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SE84C-EX Hum (Read 11629 times)
Steve_Collins
Ex Member



Re: SE84C-EX Hum
Reply #30 - 02/23/05 at 12:28:32
 
Hi Yoda
You'll find Eddie's reply here.
http://www.decware.com/cgi-bin/yabb/YaBB.cgi?board=zen&action=display&num=110536...

In my opionion upgrading the 1000uf cap to a high quality/expensive one doesn't make hardly any difference to the final sound quality.
Eddie suggested either a 470uf or a 330uf to see if it reduced the hum. I did have a 1000uf blackgate fitted, but after trying the no cap cure ( i.e. removing the 470uf ) I've put in a basic rubycon 330uf that I had and the sound has not suffered when compared to the 1000uf Blackgate or the 470uf Evox/rifa.
IMO the biggest bangs for your buck happen when changing the 84C to CSex by fitting
Audionote .1uf copper foil coupling caps
Vishay dale 1k and 150ohm power resistors + RCA5V4G
EX transformers
1k Audionote tantalum resistors on input tube
1k Allen Bradley on power tubes ( not 330ohm as SE84CS )
Mullard EL84's
Input tube of your choice

I've found that as the internals of the amp were improved the SV83's and 6N1p sounded better too, and I could quite happily live with them if EL84's etc never existed. When you couple the 84CSex to HDT drive units you really get the magic going. BTW, HDT's need every hour of the 220 hour break in period. But beware, the combo demand 1st class sources.

All the best
Steve

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144cubits
Ex Member



Re: SE84C-EX Hum
Reply #31 - 02/24/05 at 14:28:11
 

 [quote author=StingRay  link=1105725338&amp/15#28 date=1109126378]I ordered two very high quality caps per the recommendation of our resident guru.  If there is an improvement after I install mine, you're welcome to the spare. [/quote]

Stingray you are the man! I would be happy to remunerate you for any caps, resistors you find that will bring the magic back...just one request is there any place I can find pix of the procedure. I know everyone says how easy it is and how you should dive in, but I've only done one DIY in the past  but I had pix that showed what was what.

Any help would be appreciated.
8)
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StingRay
Ex Member



Re: SE84C-EX Hum
Reply #32 - 02/24/05 at 16:05:06
 
I took a pic when I snipped the Xicon out, and I'll take pics when I put the Elna Cerafine in.
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144cubits
Ex Member



Re: SE84C-EX Hum
Reply #33 - 02/24/05 at 17:42:16
 
Thanx! Looking forward to hearing your impressions.
Fred 8)
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StingRay
Ex Member



Re: SE84C-EX Hum
Reply #34 - 02/26/05 at 19:27:15
 
I've installed the new cap, and I'm pretty much back to where I started.  There is a slight improvement in the hum, and possibly a slight improvement in the sound.

From my experience there are things I think and things I know regarding the Select EX hum and cap upgrade:

Here's what I know:

1.  The hum values on my amp are not within spec.  They are 2 to 3 times higher than the specification.
2.  Removing the cathode resistor bypass cap improved the hum, but attenuated the low end response of the amp.
3.  Replacing / upgrading the stock Xicon 1000uf cap does little to improve the hum problem or improve the fidelity of the amp.  There are slight differences (hum is now at about 3.9 mv, and the amp may sound slightly better).

Here's what I think:

1.  Leave the amp alone.  Trying to fix the hum issue by fooling with the cathode resistor bypass cap causes more harm than good.  It apparently masks the 120 hz hum by attenuating the low end response of the amp.
2.  Even with the slight improvement I noted, it was a lot of trouble for very little improvement.
3.  The hum specification for the Select EX amp may need to be re-written.  With two amps confirmed at 4.5 mv, the specification of less than 1.5 mv may not be accurate.


FWIW, the hum is not a problem at normal listening levels.  It is only apparent between tracks.

I have pictures and a description of my findings at:

http://www.geraldcurtis.com/selectex_mod.htm

Jerry



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Steve Deckert
Administrator
*****


If the 1st watt
sucks why continue?

Posts: 2402
Re: SE84C-EX Hum
Reply #35 - 02/26/05 at 20:20:59
 
We have been modifying the SE84C and SE84CS amplifiers with "EX" transformers for over a year and have even built several that way as brand new amps.  Hum on these amps has always been within spec.  Spec on an EX modded amplifier is 2mv or less.

It has only been recently that a few people have reported hum issues.  

If you have an EX modded amp that has excessive hum you should consider sending it in to be checked out so we can find out what happened.  If you do, please include the tubes that you use with the amp.  

By design this amp will have a higher hum figure then a non-ex modded amp.  Example, a stock Select can be 0.7 mv and most are.  An ex mod usually bumps it up to 1.7mv which is very acceptable.

For those of you who are trying different things on your amps to reduce hum, first find out where the hum is coming from.  Pull the input tube out of the amp completely and then measure the outputs with no speakers connected, volume control all the way down.  Wait for it to warm up for several minutes.  If the hum you measure is below or at 2mv, then the power supply and output stage is not the cause of your hum problem.

If this is the case, you should roll input tubes and see if all of them behave the same.  If so, raising the bias resistors total value by 1K on each half of the tube will drop the hum by at least 1 mv or more.

Removing or lowering the value of the bypass cap on the output stage cathode circuit can also drop the hum by 1 mv or more as can adding another 40 uf to the second section of the power supply filter. I consider both of these options last resorts.
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StingRay
Ex Member



Re: SE84C-EX Hum
Reply #36 - 02/26/05 at 20:42:35
 
Steve,

Thanks for your input on this thread.  

Quote:
An ex mod usually bumps it up to 1.7mv which is very acceptable.


I agree.  I got hum down to 2.3 mv by removing the bypass cap, and it was very acceptable.  1.7 mv would be just about perfect on my 94 db/w/m speakers.  Unfortunately, with extended listening, the amp sounded too thin.

Quote:
Removing or lowering the value of the bypass cap on the output stage cathode circuit can also drop the hum by 1 mv or more as can adding another 40 uf to the second section of the power supply filter. I consider both of these options last resorts.


I am now familiar with the impact removing or lowering the value of the bypass cap has on the amp.  In addition to lowering the hum, what impact would another 40 uf on the second section of the power supply filter have on the amp?  Would it impact the sound, rectifier choices, etc?  Also, what would your choice of cap be if you decided to add it?


Thanks,

Jerry
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« Last Edit: 02/26/05 at 20:47:11 by StingRay »  
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144cubits
Ex Member



Re: SE84C-EX Hum
Reply #37 - 02/28/05 at 14:02:52
 
Hey Jerry thanx for your determination on this EX Hum issue. Your help has been invaluable. I sent you a message on your forum.

Fred 8)
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StingRay
Ex Member



Re: SE84C-EX Hum
Reply #38 - 03/01/05 at 00:49:27
 
Happy to help.  This forum has been a tremendous asset for me, and I'm glad to hear you're enjoying it.  Posted a reply.
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StingRay
Ex Member



Re: SE84C-EX Hum
Reply #39 - 03/12/05 at 16:37:47
 
Latest update.  I am thinking about reworking the PS filtering in my Select EX, but I wanted to try a few inexpensive / easier fixes first.

Today, I added a 30uf cap in parallel with the second stage of the PS filter.  This was easy to do, and the hum is now down to about 2.8 mv.  I'm going to do some listening to see if it slowed anything down.  

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decwarite
Ex Member



Re: SE84C-EX Hum
Reply #40 - 03/13/05 at 04:02:03
 
I have just come to some similar conclusions regarding grid stoppers.  Definitely, tantalum on the input tubes and allen bradley on the output tubes.  I thought the dale aluminum body power resistors  sounded thinner than the regular dale ww black body power resistors.  They sport a lower power rating but sound better to my ear.  I've taken the bypass cap off my amp and it sounds great.  Believe it or not the xicon cement resistor that came with my "c" sounds the best on the cathodes over the mills and dale aluminum body.  It has a richer and my lively sound.
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144cubits
Ex Member



Re: SE84C-EX Hum
Reply #41 - 05/07/05 at 02:42:47
 
[quote author=StingRay  link=1105725338/15#28 date=1109126378]If there is an improvement after I install mine, you're welcome to the spare.

[/quote]

Jerry, I now have about 300 hours on my EX and have changed all of the tubes. It seems that you are absolutely right. The tradeoff of the lower hum is not worth the lack of presence and thin sound.  :'(              

I would like to get to where you are. If your offer is still good I'd be happy to accept it...let me know.  
Fred
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proud_indian
Ex Member



Re: SE84C-EX Hum
Reply #42 - 05/07/05 at 20:34:51
 
[quote author=144cubits  link=1105725338/30#41 date=1115430167]

Jerry, I now have about 300 hours on my EX and have changed all of the tubes.  
Fred [/quote]

While I have no hum problems ( you cant even hear it with your ear to the speaker) 300 hrs is too little. Iam 600 hrs up, and feel that the Ex Mod really cleans up its act around 500 hrs. I still find the highs a bit sharp, but I think I will ride it out. To get over this burn in, I looked at it as a PC burn in, after Eddie mentioned to me about the length of wire used. But then, I had changed all three transformers. Another advice from him was that I was using 96 db Lowthers. Less load . So I pulled out a pair of 88 db speakers, put a blanket over it, and turned up the volume every day I was at work. Dont know what helped, but I am glad I got over the burn in. What a pain.
shreekant
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proud_indian
Ex Member



Re: SE84C-EX Hum
Reply #43 - 05/07/05 at 20:36:02
 
I forgot. A good time to burn in all your new tubes, while you wait out the burn in.
shreekant
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proud_indian
Ex Member



Re: SE84C-EX Hum
Reply #44 - 05/18/05 at 09:00:33
 
I am now around 800 hrs on the Ex Mod. It does take a very long time to break in. Even now, at higher volumes, it shouts a wee bit. But otherwise,it is quite amazing.
At very low volumes, the detail is something to hear and belive. Absolutely stunning. It drives the lowthers with such ease now, that it can get really loud. I find that I listen at a much lower vol than I used to.
With the LF kicking in, the mid's that used to be highlighted, have mellowed quite a bit. Iam happy that I took the decision to go in for the Ex. For those who have 8 ohms, don't even think twice. Its better than any other tweek that you can do. No cable or tube costing 100 bucks can even THINK of compairing itself to the EX. Sonic's wise, its miles ahead.
A modified shreekant Smiley
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