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Help biasing deluxe reverb

Discussion in 'Amp Central Station' started by Matthewdormer, Apr 13, 2021.

  1. Matthewdormer

    Matthewdormer TDPRI Member

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    I live in the uk and I cannot buy a pre-built plate current meter for less than £100. I don’t want to spend that amount on something I can do with a multimeter. Although I am not entirely sure how. And I have no access to an amp tech currently.

    Currently I have new tung sol 6v6 and a new jj rectifier in my 68 custom DR. I have turned the bias down a decent bit and the tubes are running cold. I now need to set the bias to a more useable level. But how I will do that I do not know.

    So how do I measure my plate current with a multimeter. I am led to believe that in order to directly measure it I would need to have a 1 ohm resister. Which I don’t have. Using this site https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm I am planning on using the first method that he talks about. Which means I must measure the voltage through the tubes and the voltage from the output transformer. Although before I do this there are a few things I want to clear up.

    So step one says to measure the voltage between ground, and pin 3 on both tubes, I think I will be able to manage that assuming that that is all I need to do for that step.

    step 2 is to measure the output transformer voltage. I am not sure how to do this. He says to measure the voltage at the center tap, but in step one he said that the center tap of the ot is ground. So what 2 points do I need to measure between for this ? Does he mean the center tap to the output of the transformer ?

    3rd step he says to measure the OT resistance, I assume this will use the same points of contact as to measure the voltage of the OT, but he says to discharge the filter caps, how do I do this.

    After I get these measurements I am happy that I will be able to bias my amp safely. I really just need steps 2 and 3 cleared up thanks.

    I am aware that I should one of my hands away from the amp at all times. I will do so.

    any advice would be great thanks
     
  2. 2L man

    2L man Tele-Meister

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    Measure both output transformer primary resistances. No need to remove tubes because their anodes don’t have resistance.

    Power amp and let warm. Then measure voltages over both primarys and both pin 3 to ground. Then you can calculate currents (voltages of primarys : resistances of primarys) and tube plate powers (pin 3 voltages*currents)
     
    Last edited: Apr 13, 2021
  3. Paul G.

    Paul G. Friend of Leo's

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    Hooboy. You don't seem to know a lot, and I recommend you bite the bullet and buy or borrow one of those plug in tools or get a tech to bias your amp.

    There are dangerous voltages in your amp, and you need to be measuring them. Not discharging the caps a hazard. Discharging them improperly is a hazard. Oh, the Center Tap of the output transformer is most assuredly not at ground.

    I'm not comfortable going further than that.
     
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  4. 2L man

    2L man Tele-Meister

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    Your techs instructions are better than what I wrote when there opt primary resistances are measured when it is warm and they might be different than not warm resistances are.

    Remove mains power leaving standby on discharges capacitors. It is safe to measure opt resistances after tubes have cooled few seconds but put probes only over primary coils. There might be some voltage on capacitors left because tubes quit working earlier. But there is no coltage over primary coils anymore and resistance measuring is safe. But they do not change if you spend time measuring that capacitors have discharged.
     
  5. Matthewdormer

    Matthewdormer TDPRI Member

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    If I buy one of those tools it will only tell me cathode current, I would still need to open the chassis to get plate voltage would I not
     
  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    If there is voltage on the filter capacitors there is voltage on the OT primary.

    Learn how to safely remove and test voltage on the filter caps.

    Learn how to safely work on an amp while the amp is *on*. It can be done hands free.

    Then you can proceed with biasing the amp.
     
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  7. theprofessor

    theprofessor Poster Extraordinaire

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    Certain bias probes will tell you both plate voltage (VDC) and plate current (mA). One readily available in this U.S. is this one: https://www.eurotubes.com/store/pc/Eurotubes-Pro-One-Bias-Probe-for-octal-power-tubes-p144.htm

    Yes, it is pricey, but you wouldn't ever have to open up your amp for biasing. You'd just need to switch tubes from one socket to the other, since you'd likely only have one probe of this sort. Perhaps someone in the U.K. or mainland Europe sells one like it?
     
  8. schmee

    schmee Doctor of Teleocity

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    Yes, but for $15 it's a fast track to ma reading, then measure plate voltage and set the bias to what you want. Rather than going through all this OT measurement hocus pocus etc. It also allows you to quickly check the bias setting if you are thinking it doesn't sound like yesterday or last month.

    For every 25V plate voltage you are only changing the bias maybe 3 ma.... and it's very judgmental where you want it anyway. by comparison The spread between hot and cold bias may be 14 ma or more.
     
    Last edited: Apr 13, 2021
  9. 2L man

    2L man Tele-Meister

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    Yes there might be but there is no voltage over primary coils so resistance measuring is safe!
     
  10. EsquireBoy

    EsquireBoy Friend of Leo's

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    EDIT: I’ve just recalled the deluxe does not have a bias tremolo, so all I said about the PRRI does not apply unfortunately :oops:

    Last time I had to bias a PRRI, I did it by ear, relying on the tremolo intensity. The hotter the bias, the weaker the trem.
    Be sure you get a healthy intensity on your trem and you know the bias is within the safe zone, and if the amp sounds good to you, then it’s probably good to go.
    Just use an insulated screw driver through the tiny hole in the chassis, this way you’ll be safe.

    By no mean am I saying it’s the ideal way of doing it, but it works ok and, most importantly for a neophyte, it’s perfectly safe.
     
  11. Matthewdormer

    Matthewdormer TDPRI Member

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    I bought one of these to use with a multimeter.
    https://ibb.co/Nrrs9Lv
    https://ibb.co/C1Lg9gX
    Will this work fine ? Somehow I don’t believe it to be as easy as just plugging this thing in and reading it off a multimeter? Am I wrong
     
  12. theprofessor

    theprofessor Poster Extraordinaire

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    That will give you plate current, but it will not give you plate voltage.
     
  13. Matthewdormer

    Matthewdormer TDPRI Member

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    Okay, I hope current will be enough for now with a conservative guesstimate of the voltage. How should I go about getting the voltage without dying? Literally no one in Europe or uk sells a tool that measures voltage.
     
  14. theprofessor

    theprofessor Poster Extraordinaire

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    This is not really adequate, but you could go on an average of a what a Deluxe Reverb of your exact type typically runs on your typical wall voltage there. You'd have to ask around to see what others have found.

    But really, you can bias by ear and eye. Turn the pot all the way to its coldest setting, and start from there. Play through the amp, turn a bit, play through again, turn a bit. You'll hear the sound fill out some and start to sound warmer. Keep an eye on the power tubes as you get higher on the sweep of the pot, just to be sure the plates aren't turning cherry-red. As long as it sounds good and the power tubes aren't red-plating, you should be good to go. If your probe tells you the current draw of each tube, that information can be used to corroborate an acceptable range of what you're hearing and seeing. You're not really shooting for a number as much as you're shooting for a sound. The numbers are more like outer limits.
     
  15. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    Pasted text from Ebay below :

    Unique bias probe to measure cathode current and plate (anode) voltage in one go. 23mm diameter allows this to fit into recessed sockets.
    Va = Black-Blue: Anode voltage via 100:1 divider ( 1V==100V)Ic = Black-yellow: Cathode current 1:1 ( 1mV == 1mA)

    You can quickly estimate anode tube dissipation as 0.9 x Va x Ic to see if your bias is set safely.
    Maximum operating voltage: 550VDC
    Maximum operating current: 250mAUse only to check bias under idle conditions.
    Not to be used when developing power as voltage rating maybe exceeded.
    A DVM is required - not supplied.Bias current accuracy 1%Anode voltage accuracy 2%

    The use of the tool could expose you to high voltages. Never handle the tool or connectors without the target socket being completely de-energised. Only for use by suitably qualified technicians.

    End of pasted text :

    Yep, that'll do.

    https://www.ebay.co.uk/itm/1pc-Tube-amp-bias-tester-for-el34-6v6-6l6-kt88-6550-kt120-7024-mA-test-mode/202978874031

    I use a couple of ones like the above for Octal valves, and I have a pair for EL84 too. It gets a bit messy juggling the multimeters, but with carte and an eye to not getting zapped ( important ) they get the job done.
     
  16. schmee

    schmee Doctor of Teleocity

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    Just be very careful and measure from Pin 3 of the power tube to the chassis. Or from the Center tap to chassis if you know what that is.
     
  17. theprofessor

    theprofessor Poster Extraordinaire

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    Ah, ok. Well that's cool. I didn't know something so compact could do both! Looks like you're set!
     
  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    It looks like it will work.

    Read mV DC on your meter on the yellow terminal and black terminal. This will equal the mA Plate and Screen current from the cathode of the tube. Example .021 V = 21mV = 21mA =.021 A

    Read VDC on your meter on the blue terminal and black terminal. This will equal 1/100 of the plate voltage. Example 4.1 Volts equals 410 VDC plate voltage.

    0.90 = an approximation of the Plate current - Screen current.
    14 is the given Wattage Rating of a 6V6 tube.

    Multiply the Plate Voltage x the mA current x 0.90 = Approximate Plate dissipation per tube in Watts. Example 410 x .021 x .90 = 7.75 Watts. Divide 7.75/14 = .55 = 55% Plate Dissipation
     
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  19. Matthewdormer

    Matthewdormer TDPRI Member

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    That clears it all up for me, Thankyou
     
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  20. SoK66

    SoK66 Tele-Afflicted

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    A number of years ago there was a bias probe being sold called the CompuBias. Gives plate voltage, curent and dissipation readings. Unfortunately the guy went out of business but there may be a few ouit there being sold used. I love mine.
     
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