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Runaway bias - please HELP!

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Posted on June 7, 2014 at 13:25:47
DexysMidnightRunner
Audiophile

Posts: 162
Location: Florida
Joined: January 5, 2001
 A really strange problem with my pair of Antique Sound Labs ASL 1006(845) DT monobloc amps, which have been working perfectly since I bought them (used) a couple of months ago. A couple of days ago one of the amps suddenly, for no apparent reason, failed to hold bias. I have been biasing them at c. 850, per advice from inmates here. These are the older horizontal style 1006s, not the newer vertical style; mine have 3 6SN7s, 1 KT66, and 1 845 per chassis, two switches - on/off, and operate/stand-by - and a digital readout meter for biasing.

Upon turn-on, the affected amp was showing bias as c. 750. Slight turn of the bias pot to raise it, and bias started rising...and rising...and rising, with no stopping. Through 800, 900, 1000, and so on, rather quickly; by the time I rotated the pot in the opposite direction to lower bias, it was over 1600 and still going up. A slight (by 'slight' I mean less than 1/4 turn) counter-rotation to the starting point, and bias immediately fell back to c. 750 - it did not go back through the numbers as it did while rising, but defaulted, as it were, to the 750 point. Another attempt to get it to correct bias, and the same thing happened - except this time the bias meter went all the way past 1900, before suddenly displaying the single digit "1" - which I am guessing is the meter's way of saying error. Turn down yielded the same immediate reversion to 750.

Oooookay; could be a tube - I'd replaced the 845Bs with a different new pair of 845Bs, sourced from Sophia, shortly before this problem started. Put in the older 845Bs, which on turn-on had an initial bias of c. 585. One amp took and held at 850; the one with the problem, though, exhibited the same behavior. A 1/8 turn of the bias pot, and bias started zooming up without stopping.

Anyone ever have the same sort of issue? Any thoughts on diagnosing and repair? Could it be that the pot itself has gone bad? If so, with what should it be replaced? The manual schematic just identifies it as "100 1W." 1 watt I get...but what's 100, when referring to a variable resistor?

Any help and advice is GREATLY appreciated! No music :-( in the house until I get this fixed.
-----

If you haven't heard it, you don't have an opinion.

 

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RE: Runaway bias - please HELP!, posted on June 7, 2014 at 20:35:06
Blackdog
Manufacturer

Posts: 1505
Location: Ontario
Joined: March 20, 2006
 It is quite possible you have a bad bias pot. However you're looking at the wrong pot. The 100 ohm pot is a hum reduction pot. What you need is a 50k multiturn pot. Check for bad connections first.

A word of warning here. There is nearly 1000 volts inside this amp. If you don't know what you're looking at, don't go in there. Find someone to fix it for you.
Dan Santoni

 

sounds like a runaway tube, posted on June 8, 2014 at 07:26:10
FenderLover
 Either the tube, itself, or some possible parasitic instabilities.

 

RE: Runaway bias - please HELP!, posted on June 8, 2014 at 23:06:21
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Let me ask you,when the bias current or voltage goes up on the meter and becomes sporadic,does the plate of the tube get that reddish/orange glow? If it does,the bias pot is suspect or maybe a coupling cap that feeds the control grid of the 845..Take the 845 out of the bad amp and goes to pin one which is the control grid.Put your meter on DCV on the highest scale and then clip your neg lead to ground on the chassis somewhere.Turn the amp on and see what the negative bias voltage is at pin one with reference to ground.It should be a negative DC voltage reading.If it's not,you are in the wrong pin.Once you get the negative reading,leave the meter probe on pin one and move the bias pot up and down to see if the negative voltage goes up and down depending way you turn the pot.If its real sensitive,change the pot.If that doesn't do it,the tube or the cap is suspect and I have seen more than one bad tube in a row.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Runaway bias - please HELP!, posted on June 9, 2014 at 14:27:51
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002
 "Upon turn-on, the affected amp was showing bias as c. 750. Slight turn of the bias pot to raise it, and bias started rising...and rising...and rising, with no stopping. Through 800, 900, 1000, and so on, rather quickly; by the time I rotated the pot in the opposite direction to lower bias, it was over 1600 and still going up. A slight (by 'slight' I mean less than 1/4 turn) counter-rotation to the starting point, and bias immediately fell back to c. 750 - it did not go back through the numbers as it did while rising, but defaulted, as it were, to the 750 point. Another attempt to get it to correct bias, and the same thing happened "

That sure sounds like a bad pot to me.

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

 

RE: Runaway bias - please HELP!, posted on June 10, 2014 at 18:20:57
DexysMidnightRunner
Audiophile

Posts: 162
Location: Florida
Joined: January 5, 2001
 Thanks, all, for chiming in with thoughts. Tash from Divergent Audio thinks it's likely a bad pot winding (apparently this has happened before), and I'm arranging to get a replacement from him. Should take 5 minutes to remove the old and install the new. Dan, you're 100% correct about the value of the pot (except, while the schematic shows the bias pot to be 50K, the actual part used is marked as 22K, and Tash says 22K is right); I don't know how my brain swapped VR1, the 100 Ohm hum pot, with VR2, the bias pot.
-----

If you haven't heard it, you don't have an opinion.

 

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