REF150 amp just blew two small resistors

[birdland]

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

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.



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[jdandy]

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.

[Cohibaman]

Originally Posted by jdandy

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!

[birdland]

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.

[enatai252]

Originally Posted by birdland

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.

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

[birdland]

Originally Posted by enatai252

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

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!

[jakegt3]

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.

[birdland]

Originally Posted by enatai252

...

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

Regarding the large Teflon caps, please note that when Audio Research responded, they did mention the following which sounds like a note to themselves than to me.

"Photo shows he has the older Teflon coupling caps that are replaced under the ECN. Suggest factory service, if possible, so we can replace these failure-prone caps."

So you may want to take that under advisement.

[jdandy]

Originally Posted by birdland

Regarding the large Teflon caps, please note that when Audio Research responded, they did mention the following which sounds like a note to themselves than to me.

"Photo shows he has the older Teflon coupling caps that are replaced under the ECN. Suggest factory service, if possible, so we can replace these failure-prone caps."

So you may want to take that under advisement.

Walter.......Well there you go. Pack that REF150 up and ship it back to the factory. It'll come back in better condition than when you got it. Maybe ARC will be able to square up the rear chassis dent while they're at it.

[birdland]

Originally Posted by jdandy

Walter.......Well there you go. Pack that REF150 up and ship it back to the factory. It'll come back in better condition than when you got it. Maybe ARC will be able to square up the rear chassis dent while they're at it.

Dan - I have always used my local technician to repair my ARC amps. This way I didn't have to ship the amp and expose it to possible shipping damage.

However, there are a number of things wrong here and it would be a good idea to have the amp checked out by Audio Research. So I was leaning towards sending this amp back to Audio Research. I just have to discuss this with them so they know exactly what I want done and if they can fix the dented rear of the chassis - how much it would cost. I have the feeling they may have to transfer the guts of the amp to a new chassis. If this is the case, I might not have them do this. Of course, if I ever want to sell this amp, it would make it easier to sell if the right binding posts weren't coming out of the rear at a slightly odd angle.