JVM410C (& Other 100W Marshall) Bias Question

Discussion in 'Marshall Amps' started by Leperclown, Oct 22, 2021.

  1. Leperclown

    Leperclown Member

    Joined:
    Sep 14, 2021
    Messages:
    17
    Likes Received:
    47
    Hi all,
    I recently bought an Amptata (4 probe) bias testing unit. This will save having to pull the amp out of the shell to test bias, but I will still have to pull the amp in order to adjust bias if required.

    I plan to get external bias test and adjustment points fitted (as I had done on my old JVM410 head).

    A month or so ago, I fitted new pre (Tungsol, but original Marshall ECC83 left in Cathode Follower position - V8) and power (Mullard EL34) tubes. I set the bias at 72 mVa a side (36 per tube). I was happy with the sound.

    On receiving the Amptata unit this morning, I tested the individual bias of each tube and got the following results:
    Initial test (after 5 mins warmup) - V1: 35, V2:38, V3:39, V4:38
    Second Test (after 15 mins) - V1: 36, V2:39, V3:40, V4:39
    Third Test (after 30 mins) - V1: 37, V2:40, V3:42, V4:40
    The readings stabilised after the third test

    Now the questions:
    1. Is it normal that a bias reading would fluctuate over time as above?
    2. Are the third test readings within safe (or normal) boundaries of operation?
    3. I believe V1&4 and V2&3 are pairs. Do the above readings mean the tubes aren't properly matched, or are the tolerances acceptable?
    4. Should I open up the amp and re-bias or just get over it?
    5. Is there a reason why the location of the bias test point would cause a variation. I had read somewhere that the bias reading directly from the tube was more accurate than the reading from the internal test points, but this could just be marketing BS.

    Cheers & many thanks for any input

    Paul
     
  2. Jethro Rocker

    Jethro Rocker Well-Known Member

    Joined:
    Mar 11, 2011
    Messages:
    10,126
    Likes Received:
    15,343
    Location:
    Saskatoon, Canada
    1. Yes it will vary, wait until stable, which you did.
    2. Yes although I think it could be a bit lower so no 40s in there but it will vary based upon plate voltage. Personally I woild drop it a bit. It really isn't as drastic as it can be made out to be. If the tubes are not red plating or too cold amd it sounds good, done. Tubes might wear out faster as set hotter.
    3. That's mathced close enough. Some info I read says within 10mV. So close enough.
    4. Depending on P voltage, maybe set a bit lower.
    5. Not a clue.I use a Eurotubes bias probe on the tube sockets.
     
  3. Leperclown

    Leperclown Member

    Joined:
    Sep 14, 2021
    Messages:
    17
    Likes Received:
    47
    Hey Jethro Rocker,
    many thanks for the rapid & very helpful responses. I will re-bias the JVM tomorrow.

    Much appreciated!
    Paul
     
  4. MonstersOfTheMidway

    MonstersOfTheMidway Well-Known Member

    Joined:
    May 1, 2008
    Messages:
    5,380
    Likes Received:
    3,354
    How do you like the Eurotubes bias probe? I've not run into too many people who use this particular devices, but I've been curious.

    The other bias probe unit that caught my interest is designed by The Tube Depot. Not sure how I feel about that one, but it's on my radar.

    I've biased my amps with the simple multi-meter and alligator clips without any issues, but I'd like to try something that poses a little less risk yet still be very accurate.

    Any info would be helpful. Thanks for the time. Oh, and keep up the great photographs.
     
    Leperclown and Jethro Rocker like this.
  5. Leperclown

    Leperclown Member

    Joined:
    Sep 14, 2021
    Messages:
    17
    Likes Received:
    47
    I recently bought this on Ebay (sent from China, free post and very quick):
    Amptata V8 Valve Tester Bias Current Checker

    Simple to operate, and works very well. Allows bias to be checked on four octal power valves. Press the 4 buttons individually. It only cost around USD75, which is great value.

    I successfully checked the bias on my JVM410C (picture below) and on my Egnater Tweaker 88 (KT88's).

    A lot easier than pulling the amp from the shell.

    Highly recommended.

    Cheers
    Paul

    Bias Probes.jpg
     
  6. Jethro Rocker

    Jethro Rocker Well-Known Member

    Joined:
    Mar 11, 2011
    Messages:
    10,126
    Likes Received:
    15,343
    Location:
    Saskatoon, Canada
    It only does one tube at a time but shows PV and bias current so it is really easy to bias. I see that Amptata one above shows 4 tubes at once but still no plate voltage. I never had the courage to start metering PV with a MM thus the probe. Very happy with it. As for accuracy, who knows? I have no way of checking it but since it reads off the pins of tube can't see how it could be wrong.
    Can't say about other units, never looked and at the time I was being cheap.
    Cheers on the photos! I need to shoot more...
     
  7. ken361

    ken361 Well-Known Member

    Joined:
    Aug 30, 2009
    Messages:
    4,295
    Likes Received:
    4,588
    Location:
    detroit rock city
    I brought the probe for my deluxe reverb works great.
     
  8. ken361

    ken361 Well-Known Member

    Joined:
    Aug 30, 2009
    Messages:
    4,295
    Likes Received:
    4,588
    Location:
    detroit rock city
    Have a link
     
  9. solarburnDSL50

    solarburnDSL50 Well-Known Member Platinum Supporting Member

    Joined:
    Aug 8, 2008
    Messages:
    18,076
    Likes Received:
    14,904
    Location:
    Wetville
    I've been using Amp Head dual bias probe that also shows plate voltage. Have 2 of them been using for years. Don't think they make them anymore.

    Cool to see these other offerings too.
     
  10. Leperclown

    Leperclown Member

    Joined:
    Sep 14, 2021
    Messages:
    17
    Likes Received:
    47
    ken361 likes this.
  11. ken361

    ken361 Well-Known Member

    Joined:
    Aug 30, 2009
    Messages:
    4,295
    Likes Received:
    4,588
    Location:
    detroit rock city
    Thanks
     
    Leperclown likes this.
  12. Leperclown

    Leperclown Member

    Joined:
    Sep 14, 2021
    Messages:
    17
    Likes Received:
    47
    Hi all.

    This morning, I pulled the amp (JVM410C) out of the shell and re-biased.

    I decided to used two approaches:
    1. Multimeter, using the internal test points (caliper probes as in the picture work really well)
    2. Amptata V8 Bias Probes (fitted between tubes and sockets)
    I thought I would try to correlate the readings from the two methods.

    Notes:
    1. 72 mVDC each side (or 36 per tube) has worked well for me in the past, so that is what I aimed for (Santiago recommends 70, I believe)
    2. V1 & V2 are controlled by the right side test points & bias adjuster pot (per picture below)
    3. V3 & V4 are controlled by the left side test points & bias adjuster pot (per picture below)
    4. It took an hour and a half of running the amp before I was satisfied that the bias readings had stabilised
    Final Bias Readings

    Amptata V8 - V1:33, V2:36, V3:36, V4:34 Multimeter - V1&V2:72.0, V3&V4:72.3

    I am an accountant, not an electronics engineer (so cut me some slack), but there appear to be some interesting findings out of this:

    1. 33 + 36 does not equal 72, but at 69, it is close enough (note that the Amptata does not do fractions of a MV, so the gap could be a little less than 3mV)
    2. 36 + 34 does not equal 72, but at 70 it is close enough, again, the gap could be a little less than 2mV
    3. Going forward, I can confidently check the bias without opening up the amp, using just the Amptata, now that I have a solid baseline (multimeter & Amptata)
    4. There is an interesting video on Euro Tubes that compares the readings from external bias points on Bugera and Peavey amps. The differences were very significant - his advice was not to risk using these manufacturers' external bias points for bias readings, use probes fitted between the tube and socket
    Cheers
    Paul


    Biasing 2 Approaches.jpg
     
    Jethro Rocker and Benighted like this.
  13. Benighted

    Benighted Member

    Joined:
    Sep 30, 2020
    Messages:
    70
    Likes Received:
    75
    Location:
    Illinois
    Thank you for the great write up.

    Apmtata has a probe without display (to be used with a multimeter). It would be interesting to compare the results using the same multimeter with the amps test point and an external bias probe. My gut feeling is multimeters are better calibrated from the factory than most of the the bias probes sold on ebay/amazon.

    Could you please share a link to the caliper probe you use?
     
  14. Jethro Rocker

    Jethro Rocker Well-Known Member

    Joined:
    Mar 11, 2011
    Messages:
    10,126
    Likes Received:
    15,343
    Location:
    Saskatoon, Canada
    To be completey accurate one should know the plate voltage to do the math. But it is close enough for rock n roll.
     
    Leperclown likes this.
  15. Leperclown

    Leperclown Member

    Joined:
    Sep 14, 2021
    Messages:
    17
    Likes Received:
    47
    Hey Jethro Rocker,
    how do you determine plate voltage without the Euro Tubes Probe & double screen meter?

    Cheers
    Paul
     
  16. Leperclown

    Leperclown Member

    Joined:
    Sep 14, 2021
    Messages:
    17
    Likes Received:
    47

    https://www.ebay.com/itm/3935421699...%2BStVIhIg%3D%3D|clp:2334524|tkp:BFBMjoDD2ZZf

    Or Google "Multimeter Tweezer Probe"

    Cheers
    Paul
     
    Benighted likes this.
  17. Jethro Rocker

    Jethro Rocker Well-Known Member

    Joined:
    Mar 11, 2011
    Messages:
    10,126
    Likes Received:
    15,343
    Location:
    Saskatoon, Canada
    Hell if I know. I don't poke around in there with a multi meter! Someone will know... but it isn't me! Sorry...
     
    Leperclown likes this.

Share This Page