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Jeffrey D. Medwin

Changed Ra and Rk on Dyna MK 3 tube amps

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On several occasions up here, I have suggested that instead of tube rolling, one should FIRST upgrade the cheap Ra ( Resistor anode, or plate resistor ) and cheap Rk ( Resistor Kathode, or cathode resistor ) on the FIRST / INPUT tube in your tube amp, that your Manufacturer used as a cost containment basis. 

 

It is ALWAYS a cost compromise there, by the Manufacturer, VS what is easily possible to gain in audible performance.

 

There is MORE to gain sonically, with some intelligent type of PROPER RESISTOR SELECTION and IMPLEMENTATION, than any tube rolling will get you, and at 1/4 to 1 /10th the cost.

 

For Ra spots, I  prefer the use of two 2 Watt quality resistors, very closely matched to EACH OTHER :  0.1 % to 0.01% is OK enough.  Use double the Ohmic value desired, when paralleling two Rs.  For Rk spots, one can use either one 2 Watt Rk, or two 2 Watters in Parallel, depends on budget - and how good you want to hear it.

 

So, I have this  nice-guy audio friend in New York area, owns Dyna MK 3s, for 20 years now.  I bought for him, from eBay, discontinued N.L.A. Corning ( CGW, Corning Glass Works )  2 Watt resistors, for BOTH of his MK 3s, in all the more critical spots, for only about $15.50 shipped to him. The new Rs were considerably larger :

 

                                  1063051993_Ra(1)SNIP.thumb.jpg.96e94ecb0b659462a95f64c389f02fd0.jpg

 

 

 

Room is very tight on the Dyna MK 3 stock board, and here is how he ended up adding two CGW 2 Watters in parallel , vertical, for the new Ra.

 

 

1761160068_uprightUSEEDITED.thumb.jpg.f583ee48329d96bf7b35c35d6a134bc3.jpg

 

 

Here is his report, the report that counts the most, his listening report, sent to me late last night, with only the Ra re-done, upgraded  :

 

1717272965_FirstINITIALVerbalizedreportoneof2.thumb.JPG.3fc43cd0aa8956a7bbf2a273b98eb67a.JPG

and three hours ago, with Ra AND Rk upgraded  :

 

134866938_SecondreportRaandRk.thumb.JPG.5ac521ac45baa02d78d9a04e583969f6.JPG

 

Ohhh, I almost forgot, I sent him about a foot, ( 30 cents worth - surplus priced ) of m22759/11 20 wire, and he augmented the board trace, from the upgraded Ra to PINS 6 and 2, which is the plate of the input tube and its direct couple to the grid of the phase splitter.   It was less than 1 1/2 inches of the the stock Dyna's printed circuit board trace - that benefits from this augmentation.  It is " the " sensitive span in that amp's circuit.   These mods will apply - always with improved listening results, ( and varying in degree ), when applied to all commercially-made tube amps.

 

 

Mech Engr Vic has begun a similar mod, for his DYNACO ST-70 series II , and he is partially done, ask him what he heard, just doing the Ra and a small amount of circuit trace augmentation, about two inches.

 

    70080104_Pins6and9rewiredfromRaedited2.thumb.jpg.641a14101315a2b1b3701eca80b44ebd.jpg

 

 

 

 

Well, you now see what to do !!   Have fun, listening to all your favorite L.P.s or other sources

 

Jeff  Medwin

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17 hours ago, Jeffrey D. Medwin said:

It is ALWAYS a cost compromise there, by the Manufacturer, VS what is easily possible to gain in audible performance.

 

 

17 hours ago, Jeffrey D. Medwin said:

These mods will apply - always with improved listening results, ( and varying in degree ), when applied to all commercially-made tube amps.

With the exception of those two statements above, pretty good post

 

I'm no expert, but when a parts change, across all products with an unqualified "always" I always raise my eyebrows.

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1 hour ago, dwilawyer said:

 

With the exception of those two statements above, pretty good post

 

I'm no expert, but when a parts change, across all products with an unqualified "always" I always raise my eyebrows.

 

It makes sense to me Travis, in that the INPUT tube is the most sensitive in the whole amp, and if the audio signal is compromised in any way there, it is never made up, as it goes through the amp's remaining circuitry.  The blanket recommendation is done because Manufacturers work within a budget, so their amps are competitively priced, so they can stay in business. 

 

I can only think of few exceptions to my statements, that I am aware of.

 

The gist of the Thread was meant to be, " this is low hanging fruit ", try it, but do it right, and pick the correct resistors to install.  

 

Jeff 

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53 minutes ago, Jeffrey D. Medwin said:

I can only think of few exceptions to my statements, that I am aware of.

 

Then it isn't "always", maybe "commonly" or "frequently" but not always.

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I made this modification (matched and paralleled Roederstein mk8's at the driver tube plate resistor position) and did notice an improvement in the overall sound of my Dynaco ST-70 series II. I also replaced the powers tube's bias resistors with a paralleled and matched pair of Audio Note Tantalum film resistors and also heard an improvement in sound plus an improvement in the stability of the bias circuit which was an unexpected plus. Better resistors seem to improve the "smoothness" of the sound (that's the best way I can think of to describe what I'm hearing, maybe "warmness"?). I don't know what specifically they are doing to the signal, but I have noticed similar improvements when upgrading resistors in speaker crossovers. I know many will say resistors have no effect on sound quality as long as they're in spec but repeated replacements have yielded similar results. I know Jeff likes the Roedersteins, and Deang practically swears by Mills resistors, but I've been happy with several brands of higher end ones including Mundorf, Amtrans, and Rikenohm. 

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6 hours ago, MechEngVic said:

I made this modification (matched and paralleled Roederstein mk8's at the driver tube plate resistor position) and did notice an improvement in the overall sound of my Dynaco ST-70 series II. I also replaced the powers tube's bias resistors with a paralleled and matched pair of Audio Note Tantalum film resistors and also heard an improvement in sound plus an improvement in the stability of the bias circuit which was an unexpected plus. Better resistors seem to improve the "smoothness" of the sound (that's the best way I can think of to describe what I'm hearing, maybe "warmness"?). I don't know what specifically they are doing to the signal, but I have noticed similar improvements when upgrading resistors in speaker crossovers. I know many will say resistors have no effect on sound quality as long as they're in spec but repeated replacements have yielded similar results. I know Jeff likes the Roedersteins, and Deang practically swears by Mills resistors, but I've been happy with several brands of higher end ones including Mundorf, Amtrans, and Rikenohm. 

 

 

Mills makes a large 2 Watt resistor called an MR-200. and Michael Percy stocks them.  They are not inexpensive.  I'll post the Mills selection from an old Percy catalog below.

 

1171653711_MR-20011-17-19.thumb.JPG.37e5eb411a938c539de5f2a42705c2a1.JPG

 

Anyway, the Mills MR-200 series, IF you can fit them into any existing amp, are about as good as I have personally heard, used in state-of-the-art 2A3 amplifiers. I've listened at RMAF shows from about 2006 to 2011, to several very special tube amps using MR-200s.   Boy, they have a fabulous high end, and they just sound " right " -  " special " as a plate resistor.

 

Deang's use of Mills, for Speaker Crossovers, would likely be Mills MRA-5 and MRA-12 types, a very nice sounding wire wound.  I like his choice, as a crossover R. .   I use  MRA-5s, and mostly MRA-12s in power supply circuits in amp builds. 

 

But for a Plate Resistor that me ( and /or ) a friend has tested critically  :

 

 the SILVER AUDIO NOTE 2 Watters, and

 

 the Mills MR-200 2 Watters ,  are at the top of the heap, ............sonically. 

 

But the Audio Note Silvers are what ?, about $45.00 each, whereas the MR-200s are shown in Percy's catalog, at around six dollars each.  The MR-200s have a large surface area, so the need to use two MR-200s, same value, tightly matched and paralleled to each other does not really apply as much , VS  using a single MR-200.

 

On the bargain side, vintage N.L.A. Corning two watters might sonically rate as an "8 out of 10".  There is an eBay seller, who has old stock, but not in a wide assortment of values.  eBay search term is " 5 pack Corning"  .  He charges ONLY $2.00 a pack. Five resistors in a pack, only 40 cents each for a 2 Watt Corning.

 

I should explain, when you listen to a tube amp, you not only hear the tube, but you are also hearing its Plate and Cathode resistor ( Ra and Rk ), with an audible effect.  Why else would companies make, and people buy, 6 dollar and 45 dollars a piece resistors??  

 

If you are wondering  :   a good 2 Watter will sound superior, when paralleled and TIGHTLY matched to another, than that single 2 Watter alone !!.  A 2 Watter will sound better than similar and smaller 1 and 1/2 Watter, is also a result we consistently hear.  Seems like " overkill ", using two 2 Watt quality resistors, on the Cathode of an 12AX7 type input tube, operating at 1.5 VDC and 1 mA. of current ( 0.0015 Watts dissipation ) .  However,  in any good amp, the Rk resistor implementation is HIGHLY audible, and , along with the tube's Ra,  both are key optimization areas. 

 

Look below at my 2019 KT88 DC stereo amp build, partially done, and you will see photos of this precise implementation of paralleled 2 Watt resistors for the input tube, Ra and Rk :

 

1403611102_EDITEDWIRINGKT88FrontEndREV2.thumb.jpg.9e9a813c833b335680e40847dcd00618.jpg

 

 

Jeff Medwin

 

 

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Blanket recommendations of anything are to be viewed with a great deal of suspicion, and this applies to tubes as well as other components.  In the applications noted above, matching paralleled resistors to a tolerance of 0.01% is ludicrous—and I urge people not to waste their time chasing sonic nirvana by doing that.  In audio, I often find that personal expectations create a “mind over matter effect” which may explain why people claim to hear dramatic differences in sound by using modifications which can not provide the claimed effect.

 

 

Maynard

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Hello All,

 

Mike Stehr brought up a VERY good point, the SAFETY issue of me showing Mark Z's resistor install, and inadvertently, me suggesting others to do-so like Mark Z. on this, a Public Forum.  Until Mike pointed it out, the thought of lessened-safety from shocks, was something myself, and others, did not properly consider . 

 

I apologize for not thinking enough of that facet of the work I pictured, done by Mark Z.  His work was only done in the past several days, and often, one has to think, or rethink about a modification's execution, before they discover an optimal way to execute  it.  That certainly was the case here. 

 

Frankly, I was just "taken" by the fact that Mark had squeezed the two resistors in, and that he was so pleasantly surprised and pleased with the resultant sound.  Mike Stehr is 100% correct.    And so are the Moderators.  And I now see why the Moderators have been witholding my posting on this issue.  That is very proper.

 

My own resistors, and most other amplifier's resistors, are usually located below and inside the chassis.   Not so, with the DYNA Mk3 and the DYNA Stereo 70 , both which I pictured above.

 

So - to not ruin the continuity and important intended message of this thread, ( the sonic benefits of paralleled 2 Watt resistors, as an Ra and Rk ) lets leave Mark Z's picture intact, of his initial  Dyna MK3 Ra modification. 

 

Let us consider that Mark 3 DYNA  photo as a "shining example", of how NOT to do such a mod, on a SAFETY basis !

 

I would suggest, if someone is modding any amplifier whose resistors are exposed to touch, that all such added resistors be installed on the bottom of the board, below deck, and ......thought should be applied as to keeping the lead lengths as short as possible.  Mark Z, if you are reading this, carefully consider unsoldering those Plate Resistors, and simply installing them below the board. 

 

The correct execution for a DYNACO -type amps with exposed resistors, is the other picture I posted of MechEngVic's double 2 Watt ST70 install.   Dyna amplifier owners who elect to mod the Ra, should ONLY implement the photo of Vic's install.  I do not think Vic fully-considered safety, but indeed, his execution turned out to be is as good as it needs to be !!.

 

Now, allow me to address the tight matching of resistors in parallel. 

 

The range of matching resistors I suggested was  0.1% to 0.01%.   0.1% matching  is within the range of many of the better DMMs people might own.

 

The main reason why I even mentioned 0.01% Maynard, was to impress upon readers, that it is better to have the " sameness" of the two resistors serving this one function, to be as close in Ohmic value to each other as possible. 

 

The reason for matching two resistors closely is because a skewing of the sound occurs when they are un-matched, ( assuming any decent SE amp ) .  How?  Because the signal is being developed across the Ra or Rk,....................... by two dis similar resistances.  One continually is out-of-synch with the other.   Perfect matching, is ideal.

 

When applying Ra and Rk modifications, particularly with higher mu ( gain of 50 to 80 times ) input tubes, whatever is done to the input stage is of importance.  And .....it becomes audible.  So, it is a very good procedure to match Ra and Rks in parallel, as best as one can.

 

There is one other, little known, never discussed example of skewing in SE design.   This occurs when implementing B+ filters to the Output tube.  This happens when  L1 is not the same as L2 ( in a L1/C1/L2/C2 B+ filter ).  When the two Ls (  power supply chokes  ) differ,  there is an odd-sounding / sonic skewing degrade, one hears !!  This undesirable sonic effect is easily avoided, by simply making L1 the same as L2, in any such filter.

 

I wouldn't worry too much about resistor matching differences between  0.1% and 0.01%.  Lets not "lose the forest for the trees." 

 

The main point is, improving the Ra and Rk as clearly outlined, produces a sonic gain in virtually any tube amp circuit I have built.  That old dusty  Dyna MK 3 is hardly a state of the art SE amp, is it??   Nor was MechEngVIc's own Dynaco ST-70 series II, which he has thoughtfully modified.

 

We can all tell from their comments,  both these guys HEARD an improvement, by doing this simple change.  I have Vic's email reports, of his first and second modification impressions.  He added the two Rs first, and then, later, an under 2 inch wire run of 20 AWG m22759/11, from his new Ra,  to the pins of the tube's socket.  Both of Vic's email testimonies will follow.. 

 

In this thread's first post, I bought two packets of 560K 2 Watt Cornings for Mark Z.  Ten resistors, for $4.00, a  lovely bargain for 2 Watt Cornings.   Mark Z  took the time to Ohmiate the ten and came up with  two matched pairs.  He owns a decent Fluke 110 Digital Multi Meter, so I would assume he got his pairs matched to about 0.1%.   

 

Vic spent 50 cents a resistor " extra " , and had Mr. Michael Percy match his two pairs.  Percy advertises, matching to 0.1% or better.  He has superb equipment to do so.   I match my own pairs, one to another,  using a Fluke 8060A, a 4 digit , or is it 4.5, digital Multi Meter.

 

The whole topic of unusually close matching is of minor importance in my mind.  It has become a simple distraction -  to the intent of this thread. 

 

What IS of importance, is that audiophiles who are tube rolling up-till-now, ought to be made aware to consider FIRST getting their tube's Ra and the Rk optimized, to have the best overall sonic result and enjoyment.  Why?  We are always hearing both the tube and it's associated Ra and Rk resistors. 

 

I am not picking on any one manufacturer, but as one example, I cant help but think, each and every INSPIRE amp, built to sell " at a price " on eBay, would benefit audibly from this.  All resistors are safely below the chassis. People can report their modification listening results up here.  Any questions, P.M. me.

 

The intent of this thread has been to inform music lovers, and tube rollers.. This thread's information was very likely unknown previously, to many Forum Members.  Now, Members can choose a path, or negate it.   " You can bring a horse to water ", comes to mind.   Amps are very important, the weakest link in the whole audio chain.  Have fun everyone, ....enjoy !!  See below, Vic's emails.

 

Jeff Medwin

 

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Here is Vic's first email of what he heard,  changing the Ra resistors only,  no wire augmentation.  ( He meant to start a thread, and after Mark Zs comments this week, I initiated this double 2 Watt resistor and wire topic, so others may gain benefit ).

 

Mr. Medwin,

 

I think you're on to something here.

 

I bought the resistors from Mr. Percy (and some solder) and received the package today. I installed the resistors.

 

The first thing I noticed was a fuller upper bass and lower mid-range which is always welcome. I also noticed what sounded like a somewhat rolled-off high end, something I was not expecting nor did I want. But I quickly realized that what I was hearing was not rolled off highs but better separation of the high frequencies. I played a song that usually had a convoluted high end and immediately noticed better imaging of cymbals, voice and guitar. A song that I had grown tired of was once again a pleasure to listen to! Then I thought I'd try some piano.

 

In my experience, and I think for others also, piano is one of the hardest instruments to reproduce well. Even when other instruments sound bright and full, piano can be muddy. I have made some great sound improvements by upgrading capacitors and tubes, but haven't been able to make significant strides into improving my system's piano reproducing abilities. Your resistor modification has made a noticeable improvement in piano playback. That initial roll-off I thought I was hearing at first is actually an improvement in the high end, and is what I think, making the piano really stand out. Even though I thought I was getting roll-off, the piano sounds brighter! I have to keep pausing from typing this because I keep hearing information that just wasn't there before.

 

I would like to hear a more in-depth explanation of what is going on here. I have a basic understanding of how tubes work, but I'd like to know more of when you say you are hearing the tube and the resistor. Overall, I am happy I made the mod and and grateful to you for suggesting it. My only regret is that the resistors didn't fit on top of the circuit board, I had to mount them underneath. I have paid much more for much less improvement.

 

I'm going to start a new thread soon in the technical modifications section where hopefully you'll explain this all to us in a lot more detail.

 

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After adding about five cents worth ( under 2 inches per channel ) of m22759/11 20 ( real and good ) wire to his Dynaco ST-70  Printed Circuit  boards, ( augmenting the Printed Circuit board's connection of the new 2 Watt resistors - to the tube socket pins ) - here is Vic's report  back  to me :

 

1994952734_Testimony2-wirerunJPEG.thumb.jpg.b7f84130ce773f6de85e9a9e128105ee.jpg

 

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I've got a couple of questions. First were the original resistors in spec cause I know they have a tendency to drift higher over time.  Also, I noticed in a couple of the pictures the resistors were installed upright.  Was this to simply fit them in a small space?  I ask, because most every resistor I see in a factory build lies flat on the chassis and I presume this is done for a reason, probably relating to noise.

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On 3/29/2020 at 9:22 AM, thebes said:

I've got a couple of questions. First were the original resistors in spec cause I know they have a tendency to drift higher over time.  Also, I noticed in a couple of the pictures the resistors were installed upright.  Was this to simply fit them in a small space?  I ask, because most every resistor I see in a factory build lies flat on the chassis and I presume this is done for a reason, probably relating to noise.

 

Decent questions !!

 

He could measure the ones he took out, I'll ask him to if he didn't discard them, but that is NOT the thing that is going on here at all thebes !!   

 

The stock spec is 270K, and the two 560K in parallel is 280K.   That is a 3.7% higher difference .  Not any problem.  I assume Dyna resistors are meant to be 5% or better, LOL, maybe even 10%. 

 

No thebes, we are talking about the quality of the resistor, and how it impacts your own final aural experience, and how to optimize a tube amp on an aural-listening basis.

 

Who cares if it measures the exact 270K,  and is a cheap carbon comp., and it sounds crummy ?? 

 

This factor you bring up is very minor, compared to resistor quality. I'd guess "four to one" in importance, in favor of resistor quality.

 

Yes, Mark Z applied considerable thought and made several attempts to fit the large Rs into the small space.  We know now, he should have installed them on the bottom of his exposed P.C. board, to maximize safety.

 

A resistor will operate in any orientation. 

 

A horizontal resistor will allow for better cooling, but cooling is hardly a factor, when using two 2 Watt Rs, in place of a 1 Watt stock R !!  Hey, please look at the photo of my KT88 2019 amp ( in a post above ).  I position things THREE dimensionally in that DIY build, and all eight of those high quality 2 Watters in the photo..... are vertical. 

 

Most people routinely think and build amps in two dimensions, and AFAICT, there no real reason to do that !!  Three dimensional layout is preferable to me. 

 

Jeff 

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2 hours ago, thebes said:

First were the original resistors in spec cause I know they have a tendency to drift higher over time. 

 

Being that the other resistors on the board are carbon composition, it would be a safe guess that the original resistors were carbon comp and had drifted.

 

2 hours ago, thebes said:

Also, I noticed in a couple of the pictures the resistors were installed upright.  Was this to simply fit them in a small space? 

 

It was explained and obvious from the image that room was the factor.

 

2 hours ago, thebes said:

I ask, because most every resistor I see in a factory build lies flat on the chassis and I presume this is done for a reason, probably relating to noise.

 

I don't know about noise, but having parts oriented flat on the circuit probably minimizes the risk of being shocked. If one happens to touch the tube for some unknown reason and mistakenly contacts those leads on top of the resistors, they'll probably get a good jolt.

It may have been wise to put some heatshrink over the exposed leads.

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There are other reasons not to vertically mount resistors on open pc boards (i.e. not installed within a shielded enclosure).  Mike already mentioned one reason- possible shock hazard (certainly a risk in regard to a resistor in a plate circuit).  The other is that of possible RF pickup by the leads and resistors themselves!  This wasn't an issue back in the day before we had devices operating well up in the microwave frequency range.  Now,  the wavelengths are so short (take a look at the proposed frequencies at which 5G will operate) that excessively long component leads, and resistors far longer than the originals, can actually act as antennas.  My advice is to stick with resistors of the same wattage as those for which the pc boards were designed to keep them horizontally, and closely, mounted on the board as originally intended.  Metal film resistors are wonderful replacements for the old carbon composition types and are totally reliable and cheap. 

 

Tubes are noisy devices and I have always found that tube generated noise, if excessive, is much more of an issue than the noise created by resistors.  For most listeners, neither is a problem.

 

 

Maynard

 

 

 

 

 

 

 

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All I'm going to add some really good carbon film resistors are available today they sound great and do not drift enough to talk about. I personally do not like metal film resistors in vintage amps.  

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11 minutes ago, NOSValves said:

All I'm going to add some really good carbon film resistors are available today they sound great and do not drift enough to talk about. I personally do not like metal film resistors in vintage amps.  

Hey Craig, Nice to see you Hope  all is well  Joe

 

On with the show 😃

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On 3/29/2020 at 9:22 AM, thebes said:

I've got a couple of questions. First were the original resistors in spec cause I know they have a tendency to drift higher over time.  Also, I noticed in a couple of the pictures the resistors were installed upright.  Was this to simply fit them in a small space?  I ask, because most every resistor I see in a factory build lies flat on the chassis and I presume this is done for a reason, probably relating to noise.

 

 

Mark Z just measured the stock Dyna MK3 plate reisitors he removed.   Schematic calls for 270K.

 

One was 272.5 K Ohms   ( less than 1% )

 

Other was 279.0 K Ohms   ( 3.3% ) 

 

Not too bad, after 40-50 years.

 

New ones, just installed, are 280K .     + 3.7 %

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On 3/30/2020 at 2:52 PM, NOSValves said:

All I'm going to add some really good carbon film resistors are available today they sound great and do not drift enough to talk about. I personally do not like metal film resistors in vintage amps.  

"So do these miracle resistors have a name, or are thy a trade secret?", he said disingenuously.

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Interesting t

20 hours ago, Jeffrey D. Medwin said:

 

 

Mark Z just measured the stock Dyna MK3 plate reisitors he removed.   Schematic calls for 270K.

 

One was 272.5 K Ohms   ( less than 1% )

 

Other was 279.0 K Ohms   ( 3.3% )

 

New ones are 280K .     + 3.7 %

Interesting test results, and his ears tell him there is a positive difference.  Obviously I can't weigh in on the scientific aspects of it.

I only mentioned the obvious, because , well it was obvious.

 

I have also observed in older solid state amps, double -sided boards, which are a bit 3 dimensional to me, not to mention having seen boards stacked upright in a row inside some SS receivers I've opened up, with no obvious harm to the signal or sound. However, I almost never see upright resistors in any anything tube I can recall, although most of what I've played with has been point-to-point wired, not circuit boards. Given the space restrains inside some of them, you'd think if was OK to mount them upright then they'd a probably done it. Especially since various electrical geniuses designed this stuff in the first place. And that is the basis for my reasoning, not knowledge certain.

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On 3/31/2020 at 4:13 PM, thebes said:

Interesting t

Interesting test results, and his ears tell him there is a positive difference.  Obviously I can't weigh in on the scientific aspects of it.

I only mentioned the obvious, because , well it was obvious.

 

I have also observed in older solid state amps, double -sided boards, which are a bit 3 dimensional to me, not to mention having seen boards stacked upright in a row inside some SS receivers I've opened up, with no obvious harm to the signal or sound. However, I almost never see upright resistors in any anything tube I can recall, although most of what I've played with has been point-to-point wired, not circuit boards. Given the space restrains inside some of them, you'd think if was OK to mount them upright then they'd a probably done it. Especially since various electrical geniuses designed this stuff in the first place. And that is the basis for my reasoning, not knowledge certain.

 

 

Lead length becomes a problem, IF, all else on the board is placed two dimensionally. 

 

I won't use PC boards in my builds, wire is usually better performing, real wire.  In some circuits, applications, however, a board could be the best solution.   

 

My early-model Slagle Autoformer Attenuators, in stock form, were not fully satisfactory "to me" - with that early stock PC board  and budget - based switches, until.... I did some nice PC board wire augmentations.  Used m22759/11 20 AWG soldered over a few key existing traces. 

 

For my next build, my personal-use phono / line preamp, I will hard-wire each channel's  Slagle Autoformer, which has 12  taps, point-to-point,  to a Grayhill silver-contact 12 position ( shorting, MBB)  rotary switch, using  only silver-content hookup wire.  That, should be good,  and likely satisfy my present and future needs and expectations.

 

These two channels of Slagle Attenuators will fit INSIDE the new DIY preamp chassis. A DIY preamp with Attenuators inside it -  only requires using one interconnect, to the amp's input RCA jacks.  Mr. Dave Slagle was very nice to me, in assisting with hook up information.  His recent offerings feature new  product advancements, compared to these early versions I have, ( which were recently given to me as a gift) .  Cool dude... is Dave.   Smart .

 

Jeff

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On 3/31/2020 at 5:00 PM, thebes said:

"So do these miracle resistors have a name, or are thy a trade secret?", he said disingenuously.

 

 

  No trade secret at all! I like Stackpole because they make a nice 2 watter for plates. I also like the green Kiwame (or cheaper and the true manufacturer KOA Spear). The problem with either is they are getting hard to source. I used to buy the 2 watt versions of the Stackpole from Handmade Electronics http://www.hndme.com/ but he has stopped selling them. I purchased a boat load of the common 2 watt values I use from him at clearance. The 1/2 watt versions are readily available I believe Digikey sells them. 

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LATEST RESISTOR UPGRADE REPORT........

 

Mark Z. has changed the in-series GRID-stopper resistors on his KT88 Finals tube socket ( Dyna MK 3s ).  He went from 1,000 Ohms Carbon Comp ( stock ) to a 2 Watt CDE / Corning Glass Works 820 Ohm metal film resistor. ( gray, red, brown - 8 - 2 Xs 10 )

 

Mark also just augmented traces on his Dynaco PC board, from the coupling cap, to each KT88's control grid..   He added real wire there ( m22759/11 20 ) soldered over the PC traces, and he used this same high-quality Mil Spec wire, from the coupling caps' end-lead, across the PC board, and to each octal tube socket.  Less than three inches of wire was needed, for each channel. ( the white teflon insulated wire in photo ). 

 

The amp's full audio signal, is carried by that span of wire - and it goes through a grid-stopper resistor,  ( if a grid-resistor is present, in the design.)  This is a sensitive circuit mod area, common to tube amps, SE or P-P.  We seek minimal loss of music information, while passing the music's signal through the circuit.

 

Here is what Mark's DIY upgrade work looks like, when done :

 

 

                    219121667_4-7-20GridStoppers1Kto620Ohms.JPG.d965010def17015e682c0711f9802222.JPG

 

 

After buying the Corning resistors off of eBay, and the Mil Spec copper stranded /silver plated wire likewise, total parts cost for the changes above is only about $2.00 -  and maybe 50 cents pro-rated as shipping.  

 

And here is what Mark has just described , his own initial listening reactions  :

 

                     1029965771_GRIDstopperLISTENINGEDITED.thumb.jpg.d4210c27c6d1a48c238e42a20070ef1e.jpg

 

Mark has other conservative mods to implement, to get this amp cooking more-so.  I wanted to share this wire and grid stopper upgrade with you all.  This modification applies to the majority of amps.  Parts MUST be expertly chosen !!!!  Work must be fastidious. 

 

For the record, none of my own DIY designed triode circuits use grid stoppers, maybe a ferrite bead, at the tube socket's pin, if it was necessary !!  The less parts the signal travels through, the more intact is the music at the loudspeaker. 

 

Jeff

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