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SE84C-EX Hum (Read 11692 times)
StingRay
Ex Member



Re: SE84C-EX Hum
Reply #15 - 02/07/05 at 19:47:26
 
4.5 mv of hum is the measurement I made with my DMM.  I don't have the extra digit of resolution and it was floating between .004 and .005 so I called it 4.5mv.

Looking forward to your comments.

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144cubits
Ex Member



Re: SE84C-EX Hum
Reply #16 - 02/09/05 at 18:59:25
 
Got the unit back today and the hum is significantly better. I 'm told that some hum is virtually unavoidable with tubes particularly with sensitive speakers. As long as it doesn't interfere with the music I'm OK.

I will allow the unit to break in some more and roll some tubes to give you gents a proper posting of my thoughts in the near future. For now it appears my crisis is solved.

Thanx for all of your help guys!
Fred 8)
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StingRay
Ex Member



Re: SE84C-EX Hum
Reply #17 - 02/10/05 at 02:49:33
 
I made a series of measurements of hum on my Select EX.  Following Steve's recommendations in his paper on hum, I measured between 3.5 mv and 4.6 mv of hum depending on my tube combination.  The lowest hum (at 3.5 mv) was measured without the input tube in place.  

I emailed Steve for advice, and he recommended that I remove the 1000 uf bypass cap on the cathode resistor for the output tubes.  I performed the very minor operation, and put the amp back in service.  With all tubes installed, the hum had dropped to 2.3 mv and is now essentially a non-issue.

The interesting part is that in addition to cutting the hum in half, the operation has changed the sound of the amp for the better (in my system YMMV).  I didn't like the results so much with the stock SV-83's in my system.  They sounded a little thinner and almost too detailed.  The JJ EL-84's were a completely different story.  The weight and presence was back, but the detail in the mids and highs were much improved.

One nice thing about this mod is that anybody can do it.  There is only 1 cap to clip out, and it is piggybacked on top of the cathode resistor.  It is very easy to reach, and takes only about 5 minutes to remove it from the circuit.  It would be almost as easy to add it back in if you didn't like the change.  

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« Last Edit: 02/20/05 at 22:31:01 by StingRay »  
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dc007
Ex Member



Re: SE84C-EX Hum
Reply #18 - 02/10/05 at 13:06:09
 
Stingray
What about the SE84C EX.....would that be the same fix as with the Select EX?

Regards...Mike
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StingRay
Ex Member



Re: SE84C-EX Hum
Reply #19 - 02/10/05 at 13:35:25
 
Mike,

I believe the fix would apply to the SE84C EX as well, although I didn't specifically ask Steve that question.  Steve also mentioned that I may be interested in changing the 100 ohm 25 W Dale resistor to 150 ohm.  At this point, I'm not planning on doing that, but additional listening and a description of what the increased resistance will do to the circuit may change my mind.

Jerry
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144cubits
Ex Member



Re: SE84C-EX Hum
Reply #20 - 02/10/05 at 14:39:28
 


[quote author=StingRay  link=1105725338&amp/15#17 date=1108003773]

The interesting part is that in addition to cutting the hum in half, the operation has changed the sound of the amp for the better (in my system YMMV).  I didn't like the results so much with the stock SV-83's in my system.  They sounded a little thinner and almost too detailed.  The JJ EL-84's were a completely different story.  The weight and presence was back, but the detail in the mids and highs were much improved.
[/quote]
Stingray, I rolled some tubes last night and noticed an immediately change also. I replaced the stock rectifier with a JAN Cetron and the input tube with a Mullard 6dj8. Big difference. With the stock tubes the detail was there in spades but the instruments tended to drown out voices. When I replaced them the voices came back and the overall sound softened albeit with less detail.

I'll keep rolling until I find the right mix. I'm trying to get some Mullard EL84's but man are they expensive. I think I'll go the JJ route and see what happens.

I'm open to any suggestions as I'm looking for a smooth, holographic sound. C'mon tube rollers I know you're out there!

Fred 8)
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StingRay
Ex Member



Re: SE84C-EX Hum
Reply #21 - 02/10/05 at 16:40:41
 
My two favorite combinations to date are:

NOS RCA 5U4G Rectifier
JJ EL-84's
Phillips JAN 6922 input

or

Stock Ruby Rectifier
JJ EL-84's
Phillips JAN 6922 input

I find these combinations to be exactly what you describe.  Smooth, detailed and holographic.  As a matter of fact, I would use this configuration as a benchmark for comparing other amplifiers.
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144cubits
Ex Member



Re: SE84C-EX Hum
Reply #22 - 02/10/05 at 17:06:04
 
Stingray, Thanx again for your suggestions. One question, since I have the C with the EX mod I can't use the 5u4 family of rectifiers that's why I bought the JAN Cetron. Any suggestions? I'm going to try your combo of JJ's and Jan Philips and see what happens.

As far as DIY is concerned, I've only done one project in my life and it turned out well, but one project does not make a successful DIY'er. That's why I didn't attempt it.

I agree with you that Steve has been a gentleman in this whole process and through it I've met a number of very helpful individuals such as yourself that has made this bumpy road a lot smoother.
 
Thanx again for all of your continued help. I'll keep you all posted.
Fred 8)
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dc007
Ex Member



Re: SE84C-EX Hum
Reply #23 - 02/10/05 at 19:36:01
 
144

Try a RCA 5y3gt....inexpensive and very good in the EX modded C

IMHO
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144cubits
Ex Member



Re: SE84C-EX Hum
Reply #24 - 02/11/05 at 12:46:36
 
Thanx DC. I'm busy collecting tubes but will allow the amp to break in some more before doing any serious rolling.

Fred 8)
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StingRay
Ex Member



Re: SE84C-EX Hum
Reply #25 - 02/20/05 at 22:46:34
 
Update.  I may have jumped the gun by reporting too soon on my impressions of my EX after the hum reducing cap-ectomy.

On the material I was listening to at the time, I noticed what I perceived to be an increase in detail.  On other material, I noticed a thinning in the bottom end.  This thinning is especially evident on the SV-83's, but not as apparent on the JJ's.  Today, I attached some aligator clips to the ends of the cap that I snipped out so I could do some A/B comparisons with and without the cap.

So far, with the cap, the hum goes up, but the amp sounds fuller.  Without the cap, the hum goes down, but the amp sounds thinner.  I am going to do this with some other brands and values of caps to see what happens.  The stock cathode resistor bypass cap is a Xicon electolytic and looks a little out of place in a circuit that uses higher quality parts throughout.  I'll update this thread as I learn more.
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Steve_Collins
Ex Member



Re: SE84C-EX Hum
Reply #26 - 02/21/05 at 12:41:33
 
Hi Stingray
I too tried removing the 1000uf cap, actually in my case it was a 470uf cap following Eddie's hum reducing suggestion, but I found the same as you. No hum, well 1.2mv, but the sound became super clean, recessed, and just lacking in body, presence, call it what you will the involvement factor had gone. I put the cap back and added the extra power supply capacitance to keep the hum down and the system is back to normal i.e. slight hum but sounding great. BTW, mine is a CSex diy upgrade from the C. I picked up on the suggested 5V4G rectifier and managed to find a NOS RCA version. Coupled with ex factory Mullard EL84's and Tele E188cc this rectifier sounds better than the Mullard GZ32, Sylvania 5U4G, RCA 5R4 that I've tried so far, so like others before me I can recommend it. BTW speakers are HDT drive units in non hdt cabs.
All the best
Steve
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144cubits
Ex Member



Re: SE84C-EX Hum
Reply #27 - 02/22/05 at 14:32:38
 
Stingray
 [quote author=StingRay  link=1105725338&amp/15#25 date=1108939594]
On other material, I noticed a thinning in the bottom end. [/quote] 

As I am new to the Decware "sound" I am not quite sure what I should or should not be hearing. I did notice that bass was not as full as I remember but I attributed it to the amp still breaking in. The amp now has about 70 hours and sounds good but yes recessed with voices not as palpable as I've heard in other amps.

Keep us posted Stingray as I would like to hear what these amps are truly capable of.
8)
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StingRay
Ex Member



Re: SE84C-EX Hum
Reply #28 - 02/23/05 at 02:39:38
 
If you're bypass cap was removed and not replaced, I don't think you're getting the full Select experience.

It seems that removing the cap attenuates the lower frequencies making the hum less of a problem, but trimming off the bottom end response in the process.  I decided the tradeoff is not worth it, and I'm working to get a replacement cap that will be an overall improvement.  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.

If it turns out that my amp has to have double or triple the specified hum to sound full, I guess I'll live with it.  I'll post the results after I install the new cap.
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Yoda
Ex Member



Re: SE84C-EX Hum
Reply #29 - 02/23/05 at 05:24:37
 

[quote author=Steve_Collins  link=1105725338&amp/15#26 date=1108989693]
Hi Stingray
I too tried removing the 1000uf cap, actually in my case it was a 470uf cap following Eddie's hum reducing suggestion, but I found the same as you. Steve
[/quote]

Where is this post from Eddie...he has so many!

Matt
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