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Thread: Biasing a valve amp

  1. #1
    Join Date: Sep 2017

    Location: Middlesbrough

    Posts: 8
    I'm Stephen.

    Default Biasing a valve amp

    Hi
    I am seeking advice regarding the setup of my first valve equipment. I bought a pair of 6g4g monoblocks from Vinylnvalves almost two years ago and embarrassing have only just gotten around to setting them up. There is some hum from the amps even after warm up and I understand that this could be due to the bias not being set correctly. I have read/watched various biasing guides online but they reference parts that are not present on my amps. I have an 1 external pot for adjustment but two output valves, does this alter the idling current to both or alter the balance between the two? Is a shunt probe (valve seat insert) necessary as there are no dedicated bias points? Sorry if this seems like elementary stuff, but I get the impression that I may break something/someone/me if I start poking around in an uninformed fashion.
    Any help would be greatly appreciated
    Steve

  2. #2
    Join Date: Dec 2017

    Location: Ontario, Canada

    Posts: 54
    I'm Dan.

    Default

    First off, be careful. Not having external bias points means you must open up the amps to get at the points. There will be lethal voltages, so if you are not comfortable doing this get help from someone who is familiar with tube amps.
    Having one pot for each pair is sub optimal, and means your tubes need to be matched for current gain. Regardless of bias if the tubes aren't matched you may get hum due to imbalance of the output transformer.
    Before you can continue you need to know the recommended procedure, bias points and the current.

    Did the manufacturer give you a bias procedure? If not contact them for it. If they won't or can't then it needs to calculated and this opens a whole new set of procedures.
    Dan Santoni

  3. #3
    Join Date: Sep 2017

    Location: Middlesbrough

    Posts: 8
    I'm Stephen.

    Default

    Thanks for the quick supply Dan
    The bias pot is external which is the only reason I am attempting to do this myself. There is no-one in the Teesside area that advertises amp repairs, plus I think knowing how to bias the amp will pay dividends in the long run. The amp appears to be made by an enthusiast sand as a result there is no manual. The previous owner carefully labelled each valve with its corresponding socket which indicates that they are indeed matched pairs. I suppose my question is then is there a generic biasing procedure for TWO power valves and ONE bias adjustment pot and no bias points? and can it be explained to a numpty?

    The specs for the 6b4g valves are as follows
    http://www.r-type.org/pdfs/6b4g.pdf

  4. #4
    Join Date: Mar 2008

    Location: Leeds

    Posts: 12,884
    I'm inthescottishmafia.

    Default

    Is this an SE amp or PP?
    “Music has always been a matter of energy to me, a question of fuel. Sentimental people call it inspiration, but what they really mean is fuel. I have always needed fuel. I am a serious consumer. On some nights I still believe that a car with the gas needle on empty can run about fifty more miles if you have the right music very loud on the radio”

    Hunter S Thompson

  5. #5
    Join Date: Mar 2008

    Location: Leeds

    Posts: 12,884
    I'm inthescottishmafia.

    Default

    Are you sure this is a biasing point?

    It may be a hum bucker. Does the hum alter if you twiddle it?
    “Music has always been a matter of energy to me, a question of fuel. Sentimental people call it inspiration, but what they really mean is fuel. I have always needed fuel. I am a serious consumer. On some nights I still believe that a car with the gas needle on empty can run about fifty more miles if you have the right music very loud on the radio”

    Hunter S Thompson

  6. #6
    Join Date: Apr 2012

    Location: London

    Posts: 611
    I'm James.

    Default

    Upload a picture before you do anything. We don't know if this is a monoblock, stereo, SET, PP, PSE, really it's just a shot in the dark so far


    Sent from my BLA-L09 using Tapatalk

  7. #7
    Join Date: Sep 2017

    Location: Middlesbrough

    Posts: 8
    I'm Stephen.


  8. #8
    Join Date: Jun 2015

    Location: London/Durham

    Posts: 3,502
    I'm Lawrence.

    Default

    Given the history in that thread hopefully something can be worked out for you

    Sent from my POT-LX1 using Tapatalk

  9. #9
    Join Date: Nov 2011

    Location: Seaton, Devon, UK

    Posts: 3,342
    I'm Adrian.

    Default

    Quote Originally Posted by Dorisandjess View Post
    Hi
    I am seeking advice regarding the setup of my first valve equipment. I bought a pair of 6g4g monoblocks from Vinylnvalves almost two years ago and embarrassing have only just gotten around to setting them up. There is some hum from the amps even after warm up and I understand that this could be due to the bias not being set correctly. I have read/watched various biasing guides online but they reference parts that are not present on my amps. I have an 1 external pot for adjustment but two output valves, does this alter the idling current to both or alter the balance between the two? Is a shunt probe (valve seat insert) necessary as there are no dedicated bias points? Sorry if this seems like elementary stuff, but I get the impression that I may break something/someone/me if I start poking around in an uninformed fashion.
    Any help would be greatly appreciated
    Steve
    Just a thought you say you only just set them up, were the main output valves marked left and right and/or which amp they came from, or were they transported to your home with the valves left in the amps? If they were marked and you know which pairs are which make sure you have them in the correct sockets(left/right) as the bias would have been set up for them in that configuration.

    If you have twiddled the hum pots then what I am about to suggest below may not work or be worth doing. Having had a pair of push/pull 300B's it can be tricky to get the lowest hum. But in my experience if you know which is the bias pot (possibly the one on the top between the two output valves) you should be able to hear it lower as you tweek it. If you can get advise form the designer then this would be best.

    If you do what I suggest below whatever you do make sure you switch off before removing the valves and swapping them, and remember they get too hot to handle so take your time.

    Swapping them may improve the hum issue, but is likely to be hit and miss unless you are methodical. There are several permutations as you have 4 valves and two amps, i.e. 3*2*2amps*2(left and right), so you could start with one amp and select one valve, mark each A,B,C and D. So the process would be put A in left, B in right and power on and assess asses the hum, switch off and put swap A and B from left to right, switch on and asses hum. Then A in left and C in right, and repeat as before, then with A and D. Then repeat with B and C, B and D, finally C and D. Hopefully you will find the pair in the correct configuration which are quietest. You will then have two valves for the other amp that can go in only two ways, and in theory one way should be relatively hum free.

    If you already have twiddled the pots and did not not mark or know the original pot settings then it will be a matter or setting the bias and as others have said.
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  10. #10
    Join Date: Sep 2017

    Location: Middlesbrough

    Posts: 8
    I'm Stephen.

    Default

    Thanks again for all the replies, your advice certainly isn't taken for granted.
    The four valves an sockets were labelled v1 v2 v3 v4, the previous owner had taken care to make things as fool proof as possible. They obviously hadn't foreseen me. I haven't touched the bias pot yet as I was conscious of my own ignorance, I this something that can be done by ear?

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