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This is the importance of test equipment in many caaes.

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Posted on November 26, 2014 at 18:13:20
Michael Samra
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Location: saginaw michigan
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 I test for these things being I have similar equipment but notice what good grounding does such as what the McShane upgrades do but just improving the ground on the one cap improved signal to noise by almost 30db to 40db plus.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

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RE: This is the importance of test equipment in many caaes., posted on November 28, 2014 at 09:30:24
gkargreen
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Posts: 1562
Location: DC
Joined: February 5, 2005
 yes, I have been watching David for some time now, I like how he uses his gear to test for and then find some issues with vintage gear. I just read an article by Chris @ Chris's Tube Haven that I found while searching out Scott 299A mods. He did a frequency sweep of the output of a 299C model and found the high freqs to be severely dropped off, primarily due to the use of the 500K vol pot in the preamp circuit. I have been setting up a test gear arrangement to do similar investigations and establish baselines for vintage gear and thus have some ideas as to why some gear has that rolled off sound and attempt to get that back. Michael, any suggestions as to what to read/do to get a better picture of what is going on inside these vintage integrated and therefore a means to improve areas the amps are deficient in? It seems as though high freq. roll-off is a common area...

 

Nice, Mike..., posted on November 28, 2014 at 14:15:34
Jay Buridan
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Location: Michigan
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 Still want you to modify my Wavelength monoblocks, but too much other stuff is getting in the way of my driving them up to your house. Happy Holidays.

"Horse sense is the thing a horse has which keeps it from betting on people. "
― W.C. Fields

 

RE: This is the importance of test equipment in many caaes., posted on November 28, 2014 at 14:43:50
Eli Duttman
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Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000





IMO, barking up the CMiller interaction tree is always a good idea. Lots of those vintage "integrateds" used 500 K or even 1 meg pots. to ease the burden on 12AX7 based phono sections. The uploaded original RCA design, with its load restriction warning, is a good illustration.

Obviously, more than simply reducing the value of those pots. is needed. However, the tweak of a ZVN0545A source follower that I employed in the tweaked RCA design goes a LONG way towards dealing with inability of the 'X7 triode to drive a load.


Eli D.

 

RE: This is the importance of test equipment in many caaes., posted on November 28, 2014 at 15:07:47
gkargreen
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Posts: 1562
Location: DC
Joined: February 5, 2005
 so by using the upgraded design, a lower value pot can be used in place of the 500k pots and thus provide a greater linearity into the AF amp/driver stage, am I understanding that correctly, Eli?

 

RE: This is the importance of test equipment in many caaes., posted on November 28, 2014 at 16:09:26
Eli Duttman
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Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 The grid to ground resistance (the pot.) combines with the Miller capacitance to form a low pass filter, as in rolling HF info. off. Drop the value of the resistance and the "corner" freq. of the filter rises, hopefully well outside the audio band.

Pentode power section I/P stages, with their vastly smaller CMiller, are not as vulnerable to the info. loss. Both Fisher and Scott used voltage amplifier DC coupled to "concertina" phase splitter small signal toopology, but Scott used pentode voltage amplifiers and Fisher used triodes (see Scott 299C/D and Fisher 500C). MOSFET buffering the phono section is beneficial in both cases, but Fisher stuff benefits more. The benefit in a Scott lies mostly in making the recording O/Ps highly usable.


Eli D.

 

RE: Nice, Mike..., posted on November 28, 2014 at 17:51:17
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Not a problem..It will have to be after Xmas tho because I'm still getting caught up from my post surgery issues this summer where I was incapacitated from doing anything with any conviction..Now I'm ok and getting caught up but I'm getting my obligations caught up.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: This is the importance of test equipment in many caaes., posted on November 28, 2014 at 18:55:27
gkargreen
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Posts: 1562
Location: DC
Joined: February 5, 2005
 thanks, Eli, that makes a lot of sense, in fact, we did RLC filters just over the last couple of weeks in circuits class, and what you are telling me fits right in! So I am guessing in my Scott 299A the change in the pot resistance would be minimal, but if you had used a triode for the voltage amp, it would make a lot more difference. That is what Chris had done, wired the pentode for triode operation in his Scott 299C...

 

RE: This is the importance of test equipment in many caaes., posted on November 28, 2014 at 18:57:41
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Randy
As far as the test gear goes,the first thing we want to test for is a good 1khz square wave in the amplifier section.I always go for 10khz as well but 1khz is fine.
Then you want do a frequency sweep of the linestage preamp with a low distortion generator or oscillator and if you don't have that just get a stable signal generator and see how many DBs it rolls off at 20khz and beyond. Eli already has the phono stage worked out and yes,the 500k volume put is an issue and even Wheezer told me this when I showed him Eico circuit.He was telling me to use a 100k and it worked out well. So you are on the right track and just get familiar with your test gear and if you need help,we are all here.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: This is the importance of test equipment in many caaes., posted on November 28, 2014 at 20:08:27
gkargreen
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Posts: 1562
Location: DC
Joined: February 5, 2005
 thanks, Michael and Eli, you guys are the best! BTW, one thing I had read was to upgrade/replace the resistors in the phase splitter, is there any particular types of resistors to use, like metal film for the cathodes and carbon film for the plates, etc.? thanks!

 

RE: This is the importance of test equipment in many caaes., posted on November 29, 2014 at 06:53:12
SteveBrown
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Posts: 2454
Location: Portland, OR
Joined: November 14, 2002
 Hi, sounds like you're already looking at the theory here. But I found this helpful easy to understand link. You can see how a high gain triode like a 12AX7 (frequently found in early integrated amps) would make this issue even worse.

 

RE: This is the importance of test equipment in many caaes., posted on November 29, 2014 at 08:24:47
gkargreen
Audiophile

Posts: 1562
Location: DC
Joined: February 5, 2005
 thanks, Steve, that was very informative! I guess that is why I see a lot of pentode/triode tubes in the AF/splitter sections of integrated amps. I am guessing that the 500k pots were pretty common, necessecitating the use of a pentode/triode...

 

RE: This is the importance of test equipment in many caaes., posted on November 30, 2014 at 16:23:43
GEO
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Joined: April 7, 2000
Contributor
  Since: September 9, 2000
 Where are the trust your ears folks?

 

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