1954 Fender 5D6 4x10 Bassman bias questions

Discussion in 'Amps and Cabs [BG]' started by Toppscore, Feb 11, 2016.

  1. Toppscore


    Dec 7, 2015
    NOTE: The first two photos below are of my 1954 5D6 Bassman this post is regarding.
    The remaining six photos are of other 1954 5D6 Bassmans for comparison and reference.

    Hi. I have a 1954 5D6 Bassman with an added/upgrade bias modification from fixed to adjustable bias control. Notice the white sprocket gear to the right of the power transformer. I understand that it can easily be removed and replaced with a correct fixed resistor value in the bias circuit. I plug the 5D6 into a GR GenRad General Radio metered variac transformer at 105 volts.

    1) Would one or two resistors be needed to replace the adjustable bias component?
    I notice two resistors in that tab board area in other pictured 1954 5D6 circuits.
    They all seem to vary slightly.

    2) What is the "orange" component to the LEFT of the white plastic sprocket?
    The orange component is original factory installed. I included photos of other
    1954 5D6 Bassman circuits. Some 5D6's have the original orange IC,
    some do not and one is missing. All of the fixed bias 5D6s have
    at least two resistors in that area.

    3) If I run the amp at 105 volts, will it be good to reinstall the origional fixed bias?
    I have not seen other early 1954 or 1955 Bassmans with this adjustable bias mod,
    so why have it on this one running at 105 volts? What do you think?

    Thank you ahead of time for you response.

    <a href=""><img src="http://i.imgur.com/ZU8Ur2g.jpg" title="source: imgur.com" /></a>

    <a href=""><img src="http://i.imgur.com/HNW36ti.jpg?1" title="source: imgur.com" /></a>

    <a href=""><img src="http://i.imgur.com/i1HCdhC.jpg" title="source: imgur.com" /></a>

    <a href=""><img src="http://i.imgur.com/ilsszDd.jpg" title="source: imgur.com" /></a>

    <a href=""><img src="http://i.imgur.com/IbqhwAP.jpg" title="source: imgur.com" /></a>

    <a href=""><img src="http://i.imgur.com/1DyWHVD.jpg" title="source: imgur.com" /></a>

    <a href=""><img src="http://i.imgur.com/CvVln1H.jpg" title="source: imgur.com" /></a>

    <a href=""><img src="http://i.imgur.com/Z5NvF9m.jpg" title="source: imgur.com" /></a>

    <a href=""><img src="http://i.imgur.com/60arxAH.jpg" title="source: imgur.com" /></a>

    Attached Files:

    Last edited: Feb 13, 2016
  2. rufus.K


    Oct 18, 2015
    The resistor value (s) are a result of measurement of the bias voltage you're getting. What is the voltage you have, and what is the spec value. From those you can determine the resistor value.
    Adjustable bias is a great thing, I assume someone put in a trim pot, maybe with a resistor?
    If you undervolt it, your numbers will be off too. Why run it at 105vac?
    Toppscore likes this.
  3. Toppscore


    Dec 7, 2015
    Thanks Rufus. Since it has an adjustable bias mod, already, it can stay. But, if amp repair takes it out, guessing it'd be replaced by a single correct resistor ~ taking the 5D6 back to original factory spec. The bias mod helps tubes to be balanced and last longer, but is not original and I've not seen adjustable bias mods installed on other early Bassmans.

    As I understand, typical 1954 residential homes had about 100v to 110v appliance outlets and that modern homes are 120+ spiking volts. 1950s Fender amps were supposedly designed for 110v, so I try to avoid modern 120+ volt home circuits ~ and I understand commercial locations' power may be even a bit stronger ~ so I use a slightly lower power source ~ hoping to save the amp's electronics. Am I off-base with that thinking?
  4. rufus.K


    Oct 18, 2015
    Last edited: Feb 12, 2016
    Toppscore likes this.
  5. rufus.K


    Oct 18, 2015
    The bias mod isnt to make the tubes be balance and last longer, its made to make the job of biasing the amp tons easier.
    Since theyre so rare, I see why one would want to bring it back to stock. However, IMO its nice to leave it so bias voltage can be done easy.
    I believe what I highlighted in red is the bias circuit.

    You will not harm the amplifier running it at modern voltages most likely. Has the amp been checked out by a tech?
    Toppscore likes this.
  6. rufus.K


    Oct 18, 2015
    Can you read resistor color codes?
    Its feasable that the r47k was the one turned into a trim pot, maybe the r6.8k.
    The second device could also be a balace pot replacing the two 220k's to balance the push pull 6l6's.
    Take photos of yours.
  7. beans-on-toast


    Aug 7, 2008
    You have a very nice amp.

    To add to what was said....

    In general, amps require maintenance. This includes replacing power supply and bias resistors and electrolytic capacitor components, installing a three conductor power cord, and replacing tubes. Unless it is a museum piece, this doesn't affect the value of the amp because you are doing this to make it usable. Keep all the old parts as any of these mods can be undone.

    1) The bias circuit consists of 6.8K and 47K resistor, a 100uF @ 25V capacitor, and a diode. Normally, to adjust the bias, the 47K resistor is changed. This involves unsoldering the old resistor and replacing it with one of a different value. Replacing it with a pot makes life easier and is considered a good thing by many. Normally, when the 6L6G power tubes are replaced, the bias is adjusted to optimize their performance. Not having the bias set properly can result in a sterile sounding amp or one that runs too hot, distorts easily, and wears down the tubes quicker.

    Normally when a pot is installed, the 47K resistor is replaced with a pot and a series connected resistor. The extra resistor is there in case the pot is adjusted to the end where it is zero ohms, it leaves some resistance in the circuit. Here is a sample circuit. The values could be different in your amp.

    adjustable bias resistor.jpg

    2) The orange part is a selenium rectifier and is part of the bias circuit. Normally this part is replaced with a better alternative, a silicone diode such as a 1N4007. Selenium rectifiers have a shorter lifespan than a diode, they run hot, and are prone to failure. I usually replace them.

    3) Running the amp at 105 VAC will lower the power supply voltages, giving the amp less headroom and making it distort sooner. Guitar players like this, the so called brown sound. It is important to have the bias set at the voltage that you are running the amp at. So if you want to run it at 105VAC, have your tech set the bias at that voltage.

    Back in the 1950's voltages were not consistent in the US. Some areas had 110, 115, 117, etc. In the early 50's 110 and 115 were common, at the end of the decade 117 VAC appeared on the Fender schematics. The amps were designed to run on a range of line voltages.

    Rather than run the amp at 105 VAC, I'd run it at 110-115 VAC to get the most headroom out of it. That's just my personal choice, doesn't mean that it is right.

    I would use 6L6GC tubes as they are easier to obtain. The later amps used a 56K bias resistor rather than the 47K. What resistor you use depends on the bias that the tube requires.
    Toppscore and rufus.K like this.
  8. rufus.K


    Oct 18, 2015
    You're getting good info here OP
  9. Toppscore


    Dec 7, 2015
    OK, Rufus and Beans ~ thank you for the great information.

    Photos are posted ~ first two are of my 5D6. Apparently the tech "installed the trim pot in parallel with the bias adjustment resistor that was already there. So the original resistor is still installed. However this resistor is really an item that is sometimes replaced when a new set of power tubes are installed." The adjustable bias modification was installed during 1996. The amp was just serviced during 2015 at Austin Vintage Guitars.

    Thanks for confirming the red IC is a selenium rectifier diode ~ which is 1954 Fender factory original. Some of the other amps photo'd also have this red selenium diode as well ~ some do not.

    Thank you for the bias explanation. I will set the variac at 110v. As you mentioned, one of the photos does show a potentiometer installed where the selenium rectifier was located (there are before and after photos). I'll leave the adjustable bias mod installed, since it is an easy fix back to original spec. Definitely will leave the maintenance to pro techs.

    QUESTION: Is the white plastic sprocket a unique type of bias mod component? I have not seen it before. Is there a variety of bias mod components and the sprocket is one of many? PLMK. Thanks again
    Last edited: Feb 13, 2016
  10. beans-on-toast


    Aug 7, 2008
    The white pot is not unique to bias. It is called a trim pot and is a standard component. There are different types of trim pots. The one that you have is a single turn. Others are multiturn, 10 turn, 25 turn, etc. A multiturn is better in that finer adjustments are possible and the pot is less susceptable to moving as the amp is bounced around during transport. But the type in your amp is common in bias circuits.

    The amp with the larger pot installed probably located it there because there was a hole in the chassis once the rectifier was removed. Some amps use pots like this for bias. The advantage is that they have a locking nut on the top that prevents the shaft from moving once it is set. A nice feature.

    Installing the pot in parallel to the bias resistor will allow you to lower the overall resistance from zero to the parallel combination of the value of the pot and the original resistor. This is a fast and simple way to mod the amp. This can be dangerous if you adjust the pot so that the bias resistor is effectively zero. Having the second series resistor in series is safer and how companies do it that include adjustable bias in their amps.

    I wouldn't worry about what they did. If your tech is careful when setting the pot, it isn't a big deal.

    So there are different ways of doing the same thing. Some cost more than others to implement.
    Toppscore likes this.
  11. Toppscore


    Dec 7, 2015
    Thanks again. I'm on my way. Also am getting the Weber Hip Vintage Amp book that has a chapter on Bassmans.
    beans-on-toast likes this.
  12. BassmanPaul

    BassmanPaul Inactive

    Don’t waste your money. Rufus has provided all you need to work on that amp. Bias voltage should be in the range of -40 to -45V DC.
    Last edited: Feb 13, 2016
    rufus.K likes this.
  13. rufus.K


    Oct 18, 2015
    I would not use the variac.
  14. BassmanPaul

    BassmanPaul Inactive

    OP, just plug the amp in to the standard 120V power. Supply voltage regulation was not that great when the amp was made and varied a lot from area to area anyway. An amp of that age should have had the caps upgraded by now.

    Just as a point, your amp is fixed bias. The fact that the voltage is adjustable doesn’t change that.

    The bias rectifier should be upgraded to silicon as mentioned above.

    I really feel that you are overthinking this and using a VariAC is way too much bother and could be dangerous to your amp if misadjusted. Amps were designed to be plug and play. I humbly suggest you do just that and enjoy what comes out.
    Toppscore and rufus.K like this.
  15. B-string

    B-string Supporting Member

    I will use a Variac at listed testing voltage and for over-voltage stress of electrolytics after replacement (better to have a pop with me than a customer on a funky gig). After voltage checks are done any changes in line voltage are pretty much proportional across the unit. Listening tests get what the wall has to give.
    rufus.K and Toppscore like this.
  16. Toppscore


    Dec 7, 2015
    Thank you for the comments. I've owned other amps, but none with a bias modification and never gave the mod much thought. I've read about the Blackface mods, AB165/AA864 mod, Paul Rivera mods and Bruce Zinky designs ~ that's it.

    Thanks again ~ I understand the bias mod is simple and a good mod for protecting the amp. This 5D6 was "tuned-up" in 2015 by Austin Vintage Guitars. The original 1954 red colored selenium diode did not need to be changed. The 1996 installed bias mod is good to go. I'm very happy to find that the mod is reversible if need be. I'll just let it play as is ~ probably as is since 1996. I'll have to check the receipts/history to see when the cap job was done and tubes changed.

    I bought the quality rebuilt General Radio GenRad W5MT3VM Variac to protect tube amps. I can set it at 115v ~ good to know. Just do not want vintage amps blindly plugged into just any hot circuit - lol.
  17. Toppscore


    Dec 7, 2015
    Rufus is great and generous. Cheers!
  18. Toppscore


    Dec 7, 2015
    Just wondering . . . . . not that I'd do it . . . .
    the 5D6 is original with 6L6, 5Y3 and 12AX7/12AY7 tubes
    and later tweed Bassmans are designed with different power/rectifier tubes.

    Who would change to different tubes and why?
    Why mess with a good design?
    Maybe try changing the preamp tubes for different tones?
    Are 5F6A Bassmans "tube-flexible"?
    Are the 5D6, 5D6A, 5E6 and 5E6A non-flexible?

    Just wondering what others do or have done ~ information only.
    I'm not touching this Bassman. Scary thought - lol
  19. B-string

    B-string Supporting Member

    If you are a skinny string player that likes early distortion the 12AY7 is a lower gain tube, installing a 12AX7 in its place will overdrive the preamp "sooner". Change the rectifier tubes (5Y3) for a more stout 5U4G or GZ34 and the tone (sag) changes, installing SS rectifiers (octal plug ins are available) reduces the power supply sag to that of PT saturation (also raises B+ voltages).
    Toppscore likes this.
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    Primary TB Assistant

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