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5f11 bias resistor

Discussion in 'Shock Brother's DIY Amps' started by Linkjr, Aug 16, 2017.

  1. Linkjr

    Linkjr TDPRI Member

    Age:
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    Hi Guys im new to the forum, i have a 5f11 clone amp but the 2 power tubes are red plating so i thought i would change the bias resistor to compensate.

    Looking at some of the similar issues here the general consensus is to put in a 1k-5k resistor unfortunatly mine doesnt look like any one elses here and it came with a 800k resistor in place in this position any advice would be much appreciated!
     

  2. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
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    Mar 1, 2010
    Kent, OH
    5F11 is fixed bias. Does yours have a bias pot you can adjust?

    If there's 800k range resistor there, then you probably don't have enough bias voltage. Schematic looks like 10k (if that's the resistor you are talking about)

    We'll need your Plate voltage and cathode current to bias this up proper. Do you have any 1-ohm resistors you could tack from cathode to ground on your power tubes?

    Can you get a pic of the bias section? The 5F11 schematic has a less than ideal bias supply would like to see what liberties the builder took...800k range resistor would seem strange.
     
    Last edited: Aug 16, 2017

  3. Linkjr

    Linkjr TDPRI Member

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    image.jpg Hi Commodore yes there is a 10k bias pot
    Below are pics of the resistor and it was located below the diode in the circuit image.jpg
     

  4. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    .
     
    Last edited: Aug 16, 2017

  5. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

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    OK, you've got an adjustment pot there. That's good. And probably 10k. That color code is brown-black-yellow-gold, suggesting 100k. It's possible that this is tapped off of the HV, not a dedicated bias tap. In that case, 100k might be the right value.

    Can you you get a pic of the power transformer leads and where they connect (zoom out form the earlier pic?

    Also, can you get a pic of the power tube sockets? Would like to see if there's a 1-ohm resistor there for bias checking.

    Tentatively, you would be looking to:

    1. Replace the 100k with a 100k metal film (or carbon film) resistor 1W is OK.
    2. Turn the bias pot all the way CW
    3. Fire up amp, watch for redplating
    4. If no read plating, measure cathode current. With a 1-ohm resistor this means using your DMM to measure mV. That will also be your cathode current. Place your DMM leads on either side of 1-ohm resistor and read it.
    5. Multiply that value (in amps) by your plate voltage. This will give you your plate current (and screen current but ignore that for now).
    6. That should be no more than 70% of 12 --8.4 watts. (12 is a safe max dissipation value for a 6V6). If your tubes are EH, or JJ you could assume 14 as your max plate dissipation.
    7. So basically you tweak that 10k pot until your plate dissipation is around 70% of max. The pot changes the bias voltage on your grids.
     
    Last edited: Aug 16, 2017

  6. Linkjr

    Linkjr TDPRI Member

    Age:
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    Thanks for looking into this Commodore, and thanks for the helpful suggestions will get some more pics tommorow.

    Sorry my pic of resistor is not so clear but resistor is actually grey black yellow gold,

    With the bias pot set to minimum when the resistor was still in there there was less red plating but still present.
     

  7. dan40

    dan40 Tele-Holic

    658
    Aug 19, 2015
    Richmond Va
    Has the amp worked properly for awhile and then started redplating or was it recently built and has been redplating the whole time?
     

  8. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    A leaking coupling cap on the power tube grid can cause red plating. Can you measure the voltage on the power tube grid?
     

  9. Linkjr

    Linkjr TDPRI Member

    Age:
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    image.jpg image.jpg image.jpg The amp was red plating when i got it but did not know that this was not a good thing as it sounded great, found out some more about amps and came here.

    Here some more pics of the power tubes and transformer leads
     

  10. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

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    OK the bias supply is tapped off of one leg of the HT. I'm going to assume your PT is close to original spec which would be 330-350 on each side. So with that in mind, that resistor you pulled should be ~ 180-220k. Install something around there. A lower resistance is going to give you MORE negative bias voltage which is going to be safer for your tubes. So you could start with 100k. The bias pot is going to allow you to adjust the range of the bias voltage. It's probably 10k, and may not give you enough span to set your bias. But we gotta start somewhere.

    Once done, fire up the amp. Get us a plate voltage (pin 3 on each power tube) and a grid voltage (pin 5 on each power tube). And check for red plating. If there's no red plating, then you need to check plate dissipation, and for that you need plate current.

    You will need to measure plate current using an OT method...unless you want to tack 1-ohm resistors from pin 8 to ground on your power tubes.<-- and that's what I recommend doing. But if you want to proceed, you've been warned ;):

    You must be careful when doing this, I've slipped with a probe and fried a humdinger pot when doing this and I've also fried the fuse in a very expensive DMM which was a special order fuse. (Shorted pin 8 to pin 7, I think). Below are instructions courtesy of: http://www.aikenamps.com/the-last-word-on-biasing
    • The plate current can also be measured using the "shunt" approach, where an ammeter is paralleled across each side of the output transformer. Since the internal shunt resistance of the ammeter is usually small in comparison to the resistance of the primary winding of the output transformer, most of the current is diverted through the ammeter, giving a fairly accurate reading of the actual plate current. This can also be dangerous because of the high voltages involved. One slip of the probe, and your expensive output transformer primary is shorted to ground through the low resistance of your multimeter. At best, you will blow the fuse in the meter. At worst, the output transformer primary winding will burn out in order to protect the multimeter fuse. This method is also inaccurate to varying degrees depending upon the make/model of the meter used. Some digital multimeters have fairly high internal shunt resistances (particularly on the lower current ranges), which will result in a reading that is lower than the actual plate current. This can result in setting the actual bias current too high, which can cause premature tube failure. Note that the shunt current measured on each side of the output transformer will be the total current drawn by all the tubes on that side, so if there are two tubes on each side, you must divide the measured shunt current by two. Remember for this, you are setting your DMM to mA! That requires you to put the red lead in a different hole. -

    Here's another safer method courtesy of Aiken:
    • The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side.
    You could also get something like this: http://biasking.com/ I have one, they aren't cheap and they only work for octal power tubes. But I didn't know any better when I was less experienced (and had more money) than I am now. So I never use it now because I use 1-ohm resistors instead, or God Forbid, the OT shunt method. :p

    One last edit: If it sounded great and wasn't crackling and blowing power tubes, it might not have been redplating...the filaments are supposed to glow orange/red. So you'll certainly see orange/red glow in normal operating conditions.
     
    Last edited: Aug 17, 2017

  11. Linkjr

    Linkjr TDPRI Member

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    Wow thanks Commodore i believe you were also right about the 100k resistor not 800k as i said it was that i pulled put- Im having trouble seeing the colour brown on the brown resistor.
    Will not be doing the shunt method! Orderered some resistors today and will order some 1ohms

    1 question if it was a 100k i pulled out and bearing in mind what you said about neg bias voltage might i need some lower value than 100k resistor in place to stop it red plating again?
     

  12. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
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    Well that depends. Did you try to bias it with the potentiometer? Did you apply max -ive bias voltage possible with the pot?

    First thing is to double check the value of that pot. I can only see a 1 and a 0, so I assumed 10k. If that resistor to ground on the pot is red/red/orange/gold, that's 22k. So that means at minimum, there would be 22k ohms to ground with the bias pot. The pot simply adds up to 10k additional resistance as you rotate it.
     

  13. Linkjr

    Linkjr TDPRI Member

    Age:
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    Aug 16, 2017
    Uk
    Yes it is a 10k pot and i did - it was shipped to me with bias set where i put red marker on pot around 3/4 clockwise but it was red plating much less when the the bias pot was set as far counter clockwise as it will go and also better range of tremolo but the amp sound was a bit dull.
     

  14. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
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    Mar 1, 2010
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    I'm not seeing the 220k grid leak resistors on your amp. I see the 1.5k grid stoppers. Fllowing what I think is your bias pot, I don't see the 220k grid leaks. Are they there?

    edit...can you just get a quick shot of your whole chassis.

    Also it's hard for me to see what's feeding the grids on your power tubes. One power tube I clearly see a yellow wire which I assume comes form a 220k, but I don't see a lead to the other one.
     
    Last edited: Aug 17, 2017

  15. Linkjr

    Linkjr TDPRI Member

    Age:
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    Uk
    image.jpg There are 2 x 220k resistors in v shape in the centre of the circuit under where the tone pot is in the chassis, heres the whole chassis there is a red wire coming off of pin 6 of the 2nd from left 6v6
     
    Last edited: Aug 17, 2017

  16. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    OK good. Looks like a pretty sanitary build there. Is that bias resistor burnt?

    Just ruminating here. Did you ever measure plate voltage? If so what did you get?

    If that is a 325 or 350-0-350 PT, then your voltages may be sky high if that's a GZ34 or a modern 5Y3 (which behave more like a GZ34). What's your rectifier tube? EH power tubes right? Those are my favorite 6V6GT these days.

    And if you are ordering resistors, you might consider some 470 or 1k 5W screen resistors for your power tubes. There's a low mass 5W that is more like a 3W size that I like to use on screens. 3W will likely be fine, BTW, but it doesn't hurt to overengineer there.

    The greenies on the power tubes below are 5W low mass resistors From here: https://www.tubedepot.com/products/...und-improved-performance-screen-grid-resistor [​IMG]
     
    Last edited: Aug 17, 2017

  17. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    My 5F11 I finished recently had a 315-0-315 PT and I used the high voltage tap. My bias setup went from rectifier, 100k resistor, diode, and I paralleled two 33k resistors on a 10k bias pot.

    Here is a pic of what I used, with the 33k paralleled with another in the end, so 16k plus the 10k pot.
    5f11 bias.jpg
    5f11 bias 2.jpg
     

  18. Linkjr

    Linkjr TDPRI Member

    Age:
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    Aug 16, 2017
    Uk
    My 5y3 is a JJ Tube, yeah came with those electro harmonics power tubes
    ive not measured plate yet, im not sure about transformer, dont suppose there is any way to tell without removing the chassis from the cab? Transformer is Heyboer
    Those screen resistors you mentioned are they to improve sound? Or are the 2 resistors i got currently on the tubes ok?

    Thanks Jsnwhite, cant see how youve 'paralleled' the 33 k resistors are they both coming off one leg of the bias pot?
     
    Last edited: Aug 18, 2017

  19. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    I only had one on there at the time in the picture, but I ended up adding another with it off that lug on the pot and grounded to the back. I didn't have anything smaller, so those worked out to 16.5k.
    Your setup appears to have the resistor on the opposite lug as I did, and you have an extra wire - mine has the center lug connected to the diode, and the same eyelet leads to the negative side of the bias cap as well as to the depth pot, which I think is how the original layout was depicted.
     

  20. Linkjr

    Linkjr TDPRI Member

    Age:
    47
    23
    Aug 16, 2017
    Uk
    I have fitted a 68k resistor in place of the 100k and turned bias pot all the way cw measured plate and grid
    Both power tubes pin 3 to ground is 383v
    And both have pin 5 to ground at 31v

    My pin 8 already has a connection to ground do i need to unsolder this and put the 1ohm resistor between the 2?
     

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