So I purchased a used Audio Research VT130 years ago. Now it blew a resistor , caused by a blown tube no doubt. What’s weird is the color code on the carbon resistors. These are 100 ohm resistors … why orange bands? Okay, I’m guessing because they can’t be brown since that is the color of the resistor? Does anyone know where I can get identical resistors for this amp?
It appears R31 blew up.
It appears R31 blew up.
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Overheated resistors may change color. These high power tube amps with encased tubes really scare me and heat stress the components beyond my level of comfort. I second Chris that all these (g2?) resistors should be replaced. First things first, I would check if the caps are bad, if the drop across R38-9? is reasonable and then obviously replace all the bad parts. Old carbon comp resistors are know to STB every so often and for no particular reason. So tubes maybe ok.
Close enough values of Allen Bradley carbon resistors should be fairly easy to locate on ebay or even on this forum in the swap meet section.
Looking at the poor general state of the solder joints, it seems to me the amp could use a little bit more in-depth refresh.
Close enough values of Allen Bradley carbon resistors should be fairly easy to locate on ebay or even on this forum in the swap meet section.
Looking at the poor general state of the solder joints, it seems to me the amp could use a little bit more in-depth refresh.
Thank you both for the info. The Tubes were only about 3 weeks old when the resistor blew. Good to know that the tubes may be okay. Is there anyway to check the tube to see if is good or bad with buying a tube tester? I looked online about it but haven’t found anything.
What wattage of resistors would you recommend? I suspect these are 1-2 Watts. I’ll look for a full schematic as well which should specify that.
What wattage of resistors would you recommend? I suspect these are 1-2 Watts. I’ll look for a full schematic as well which should specify that.
I have used quite some of ARC's amps, and the one I miss the most are VT-130SE, so take good care of it.
Had both VT-150SE and VTM-200 that was supposed to be better, but they weren't upgrades sonically to me.
VT-130SE has the perfect blend of musicality and clarity with excellent transient attack of acoustic instruments.
I would buy one again if I could find it here in Sweden where I live.
During the years I also repaired a few of ARC tube amps for myself and friends, and the 100 ohm resistor are a common repair issue.
In later released amplifiers ARC have changed to 3W wirewound resistors, and when asking for replacements for the 2W carbon you have, they will send 3W wirewound resistors.
I think you safely can use a nice 3W wirewound resistor in this position without a sonic degradation.
Unfortunately I have to say that in my experience there is a faulty tube when this resistor blows.
Measure resistance between pins of all your output tubes.
Eventually you will find a short or unstable value that indicates which tube differs from the others.
Had both VT-150SE and VTM-200 that was supposed to be better, but they weren't upgrades sonically to me.
VT-130SE has the perfect blend of musicality and clarity with excellent transient attack of acoustic instruments.
I would buy one again if I could find it here in Sweden where I live.
During the years I also repaired a few of ARC tube amps for myself and friends, and the 100 ohm resistor are a common repair issue.
In later released amplifiers ARC have changed to 3W wirewound resistors, and when asking for replacements for the 2W carbon you have, they will send 3W wirewound resistors.
I think you safely can use a nice 3W wirewound resistor in this position without a sonic degradation.
Unfortunately I have to say that in my experience there is a faulty tube when this resistor blows.
Measure resistance between pins of all your output tubes.
Eventually you will find a short or unstable value that indicates which tube differs from the others.
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With G2 disconnected it cannot accelerate the beam, so as petertub says the valve becomes a (fairly poor quality) triode. But the valve is not damaged if the G2 stop is fused open quickly enough, so may live to fight again after the causal problem is fixed. Resistors are cheap, output valves are expensive, gotta fix it anyway, that's my line of thought.
All good fortune,
Chris
All good fortune,
Chris
You guys got me curious, so I tried a well worn (45yo!) RCA 6L6GC on my Metrix U61B analyser: it's still valiant spec wise:
Va = Vg2 = 250V
Vg1 = -14V
Ia = 64mA (from 72mA new)
Ig2 = 5mA
Gm = 6mA/V
Disconnecting the G2 cuts the tube off completely, even when I turn Vg1 down to 0, not conducting, nada.
The moral of the story is:
G2 resistor as fuse is completely valid
G2 disconnected gives a degraded triode is not
Va = Vg2 = 250V
Vg1 = -14V
Ia = 64mA (from 72mA new)
Ig2 = 5mA
Gm = 6mA/V
Disconnecting the G2 cuts the tube off completely, even when I turn Vg1 down to 0, not conducting, nada.
The moral of the story is:
G2 resistor as fuse is completely valid
G2 disconnected gives a degraded triode is not
Didn't say it was a really great triode, insert smiley-face emoticon. In multigrid valves the second grid tends to get located where the anode would be if that G2 wasn't hogging the lane, monopolizing the conversation, just generally showing its ***, so the anode gets pushed so far away that it depends on a properly accelerated beam to get its gig done.
Type 6L6 was one of if not the first beam tubes, that bypassed the Philips patent on the pentode. Of course, you know all of that. Just saying that petertub's description is pretty solid.
All good fortune,
Chris
Type 6L6 was one of if not the first beam tubes, that bypassed the Philips patent on the pentode. Of course, you know all of that. Just saying that petertub's description is pretty solid.
All good fortune,
Chris
I would go back to basics and check/ replace everything that needs replacing. Cc resistors are cherry on the cake. Best to start with some reliable and easy to find 3w metal film res. Check the power tube bias current and go from there.
My advice would be to check electrolytic caps that have been exposed to high temperatures inside the case for decades. Maybe even some film caps may be degraded.
Good luck!
My advice would be to check electrolytic caps that have been exposed to high temperatures inside the case for decades. Maybe even some film caps may be degraded.
Good luck!
Flex2, thanks for sharing about your experience with your VT130 … I’ve enjoyed this amp so much. I’ve had expensive amps, mostly solid state or hybrid but this is my favorite. I hope to get it running again soon.
I’ll give real consideration to the 3 Watt wire wound resistors … seeing as it’s hard to find exact replacements for the Allen Bradley 2 Watt, and not wanting these to blow again. Yeah, I wouldn’t expect it to degrade the sound there.
Thank you all the great input … time is a real constraint right now. I will replace the resistors and the tubes in that channel. My understanding is that matching isn’t mandatory but is recommended for best sound. I have a working older set of tubes I’ll test first in that channel after I replace the resistors.
About checking the caps, is there a good way to do that ? Just see if the hold a charge by placing a battery across them?
The right channel worked fine after the resistor blew, but the left channel gave out completely with some quick speaker pops … after which I quickly turned off the amp.
About checking the caps, is there a good way to do that ? Just see if the hold a charge by placing a battery across them?
The right channel worked fine after the resistor blew, but the left channel gave out completely with some quick speaker pops … after which I quickly turned off the amp.
So, I pulled the resistors on one channel.
ordering this non-inductive wire wound resistor:
https://www.digikey.com/en/products/detail/ohmite/WND100FET/3114550
and this Vishay-Dale:
https://www.digikey.com/en/products/detail/vishay-dale/RS02B100R0FE70/1166406
I'm just going to go ahead and order both of these ... any votes on which to use? Never used Ohmite before, seems like a good choice as it is a non-inductive winding. Any complaints with Ohmite, reliability or such? I saw no inductive spec on the Vishay but it is likely fine too. thoughts?
Edit: I see the Ohmite is rated at 150 degrees C versus 250 for the Vishay. The tubes do get very hot though and these resistors are close by.
ordering this non-inductive wire wound resistor:
https://www.digikey.com/en/products/detail/ohmite/WND100FET/3114550
and this Vishay-Dale:
https://www.digikey.com/en/products/detail/vishay-dale/RS02B100R0FE70/1166406
I'm just going to go ahead and order both of these ... any votes on which to use? Never used Ohmite before, seems like a good choice as it is a non-inductive winding. Any complaints with Ohmite, reliability or such? I saw no inductive spec on the Vishay but it is likely fine too. thoughts?
Edit: I see the Ohmite is rated at 150 degrees C versus 250 for the Vishay. The tubes do get very hot though and these resistors are close by.
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