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

BTW, the tubes are relatively new and have less than 500 hours on them.

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.

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.

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. :yes:

It was the bleeping battery! :sigh:

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. :music:

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.

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.

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.

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.

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