Mills MRB-12 failure

184.76.252.79

Posted on March 8, 2015 at 07:27:50

Abacus Music
Audiophile

Posts: 97
Joined: February 13, 2012

My 300B amp developed an intermittent crackling noise in one channel. Investigation led to signal tube anode power resistors (two in parallel), both MRB-12, the old style black ones. The 20k resistor measured ~39.2k. No physical damage was observed, no spots or burns.

What could have happened there? Could Mills fail due to thermal stress?

I will be replacing them with the new MRA-12 since I can't find any MRB's in any of the usual spots - hoping they sound as good.

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RE: Mills MRB-12 failure

Abacus Music

12:13:31 03/20/15

RE: Mills MRB-12 failure

mach1

09:59:15 03/11/15

RE: Mills MRB-12 failure

Caucasian Blackplate

11:38:10 03/08/15

3

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RE: Mills MRB-12 failure

Abacus Music

12:13:31 03/20/15

RE: Mills MRB-12 failure

mach1

09:59:15 03/11/15

RE: Mills MRB-12 failure

Caucasian Blackplate

11:38:10 03/08/15

3

RE: Mills MRB-12 failure

Abacus Music

11:50:41 03/08/15

2

RE: Mills MRB-12 failure

Caucasian Blackplate

11:52:17 03/08/15

RE: Mills MRB-12 failure

Abacus Music

11:57:44 03/08/15

RE: Mills MRB-12 failure

drlowmu

08:41:50 03/08/15

3

RE: Mills MRB-12 failure

Abacus Music

08:53:13 03/08/15

2

RE: Mills MRB-12 failure

drlowmu

10:29:38 03/08/15

RE: Mills MRB-12 failure

Abacus Music

11:05:41 03/08/15

RE: Mills MRB-12 failure, posted on March 8, 2015 at 08:41:50

drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005

(1) Can you read / see the Ohmic rating on the body of the "failed" one ?

(2) How does the "other" resistor in the paralleled pair measure, in
comparison to the 39.2 K one?

(3) Does the schematic designate "20K" as a TOTAL value of both Rs, while showing two MRB-12s? If so, then each could easily be 40.2K.

The original designer could have paralleled two equal value MRB-12s, in this case two at 40.2K, a standard Mills Ohmic value.

Did you also measure the second R and can you see the rating on the bodies of both ?

Go from there. I'm outta here.

JM

RE: Mills MRB-12 failure, posted on March 8, 2015 at 08:53:13

Abacus Music
Audiophile

Posts: 97
Joined: February 13, 2012

:-)

it reads 20k on the resistor body...

two parallel resistors are 20k and 33k, both MRB-12, the second one measures 33k as prescribed.

RE: Mills MRB-12 failure, posted on March 8, 2015 at 10:29:38
drlowmu Manufacturer Posts: 9730 Location: East of Kansas City Joined: January 10, 2005	Wow !! I never run them over 6 Watts, usually 4-5 Watts. Likely something else failed in the circuit, a tube shorting, etc, and placed extra stress on the R. Anything can occur, and you are living proof it does ! Surprising to me. Jeff Medwin

RE: Mills MRB-12 failure, posted on March 8, 2015 at 11:05:41

Abacus Music
Audiophile

Posts: 97
Joined: February 13, 2012

There is one thing that may have caused the failure. There is a fat Mogami signal cable that runs from the back of the amp to the input pin of the tube, past those resistors. The insulation of that cable at one point was touching that 20k resistor as it is apparent from the burn marks on the cable. It is possible that PVC material somehow penetrated the silicon skin of the resistor and managed to "incinerate" the resistive element or contaminate the end cap causing the resistor to fail.

Usually when resistor burns you can definitely tell that it had burned, the visuals should be there. In my case the resistor looks perfectly normal, no cracks, spots, discoloration or smell. Need to do a detailed autopsy. Mystery!

RE: Mills MRB-12 failure, posted on March 8, 2015 at 11:38:10
Caucasian Blackplate Industry Professional Posts: 8313 Location: Seattle Joined: June 18, 2004	How much voltage is supposed to appear across that resistor?

RE: Mills MRB-12 failure, posted on March 8, 2015 at 11:50:41

Abacus Music
Audiophile

Posts: 97
Joined: February 13, 2012

~250V.

20k||33k = 12.45k

250V/20k = 12.5 mA

12.5 mA x 250V = 3.125W

RE: Mills MRB-12 failure, posted on March 8, 2015 at 11:52:17
Caucasian Blackplate Industry Professional Posts: 8313 Location: Seattle Joined: June 18, 2004	250V across a 20K resistor is 12.5mA, so you'll be sitting at just over 3 Watts of dissipation, which shouldn't have been a problem with a 10-12 Watt resistor.

RE: Mills MRB-12 failure, posted on March 8, 2015 at 11:57:44
Abacus Music Audiophile Posts: 97 Joined: February 13, 2012	No, it shouldn't have. The problem is someplace else.

RE: Mills MRB-12 failure, posted on March 11, 2015 at 09:59:15
mach1 Audiophile Posts: 399 Location: Brisvegas Joined: April 24, 2005	In my experience the stated power ratings for Mills resistors are way optomistic. Even in well ventilated free air they start smoking well before reaching their stated power capacity. Derate to half the stated capacity; and even take that figure with a pinch of salt over the long term.

RE: Mills MRB-12 failure, posted on March 20, 2015 at 12:13:31
Abacus Music Audiophile Posts: 97 Joined: February 13, 2012	Received new resistors from Handmade, put everything where it belongs - the amp sounds great. While on the bench I also put in a thermistor with delayed shorting relay to mitigate turn-on current rush.

SET Asylum

Mills MRB-12 failure

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