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I'm perplexed as to why one channel of my SET amp has 4.0 mv. of hum and the other has 1.2.....

75.82.140.48

Posted on July 2, 2012 at 20:55:23
Iczerman


 
Can someone..unplex me? (replex..suplex( um..skip that one..)

Thanks!

 

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RE: I'm perplexed as to why one channel of my SET amp has 4.0 mv. of hum and the other has 1.2....., posted on July 2, 2012 at 21:48:28
GTCharlie
Audiophile

Posts: 971
Location: Philippines
Joined: December 9, 2004
Heater?

 

Sounds normal, posted on July 2, 2012 at 22:48:58
commentary


 
No two DHT's have the same hum nulling capability, and the sometimes you have to select for a lower hum DHT. Also, hum pickup might be from noise at the front end with the accompanying unbalanced hum sources nearby. Also, the grounding scheme must be good in each circuit.

Low hum like the lower hum producing channel needs a lot working for it to get that low. That's why a lot of people like to just go to DC filaments.

One thing you can do is balance the levels in each channel for approximate same level and maybe even for a small null at the listening position.

 

RE: I'm perplexed as to why one channel of my SET amp has 4.0 mv. of hum and the other has 1.2....., posted on July 3, 2012 at 00:44:37
drlowmu
Manufacturer

Posts: 8249
Location: East of Kansas City
Joined: January 10, 2005
Are the Finals DHT tubes or indirectly heated.?? Need to know that. Are your filament heated with AC or DC ?

If DHTs on AC : swap tubes between channels and remeasure hum to see how much of that is due to tube differences, which with DHTs can easily account for these measured differences. Report back to us please.

One other thing, a dead quiet DHT amp is also often a dead sounding amp !!

Jeff Medwin

 

RE: I'm perplexed as to why one channel of my SET amp has 4.0 mv. of hum and the other has 1.2....., posted on July 3, 2012 at 13:39:42
Iczerman


 
DHT's-AC heated-Individual power trannies each channel.Centertap used for cathode biasing.

 

RE: I'm perplexed as to why one channel of my SET amp has 4.0 mv. of hum and the other has 1.2....., posted on July 3, 2012 at 14:29:42
drlowmu
Manufacturer

Posts: 8249
Location: East of Kansas City
Joined: January 10, 2005
OK, perfect ....so SWAP the Finals tubes and remeasure, does it go with the tubes any ?? You are "there" to find out something !!

Cheers,

Jeff

 

RE: I'm perplexed as to why one channel of my SET amp has 4.0 mv. of hum and the other has 1.2....., posted on July 5, 2012 at 16:19:20
Iczerman


 
The humm is still there..but measures lower...must be a wire routing thing.

 

RE: I'm perplexed as to why one channel of my SET amp has 4.0 mv. of hum and the other has 1.2....., posted on July 6, 2012 at 00:22:52
drlowmu
Manufacturer

Posts: 8249
Location: East of Kansas City
Joined: January 10, 2005
It will be partially a result of the 2A3 tubes themselves, different ones have different mVAC. Some hum is fine !!! Better than dead quiet, music playback wise !1

Jeff

 

RE: I'm perplexed as to why one channel of my SET amp has 4.0 mv. of hum and the other has 1.2....., posted on July 9, 2012 at 14:10:33
caffeinator
Audiophile

Posts: 1392
Location: Pacific Northwest
Joined: August 22, 2003
Make sure to short the inputs before measuring - I remember finishing a pair of amps and being aghast at the amount of hum until I realized I'd forgotten to do that.

My two identical monoblocks had different hum levels - it's not unusual - can't recall the amount, but some was due to tube variation, the rest I couldn't find - the wire routing was nearly identical on the two, so I'm guessing it was simply tolerance stackup of component values or something else - maybe sunspot activity.

I plugged 'em in and started listening to some music. Two beers later, I couldn't remember which one was the hummer...

 

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