ONLY 12au7 EH's , no other 12au7's, work in Rickenbacker M-11

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mbruffey, Jun 18, 2005.


  1. mbruffey

    mbruffey Member

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    I realize I have a separate post about the Rick amp, but this one's got me really puzzled. Maybe you can help?

    I have a 1957 (?) Rick M-11. It was my dad's. We last turned it on in the late 70's. It would run about 10-15 minutes then go into distortion like crazy and eventually "honk" out.

    The other day, I decided to have an amp tech look at it. He replaced some coupling caps and a resistor and changed the power and pre amp tubes. Amp sounds fine on his bench.

    I have plenty of pre-amp tubes at home, so I offer to save self a little cash by using my own pre-amp tubes (1 12ax7 and 1 12au7). Insert my pre-amp tubes at home. Amp sounds like a bee in bottle 5 minutes after I turn it on. I try all kinds of knob settings, but on every note and especially chords I get all this distortion/fuzz, even at low volumes.

    Take amp back to tech. Tech very kindly re-inserts his preamp tubes. Suddenly all is well! Tech shows me how much difference the EH 12au7 makes. I stare in disbelief. I go home red-faced and with a lighter wallet.

    Well, after all this, I tried about 5 different 12au7's, including NOS, used, blackplates, greyplates, etc., etc., in this amp and ONLY the EH gives a clear tone.

    The EH tests about 20% weaker on my EICO 667 than the NOS tubes. I conclude that the distortion, then, is due to higher mu of the NOS generation. This CANNOT be right, methinks, because the amp was designed in the period when all these tubes would have been a common replacement.

    In a couple of places I have read some claims that the 12au7a might NOT always be a suitable replacement for the 12au7, and the labeling on the amp does specify 12au7, although it does not offer warnings about subbing a 12au7a. Is there really a significant enough difference between the 12au7 and the 12au7a to cause such a problem?

    I have tried various 12ax7's in combination with the various 12au7's and nothing seems to affect distortion levels. Also, I did test with different power tubes too. No change. The tech even replaced the 5Y3 when I went back because a new one raised the voltage levels a little, so that does not seem to be part of the equation.

    Am I missing something here, or am I right to suspect there's still something amiss in the 12au7 circuit that's causing the distortion with the NOS tubes?

    Thanks for any help,

    Mark
     
  2. John Phillips

    John Phillips Member

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    You are right, and your tech is wrong - and you should get your money back on the "repair", since he didn't fix it.

    If the amp becomes unstable with the correct specification tube, there is a fault with the amp - I don't know what exactly, but that's not the point.

    Putting an under-spec modern cheap tube in to "cure" the problem is trying to make a right out of two wrongs. What did he do with the old tubes? I hope you got them back.


    If the amp was last turned on in the late 70s, it needs all the electrolytic caps replaced for a start, even if they appear to be "OK" - they simply break down with time, especially if not used, and twenty years is a commonly-accepted life expectancy in any case... even if they were brand new when it was last used, I'd suspect they need changing - but this amp is nearly fifty years old! It's unlikely that coupling caps or resistors would have degraded in the same way though - even the old tubes should have been fine, unless they were on the way out back then.

    Time for a new tech I think... he did some stuff that might well not have needed doing, didn't do some stuff he should have, and charged you for not fixing the amp - then when the fault came back, sold you some below-spec tubes you don't need.

    Just my opinion.
     
  3. mbruffey

    mbruffey Member

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    I did get all the tubes and all the other parts back, BTW.
     
  4. mbruffey

    mbruffey Member

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    OK, I got up enough nerve to get in the amp and test some resistor values, since I at least know something about how to test those (don't know how to test caps though).

    This little amp has about a dozen small resistors and two great big brown ones. I found 6 (small) that appear way out of tolerance.

    Two of the six read high and one of these two appears to be breaking down: it reads anywhere from 51,00-92,000 and constantly changes with varying pressure on the leads. Must be cracked.

    I'll describe here a little more. Pins 1 and 7 of the 12au7 are tied together. I think these are the plates. A red wire comes off these and over to the separator board, passing through the faulty resistor I described (inline with a second resistor). I can't really tell its relationship to the coupling caps, but it passes by one. Then it disappears into the separator board and I can't trace it from there. The only other similar looking red wire comes out from under the board at a different location and goes over pin 4 on the 6v6's--grid 2, but it's just a guess whether or not it's really the same circuit.

    OK, now you can pat me on the head and say "nice try little boy," because it's obvious I'm a novice at understanding tube amps--but I CAN learn, given the opportunity.

    Now, I reason that "stuff" is trying to get off that plate in the 12au7 thru that red wire and is getting hung up because there's a flaky resistor with way too much resistance in it's way. That means there might be too much "stuff" flying around in the tube, hence distortion. With the lower mu 12AU7, a la Electro-Harmonix, the circuit isn't pushed into distortion, but wuth the higher mu NOS 12AU7A's the circuit _is_pushed into distortion.

    Again, I'm sure I could be totally all-wet here, but at least I am _trying_ to apply the limited understanding I currently have.

    Maybe someone will be willing to offer a little further instruction . . . .


    If anyone is interested I can post a pic of the guts of the amp, show where all the resistors in question are, and post exact measurements.

    Sorry I don't have a schematic, so I'm guessing on some of the connections hidden by the paper board.

    Thanks for any who may feel inclined to help.

    Mark
     
  5. doctord02

    doctord02 Member

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    you cant really test resistor or caps in the circuit; you'll have to remove/desolder the suspect parts and measure them outside of the influence of the other parts of the circuit.

    the only two resistors that looked suspicious to me are the grungy looking ones on the input jacks...

    we also dont know anything about the quality/condition of the tubes in your stash; they could be defective or out of spec...
     
  6. mbruffey

    mbruffey Member

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    Well those two grungy resistors at the jacks are supposed to read 68K, but both read EXACTLY 37.4K. I wonder if that could really be the cause of the problem.

    If you happen to be a tech, I'll take your word for it, but I didnt think checking resistance from point a to point b with nothing between would matter in or out of circuit. But I can see, I suppose, where multiple connections in the circuit could affect the readings . . . . As I said, I'm not a tech.

    There is no doubt that the one resistor I described is subject to changing values based on physical force applied to the leads.
     
  7. mbruffey

    mbruffey Member

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    I did check one of my 12au7's in an eico 667. Tested good, 20% better than the EH 12au7. I suppose it's possible all 5 of my 12au7's are out of spec. I could check them in one of my other amps to verify, I suppose.
     
  8. mbruffey

    mbruffey Member

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    Well those two grungy resistors at the jacks are supposed to read 68K, but both read EXACTLY 37.4K.

    Is this an example of resistors "in parallel" (as I just learned from a little research)?

    If so, and if you know the values of each are supposed to be identical, is there a formula to use to avoid having to remove them from circuit?
     
  9. mbruffey

    mbruffey Member

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    Supposing you had a circuit with a 100 ohm and a 1M ohm resistor in parallel, nothing else. If you measure resistance in that circuit, will you always get the measurement from the path of least resistance? If that does happen to be the case (wouldn't that be nice), then you could still assume that a high reading would still indicate a defective unit.
     
  10. doctord02

    doctord02 Member

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    for grins, try putting a jack in one of those sockets and remeasure those resistors. a non connected guitar cable or patch cable will do just fine.
     
  11. doctord02

    doctord02 Member

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    actually, now that I look at that picture, that may not make any difference.

    those resistors are connected to the sleeve part of the jack on one end, and are wired together on the other end. The sleeve of both jacks are grounded to the chassis.

    This means that yes, you are reading exactly half the actual resistance of the resistor, because even tho you are only measuring across one resistor, the other resistor is in parallel.

    37.4 + 37.4 = apx 74.8K each... so the resistors have drifted a bit, which is not supprising for old carbon comp resistors. Of course the only way to know precisely what each one measures is to remove it from the circuit.
     
  12. mbruffey

    mbruffey Member

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    OK, that's very helpful. I interacted extensively with an electronics kit from Edmund's Scientific when I was 10-11 years old, but it takes a while for all that stuff to come back. It had all these loose resistors and diodes and transistors that you had to assemble into a circuit--wish more of it had stuck!

    In the case of the input jacks, would it be true that since the resistance has drifted higher (unless they've always been high--they're 10% tolerance--680+68=748), the gain from the pickups would be reduced thus reducing the drive on the circuit? Probably would lead to cleaner tone?
     
  13. John Phillips

    John Phillips Member

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    That is a nice try. It may even be the correct one...

    The first thing I would do is to replace that resistor. You can't always accurately measure values in a circuit, since other parts may be in parallel (maybe not intentionally so, if they're also faulty) and alter the readings, but with some you can, or at least get a reading which will tell you whether the problem is even likely to be there before removing the part.

    That's exactly the sort of thing to look for. Pins 1 and 7 are the plates on a 12A*7 tube, yes - so that resistor is the plate resistor, which directly supplies current to the tube and largely controls the gain in that part of the circuit. A higher value (which is what typically happens with a failing carbon-comp resistor) increases gain and could cause instability. If it's not only high but cracked, hence intermittently changing or going entirely open-circuit, or producing a "diode-like" effect (they can) that would probably explain the symptoms... and why substituting a lower-gain tube will "fix" it.

    FWIW, this is how I learned about fixing tube amps too - I had a 'proper' electronics education, but in service work there's no substitute for hands-on experience, as well as some basic theory and knowing how to logically work through a problem.

    This is an extremely simple amp, and an ideal starting-place for learning how to be a tech! Don't give up - even if it's not that resistor.
     
  14. mbruffey

    mbruffey Member

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    Thanks much for your kind reply, John. You know, looking at the schematic for the different version of the amp (see earlier post) assuming much is the same, less different preamp tubes and circuit voltages and resistance, I am pretty sure that red wire does go over to the control grid 2 on the 6V6s. I'll take your advice and change the resistor, or try to talk the tech into doing it maybe . . . . Sincere thanks, Mark
     
  15. mbruffey

    mbruffey Member

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    OK. I changed out the suspect resistor with very little noticeable difference. The sound with the EH 12au7 may be a tad grainier. Sound with NIB 12au7a is still like a wasp in a bottle, maybe a bit worse than before. Next?
     
  16. mbruffey

    mbruffey Member

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    After changing the most suspect resistor (the one changing values) with little noticeable difference, I dug in and took some more measurements. I also re-soldered a bunch of joints. I think the only ones I didn't re-melt were the pre-amp tube joints. I also took up the circuit board so I could look underneath in the absence of a schematic. I think I was successful in disconnecting at least one end of each circuit where I took a measurement.

    I'll refer to pics, looking in with the TOP of amp facing me.

    http://home.comcast.net/~mbruffey/ebay/ric_03.jpg


    First, the resistors.

    (a) ypo alongside the leftmost yellow cap. Should measure 47k, measures 70k.

    (b) rry btw yellow caps, connected to left cap lead. Shoud measure 220k, reads 1.2m. (see these in a different pic)

    http://home.comcast.net/~mbruffey/ebay/ric_07.jpg

    (c) rry btw yellow caps, connected to right cap lead. Shoud measure 220k, reads 758k.

    --(b) and (c) are tied together. The blue leads from the south end of those two yellow caps go over to the LH volume and tone/on-off controls, btw.

    The rest of the resistors on the board all seem to measure within spec.


    Second, an unidentified object that I think is a resistor.

    It's a little doodad shaped like a pancake between the LH volume and tone control. It has 4 dots on it, either:

    yellow, purple, brown, black,

    or

    yellow, light blue, brown, black

    This object appears to be breaking up a little where one of the leads enters. Dry stuff flaking off and a visible "hole" where lead enters the pancake. Help?


    Third, since my tester has a "cap" measurement capability, I thought I would at least report what it reads for each. Obviously, I tested these with power off.

    Five of the six measured about 60nf, but the RH yellow cap measured 76.5nf.

    Again, I'm not experienced enough to know if these measurements are meaningful, but figured I would report, at least.


    Fourth, measurements of the parts the tech replaced (2 600 vdc and 2 smaller 200 vdc caps, and 1 470k resistor)

    I notice he replaced both sizes of caps with the same sized orange drops--does that mean anything?

    Anyway, old caps:

    (a) 70 nf
    (b) 73.7 nf
    (c) 75.4 nf
    (d) 76.6 nf

    old resistor:

    should be 470k, measured 494k, but does show a crack



    That's about it.

    I did NOT check the cap can. Should I just test it with the "cap" test on my meter and report findings? It will be a real project to get into it. If I had specs, of course, I could just bypass it easily, I understand. I does look like it's been messed with before.

    Mark
     
  17. doctord02

    doctord02 Member

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    "Second, an unidentified object that I think is a resistor.

    It's a little doodad shaped like a pancake between the LH volume and tone control. It has 4 dots on it, either:

    yellow, purple, brown, black,

    or

    yellow, light blue, brown, black

    This object appears to be breaking up a little where one of the leads enters. Dry stuff flaking off and a visible "hole" where lead enters the pancake. Help?"

    -----------------------------

    This is a capacitor. If you have a schematic, look and see if you can determine the values. It does sound like it needs replacing. A picture would help, if you have one.
     
  18. r9player

    r9player Silver Supporting Member

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    So far from what it sounds like you have a ton of stuff way out of spec. (assuming measurements were done correctly)
    At this point in time ...
    1. Amp goes to a reliable tech for a thorough and complete repair
    2. You become mr. Hobbyist amp tech and you start playing around, scrounging for proper parts etc.
    3. You give up and sell it off .. give it away ...

    One thought, since the amp is fully functional, did you measure plate voltages etc? Not sure if you have any guide line to tell you what they should be though. But if you think you are getting too much gain, that would sound like the bias is too hot? Which possibly could show in a higher voltage on the plates. (Someone stop me if I am just dreaming this up!) Best of course would be abilty to measure the current .. but that is a lot trickier.
     
  19. mbruffey

    mbruffey Member

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    3. You give up and sell it off .. give it away ...

    I will keep it because it was my father's. Probably keep pecking away with diagnostics until properly repaired or fried . . . .
     

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