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Thread: musicman rp-65 bias

  1. #1
    Senior Member Mars Amp Repair's Avatar
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    musicman rp-65 bias

    say,
    Anyone know how to adjust the bias on a Musicman 65? Looks like they control it through 2 transistors in the cathodes of the 6l6's.

    At idle, the centertap of the OT measures increadibly low like 4ma, however the amp behaves rather normally.

    I'm thinking this arrangement somehow tracks the bias through it's power range, so you don't need to cook the tubes at lower power levels...not sure.

    I see it is basically set up as cathode bias with a lower positive voltage on the grids than the cathode equating to about -40vdc between g-1 & K.

    That sounds about right, but I don't quite know where (how) they want you to adjust this 'tracking' circuit', if that is indeed what it is.

    thanx, g

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    If you're working on a Music Man this will be your best resource:

    http://www.musicmanamps.com/

    You need to identify the amp by "chassis number", probably stamped on the serial number plate if I remember correctly. Possibly 2165-RP or 2165-RD.

    Somewhere in that site there is just about everything you'd ever want to know about those unusual amps. If not in the documentation then in the forums.

    Have fun!

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    Old Timer Amp Kat's Avatar
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    I believe it's 25 mv's across the 3.9 ohm emitter resistor Glen. Of course you can increase it to your liking but yeah your right that it's with respect to the Cathode but I wouldn't say it's Cathode bias but more of a Cathode injection or current source. Very low current flow at quiescent but supplies more as it's asked for it or tracking as you say. The Musicman sight is pretty good but I've seen some differences of opinions mainly on the HD130 that were a little off and the sight is under construction so the official pages of correct bias are missing but I'm sure someone like Enzo has the original Music Man aka Ernie Ball biasing procedure. I've seen it and remember 25mvs. FWIW everyone of these I get have the original silver Mallory's under the hood and blue type on the main board and they have to be 40 + years old and some still sound pretty good but starting to hum and do freaky noises.

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    KB

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    Looks like Amp Kat is right on the money (good memory!).

    I did find this through the forums, though parts of the site do appear to be under construction:

    http://www.pacair.com/discus/message...tml?1171360675

    Based on my (not all that recent) experience you should also check the values of the zener dropping resistors for the low voltage supplies. I seem to remember having to replace those in many of the amps I saw due to large drift in value. Ran across a few out-of-spec zeners too.

    Edit: Aha - found the good stuff:

    http://pacair.com/mmamps/Documents/M...tins%20All.pdf

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    Last edited by Mark Black; 03-29-2007 at 08:42 PM.

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    Senior Member Mars Amp Repair's Avatar
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    BEAU-TI-FUL! Just what I needed. That is a strange ckt. Thanx too for the history regarding those dropping resistors. Always nice not to have to learn via a rework.
    It still amazes me how many manufacturers use so much current & generate so much heat instead of just using a simple 3 legged regulator...even these days.

    g

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    Lifetime Member Enzo's Avatar
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    The reason these are so cold it the amp is running pretty much in class B. They idle near cutoff. They idle at about 6ma.

    The adjustment is 25mv across the 3.9 ohm emitter resistor. If they are not equal, set the lower one to 25mv as long as the higher one does not go over 55mv.

    Note: use only 1458 op amp to drive this. Do not use 4558 or others.

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    Senior Member Mars Amp Repair's Avatar
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    Yep,
    Thanx Enzo...got all the info from the Musicman link that Amp kat supplied. Interesting design...I'll bet those tubes last forever idling in class B. Do you know if there was any inherent problems with this design? Seems like a good idea at first blush...g

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    Old Timer Tom Phillips's Avatar
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    Welcome to the world of Class B output circuits Glen and pity the tech who tries to adjust the idle bias current as if it was a class AB1. I don't know if you would call it an inherent problem but performance will degrade faster as tubes age and anything else happens to change the bias setting. This is more of a problem for Hi Fi amps because the cross over distortion can quickly increase. On the other hand, you can get more power, like up to 70Wrms out of a pair of 6L6s. That Musicman site is a lifesaver.

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    The circuit in question is a cascode. The transistor is a gain stage and output tubes are working in grounded grid. This circuit offers high bandwidth and low distortion.

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    Senior Member Mars Amp Repair's Avatar
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    OOOO, grounded grid...I can relate that back to my schooling on common base ckts. As I recall low input impedance, but as you mentioned increased bandwidth. Interesting...g

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    http://www.sound.westhost.com/amp-basics5.htm#5.5

    Here's a bit on cascode circuits. You get the best of both worlds: high input impedeance and increased bandwidth. But seeing as this isn't a concern for guitar amps, Musicman surely did it to squeeze more power out of two tubes like Mr. Phillips was saying.

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    Senior Member Mars Amp Repair's Avatar
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    oops,
    thought that arrangement would give a lower input impedance...better go back & refresh the memory....thanx, g

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    A common base circuit by it's lonesome has a low input impedeance so your memory is still good

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    Quote Originally Posted by Ptron View Post
    http://www.sound.westhost.com/amp-basics5.htm#5.5

    Here's a bit on cascode circuits. You get the best of both worlds: high input impedeance and increased bandwidth. But seeing as this isn't a concern for guitar amps, Musicman surely did it to squeeze more power out of two tubes like Mr. Phillips was saying.
    Nelson Pass did some work with cascode power amps. Here's his white paper on that work.

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    Noodle of Reality Steve Conner's Avatar
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    I always liked that Music Man cascode circuit, from a tech geek perspective at least. (I'm not so sure how it would sound.) I think they did it for simplicity and economy.

    Like the old saying goes, "If you want high current out of tubes, or high voltage out of transistors, be prepared to pay for it." That means that solid state circuits, when designed for minimum cost, end up having a low characteristic impedance compared to tube circuits. (If you look at a tone stack in a solid-state amp, the pots should be about 1/10 of the resistance, and the capacitors 10 times bigger, than they would be in a tube amp.)

    The cathode is the lowest impedance point in a tube, and the collector is the highest impedance point of a transistor, so if you're going to connect transistors to tubes, the Music Man cascode circuit is kind of the natural path of least resistance. The resulting composite device has the high gain of a transistor and the high voltage handling capability (and hopefully some of the sweet sounding distortion) of a tube.

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    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Old Timer Amp Kat's Avatar
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    Quote Originally Posted by Steve Conner View Post
    I always liked that Music Man cascode circuit, from a tech geek perspective at least. (I'm not so sure how it would sound.) I think they did it for simplicity and economy.

    .
    These amps are loud and clean and used by many Violin,harmonica,keyboard and guitarist who prefer clean high power. After all in hi power mode theres over 700 volts on the plates and the preamp with it's SS circuitry can provide a pretty linear signal with not much thd+N. There are some flaws as Mark pointed out the zeners maybe should have been heat sinked 78 & 79 series regulators as they go out quite often but those transistors very rarely fail and in all reality shouldn't. I like cascodes for mic pres over guitar amps and you don't need transistors for that as an all tube works excellent

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    KB

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    musicman 112 65 rd

    Quote Originally Posted by Enzo View Post
    The reason these are so cold it the amp is running pretty much in class B. They idle near cutoff. They idle at about 6ma.

    The adjustment is 25mv across the 3.9 ohm emitter resistor. If they are not equal, set the lower one to 25mv as long as the higher one does not go over 55mv.

    Note: use only 1458 op amp to drive this. Do not use 4558 or others.
    hi man.I know this is old, 2007, but I don't get on the page enough.since you helped me out with some great info on that crate I thought I'd ask about this music man 112 65 rd G2A(6L6) solid state driver. a customer came in,gave it to me.said he found it in a dumpster.thought I could use it for parts. well it was missing tube sockets,speaker,power cord,grille,output tubes.I had everything right there on the bench.I love these so being in the market for a nice gig amp smaller and lighter than my super twin I figured it was just what the doctor ordered.so I replaced everything,including the B+ supply caps.even had a pair of vintage sylvania 6L6 tubes.sounds great but...the bias is stable but it seems like I have to jack the gain and volume in the clean channel to get a usable level.when I do there is that rustling sound like an old black face fender with a bad plate resistor. still has original op amps. I know these things have a history of breaking down and op amps have become a lot quieter. can you recommend a replacement. the over drive channel is noisy as well but the signal goes through the same IC 1 as the clean channel. 1458.thanks

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