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  #1  
Old 08-26-2018, 10:16 PM
birdland birdland is offline
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Default REF150 Tube Bias is increasing!?

Hi:

I was going to post this as a reply to the coupling caps topic but since the last post there was 2 years ago, I thought I'd post this as a new thread.

Does anyone know what failure due to the teflon coupling caps looks like?

I decided to check the bias on my power tubes in my REF150 and I found that the front tube of the right set power tubes on the left channel was reading 70mV! The last time I checked which was only a few listening sessions ago, they were are 64/65mV. The left side of the left channel were reading exactly what I set them to awhile ago.

The right channel had the same problem. The front tube of the right set of power tubes was reading 75mV!

I lowered the bias to 63mV and then checked the rear tube which was more than 64mV and then I went back to the front and it was already at 67mV.

At this point, I just turned off the power.

Could this increase in biasing voltage be due to the coupling caps starting to fail? My local tech mentioned that I should check my bias periodically because when the coupling caps fail, they do not take very long to completely fail.

Thanks,
Walter
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  #2  
Old 08-26-2018, 10:19 PM
birdland birdland is offline
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BTW, the tubes are relatively new and have less than 500 hours on them.
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Old 08-28-2018, 07:05 AM
jakegt3 jakegt3 is online now
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Are you letting the amp run for a good amount of time (at least half an hour) to stabilize before you measure the bias? The bias voltages in my Ref 150 are very stable when I check them this way. I think I've only had to make one slight adjustment in ~1000 hours of operating time.
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Old 08-30-2018, 07:36 PM
birdland birdland is offline
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Hi John:

Depends on what you mean by run for a long time.

The amp was playing music for about an hour. I then muted the pre-amp and let it settle while I was putting away the record I was playing and turning off the phono pre-amp, etc. I guess music wasn't playing for a few minutes - most likely less than 5 minutes.

Maybe, I just need to be more patient and let it settle with the pre-amp muted for a bit longer before I check the bias. I have done this many times and I have never seen this high bias voltage before. It is possible that I didn't wait as long as I should have.

I will try again but this time I will just power on the amp and let it warm up for 20 to 30 minutes before I check the bias.
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  #5  
Old 09-01-2018, 08:08 PM
birdland birdland is offline
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So, I turned on the amp with the pre-amp off and waited for 20 minutes. I checked the bias and they were HIGH again! So I lowered the voltage to 63mV. I checked a minute later and they were up again. Also, I had a very hard time adjusting the voltage. A slight push and the mVs would go up by 2 or 3 or more.

I had never seen this before. I finally started being suspicious of my multimeter. I had not changed the battery in some time. I changed the battery and check the voltages and they were all in the 40s of millivolts.

So I adjusted all of the bias voltages to 63mV again and let them settle down and now they did not change.

It was the bleeping battery!

Yes, I know that the bias voltage is supposed to be 65mV but I wanted a bit of a buffer just in case. I now have music playing and I am not worried about the bias although I will check the bias again after playing music for a bit.
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Last edited by birdland; 09-01-2018 at 08:12 PM.
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Old 09-01-2018, 08:25 PM
birdland birdland is offline
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The worse part was that the first time I checked the bias several days ago, the first tube measured as expected but the next tube I checked and adjusted showed very high bias. Hence, the battery's voltage changed sufficiently between checking the first tube and the second tube to throw the readings off. So I never thought about the battery.

Today's bias check showed all tubes reading high which made me suspicious.
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Old 09-04-2018, 06:56 AM
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Default REF150 Tube Bias is increasing!?

What kind of meter are you using? 65 mV is a pretty low voltage, so you’ll need at least a 3 1/2 digit multimeter with a 200mV or 300mV range.

A good meter will operate correctly independent of battery voltage and then provide a clear indication that the battery is low and needs to be replaced.

I use a TPI 183 - good value. Fluke also makes a good multimeter.
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Old 09-04-2018, 09:14 AM
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I understand your concern over coupling cap failures and would think this failure would cause a much larger change in grid bias. Keep in mind 5mv is only .005 volts. If I was checking a large batch of output tubes on a tube checker I would consider this normal.
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Old 09-04-2018, 10:16 PM
birdland birdland is offline
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Quote:
Originally Posted by W9TR View Post
What kind of meter are you using? 65 mV is a pretty low voltage, so you’ll need at least a 3 1/2 digit multimeter with a 200mV or 300mV range.

A good meter will operate correctly independent of battery voltage and then provide a clear indication that the battery is low and needs to be replaced.

I use a TPI 183 - good value. Fluke also makes a good multimeter.
Thanks for the reply.

I'm using a Fluke 11 Multimeter. It is an old meter and I do not remember the low battery icon showing but I could have missed it. I still have the old battery. I may put it back in just to see if icon shows.

The voltages are stable now so it was definitely the battery.

tdelahanty - I understand the .005 volts but the technician, who fixed the resistors that blew shortly after I purchased this amp used, always told me that I should not drive the power tubes any harder than they already are.
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  #10  
Old 09-06-2018, 09:49 PM
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The Fluke 11 only has a 4V scale, so if you want to measure 65 mV it won’t be very accurate down that low. It will work fine for relative measurements, though, which is probably all you need.
Tom
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C 1100, MX 151, MCD 1100, MR 80
Nottingham Dais with Sumiko Palo Santos Presentation
SurfacePro 3, JRiver, WW Starlight Platinum USB, Benchmark DAC3 HGC

MX 151, OppO BDP-95, JVC RS-500 DILA projector, 106" diagonal Stewart Luxus Screenwall Deluxe with Studiotek 130 G3 material.

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