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Old 14th February 2012, 05:35 AM   #1
WILD1 is offline WILD1  United States
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Default Audio Research M100s

Hey Guys,
Bought a pair of Audio Research M100s and there are some issues with one of them. I sent it back and when it returned it still isn't right. The little bit that I have listened to them I have loved the sound and I hate to return them although that is an option. Any input would be greatly appreciated. I am not an electrical engineer. I am a bovine herder and manage a family ranch. If you can help you have an open invitation to visit. We'll kill a cow have a barbecue and sip tequilla until the cows come home. I do have a good meter and know how to use it.
The problem I believe is in the power supply. It uses three tubes as "Regulators". One of these is a 6550 and it is the "Screen regulator" for the output tubes. Before I sent it back it was getting way too hot to the point the plates in the tube were turning red. I know it is not my imagination. When it first happened I pulled one of the 6550s [Winged C] from the other Amp and substituted it for this regulator tube [Sovtek] and yes it helped but only for a short while. While the amp was at the shop I sent some tubes to be checked and sent this 6550 as well. The test results came back and it was extremely gassy indicative of a tube running too hot. I believe the output tubes were running somewhat hot as well. It got to the point that the regulator tubes brightness was oscillating as well as the volume. In my conversation with the salesman I told him all of this and as well that the output tubes would not bias which they would not. I think he told the tech that it had issues and wouldn't bias. Now I have it back and it has KT88s as output tubes and the regulator and yes it will bias right now but volume compared to the other amp is greatly diminished and that output tube is still getting hot as a firecracker. I feel if I continue to use it that it will get to the point it will not bias and ruin these KT88s as well. I am getting so paranoid about it that I would swear that all the tubes seem a little too bright There is a schematic of the amp at this site ARCDB - M-100 . As I said any input would be greatly appreciated.
Thanks
Tom Wild
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Old 14th February 2012, 06:50 AM   #2
prakit is offline prakit  Thailand
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I have a pair of M100s too. They are among the best sounding ARC amps but extremely finicky. They are running with all devices of the regulators on their edges. Anything could go wrong at anytime. Most of the problems I experienced were on the power supplies and regulators.
I would suggest you take off all output tubes and check if all the power supplies and regulators work properly. The schematic includes the dc voltages of all regulators' output. I suspect the voltage at screen grid no 1 has gone wrong.
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Old 14th February 2012, 01:22 PM   #3
WILD1 is offline WILD1  United States
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Default Audio Research M100s

Hi Prakit,
I appreciate your input and you seem to be fairly knowledgeable about this amp. The first thing is I need to do is decide whether to fix them or send them back. Is it possible that this problem has damaged all tubes and other components in the amplification stages or will the damage just be associated with the output tubes? What and where exactly is screen grid no, 1? Is this the 6550 tube in the power supply that they are calling the "Screen Regulator" and I have talked about getting too hot? As you can see I am definitely a newbie but willing to put out some effort. If I do decide to keep them I will probably have time this weekend to look at them and perform these voltage checks. Until then.
Thanks so much,
WILD1
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Old 20th February 2012, 02:26 AM   #4
WILD1 is offline WILD1  United States
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Default Audio Research M100s

Hey Guys,
I did the voltage checks as you suggested Prakit and I had high readings for the voltage coming from the transformers and going to the 6550 regulators anode and screen. The schematic says that the voltage should be +590. I am getting 736 volts for one amp and 685 volts for the other amp. The only other items in this line coming from the transformers are two resistors and the capacitors. The resistors on visual inspection look good. I will have to pull the capacitors out to get to them if I want to check them. Could bad capacitors cause this? They are 27 years old. The strange thing is that the control grids voltages are only at 270 and 258 and the voltages at the cathodes are 330 and 299. The schematic says the voltage at the cathode should be +340 so these values are not high. yet those values for the anode of a 6550 tube are over the limit.

Dazed and confused,
WILD1

P.S. Had some boys out at the ranch today hunting wild hogs. Now that would make a good barbecue!

Last edited by WILD1; 20th February 2012 at 02:52 AM.
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Old 20th February 2012, 03:55 AM   #5
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oh bubba, you got to keep them.
the tubes and xformers are the pricey parts.

these guys could walk a blind man through a mine field. just be careful.
is that B+ voltage on the print with or without a load? without a load it will go up...that much i dont know?
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Old 20th February 2012, 04:27 AM   #6
Jim W is offline Jim W  United States
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Be extremely careful with that amp and those power supply voltages. There is plenty enough energy in those supplies to kill you several times over. The caps will retain a charge until they have time to bleed down, so the amp is dangerous for a time even after you turn it off. I don't want to sound like somebody's mom, but non-experienced people servicing hi-voltage tube gear worry me.

Great amp by the way. I really like that era ARC tube stuff.

Last edited by Jim W; 20th February 2012 at 04:30 AM.
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Old 20th February 2012, 04:58 AM   #7
Jim W is offline Jim W  United States
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Now that I'm done with the safety lecture... According to the schematic at arcdb.ws, there should be about 630V feeding the plates of the regulator tubes (through resistors). Looks to me like the 630V comes from laddering the transformer's rectified DC output. It is hard for me to picture how you're getting those high voltages right off the transformer. Could it be caps? I'm guessing not likely, but a quick look at them with a scope would tell you if they're shot. This is a really complex power supply, and I wouldn't want to troubleshoot it without proper tools (and one hand behind my back).

I've got a D90 which was re-capped a couple of years ago, along with new tubes and a general tune up, and I love the thing. I'd recommend that for an amp this old - it's worth it and it should last another twenty-some years before it needs it again.
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Old 20th February 2012, 05:04 AM   #8
WILD1 is offline WILD1  United States
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WOW!
Appreciate the input. Please keep it coming. Those B+ voltage readings are with all tubes installed and no signal. I first tested them with no output tubes installed and when I got high voltages I thought the same thing no load. But when I put the tubes back in voltages did not change. As far as working with high voltages, I had my father standing there with a walking stick so if it got me he could knock me off of it. I get bit on the ranch at least once a month by the electric fences. Eight to eleven thousand volts. It will make your teeth hurt. However it is a pulsating current with very low amperage not a DC regulated current like this. As far as the charge on the capacitors they have already drained down and I was working on the amps this evening. Less than four hours.

Thanks again,
WILD1
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Old 20th February 2012, 06:16 AM   #9
WILD1 is offline WILD1  United States
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Hi Jim,
I appreciate the input. I have been looking at a more recent version of the schematic. Not near as detailed as the one at the ARC website. Also like I said it showed 590 volts at the Regulators plate and screen grid. The voltages I gave were after the small resistors, however at ten and 100 ohms I don't think they change the voltage very much. I also checked the resistance of these resistors as well as the two before the control grid.They were all fine. I can not believe they are using this tube even at 630 volts. On a spec sheet by Tungsol for a 6550 maximum plate voltage recommended is 600 and screen voltage is 400. I am starting to believe Prakit that this power supply is running Balls out. I had another person tell me that yes they are great amps but be prepared to replace output and regulator tubes every two years. In his words "It eats output tubes." They are rated at a modest 100 Watts but at 16ohms. The little I have listened to them I think they are incredible The Bass is unbelievable for a tube amp. I still would sure like to calm down that output voltage from the transformer. If any one has any suggestions please let me know.
Hasta Luego,
WILD1
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Old 20th February 2012, 06:42 AM   #10
prakit is offline prakit  Thailand
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Hi WILD1,
Some discrepancies of values between schematic and measurement are normal. But 736 V. and 590 V is too much diff. I would check the AC V for the secondaries for both units. If they are the same, very likely the trans are ok.
Since I don't think the caps are the culprit here, the v-dropping R could be bad.
I would also check all V out from other regulators while at it.

When I first received the M100, all voltage were off the spec quite a bit. It took me a full month or so to work on this problem. I suspected the resistance drifted after years in extremely hot environment.

Agree with the words of caution. Make sure that all caps are drain before fiddling around.

One of my M100 killed a thermistor a couple week ago. Still yet find time to work on it.

Good luck
Quote:
Originally Posted by WILD1 View Post
Hey Guys,
I did the voltage checks as you suggested Prakit and I had high readings for the voltage coming from the transformers and going to the 6550 regulators anode and screen. The schematic says that the voltage should be +590. I am getting 736 volts for one amp and 685 volts for the other amp. The only other items in this line coming from the transformers are two resistors and the capacitors. The resistors on visual inspection look good. I will have to pull the capacitors out to get to them if I want to check them. Could bad capacitors cause this? They are 27 years old. The strange thing is that the control grids voltages are only at 270 and 258 and the voltages at the cathodes are 330 and 299. The schematic says the voltage at the cathode should be +340 so these values are not high. yet those values for the anode of a 6550 tube are over the limit.

Dazed and confused,
WILD1

P.S. Had some boys out at the ranch today hunting wild hogs. Now that would make a good barbecue!
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