, for component positioning and mounting.  MORE to go, next few days.

 

Ordered Powder Coating Material today. Going to do amp in a Midnight Blue Wrinkle powder coat finish.

 

 

 

 

 

P1010131 explained 4.jpg

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    3-26-19 PROGRESS Photo :

 

'Just finished drilling and tapping ( 6/32nds ) eight holes to secure bottom cover,  to chassis.  14 gauge steel. 

 

Only one Manufacturer of tube amplifiers discovered how to make an ordinary bottom cover into a Kick-Butt AUDIO ISOLATION PLATFORM.  And I try to copy that person's work !

 

After this chassis comes back from Powder Coating, I will show anyone interested, with photos,  HOW its done.  Its very cool !!

 

P1010139 Explained 3.jpg

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Decided today to add a third chassis cooling hole, under the ( raised above steel chassis like all chokes and transformers) Power Transformer's #3  position.

 

I really like this amplifier chassis to have this specific design-feature.  Three cooling holes

 

Its Biblical to do this !!    Did you know that ??!!?       Holy, holy, holy .....  AND /  OR...... Kadosh, Kadosh, Kadosh !!

 

 

 

P1010155 Fully Explained.jpg

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       3-27-19  PROGRESS Photo :

 

A first / preliminary idea - on how to position many of the amp's power supply / below-deck parts. 

 

Coming from the unseen Power Transformer ( located above deck ) there will be an 8 AWG ( equivalent ) Mil Spec wire ground, running below-deck, under C1 ( which is raised 5/8ths of an inch above the under-chassis ). This 8 AWG silver-plated copper-stranded wire becomes the main ground buss, positioned at amp's center, for the length of the amp - past the RCA Jacks. 

 

After the shared L1 / C1- C1 BYPASS , this stereo amp is to be symmetrically - constructed, channel-wise.  Notice how I've skewed on purpose, positions of the 5VCT transformer, and three chokes L1, L2, L2 under-chassis.  Why ??

 

No ( typical ) negative feedback ( is needed - or wanted ) allowed in this Pentode KT88 amplifier !!!  See Master Schematic in a post above.  

 

Each channel's Input stage  B+, and the G2 Screen B+, is individually Shunt Regulated.  A Shunt Regulator uses just ONE part, .....a large chassis - mount wirewound  Power Resistor.  KISS..... rules.

 

Swell, .........  UPS just delivered a pound of Power Coating, a favorite color.

 

"Off" this build - till Monday !!  Vacation. 

 

Comments, questions, advice, input, or encouragement to continue on ........ is welcomed.  Thanks.

 

 

3-27-19 Explained 3.jpg

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Jeff,

 

Looking good.  By the way, I did find the white paper you sent.  I had somehow missed it before.  Thanks so much for sending!  Initial layout looks good, too.  

 

....surprising other resident builders haven't responded here, but I am certainly staying tuned in to how this design works out.  Anything using -- or better -- able to use, direct coupling is of great interest to me, single-ended in particular.

 

Thank you for sharing your project here.

 

erik

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