Poll

What is amplifier bias

I think it is very important to get just right
1 (6.3%)
Close is good enough
6 (37.5%)
Set the meter to mV
2 (12.5%)
The right bias comes from a design equation
3 (18.8%)
We are measuring what is traditionally called "bias"
3 (18.8%)
Set the meter to mA (milliamps)
1 (6.3%)
None of the above,  I have a question
0 (0%)
We are not measuring what is traditionally called  "bias" but something else
0 (0%)

Total Members Voted: 10

Voting closed: 25 Aug 2014, 03:17 am

Amplifier Bias

0 Members and 1 Guest are viewing this topic. Read 9684 times.

tubegroove

  • Jr. Member
  • Posts: 64
Re: Amplifier Bias
« Reply #20 on: 19 Sep 2014, 09:31 am »
Hi enochrome
Not sure if you have since solved your issue but setting the bias on the RM10MkII is super easy.  300mV is the suggested value but that's not cast in stone.  On what aspect exactly do you need help?

Also, when running the RM10MkIIs in parallel (mono) mode, you have to pay attention to how you load the taps.  It can be a little confusing at first but if I have understood it correctly, to extract the full 70W/monoblock you should connect a speaker of half the impedance value of the tap selected (for eg a 4ohm speaker on the 8ohm tap in parallel mode).  In your case, casually looking at the Stereophile measurements of the M20s, it seems that the speaker is probably behaving more like a 4ohm nominal rather than a 8ohm load.  I would try connecting them to the 8ohm tap in parallel mode and see if that makes an improvement.  On the 4ohm tap, I believe the power will be only 1.4x the 35W stereo power and Revels are power hungry in general from what little I know about them.

Note: I use my RM10MkII in stereo mode so you may want to phone Roger/Ben and confirm the above.

enochrome

  • Jr. Member
  • Posts: 33
Re: Amplifier Bias
« Reply #21 on: 20 Sep 2014, 03:29 pm »
Thanks for replying.

I have never biased an amp before and I don't have the manual. Do I take the negative(black) connection and connect to the ground post of the speaker terminal? Then, I insert the positive(red) contact into the holes in front of the tubes; one for each tube? I then adjust bias by turning the flat head screw attentuator to read 300mv?

Sorry you guys are going to dumb down for me, but I do appreciate it.

Thanks

Freo-1

Re: Amplifier Bias
« Reply #22 on: 20 Sep 2014, 04:08 pm »
Here is a little quiz on bias.

You get to vote for as many options as you want.

The poll closing date is noted and results will be shown and discussed then.

In the meantime lets hear what people think about bias.

I think fixed bias is the best way to go.   Can't see any advantage to using cathode bias from a performance aspect.  In addition, I also think each output tube should have it's own bias pot.  The DIY amps I've had all employ this, and measure in Milliamps.

pehare

Re: Amplifier Bias
« Reply #23 on: 20 Sep 2014, 04:48 pm »
To adjust & check your bias voltage on the RM10 you use the 2 test jacks or bias pots in front of each pair of tubes.  Begin by letting the amp & tubes warm up for 20 minutes.  Each channel is biased as a pair so there is only one adjustment screw for each channel or pair of output tubes.  Left bias pot or test jack is for your negative voltmeter probe & right is positive.  Set each pair at 300mV as per manual or less for more tube life.  enjoy!

Freo-1

Re: Amplifier Bias
« Reply #24 on: 20 Sep 2014, 04:59 pm »
To adjust & check your bias voltage on the RM10 you use the 2 test jacks or bias pots in front of each pair of tubes.  Begin by letting the amp & tubes warm up for 20 minutes.  Each channel is biased as a pair so there is only one adjustment screw for each channel or pair of output tubes.  Left bias pot or test jack is for your negative voltmeter probe & right is positive.  Set each pair at 300mV as per manual or less for more tube life.  enjoy!

Is that 300ma or 30ma?   300ma seems way too high.

Update:  It IS 30 ma.  At 300ma, you would cherry the tubes quickly!  Read the link provided:

http://www.ramlabs-musicreference.com/rm10design.html

pehare

Re: Amplifier Bias
« Reply #25 on: 20 Sep 2014, 05:52 pm »
bias on the RM10 is set in millivolts so the multimeter is set for V not A

enochrome

  • Jr. Member
  • Posts: 33
Re: Amplifier Bias
« Reply #26 on: 20 Sep 2014, 06:56 pm »
Thanks pehare and everyone, that is exactly what I needed to know. I will let you know how it turns out.

Thanks again!! :D

Freo-1

Re: Amplifier Bias
« Reply #27 on: 20 Sep 2014, 09:21 pm »
bias on the RM10 is set in millivolts so the multimeter is set for V not A

So, the ratio is 10mv to 1ma.  Makes sense.  The old Dynaco amps had a scaling factor to set bias at 1.56V, which equaled the correct bias in ma.

pehare

Re: Amplifier Bias
« Reply #28 on: 20 Sep 2014, 09:42 pm »
Yes, it's measured across a 10 ohm resistor thus the current is 30mA for a 300mV reading (as per owner manual).  enjoy!

enochrome

  • Jr. Member
  • Posts: 33
Re: Amplifier Bias
« Reply #29 on: 20 Sep 2014, 10:26 pm »
So I biased the amps to 300mv with no problem. I hooked up each amp in stereo to check both channels, which indeed they both worked for each amp. I am still having the same problem, in which I need to turn my preamp to 3 o'clock or more to get reasonable volume. What's weirder is that I don't get hardly any improvement running them in mono as I do when they are each hooked up in stereo to my speakers.  :scratch:

Obvious question, if I hook it up in mono I need to use the left channel speaker connections correct? I assume so, because the mono input is the left channel one.

Are the tubes or my pre the problem? I thought these amps could run with a passive pre? I got a lot more power with my Exposure 2010s, that used a passive pre and was 70 watts solid state. I thought tube watts were more powerful than solid state?


bdp24

  • Full Member
  • Posts: 883
Re: Amplifier Bias
« Reply #30 on: 20 Sep 2014, 10:49 pm »
Giving up already, Eno? (I just saw the amp on U.S. Audiomart). Give Roger a chance to sort out your situation---it's probable something simple. It doesn't sound like a lack of power issue to me, and the amp's sensitivity can be increased by changing the input impedance-determining resistor.

Freo-1

Re: Amplifier Bias
« Reply #31 on: 20 Sep 2014, 10:53 pm »
So I biased the amps to 300mv with no problem. I hooked up each amp in stereo to check both channels, which indeed they both worked for each amp. I am still having the same problem, in which I need to turn my preamp to 3 o'clock or more to get reasonable volume. What's weirder is that I don't get hardly any improvement running them in mono as I do when they are each hooked up in stereo to my speakers.  :scratch:

Obvious question, if I hook it up in mono I need to use the left channel speaker connections correct? I assume so, because the mono input is the left channel one.

Are the tubes or my pre the problem? I thought these amps could run with a passive pre? I got a lot more power with my Exposure 2010s, that used a passive pre and was 70 watts solid state. I thought tube watts were more powerful than solid state?








I think that the red goes on the left 8 Ohm tap, while the black goes on the right common.   That is how my old McIntosh amps were. The left input has to go to both sets of output tubes, and across both output transformers.

tubegroove

  • Jr. Member
  • Posts: 64
Re: Amplifier Bias
« Reply #32 on: 21 Sep 2014, 06:46 am »
So I biased the amps to 300mv with no problem. I hooked up each amp in stereo to check both channels, which indeed they both worked for each amp. I am still having the same problem, in which I need to turn my preamp to 3 o'clock or more to get reasonable volume. What's weirder is that I don't get hardly any improvement running them in mono as I do when they are each hooked up in stereo to my speakers.  :scratch:

.....
Did you try different output taps as I mentioned in my earlier post?

enochrome

  • Jr. Member
  • Posts: 33
Re: Amplifier Bias
« Reply #33 on: 21 Sep 2014, 06:59 am »
Did you try different output taps as I mentioned in my earlier post?

Yes, I ran it this time with the 8ohm taps.

I think my speakers are just too power hungry.  :|


enochrome

  • Jr. Member
  • Posts: 33
Re: Amplifier Bias
« Reply #34 on: 21 Sep 2014, 07:01 am »
Giving up already, Eno? (I just saw the amp on U.S. Audiomart). Give Roger a chance to sort out your situation---it's probable something simple. It doesn't sound like a lack of power issue to me, and the amp's sensitivity can be increased by changing the input impedance-determining resistor.

I figured that my Revels just need too much power and that I should cut my losses and keep one for a single driver project
and sell off the other.

Freo-1

Re: Amplifier Bias
« Reply #35 on: 21 Sep 2014, 03:28 pm »
Yes, I ran it this time with the 8ohm taps.

I think my speakers are just too power hungry.  :|

Did you connect the positive connection to the 8 ohm tap left channel, and the common to the right common?  In order to get the extra power, need to use all the tubes and iron.

enochrome

  • Jr. Member
  • Posts: 33
Re: Amplifier Bias
« Reply #36 on: 21 Sep 2014, 03:39 pm »
Did you connect the positive connection to the 8 ohm tap left channel, and the common to the right common?  In order to get the extra power, need to use all the tubes and iron.

No, I used the left common. Do I need to use the right when in mono? Is that what tubegroove meant by "in parallel"?

Freo-1

Re: Amplifier Bias
« Reply #37 on: 21 Sep 2014, 06:49 pm »
No, I used the left common. Do I need to use the right when in mono? Is that what tubegroove meant by "in parallel"?

Yes, for the reasons stated above. 

Roger A. Modjeski

Re: Amplifier Bias
« Reply #38 on: 25 Sep 2014, 11:49 am »
I am a new owner of RM-10 mkii amps that I have bought second hand. I am also new to tube amps and would like to set "bias" correctly for them but I have never done this before. Any help would be appreciated. I am concerned because I have set both amps up to be used as mono's. They are set at the 4 ohm tap for "light loading". When I play music I have to crank my Pass Aleph P preamp to almost 3 o'clock to get music to play at 75db. My speakers are 87db @ 6 ohms Revel M20's. I thought 70 tube watts bridged would be enough for my small room, but it isn't, and I think something is up that is not normal  :scratch:

Any advice would be greatly appreciated.

Thanks to all for your efforts. I dont know if eonochrome has written Ben (tubeaudiostore@gmail.com) but these numbers are all out of whack and it seems no one has mentioned that he should be using the 8 ohm tap for a 4 ohm speaker. When you parallel (mono) a tube amp the voltage stays the same and the current doubles. The taps all go to 1/2 their impedance. This is true for all tube amps! It is just the opposite for transistor amps when bridged the voltage doubles and the power quadruples. This is basic electronics that I teach in my classes or you can find in books.

The setting of the volume control is absolutely immaterial as long as its not full up and not enough volume. VOLUME CONTROL SETTING HAS NOTING TO DO WITH POWER OUTPUT. Its all relative to gain of the amp and voltage of the source. The RM-10 will produce full power with 1 volt input and most CD players have 2 volts output.

It would be nice to know your listening level at 1 meter. 



Freo-1

Re: Amplifier Bias
« Reply #39 on: 26 Sep 2014, 06:36 pm »
So, did you ever get around to trying this out?