1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Deluxe Reverb Bias Question

Discussion in 'Amp Tech Center' started by 724SP, Feb 3, 2014.

  1. 724SP

    724SP Tele-Meister

    Posts:
    226
    Joined:
    Jan 26, 2008
    Location:
    Salinas, CA
    I have a hand-built (point-to-point) wired Deluxe Reverb AB763 circuit amp that is about 8 years old. Over the weekend, I decided to try a different set of 6v6 power tubes. When I plugged in my Bias-Rite meter to check voltages and currents (with the new tubes) I was very surprised at how hot this amp was running:
    Plate Voltage: 425v
    Bias current: 35ma
    Power: about 14.9 watts

    I know the 6v6 MPD is rated about 14 Watts, and in my tweed amps, I normally bias at around 70% of that (at least as a starting point, then use my ears). The thing is, I've been running this amp for the last 8 years like this. Values are about the same with the original NOS Brimars that were in it.

    So, I figured I'd lower the bias a bit and see how it sounded. Unfortunately, I found the bias pot was already set as low as it would go. What should I do? Amp sounds great...just wasn't expecting those numbers! My guess is that I should take it in and let a tech look into the bias circuit. Am I missing something here ? Anyone experienced this before? Sorry for the lengthy post, I just don't have much experience with BF amps...appreciate it!
     
  2. andyfromdenver

    andyfromdenver Friend of Leo's

    Posts:
    4,703
    Joined:
    Dec 21, 2011
    Location:
    grandpa's
    What's the power transformer?
    What rectifier you using?
    What are the heater readings?

    Maybe the PT has an optional voltage? Maybe you can sub in a lower voltage drop rectifier? Maybe JJ 6v6s will be just fine?

    You can go in maybe 3-5k increments on the fine tune resistor off the bias pot to ground. Up or down not sure (probably up to 15k maybe) :) til you can roughly center the pot at a good bias voltage.

    This is my Season of Maybes...
     
  3. Telenut62

    Telenut62 Poster Extraordinaire

    Age:
    60
    Posts:
    5,592
    Joined:
    Oct 29, 2008
    Location:
    Eugowra, Australia
    Plate volts on the ol schematic is 415v so you're not that far away.....35mA is a bit hot.
    I have mine around 26/28mA from memory.
     
  4. 724SP

    724SP Tele-Meister

    Posts:
    226
    Joined:
    Jan 26, 2008
    Location:
    Salinas, CA
    Thanks for the responses, guys. The Transformers (power, output, and choke) are all Mercury Magnetics 'Tone Clones', Mercury's direct replacements for the Deluxe Reverb. Rectifier is a JJ GZ34.

    I guess I'm more concerned about not having enough span on the Bias pot to lower the bias current a little. It seems out of balance (lots of room to go up, no way to come down). May look deeper into that bias circuit.

    Since I've gone 8 years on the same power tubes with no problems, perhaps I am worrying about nothing. I don't run this amp dimed all the time, but it usually runs with the volume at 6 or 7...definitely not bedroom level:) Thanks again for the input!
     
  5. andyfromdenver

    andyfromdenver Friend of Leo's

    Posts:
    4,703
    Joined:
    Dec 21, 2011
    Location:
    grandpa's
    I'll preface this with, I've only been at this a couple years, so bare that in mind.

    Your plate voltage is going to increase as you reduce the bias current.

    Sadly, it seems like a product of direct copy transformers which are designed to run on 117v.
    I'll bet if you operated the amp with a variac at 117, you'd think, "wow! Sounds awesome :)"

    MM is super guarded with their specs. But you can maybe try a 5u4 rectifier, it is a 3A tube so you may want to consult MM first, but I imagine they spec'd it for that. That should hopefully bring your main B+ down. Sadly, you'll loose the cool slow warm up of the mighty gz34.

    You can reduce your B+ in a few ways. With a big fat dropping resistor or with Zener diodes (I've never done the later so can't speak on that), or with a change in rectifier tube.

    I'm guessing you are trying to rotate the pot to reduce the current to like 25-26ma? And it won't rotate? You can change out the fine tune resistor to change the range as I mentioned.

    It'd be interesting to know what the heater voltage is, if it's too high that would be a cause of concern.
    Check AC one lead to each side of the heater leads on the pilot light.

    Here's a link of different rec tubes and voltage drops if it's helpful at all.

    If you have a good tech in the area, that's always a good idea and a fun trip imo :)

    http://www.300guitars.com/articles/rectifier-tube-voltage-drop-chart/
     
  6. telex76

    telex76 Doctor of Teleocity Ad Free Member

    Age:
    69
    Posts:
    16,116
    Joined:
    Jun 24, 2006
    Location:
    Fort Worth,Tx.
    My original 65 DR runs at 430v with tubes biased at 28ma.
    I've never had any problems with it running NOS tubes.

    You can change the resistor (higher value if my head is working right) on your bias pot to get you power tubes biased lower.
     
  7. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

    Posts:
    1,536
    Joined:
    Nov 20, 2011
    Location:
    Arkansas
    Why worry? You are pulling 70ma. (Two tubes) O.K. fine. 70ma quiescent dissipation is exactly on the mark for a pair of nos 6v6 class operating in AB1. Yure getting 14.8 watts, which is exactly right on for AB1 at this current. I would not hesitate one moment to run NOS Brimar (or new JJ) 6v6 at 425 plate volts.

    Both are quality 6v6. Both are rated to at least 47ma plate and 7ma screen current in class A.
    Therefore, you're not hurting them at 35ma quiescent in AB1. That is why you ran it for eight years with no problems.

    JJ6V6 datasheet shows 500vdc on the plates in pentode mode, no problem.
     
  8. Wally

    Wally Telefied Ad Free Member

    Posts:
    37,179
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    So....running those tubes in AB1 at 106 % of max plate dissipation is of no concern??? Or..in Telex76's case at 86% of max dissipation??
     
  9. Wally

    Wally Telefied Ad Free Member

    Posts:
    37,179
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Additional quesiton....when do those 6V6's begin to redplate?
     
  10. BiggerJohn

    BiggerJohn Friend of Leo's

    Posts:
    3,311
    Joined:
    Jun 1, 2009
    Location:
    California
    Sounds too hot to me.
     
  11. Knowcaster

    Knowcaster Tele-Holic

    Age:
    56
    Posts:
    619
    Joined:
    Jun 30, 2007
    Location:
    Pittsburgh, PA
    Are you sure you are turning the bias pot in the right direction? The reason I ask is that I recently reset the bias on my DRRI and as I recall I had to turn the bias pot in the opposite direction than what I though to lower the bias current. If you are all the way at the end of the travel of the pot and turn it in the opposite direction, does that increase the current?
     
  12. 724SP

    724SP Tele-Meister

    Posts:
    226
    Joined:
    Jan 26, 2008
    Location:
    Salinas, CA
    Hey Knowcaster,

    I'm going to re-check this, but I'm positive I went the other direction and saw the plate voltage go up. So positive, in fact, I can't remember:(

    That said, it seems very strange that there would be so much room to increase when the amp was so hot at 'minimum' pot setting. So, I'm going to check again. Unfortunately, my bias pot is inside the amp chassis, which means I have to pull it out of the cabinet to make any adjustments or to test this theory.

    Thanks for the input, all.
     
  13. 724SP

    724SP Tele-Meister

    Posts:
    226
    Joined:
    Jan 26, 2008
    Location:
    Salinas, CA
    Just wanted to provide an update. Dug into the bias circuit today and found the bias resistor was bad. Only resistance in the circuit was coming from the bias pot itself. Swapped the resistor and voila, back to more normal readings on the tubes (I think).

    Here's where I'm at:

    444v plate, 22mA and 9.7 watts for each tube.

    Amp sounds great, so I think I'm going to call it done. Probably re-check bias in a week or so to make sure all is where it should be.

    Wanted to thank everyone again for the help. Several of you pointed to the bias resistor, so that's where I looked first. Thanks!
     
  14. roberts67

    roberts67 TDPRI Member

    Posts:
    51
    Joined:
    Jan 14, 2014
    Location:
    Oregon
    Hello

    Glad you found the problem and fixed it!
     
  15. Wally

    Wally Telefied Ad Free Member

    Posts:
    37,179
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    724SP, congrats on the good work! So the resistor that took the 3rd lug to groound was open?
     
  16. 724SP

    724SP Tele-Meister

    Posts:
    226
    Joined:
    Jan 26, 2008
    Location:
    Salinas, CA
    Hi Wally. Yes, when I tested that resistor (just to the right of the pot in the photo), I got no resistance reading at all. I removed it and tested again, still nothing. Once I replaced it, I had the span (not sure if this is the right term) on the Bias pot I needed. I was able to get 9.7 watts from the tubes at 22mA current, which seems more reasonable (about 70% MPD for the 6v6). Amp sounds much better now!

    image-1984163322.jpg
     
  17. Wally

    Wally Telefied Ad Free Member

    Posts:
    37,179
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    IN effect, that bias circuit was ungrounded...and this is why the pot didn't act as you expected it to with regard both to bias adjustment....both in voltage and the direction of the sweep vis-a-vis the effect of the adjsutmetn.
    Question: Did you ever measure actual bias voltage? IF not, this measurement would have been a quick 'pointer' to the problem. I always take note of the bias voltage when I am adjusting things....out of curiousity as much as anything. Schematics usually give us a bias voltage,and Ilike to see how that voltage recommendation sits with the final plate dissipation figures.
     
  18. 724SP

    724SP Tele-Meister

    Posts:
    226
    Joined:
    Jan 26, 2008
    Location:
    Salinas, CA
    Hi Wally,
    No, I never measured the actual bias voltage. I see how that would have been a good, quick check of the bias circuit...thanks for that and for following up on the diagnosis! Hopefully, this thread may be of use to others, some day. Because of this, I have acquired some good schematics of the AB763 circuit, as well as a copy of Dan Torres' book 'Inside Tube Amps'. So much to learn! Again, appreciate your help and wisdom!
     
  19. Wally

    Wally Telefied Ad Free Member

    Posts:
    37,179
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Lot of good books... I like Dave Funk's "Tube Amp Workbook". He has the best and most complete assortment of FEnder tube amp schematics in book form that I have ever seen.....and he knows whereof he speaks when it comes toamplifier circutis. His Thunderfunk amps are very well regarded. He puts the information out in a manner that even a simple guy like me can understand.
     
  20. 724SP

    724SP Tele-Meister

    Posts:
    226
    Joined:
    Jan 26, 2008
    Location:
    Salinas, CA
    Sounds like the perfect book for me as well:) Much thanks!
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.