De-humming an Ampeg B-12-XY.........

Discussion in 'Amps [BG]' started by 59Pan, Dec 24, 2012.


  1. 59Pan

    59Pan Graybeard Supporting Member

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    I have posted this thread elsewhere, but not getting any response to it. Maybe it's the holidays. Maybe just a difficult fix. Either way, here's what I have:

    I am on the finishing end of a repair/restoration of a '64 Ampeg B-12-XY. It was, for the most part, a very trashy mess. I discovered what may have been the origins of the real mess. I replaced all power supply decoupling caps, as a first step. This had been already done in some manner, but with nothing matching the original schematic. That was fixed up front. All electrolytics were replaced. C36, at the bias diode, was completely missing. Major hum fix there.

    The oscillator for the 'Vibrato' caps were replaced along with a tube swap, bringing it back to life. The 'Echo' was not what I thought it should have been, but have since come to realize that the large speaker is overwhelming the two little speakers in the satellite cab. After replacing various components in that circuit, moving the speaker cab away from the main cab was the biggest fix. Then I could hear it.

    Let me state one handicap up front. And, this may very well explain how it got so messy. My amp was built different from the typical schematic found. I hand-traced everything and found the triode assignments for V7 and V8 did not match the schematic. I found a couple other people who had the same. So, I used the original schematic as a reference, and hand-traced my amp, until I had a correct schematic. I also found that my Echo footswitch was on the cathode resistor of one of the triodes, where the drawing showed it grounding the output of the spring tank. Hand-tracing done, I CAD'ed my own drawing. It is very much the same as the original, but reflects my tube assignments and component values. A lot of caps were of different values from the original schematic. I now have a schematic that I can read. The original (from the Internet) is a very poor copy.

    I moved my Echo footswitch from grounding/ungrounding the cathode resistor to that of grounding/ungrounding the spring tank output. The way mine was originally wired made a very loud pop when making or breaking the switch. On the signal path from the spring tank, it is very quiet, just like the original schematic.

    I'm very satisfied with the way the amp turned out, but I'm now trying to get rid of a loud hiss on Channel 2. When the volume is above half, it starts to become obvious. On Channel 1, I have a fairly obvious hum above half volume and some hiss as well. There was quite a bit of lower frequency hum, until I moved the Hum Control pot leads from V1 to V7. I was then able to null most of it out. If I can rid it of the hiss and higher frequency hum (which is mainly on Channel 1), I may just be satisfied with it, as-is. Meaning, if there is an subtle hum at full volume, I won't worry about it. I won't be using it at full volume. Half volume at the most. But, I would like it clean all the way up, both channels.

    I've done quite a bit of troubleshooting by grounding grids. So, here's where I am. Using a couple alligator jumpers, one end of each connected to signal ground, I have walked my way back through each channel, from the common tie point at the grid of V3-pin7.


    Channel 1 troubleshooting, Volume at full, no input connections:
    1 - Ground jumper on C2/R3c, total silence. No hum or hiss of any kind. Lifted ground
    2 - Ground moved to the junction of C1/C6, the amp is very quiet with a very subtle hum. I can live with this one. I don't believe anyone would use an amp for recording at full volume. Lifted ground.
    3 - Ground V7-pin9, no real change in hum other than my jumper is acting like an antenna, because I get a tremendous hum. Lifted ground.
    4 - I made a grounding plug and put it in the open circuit J1. No change in original hum. Removed the plug.

    I did some tube swapping. I only have the two 6D10's that came with the amp. One of them will not work in the oscillator position. It checks just as high on my B&K 707, as the other tube. I can swap the two and the Vibrato (Tremolo) works. I have quite a few 6AC10's, and in the Sylvania spec sheet, it is a higher gain tube than the 6D10. So, I put the 'good' 6D10 in V7 and a Sylvania 6AC10 in V8. I like it better, as the channel brighter than before. Not that that's what I'm looking for. But, Channel 2 has octals on the front end, and is not as bright as Channel 1. I like having one of each. I put another 6AC10 in V7. I've convinced myself the 'Vibrato' is better. So for now, the 6AC10's are in the amp.

    I haven't pursued the hiss on Channel 2, yet. I will fix Channel 1 before moving on. Unless someone has something to try on it.

    Anyway, I'm at sort of a stand still, for now. I don't know how much troubleshooting I'll get done over the next couple of days. But, I'll be back on it again, for sure Wednesday afternoon.

    Please take a look and shed some light on something that I may be overlooking. I've included a pdf schematic of my amp. Thanks for all help.

    Edit: 26Dec12 - I put the revised drawing below.

    Jack
     
  2. stiles72

    stiles72

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  3. jnewmark

    jnewmark Supporting Member

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  4. 59Pan

    59Pan Graybeard Supporting Member

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    No I haven't posted there. Is there anyway to have this thread moved to that forum? If not, I'll go over there and post a link to this one. Thanks for the heads-up.

    Edit: Just noticed that it's a closed thread. So, I'll check here for any updates. Thanks.

    Jack
     
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  6. 59Pan

    59Pan Graybeard Supporting Member

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    Thanks for the link. I'm a member there, but as I have this problem posted here, as well as two other sites, I'll give it a little time for people to check for me. It's the holidays, so responses may be slow. Thanks again, for the link.

    Jack
     
  7. JimmyM

    JimmyM Supporting Member

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  8. 59Pan

    59Pan Graybeard Supporting Member

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    Thanks. Just posted there.

    Jack
     
  9. MAMMOTHvolume

    MAMMOTHvolume

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    I'm assuming you've replaced the anode resistors with metal film?
     
  10. 59Pan

    59Pan Graybeard Supporting Member

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    I keep wondering if the two 6D10's need shielding, but then I have to look back at what eliminates the hiss and hum. Channel 1 volume at full, Channel 2 volume off, grounding the junction of the Volume pot and C2 makes the amp dead quiet. That's at the third triode of V8. Grounding the junction of C1/C6 eliminates all the hiss, but leaves an ever so subtle hum. This is at the first triode of V7. I have replaced both C1 and R6 (metal film). No change.

    I eliminated a hum on the last amp, just by re-flowing the solder on both ends of a plate resistor. That was done here, with the new component installation.

    I'm taking a welcome break, today and tomorrow (Christmas Day). Sometimes, I focus a bit too intense, and have to back up and take a fresh look. That's where I am.

    Such a fine amp, and such a small issue. More to come. Have a good one.

    Jack
     
  11. beans-on-toast

    beans-on-toast Supporting Member

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    Let's assume that you've done a close inspection of the amp, cleaned and re-tensioned the tube sockets. Sometimes one application of a cleaner isn't enough. Sometimes you need to scrub away any oxidation with multiple applications of the cleaner. I use a product called DeOxit. It is the best stuff that I've found.

    In general, hiss and hum can be tube related. I would swap another or more in and see if it makes a difference. I tube shield might help. A small metal can might work for testing purposes.


    I took a look at your schematic. I noticed that V7a pin-10 is kind of low. The B-12XY schematic shows that this should be 60V, if your wall voltage is higher than 117 VAC, this will be even higher. Your reading was 46.2V. In general, a low voltage on the plate of one triode and a high voltage on another could lead to crosstalk with the tube. Not much you can do but which of the three triodes is where might make a difference.

    I'd look at the circuit at V7a pin-10 first and see if something can be done. V7a pin-4 looks good so something odd is going on there. Reflow the solder joints, check to see if the coupling cap (C1) is leaking, check the plate resistor, R6 to see if it is up to spec.

    The readings are supposed to be made with a 20,000 ohms per volt meter, no input, volume down, tone controls at noon. If the meter is not 20,000 ohms per volt, the readings will not match the reference ones provided on the schematic. You can use a shunt resistor if the sensitivity of your meter does not correspond to the required spec.
     
  12. 59Pan

    59Pan Graybeard Supporting Member

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    Thanks for the detailed reply. Tomorrow afternoon, when I get out to the shop, I'll recheck my voltage readings. When I took the ones in the chart, the pots were as you stated. Volumes at minimum and all tone pots at 12 o'clock.

    I do have some DeOxit, but have not sprayed the connections. Will do so.

    C1 and R6 have been replaced with a new ones, and no change in the noise.

    I've been using a Fluke 8062 DMM, but I do have a VTVM that I can use to compare readings. I don't remember the impedance of either. I'll check that tomorrow.

    I'll report back with my findings. Thanks ever so much for looking it over. I really enjoy troubleshooting these Ampegs. They are so easy to get to everything in the chassis. More to come. Have a good one.

    Jack
     
  13. beans-on-toast

    beans-on-toast Supporting Member

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    It would help if you measured the voltage on the other side of the 470K resistor, node D on your schematic. Also measure D at R81 just to make sure that they are the same and there wasn't a voltage drop after the node resistor at the cap. While your measuring node D, record the voltages at nodes A, B, and C in case they are needed.

    I find it to be a good idea to measure component values before they are installed. Sometimes that 470K resistor isn't what it supposed to be.
     
  14. Codger

    Codger Supporting Member

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    Does the amp still have the stock grounding scheme?
     
  15. 59Pan

    59Pan Graybeard Supporting Member

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    Yes, it still has the original grounding scheme where the Power Transformer secondary CT goes out through the speaker cable, completes the path through a jumper in the cab, and returns to connect to the signal ground on the eyelet board.

    Jack
     
  16. 59Pan

    59Pan Graybeard Supporting Member

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    Will do. I had considered adding those measured values to my schematic, but my attention span just ain't what it used to be. I will measure and add them. And, post them here.

    Jack
     
  17. Codger

    Codger Supporting Member

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    Your schematic shows it as the primary CT doing that though the schematic doesn't really show it going through the ground to the speaker and back, rather it looks like they are both tied to a common point - difficult to know without actually seeing the layout. I wonder why it was run through the jumper - safety feature of some kind?. For my lowest hum amps I have good success tying the power transformer CT directly to the neg terminal of the first PS filter cap. Nonetheless, if this was the original grounding scheme then it should hum no more than the original, though the need for a hum balance is perhaps a clue.

    Actually though, I was thinking more whether the other grounding was original, specifically the input group consisting of the input jacks, R3, R4, and R5? Similarly the 2nd group, R8, R9, R10 and C5? Are the input jacks grounded to the chassis or floating?
     
  18. 59Pan

    59Pan Graybeard Supporting Member

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    Actually, it does show it leaving the secondary CT. Look for the ground between the secondary and the rectifier tube. On the primary side is a ground from a switched capacitor used for hum canceling.

    I do see a discrepancy in my drawing. I drew in the cabinet plug terminals, but forgot to indicate them as such or label with plug pin numbers. Will correct that this evening. Look to the left of the standby switch and you will see the two plug pins and the jumper. I'll label the jumper and pins.

    Yes, it is a safety feature, to break the B- path, and protect the amp, in the event it is unplugged from the cab.

    Grounding is original to the amp, on the input jacks. Strangely enough, there is a signal ground from the eyelet board to pots, and those are jumpered to each other. Not having the amp here in front of me, as I'm writing, I can't say exactly how the jacks are grounded, though I do know that some are dependant on chassis for a path back.

    The B12N that I just finished up had no grounds hardwired to the jacks. I added those to it, and will add them to this amp.

    I'm confused by your listing of input jacks. The first input group resistors are R1, 2, 3, & 4. The second input group resistors are R36, 37, 38 & 39. Cap C5 is part of the Channel 1 Tone Stack, which is a combination capacitor of C3, 4, 5, & 6.

    Jack
     
  19. Codger

    Codger Supporting Member

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    Yes, of course you are right. I screwed the pooch on that one.

    I group the grounds associated with the first stage to include the grounds of the cathode circuit of the first preamp. Same with the second stage the stuff in front of it and its associated cathode resistor and bypass. This is the sort of grounding described by R.G. Keen which I have used successfully to eliminate hum in an amp that had the sort of problem you described. Everything was fine until I plugged in the preamp tube.
     
  20. 59Pan

    59Pan Graybeard Supporting Member

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    Ok. Updated drawing to reflect the cabinet plug pin numbers. Look for the cloud near the main speaker.

    Too much rain this afternoon to work. Can't hear even a super loud hum, with the rain on the metal roof. Should be clear tomorrow (Thurs) afternoon. I'll report my progress.

    Jack
     

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