Results 1 to 15 of 15

Thread: Musicman 112RD Fifty Setting Bias

  1. #1
    Supporting Member Drewline's Avatar
    Join Date
    Feb 2012
    Location
    Colorado
    Posts
    223
    Thumbs Up/Down
    Received: 12/0
    Given: 19/0
    Rep Power
    8

    Musicman 112RD Fifty Setting Bias

    This is the later style amp with 2 6L6 power tubes & a 12ax7 as a limiter. The amp is biased cold. There is no pot for bias. I'm wondering how to set the bias on this.

    Thanks in advance for any info I can get.

    Click image for larger version. 

Name:	mm_rd50_mod_Page_2.jpg 
Views:	1268 
Size:	600.9 KB 
ID:	28794

    0 Not allowed! Not allowed!
    Drewline

    When was the last time you did something for the first time?

  2. #2
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    11,629
    Thumbs Up/Down
    Received: 1,544/23
    Given: 3,908/11
    Rep Power
    22
    Do you mean biased cold for a Musicman, or compared to some other number? Is there objectionable cross-over distortion?
    Musicman amps are typically biased quite cold as they are running very high plate voltages. It seems odd that they have removed the bias adjust pot in this later version of their amps. Perhaps they had a reason .
    What voltage are you reading across R51 and R54?
    If you look at some of their other models with the bias trim pot, I'm sure you could add one the same way.

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  3. #3
    Supporting Member Drewline's Avatar
    Join Date
    Feb 2012
    Location
    Colorado
    Posts
    223
    Thumbs Up/Down
    Received: 12/0
    Given: 19/0
    Rep Power
    8
    The customer is complaining that the amp distorts quickly & sounds tinny. I put it on my scope & you can see noticeable crossover distortion at 1K. The amp has a pair of Ruby Tubes 6L6.
    I measured R51 & R54. I'm reading 9.9mV & 11.2mV.

    Thanks

    0 Not allowed! Not allowed!
    Drewline

    When was the last time you did something for the first time?

  4. #4
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,369
    Thumbs Up/Down
    Received: 1,391/21
    Given: 1,178/0
    Rep Power
    14
    Quote Originally Posted by Drewline View Post
    This is the later style amp with 2 6L6 power tubes & a 12ax7 as a limiter. The amp is biased cold. There is no pot for bias. I'm wondering how to set the bias on this.
    I have the devil of a time with these. Potentially a good sounding clean amp but there's really not much "wiggle room" for setting bias electronically and believe me, I've tried all the ways. Selecting another set of tubes with a larger Pc rating may help, like 10 or more points higher not just a couple.

    Great overdrive tone though - first time I heard one it was Ace Frehley jammin' out a new song on it - finally made it to album about ten years later. Ace's take on the state of the environment: "Remember Me" was the title. Rippin' good blues.

    0 Not allowed! Not allowed!

  5. #5
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    11,629
    Thumbs Up/Down
    Received: 1,544/23
    Given: 3,908/11
    Rep Power
    22
    Do you think the crossover you are seeing is what the customer is describing as "distorts quickly" ?
    How does it sound?
    Unless these are different tubes than he was using before, I don't see how he would just notice this now. Yes it is biased very cold, but it must have been this way before.
    If the problem appeared when these Ruby's were installed, then yes, the bias is probably the issue. As Leo mentioned, putting in a set that runs much hotter would probably be the easiest solution.

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  6. #6
    Old Timer Tom Phillips's Avatar
    Join Date
    Jun 2006
    Location
    California, USA
    Posts
    3,559
    Thumbs Up/Down
    Received: 160/0
    Given: 92/1
    Rep Power
    17
    Note that Music Man doesn't use the common PP output stage we are used to. The amps are running close to class B which means that they are designed to be biased a lot colder than your average class AB amp. The signal to the power tubes is cathode injected.

    It seems strange that they eliminated the bias pot. I guess, as mentioned above, it was providing the opportunity to do more harm than good.

    Pasted in below is a copy of an old thread about Music Man bias. I couldn't find the address to this thread in the archives. A Google search for "Music Man bias" or similar should turn up lots of info. My best advise is to research and understand the circuit before you make your adjustments. Maybe the issue you are having is not caused by the output tube bias setting.

    Here is the blast from the past:

    From: anonymous
    Date: 10/30/2002 8:05 PM
    Subject: Ray Ivers ? : Music Man bias and output class ?
    Doug Roccaforte is claiming the operating class of Music Man tube amps is Class-B on the PRS forum. Some responses suggest Doug may be wrong, in calling the Music Man amps class B ? Ray, you know about these amps, what's the proper class ?

    From: Ray Ivers (RAGEray@aol.com)
    Date: 10/30/2002 9:23 PM
    Subject: Re: Ray Ivers ? : Music Man bias and output class ?
    The thing about the MM amps, like other guitar amps, is that there are different versions of the same amp model. Some of the MM amps use tube output stage drivers, some use SS. I'll just take the SS RD-100 case.
    From the RCA RC-30 tube manual, page 37:
    A Class B amplifier employs two tubes connected in push-pull, so biased that plate current is almost zero when no signal voltage is applied to the grids.
    By this definition, the MM RD-100 SS cathode-drive amp is a Class B amplifier, as it draws an extremely low value of quiescent cathode current. I've seen other Class B definitions that specify an exact 50% duty cycle per tube (zero idle current), and by those definitions this amp is not Class B.
    Is that any help?
    Ray Ivers

    From: KB (ktbullion@presonus.com)
    Date: 10/31/2002 1:42 PM
    Subject: Re: Ray Ivers ? : Music Man bias and output class ?
    FWIW I only measured 10ma's of current on the transformer shunt methode. The Cathode injection or whatever you call it is unique in it's own way and the plates usually have 720 volts on them. Extreme power is an understatement. !! I agree with Ray as well.,
    KB

    From: O'Connor (amps@londonpower.com)
    Date: 11/7/2002 7:10 AM
    Subject: Re: Ray Ivers ? : Music Man bias and output class ?
    The cascode output stage uses local feedback around the BJT to control the tube, who's cathode is driven by the BJT collector. The grid sits around +30V and the screen is at B+/2, which ends up as 235V to 350V depending on whether the amp is set to 'high' or 'low' power.
    The feedback resistors around the BJT set up the idle current, which as others have noted is very low. This extends tube life and does not really effect tone as the output stage itself is within the very powerful feedback loop of an op-amp. If the BJT/tube was biased to zero, cross-over distortion would be very low until you hit 5-6kHz. The cascode is very linear over a wide current range and a wide frequency range-- much wider than needed for audio let alone MI.
    Because there _is_ idle current, there is a cross-conduction area of class-A operation albeit a very small one. So, you would call this low-bias class-AB, or simply AB.
    The circuit will be detailed in TUT4.
    Have fun
    Kevin O'Connor

    From: KB (ktbullion@presonus.com)
    Date: 11/7/2002 9:27 PM
    Subject: Re: Ray Ivers ? : Music Man bias and output class ?
    Thanks Kevin for the explanation. Peavey has an amp called thr Heritage which is very similar to the MM except the plates have only 525 v on them. Another Poster here and I discussed the situation after this post and had concluded it AB as well. A Class B would not draw any grid current whatsoever so this amp would draw maybe a hair. The grid bias also would be equal to the cutoff value. Now class B1 would have no grid current flowing but class B2 would have some grid current flowing so could this also be Class B2 ? You mentioned in TUT that transformer drives cound be used at zero bias and other books say that parasitics can be eliminated with the trasformers or swinging chokes. Also the grid as you mentioned sits at +30 volts but the Cathode is collector driven with a pot to about +70 I think or maybe lower. Even though it's positive it would still be called negative grid bias as long as the grid is negative with respect of the Cathode and the amp sounded flabby(crossover distortion) when brought to long cutoff rates at lower positive voltage from the collector pot. ,
    KB

    From: anonymous
    Date: 11/8/2002 2:16 PM
    Subject: Re: Ray Ivers ? : Music Man bias and output class ?
    So Roccaforte once again proves his ignorance of tube theory !

    From: Carl Z (carl@zwengelamps.com)
    Date: 11/8/2002 9:04 PM
    Subject: Re: Ray Ivers ? : Music Man bias and output class ?
    At a glance I'd have said it was class B as well. However, Kevin took the time to thoroughly analyze the circuit and provides a strong argument in favor of class AB. Doug is way off the mark on a few things here and there but this is NOT something I'd jump down his throat over.
    Carl Z
    ???????????

    From: O'Connor (amps@londonpower.com)
    Date: 11/16/2002 2:15 AM
    Subject: Re: Ray Ivers ? : Music Man bias and output class ?
    To KB:
    >Now class B1 would have no grid current flowing but class B2 would have some grid current flowing so
    could this also be Class B2 ?
    There is no grid current flow in the MM circuit unless the cathode is pulled below the grid potential at +30V. Normally, one associates heavy distortion with grid current, but that is not the case here. In cathode-driven cascodes, and transformer-driven grids, the drive impedance is much lower than the grid impedance, even once the grid conducts. That low drive-Z also hastens recovery to the zero grid current condition. Transformer-driven grids were used in the Fender PS300/400 amps.
    >You mentioned in TUT that transformer drives cound be used at zero bias and other books say
    that parasitics can be eliminated with the trasformers or swinging chokes.
    The "zero bias" you speak may confuse some readers, and is only applicable in two situations: when using power-grid tubes, or when using screen drive. In the first case, a tube like the 811A or SV572B is implemented, which has been specifically designed to not need a bias supply. When Vgk=0, very liitle cathode current flows. The grid must be driven positive and consumes current for large plate current to flow.
    In the second case, the grid of a conventional tube is grounded, or tied to the screen, and the screen is driven by a ground-referenced transformer winding. When Vg2=0, no current flows. As the screen is driven positive, plate current flows but it takes a huge voltage to arrive at the usual sort of currents found in most PAs.
    Distortion in both cases is very low but you need many Watts of drive power to get output.
    have fun
    Kevin O'Connor


    0 Not allowed! Not allowed!
    Last edited by Tom Phillips; 05-14-2014 at 02:31 AM.

  7. #7
    Supporting Member Drewline's Avatar
    Join Date
    Feb 2012
    Location
    Colorado
    Posts
    223
    Thumbs Up/Down
    Received: 12/0
    Given: 19/0
    Rep Power
    8
    I'm looking into higher transconductance tubes as a solution if I can't figure out how to make the bias adjustable. Tried a pair of Mesa grey 6L6 tubes which got the bias measurement up to 15.8mv sounded better Now I'm wait to talk to the customer.

    0 Not allowed! Not allowed!
    Drewline

    When was the last time you did something for the first time?

  8. #8
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    11,629
    Thumbs Up/Down
    Received: 1,544/23
    Given: 3,908/11
    Rep Power
    22
    The older units that had the bias trim had 3.9 ohm emitter resistors and called for 25mV, so about 6mA idle current. You now have 16mV across 6.8 ohms for about 2mA. (I'm assuming you're still measuring across each 6.8R).

    I'm wondering if a bias adjust at the grids would be easier than the way they did it in the older units (adjusting bias of transistor).
    The zener D9 sets the grids around +30, increasing that voltage a bit would make the tubes run hotter. Perhaps a trimpot in series with the zener would do the trick?

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  9. #9
    Supporting Member Drewline's Avatar
    Join Date
    Feb 2012
    Location
    Colorado
    Posts
    223
    Thumbs Up/Down
    Received: 12/0
    Given: 19/0
    Rep Power
    8
    Leo,

    This is where I get confused. I usually purchase tubes from CE. If I'm looking for a hotter tube, what do I ask them for? Their ratings do not are totally different than Ruby's or Groove Tube's. High Transconductance or Low Transconductance? High Current or Low Current? When we used Mesa Tubes at Mars the color coding was a good indicator of softer vs harder. I always used Red or Yellow tubes for lower bias & grey for higher bias on non adjustable fixed bias amps.

    So my question is: Are hotter tubes Low or High Transconductance?

    Thanks,

    0 Not allowed! Not allowed!
    Drewline

    When was the last time you did something for the first time?

  10. #10
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,369
    Thumbs Up/Down
    Received: 1,391/21
    Given: 1,178/0
    Rep Power
    14
    Quote Originally Posted by Drewline View Post

    So my question is: Are hotter tubes Low or High Transconductance?

    Thanks,
    Yes. You've seen Pc and Tc numbers on Ruby, also on TAD tubes. CE puts a hand-numbered sticker on the box. First number is emission Pc and second transconductance Tc. One pretty much tracks the other. If you have numbers on the tubes you have, ask for ones a good chunk higher not just the next one up. It looks like that's what this model of amp is asking for.

    I've never seen an explanation of Mesa's color code, specifying what color corresponds to what Pc and/or Tc. Sure would be nice to know what's up with that.

    0 Not allowed! Not allowed!

  11. #11
    Supporting Member Drewline's Avatar
    Join Date
    Feb 2012
    Location
    Colorado
    Posts
    223
    Thumbs Up/Down
    Received: 12/0
    Given: 19/0
    Rep Power
    8
    I wanted to post an update on this one. Talked to the customer who said he replaced tubes about a year ago. Sounded good for a month or two & then started sounding distorted. With my "Grey" Mesa 6L6's it sounds much better so I've ordered hotter, harder tubes from CE & will see what the bias current measures with the new tubes. I'll post my results after I get the tubes.
    Thanks for all the advise.

    Drewline

    0 Not allowed! Not allowed!

  12. #12
    Senior Member
    Join Date
    Jun 2017
    Location
    Ontario, Canada
    Posts
    353
    Thumbs Up/Down
    Received: 23/0
    Given: 38/0
    Rep Power
    3
    OK, I know this a a five year old thread but I reckon I'll try to revive it before starting a new one. I have one of these RD-50's (head version). I was curious about the bias issue and came across this thread and also this one http://www.pacair.com/discus/messages/6/35.html

    In the linked thread under "Section 1" it says:

    "Adjust the bias trimpot (TR-1) until you read 25mv DC across the 3.9
    OHM emitter resistors. If there is a difference in voltage between the
    emitters of the two driver transistors, set the lower of the two to 25mv.
    The higher of the two should not exceed 55mv DC.


    I'm curious how these numbers are derived. When biasing we are ultimately concerned with the amount of current the tube is conducting. With a typical fixed or cathode biased design I use the OT shunt method to determine current and for a 6L6 it's usually in the 30mA - 40mA ball park. Using 25mV across 3.9 ohms as a baseline that works out to about 6mA (as mentioned by G1 above). It has been noted that these amps are biased cold but does 6mA actually represent the total amount of current being conducted through the tube in that scenario?

    So now to get to my own amp. Per the schematic in the original post the emitter resistors to ground are 6.8 ohm. I'm measuring 32mV across one and 50mV across the other at idle for 4.7mA and 7.3mA, respectively. At first this discrepancy concerned me but in the blurb I pasted the writer seems fine with one side being 25mV while the other is as high as 55mV. Also, when I calculated the output power before clipping I found the amp is putting out 47 W (seems about right) and voltage across both emitter resistors was right around 0.7V. So can I safely assume everything to do with power tube bias is OK with my amp?

    B.

    0 Not allowed! Not allowed!

  13. #13
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,369
    Thumbs Up/Down
    Received: 1,391/21
    Given: 1,178/0
    Rep Power
    14
    Quote Originally Posted by bobloblaws View Post
    So can I safely assume everything to do with power tube bias is OK with my amp?
    In a word, yes. Times in the past the RD50 led me on a merry chase until I understood "that's the way they are, get over it." Even with the apparent imbalance* - remember Enzo's "It's just a guitar amp." Although it was such a PIA figuring out what & what not to do, I still like the RD50 tone, they're a handy size box with a lot of holler.

    * About that balance - check the values of those resistors, they may have drifted causing your measurements to mislead you. And if not, nuttin' ta worry 'bout anyways.

    1 Not allowed! Not allowed!
    Enjoy. Every. Sandwich.

  14. #14
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    31,873
    Thumbs Up/Down
    Received: 1,726/7
    Given: 0/0
    Rep Power
    54
    The numbers were derived from the factory bias procedure.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  15. #15
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    11,629
    Thumbs Up/Down
    Received: 1,544/23
    Given: 3,908/11
    Rep Power
    22
    Sounds like it is pretty much all good.
    If you want to check on the matching, swap the tubes around. If the high number stays with one tube, they are a bit unmatched, but still considered within the acceptable range.
    If the imbalance stays with the sockets, the transistors may not be perfectly matched (also check that the screen voltages at pin4 are same). But still, your numbers are within the acceptable range.

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. setting cathode bias?
    By chuckb in forum Maintenance, Troubleshooting & Repair
    Replies: 15
    Last Post: 03-10-2014, 05:50 AM
  2. Fender Dual Showman bias setting
    By MarkusBass in forum Maintenance, Troubleshooting & Repair
    Replies: 2
    Last Post: 10-03-2011, 08:15 AM
  3. Behringer 180W bias setting
    By schraedertr in forum Maintenance, Troubleshooting & Repair
    Replies: 4
    Last Post: 08-05-2009, 03:51 AM
  4. Bias setting Ampeg V4BH
    By breogan in forum Maintenance, Troubleshooting & Repair
    Replies: 4
    Last Post: 01-29-2008, 06:48 PM
  5. Setting Bias for Ampeg SVT 6550
    By Doug in forum Maintenance, Troubleshooting & Repair
    Replies: 4
    Last Post: 08-15-2007, 02:18 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •