REF150 amp just blew two small resistors
 

Well, my used REF150 amp just blew two small resistors in its left channel. :tears:

I have attached a photo of the left channel. Anyone know the significance of these resistors?

The fuse did not blow but I turned off the amp immediately. I don't know if I should turn it back on to check the bias on the tubes.

The tubes have slightly less than 1600 hours and the last time I checked the bias which was during my last listening session, the bias was just slightly below 65mV on the master tubes.


https://www.audioaficionado.org/atta...2&d=1494813492
.
.
.
.

Walter.......DO NOT turn the amplifier back on. Nothing good will come from that. These resistors failed for a reason, quite possible a power tube issue. Without a service manual and schematic I would not attempt to mess with it. Let a service tech fix what is wrong.

+1 Sage advice!

HI Dan:

Thanks for the response.

yes I agree. I had even unplugged the power cord so there is no way the amp could power on. I have contacted my local tech and I have emailed Audio Research in case there is a known issue surrounding these particular resistors. I hope this isn't the Teflon capacitors failing. They are known to do that.

REF150 amp just blew two small resistors
 

My understanding is that all ARC amps except for the most recent models did not have a fuse in the signal path so if a tube Arc'd for some reason- resistors fail to protect the rest of the amp. They are easy to fix by a qualified tech but do require someone to solder in new resistors. ARC has always felt this extra work was worth it to keep a fuse out of the signal path and protect SQ.

This is a layman' answer based on my understanding and me owning Ref 250s, ref 150 and VS115. I do think they have changed their position on most current amps

Good luck...takes some effort but hopefully easy fix along with checking tubes

If it makes you feel better I have a ref150 with the big Teflon caps and it's still running great about six years in

Thanks for the heads up on the Teflon caps. My local tech mentioned that they do have tendency to fail and when they do it is very fast and they take the power tubes with them along with other things. I've been keeping an eye on the bias for this reason.

Also, I purchased this amp used from a dealer in New Jersey and he misrepresented the age of the tubes (said 750 but received the amp with counter at 1400 and change) and the fact that the chassis was bent at the right speaker terminals. It looks like the speaker terminals hit an object.

I had just received a response from Audio Research about the cost of replacing all of the tubes in the amp and this happens.

I'm not sure if these are the resistors which normally fail when a power tube goes. I thought there were one of the larger ones closer to the power tubes. Anyway, the resistors you mentioned do not always protect the rest of the amp. One of my VT100s (REF150 replaced 2 VT100 Mk IIs), blew all sort of components when the driver tube decided to go including a power tube. It was my fault. I did not keep track of how many hours were on the tubes. There is no tube counter in a VT100.

However, this time I've been a good boy and monitored everything and still this happens!

I also have a Reference 150, so this is of interest to me. In the ARC amps I'm familiar with, the resistor that blows is in the path that supplies bias voltage to the tubes. What usually triggers the resistor to blow is an internal arc or short within a tube, which in turn generates a big current spike through the resistor, causing it to blow. I've also heard that this is a protection mechanism that ARC uses in place of fuses in the signal path. The damage is generally limited to the bad tube and the resistor.

However, looking at the photo here, the resistors that have blown are very small ones and I'm not sure they look like the resistors in the bias path. The Reference 150 could be using some other kind of protection circuit that I'm not familiar with. It would be nice to have a schematic diagram of the amplifier for troubleshooting purposes, but as far as I know, ARC doesn't release those to customers.

To be safe, you should not turn on the amp again until it's fixed.

Thanks for all of the responses.

Audio Research responded this morning.

"These parts are the two heater-isolation 100 ohm 1/2W resistors. They are only damaged when an output tube arcs in that channel. One of the 4 output tubes in this channel has arced. Check the 1 ohm plate, 100 ohm screen and 1 ohm cathode resistors at each tube socket. One of these has likely blown open as a result of the arc. Do not use the REF150 until this has been repaired."

So I checked the resistors all around the power tubes and found one which read 304.4 ohms instead of 1 ohm. Strangely, it has not blown open and the resistor doesn't look blown like the two smaller ones.

I have responded to Audio Research with this information and the fact that the right binding posts are bent (well chassis is bent) so I can get an idea of what this is going to cost.

If you want to know who is the audio dealer in New Jersey that sold this amp to me, please PM me. I would stay away from that dealership.

REF150 amp just blew two small resistors
 

Checking resistances with the component still installed in a circuit can (and often does) give values different than the expected value. The only way you can check values properly is to lift (unsolder) one leg of the resistor.

I am very confused by this statement. The meter is measuring across the resistor.

Now that I think about it, I guess it is possible that if the resistor is open and there is a path from one end of the resistor through the circuit to the other end, I could get an actual non-zero reading. OK - got it. Thanks.