Setting bias

Discussion in 'Glowing Bottle Tube Amp Forum' started by lastRebel, Apr 6, 2020.

  1. lastRebel

    lastRebel Tele-Meister

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    So I’ve been learning an building on an amp for the past year now... and I’ve finally got it started up. Everything works as it should, but the power tubes are red-plating pretty bad. I’m trying to figure out how to set the fixed bias on this amp, and it looks like I’ll need some help. Pic shows where I think the problem is. The 10k resistors circled in green just before the power tube grids should be lower?? It showed a 1k originally, but it was suggested I change it to a higher value for tone. I’ll link the original schematic thread at the bottom too.
    11F57254-3AF4-4B17-8CAF-2F2AF865096F.png
    4987A93F-C1FB-41C4-9DD1-60D85080FBD0.jpeg


    Original thread:
    https://www.tdpri.com/threads/requesting-schematic-check.940338/
     
  2. lastRebel

    lastRebel Tele-Meister

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    Also, FWIW, I will be testing things as soon as I get ahold of a meter (my good one is at work), and I plan to replace the resistors that need to be adjusted for bias with trim pots. I’ve already ordered 2 5k pots for the ones I’m questioning, but it dawned on me that the 150k’s may play a part here too. I just need to make sure I’m on the right track, and it’s not something else that I’m overlooking.
     
    Last edited: Apr 6, 2020
  3. Dacious

    Dacious Poster Extraordinaire

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    Bias resistors are not something you can arbitrarily set by advice over the internet. A jump from 1k to 10k is a big leap.

    Their purpose is to regulate current draw. The formula for power is literally power=voltage X current. So knowing the maximum power your tubes can generate and how much regulated power is sitting on the plates you can work backwards to work out how much current you need.

    Rob Robinette who is a user on his site has a great page with helpful information. Do some reading and some measurements.

    https://robrobinette.com/Generic_Tube_Amp_Mods.htm#Adjustable_Bias

    https://robrobinette.com/Tube_Amp_Startup.htm
     
  4. lastRebel

    lastRebel Tele-Meister

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    I understand what you are saying, and thank you. I’m trying to make sure I’m looking at the right resistors to start with.
     
  5. lastRebel

    lastRebel Tele-Meister

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    I guess I should have been direct in asking “are the green circled 10k resistors where the bias is set?”
     
  6. Paul G.

    Paul G. Friend of Leo's

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    Leave the 150ks alone.

    Are you getting -30V on that bias circuit? Is the diode and filtering cap in your bias circuit facing the correct way? Remember, they should be facing towards the ground to ensure negative voltage on the grids.

    If you're redplating, you're not getting a large enough negative bias voltage, so, you need to reduce the voltage drop there, get rid of the 10K resistors altogether, then if you're still red-plating, I would start with the 33K resistor going to ground in your bias circuit, raise that and recheck your negative voltage.
     
  7. Paul G.

    Paul G. Friend of Leo's

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    To clarify, when your meter's black lead is connected to ground and the red lead on the bias, your meter should read -30Vdc (more or less). That negative sign must be there.
     
  8. Wally

    Wally Telefied Ad Free Member

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    What amp is this. I need a schematic that does not have crayons marks obscuring things. (;^)
     
  9. lastRebel

    lastRebel Tele-Meister

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    Thank you Paul. I’m looking for my old meter, but haven’t had any luck yet. I may have to wait until tomorrow when I can bring my good one home from work. This gives me a bit of confidence that I was on the right track though. I’ve been learning throughout the last year or so, but it’s not a formal (or even continual) education by any means. If I can round up another meter I’ll drop in to share my findings.
     
  10. lastRebel

    lastRebel Tele-Meister

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    LMAO... sorry friend, I used a highlighter to retrace all my wiring when I was building and troubleshooting. It’s actually a Cordovox accordion amp from the mid-late 60s. I’ve added one of Robrob’s preamp circuits to it to make a guitar amp. I’ll see if I can find the digital copy of my build schematic.
     
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  11. lastRebel

    lastRebel Tele-Meister

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  12. D'tar

    D'tar Friend of Leo's

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    This is your bias supply....

    20200406_095755.jpg
     
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  13. Wally

    Wally Telefied Ad Free Member

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    Would not the 33k resistor going to ground be a prime suspect for a change to manipulate that bias voltage? And...could not a trim pot be placed where the 22k resistor is for adjustability?
     
  14. lastRebel

    lastRebel Tele-Meister

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    Hmm... I would think so, at least for an overall adjustment. Wouldn’t that affect V2 though?
     
  15. D'tar

    D'tar Friend of Leo's

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    I would remove the power tubes until you have sufficient negative voltage to the grids.

    The 10k resistors you circled are grid stoppers and will have no effect on bias voltage.

    The 22k and the 33k in the photo above are your bias resistors. Notice the diode polarity and cap orientation necessary to get -ve voltage. The 33k can be replace with say..... a 15kr and a 25kl pot or, 22kr and 10kl pot for agdjustable fixed bias.
     
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  16. D'tar

    D'tar Friend of Leo's

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    No, the v2 .047coupling capacitors block dc bias voltage which is isolated to the grids of the power tubes. Remember capacitors block Dc.
     
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  17. lastRebel

    lastRebel Tele-Meister

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    I haven’t ran the amp since I started it up the first time and got red plates. I hope these tubes didn’t suffer much damage in that time.

    So, being the honest person I am, I need to share something. I, my friends, am an idiot. Somehow, the ground side of the bias winding wasn’t soldered in. It was hiding behind the filter caps. Let the flaming begin... haha

    Once soldered in place, I have -22.23 volts on my bias circuit. The tubes aren’t red plating anymore (in the few minutes I ran it), and at least I have a starting point to set the bias properly.
     
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  18. Paul G.

    Paul G. Friend of Leo's

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    You're not the first, nor will you be the last to overlook something like that. Happy you've got it working.
     
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  19. lastRebel

    lastRebel Tele-Meister

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    Thanks Paul, and everybody else too!
     
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  20. Johnnie O

    Johnnie O NEW MEMBER!

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    Hello Tele-Meister and fellow Musicians,

    This is my first post and request for information though I have followed TDPRI for a long, long time.

    My request is for the source of the 1960's Cordovox CG Amp and Tone Generator schematics and/or Service Manual where I can place an order.

    The schematic image in this thread appears to be of good quality that would allow me to service the Amp and hopefully restore the Tone Generator.

    Sourcing the schematics has been difficult. Thank you in advance for any assistance. I have already contacted Rick at Stereo Manuals, and am reaching out to MusicParts.com.
     
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