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Fixed bias problem "fixed" or when it rains it pours.

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Posted on January 19, 2017 at 16:14:33
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010



Hey thanx to everyone who helped me track down the problem. LSS, it came down to replacing the filament transformer which evidently was defective. I tracked it down when i was doing testing during its runaway mode and i noticed that the output from the fil trans was decreasing when it should not be affected.
Unfortunately, at the same time as this i had 2 other unrelated problems that occurred along with the bad fil trans. This really was very frustrating as it meant taking the 50 pound "Big Bertha" back and forth between home and shop.
The biggest headache was using a red LED for the driver tube bias. While this meant much higher gain vs the unbypassed resistor route it also meant that there was much more noise in the circuit. The amplification of spurrious noise may have contributed to the instability of the amp. Not certain about this but i thought it best to take up the LED biasing with a different amp.
So, i went back to the cathode resistor and the output is dead quiet. The other problem indirectly relates to the use of the LEDs. This is because i used some clear plastic tube sockets. Unbeknownst to me is that this type of socket is not heat tolerant. And while playing with leds and resistors and unknown to me one of the tube pins broke off in the socket. This set off another round of troubleshooting as one of the driver tubes would not bias up. Anyway once i changed out the broken solder lug voltages went back to normal.
Anyway, all seems to be ok now and the amp sounds very, very, excellent in every way. Thanks for the help/support/suggestions and best regards, dak

 

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RE: Fixed bias problem "fixed" or when it rains it pours., posted on January 19, 2017 at 16:45:02
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
A bad transformer is freaky. Usually lasts a 100 years or more.

 

RE: Fixed bias problem "fixed" or when it rains it pours., posted on January 19, 2017 at 17:05:27
Tre'
Industry Professional

Posts: 17263
Location: So. Cal.
Joined: February 9, 2002
Until you pull too much current through them once.


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Or hack on new lead outs! (nt), posted on January 19, 2017 at 17:18:03
gusser
Audiophile

Posts: 3649
Location: So. California
Joined: September 6, 2006
x

 

RE: What do new leadouts have to do with transformer performance?, posted on January 19, 2017 at 18:41:35
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
I have seen it done countless of times other than the additional wire marring the color scheme, i have found little to show that adding some wire jeopardized the functioning of a transformer.

 

RE:In this case it was a used filament trans.., posted on January 19, 2017 at 18:46:16
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
And that came back to haunt me. I thought that it would be a case where either it worked or it didn't. Instead it worked when it wanted to for about 4 to 10 minutes and then increased current demand sent it packing. As with everything, you learn the most by solving problems and making mistakes.

 

RE: I may have had a combo problem, posted on January 19, 2017 at 18:55:04
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
which made a troubleshooting even more challenging. I suspected the fil trans early on but was loathe to change it out since that took the most work to deal with. And in combo with the LED, which i am sure caused some instability because it made the driver tube amplify some very high frequency signals.

 

RE:In this case it was a used filament trans.., posted on January 19, 2017 at 20:04:03
Tre'
Industry Professional

Posts: 17263
Location: So. Cal.
Joined: February 9, 2002
Used = unknown

"As with everything, you learn the most by solving problems and making mistakes."

Amen to that!

Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: I may have had a combo problem, posted on January 20, 2017 at 06:41:55
PakProtector
Audiophile

Posts: 12356
Joined: May 14, 2002
LED's are not created equal, some are remarkably noisy. The low impedance, 'standard' Red LED from Agilent is among the best in terms of delivered impedance and noise. The 'high intensity' variants are all higher impedance vs the standard sort.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: Or hack on new lead outs! (nt), posted on January 20, 2017 at 07:54:43
rage
Audiophile

Posts: 792
Joined: December 17, 2010
you only have to screw up "new lead outs" once to abandon that ship.

after my failure I pretty much threw out everything I'd replaced leads on...

(chokes, filament transformers, a nice power transformer I ruined)

 

How do you know who makes them?, posted on January 20, 2017 at 09:20:56
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
I dont see any part numbers or manufacturer marks.

 

RE: What do new leadouts have to do with transformer performance?, posted on January 20, 2017 at 10:08:53
gusser
Audiophile

Posts: 3649
Location: So. California
Joined: September 6, 2006
And with 30 years of designing TV broadcast systems and equipment I have found nothing to show that replacing manufacture supplied component lead-outs is of any benefit whatsoever.

 

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