Clean channel distorting, unsure of cause

[Ravenant211]

True enough! I don't know why I didn't even think of that... guess that's what happens when you are staying too busy

I do have a second tube amp (Fender Blues Jr.) that takes 12AX7s. I can test the preamps anyway. The power tubes are very different, but I kind of expect it is the preamp section/tubes anyway.

I don't see an issue with putting a volume control on the stethoscope, but I'll talk to the guy who will be helping me build it first before I stick my foot in my mouth.

[Ravenant211]

Been a few days so I thought I would post back here while I had a minute and update the status of my repair job.

At the moment I am at a slight stand still. We decided to try the listening device ampgeek had mentioned but unfortunately the local Radio Shack lacks one of the parts, the capacitor. After checking around locally and realizing no one else has them either, we've had to order the capacitor... We managed to scrounge everything else but that is the one thing we couldn't get a hold of... we thought of running a couple in series to get up to the voltage we needed but they don't have anything other than the most basic very low voltage capacitors around here. It's a shame to have to order such a dirt cheap item, but it's the price you pay for living in a small town I suppose.

I'm hoping to have the capacitor in within a 2-3 more days and then we can build the "stethoscope" and see what we can figure out. We've pretty much done everything else we can on our own at this point so hopefully this little device will be the magic bullet. We don't have access to the equipment or the $$ to buy them to do full standard testing so this will hopefully do it.

Thanks again to all, I'll post whatever happens up here for the benefit of others and to report on with our experience of the build and use of the stethoscope. I'm hoping it will work well as that would be a really useful and cost effective device to build. It make take me a week or two with schedules and waiting on the parts order to try everything out, but I'll let you know!

[nakedape88]

Wow. I'm so glad i found someone that has the same exact problem I have, with the SAME exact amp! Can't wait to hear what happens, hopefully you can point me directly to what I need to work on/replace...

Also i was wondering if you noticed a group of bundled wires that were directly touching one of the 6L6 tubes. I'm not at all knowledgeable about amps or their parts lol, but I did notice, while changing tubes, that it burned through the casing of those wires. No damage to the actual wires, but it was a scary find, and now I'm wondering why they did such a terrible job at putting this amp together...

And I'm also curious to hear about your custom speaker cab. How much did that cost you?

[Ravenant211]

Hey Nakedape,

Welcome to the club of defunct B-52s

I didn't have the wire issue that you were talking about in my amp. Maybe at the factory someone wasn't paying attention at quality control? I actually had my amp modded a fair bit a couple years ago as it was overheating the tubes at the factory default setup. There was no adjustable bias etc to do anything about it and I was getting maybe 2 weeks out of tubes max before they started to break up on the clean channel and just sound generally bad and fizzy. If your amp is still the factory default and your getting distortion on the clean channel, I would definitely suggest having a good amp guy in your area mod the amp to at least allow you to have a bias so you can let the tubes run cooler. I don't know why they do this so often from the factory but I also have a series II Fender Blues Jr. and I had to mod that amp as well for the same reason. Among other problems, the factory setup was running the tubes crazy hot! Ever since I got my ST100A modded it was running reliably and sounded great for a long time. I had mine on the road for a 2 month tour and I suspect it got banged around at some point, thus my strange problem now.

I had my amp modded by Torres Engineering which I believe is now closed. They had a kit just for the B-52 amps because it was a common problem. While they were in there they also replaced many of the capacitors where meaningful with orange drops and improved some other parts. We've noticed while we've been poking around that some parts of the amp it seems like they skimp on and then in other areas they do things that are typically only done in high end amps... doesn't make sense that they would be so erratic but there you go.

I'll definitely keep you posted. We're almost done building the listening device that ampgeek mentioned. Had to wait on a capacitor we had to order online and then we forgot about a couple parts we needed in our rush... like the alligator clips Looks like we'll probably have the device fnished hopefully tomorrow and then can start really getting in there. I'll definitely let you know what we find. It's hard to find info about ST100A heads anywhere!

As for the cabinet. I actually rebuilt just the cabinet for the head, not a full speaker cabinet. UPS was so kind as to drop something heavy on my amp while shipping it back from having the mod done. The amp held out from the accident very well but the cheap wood they make the original head cabinet out of took the brunt of it and cracked in half on both sides. Cost wise it probably only took about $50-$60 at the most (excluding tools like a jigsaw, drill etc.) and about 2 weeks of my evenings taking my time to get the measurements right. I used the original busted cabinet to get the proper dimensions etc. Attached is a few pics of the cabinet when I was done. I also used a decent strong plywood instead of the particle board they used on the original. The cabinet is MUCH stronger now... they should have used plywood to begin with!

I'll keep you informed, hopefully we'll have the solution soon!

[Ravenant211]

Hey everyone,

Especially since Nakedape is having similar proglems I didn't want to keep everyone in suspense about what's going on with my amp repair. I hate it when I am following a forum post that I need/want information from that just seems to die when the OP gives up or has a fix.

I'm still on hold with my amp at the moment. Work for me and the guy who is helping me with the repair has just been a little too hectic the past couple weeks. We've gotten together enough to finish the listening device which didn't take too long but haven't got to fire everything up and test the amp and tool yet. I waited because I thought we could get together today and I would have more to report but that fell through due to mandatory overtime at work for him.

I know some of what I could/need to test but not everything, he just has a much more solid and thorough understanding of circuit paths and operation than I do. I haven't dropped off the earth, I'm waiting on him to play it safe and will report in when it's all done. Sorry about the delays!

[Structo]

Hope you get it fixed soon!

That's wild about them using particle board for the head cabinet.

I don't believe I have seen that before.

Although I believe Vox uses quite a bit of it or used to.

Not only is it heavy, it tends to fall apart over the years.

[Ravenant211]

Thanks Structo,

Hopefully tomorrow will be the fateful day. Yeah I was surprised too. The B-52 heads aren't what you would call a premium tube head by any means, but they weren't exactly cheap either. I was disappointed that they used particle board but I guess it's getting to be more of the norm these days.

I also have a Fender Blues Jr. Series II and while it's covered and the interior is coated heavily with black paint, I'm pretty sure it is made of particle board as well. Perhaps not as much of an issue since it's not nearly as heavy as the B-52 head, but it makes me a little nervous after my previous experience. The thing is, when I bought the plywood sheet at the store, I didn't go cheap with it at all and it was barely more expensive than the particle board they had. In fact the premium 3/4 particle board was more expensive than the middle of the road plywood I bought. The plywood cabinet I built is MUCH stronger than the old particle board one. It feels very tight and solid with no creaking like the old one all while being a fair bit lighter too!

I used tight bond along with screws for the wood joints and attached the covering material with gorilla glue. It works excellent for this as long as you follow the direction on the glue about swelling. The covering was by far the single most expensive part of the rebuild. It was a generic kinda cheap looking black before, so I gave it a little personality. All said and done was really cheap to do. I called around to get a woodworking place to do the basic box shape for me and the cheapest place I could find to cut and bevel the pieces and assemble a rectangular box with two ends open was about $300!!! It wasn't that hard and was fun to do by hand.

[Ravenant211]

So we've had success on fixing the amp... sort of! The original problem has been corrected. We went over it again and fixed any joints that looked even slightly suspicious and also went over it again with contact cleaner in all the pertinent places thoroughly. When I hooked everything back up I no longer have any distortion on the clean channel and the distorted channel is now breaking up as expected and no longer has the notes getting cut off or poor sustain.

Somehow though a new issue has sprung and I'm hoping you guys might have some suggestions on where to look first. I'm getting a slight buzzing/humming sound now when the amp is on. It's quiet in Bypass mode and doesn't start until the amp is on. Doesn't matter whether the volume is all the way down or not, it's still there and is also there when nothing is plugged into the input jacks. It sounds very similar to 60 cycle hum so I'm thinking it could be a shielding or grounding issue but I'm not sure about that. It didn't do this before. The hum gets louder as you turn the volume up but doesn't overpower the notes as you play. It's worse on the distortion channel than on the clean channel and is also worse when the amp is in solid state mode as opposed to Tube A/B or Tube A settings. Switching the ground lift switch either direction doesn't make any difference and the amp is back in it's well shielded cabinet again but still hums/buzzes. It's not a crackling sound at all, it's more like a ground loop hum. Any ideas?

Also we tried to get the "stethoscope" working to test everything and built the device to spec. We weren't able to use it though as it was building up a charge wherever you touched something that was "hot" it was then discharging when you touched something else. Neither of us felt this was a good thing and the guy who's helping me said it's also very dangerous. Maybe ampgeek could comment on a way around this, but we looked over the info on the link that was provided and there is nothing to address this issue. The guy I'm working with kind of suspected that might happen but wasn't sure until we tried it. He thinks the only way to prevent it and make the device usable is to build a separate active voltage divider (I think I got that right?) or to build some different kind of probe he was talking about that I couldn't follow what he meant, was over my head! He's not eager to try either one so without some more information from someone more knowledgeable, and without a device to counter the charge build up I have to say my personal experience with the listening device was that it was unusable. I couldn't see if it does what it's supposed to and could be dangerous so I can't recommend bothering with it at all right now.

Since we didn't have the use of the listening device we just played better safe than sorry with any even slightly suspicious solder joint and then thoroughly cleaned it and that seemed to be the trick for the one issue anyway. Sorry it took so long to get around to this, some crazy work weeks have been getting in the way. Any further advice would be greatly appreciated!

[Structo]

If you can identify it as 60Hz hum then it is in the heater circuit or the heater wires are inducing the hum into the signal path.
Check to see that the heater wiring is away from the signal grids.

If the hum is 120Hz then it is most likely the power supply and filtering.

Most of the time humming is either bad ground connections or bad lead dress but there is always the exception to every rule.

[ampgeek]

Glad to hear that your primary issue has been solved!

Charging and discharging the DC blocking capacitor in the stethoscope (0.1 uF/600V) does not preclude its functionality in any way.

Regarding that charge, at 400 V a 0.1 uF cap only stores ~0.007 joules which is insignificant from a safety perspective. Unlike larger PS filtering caps in a tube amp (e.g., ~8 to ~50 uF), small value blocking caps have little "reservoir" capacity.

Obviously, the probe end of the device must be insulated to prevent direct bodily contact with high DC voltages.

Were you ever able to get an AC signal to pass through to the listening amp?

Although the applicability here has been overtaken by events, I just want to make sure that our less experienced bretheren recognize that this is a well functioning and safe tool when constructed and used properly. That is not meant to imply in any way that Revenant did otherwise.

Cheers,
Dave O.

[Ravenant211]

Thanks for the insight guys, we'll check into that and see what we can track down. I'm wondering if we might have knocked something loose when we used the contact cleaner and that is now resting on something it shouldn't. Some of the debris I mean. Is that also possible? We'll go down the list and see what we can find.

Ampgeek, yeah we did test the device before we plugged it in and it appeared like it would function properly. We were getting visible arcing and discharges when the lead made contact with the solder joints and felt it wasn't safe to continue. The guy I'm working with had mentioned that he could build a test lead that would solve it but he wasn't in a hurry to do so... getting parts of any kind around here has turned out to be very frustrating, we're pretty much forced to go with whatever the local Radio Shack has or order online, and our Radio Shack doesn't stock much. There used to be a nice parts store for tinkerers locally but they apparently closed 2-3 years ago.

On a side, I just realized today that you have a picture posted on one of your previous posts... this is the first time I've seen it! I don't know why but this board doesn't show the pictures for me 90% of the time. Even the cabinet build pics I posted earlier have only shown up for me 2 times in all the times I've visited the forum. Your picture looks different from the pictures in the link. We used a plain old test lead like they showed in your link. The lead in your pic is definitely different than the one used on the web link. I'll show your pic to the other guy and see if he has anything that we can switch out or make based off that one.

I don't want to imply that the device doesn't work, in fact it seemed that it would work nicely based on the tests that we performed before trying it on the amp. Everything was as expected. The web link is somewhat vague on some points though and I have no experience with this kind of thing, the other guy has lots of experience with these things, but has forgotten more than I'll even know as he doesn't tinker with this kind thing as much anymore. He had some concerns that they didn't address in the link and worked some of it out himself. Looks like you're device is very robust and well built. Maybe you should put up a DIY blog about the device, looks like you would do a better job at it

[ampgeek]

Hmmmmm...I must admit that I have never seen/heard a visible discharge from my probe tip even when going directly from a high DC voltage area (e.g., tube plate) to a low DC area (e.g., tube control grid).

Here is a link to the type of probe that I am using. Nothing special really. I like being able to clip it on to a component with a single hand.

http://www.radioshack.com/product/index ... Id=2062232

The tip is a relatively large, flat bent hook of wire. Is your tip conical and sharp? Having a "focused" point of discharge like that may make a difference. Alas...I am a mere molecular biologist whose knowledge of physics is only slightly more than skin deep!

Regardless, a small discharge shouldn't hurt anything in an all tube amp sans any "static" sensitive components. I'll bet it would make a pretty good "bark" on a tube grid! You wouldn't hear it though since the amp being tested is firing into a dead load.

Cheers,
Dave O.

[Ravenant211]

Hey Ampgeek,

He took one look at your pic and saw what he did wrong. Your cable going to the probe has the shield removed, which he says makes perfect sense. He wired it with the shield on the probe, just comes from how we've sandwiched in time to work on the amp. We've been trying to make the equipment and test the amp in rush for a just a few minutes every time we can get together. He's been working 60-70 hour work weeks as well and I don't know enough to know better, he pretty much just fuzzed out from overwork when he made the cable. He said a quick fix there and we should be fine with it working as you describe.

Thanks for the pic, I'm glad it showed up for me this time! I never knew you had posted that... Otherwise he built his pretty much identical to yours based off the pic you posted so we should be good.

We didn't get to test the hum issue much today, but we tried a couple things to remove external causes while he was here. We both believe it's a 60 cycle hum and he's wondering if maybe when we discharged the improperly made test probe if we might have done something to the filters on the rectifier stage. We're gonna look at the easy things first next time he's over with a meter and then if that doesn't solve it we'll fix the cable for the stethoscope and use it to test further.

You should see if you can have your pic added to that web link. Even if it's just a picture it was more informative than the whole article that was written by whoever. The guy who's working on my amp said that they used to make these devices in the old days too and called them "snoopers" rather than stethoscopes. Thanks again, I'll let you guys know the end result of this second issue as well.

[vibratoking]

Pics aren't viewable unless you actually login.

Don't know if this has been mentioned as a possible source for hum. I haven't read the whole thread. Do you have a CT on your heaters? If not, do you have an artificial center tap? If not, you may want to add one.

[Ravenant211]

Thanks for the heads up VibratoKing,

We're getting back into the amp on Thursday and we'll check about that as well. I would look right now but we put the amp back in it's cabinet wondering if the wiring was picking up noise when it wasn't in a shielded environment. For some reason still unknown to me I proceeded to mindlessly screw the amp back into the cabinet instead of leaving it loose. It uses those thread through kind of screws that come up from the bottom of the cabinet, pass through the amp chassis and come out the top of the chassis slightly and it's a major pain to get in and out of the cabinet.

I'll have it out by Thursday and ready to go for round two and we'll definitely add your suggestion to the list. Thanks again!

As for the pictures not showing, maybe it's something to do with my browser then. I always log in (have to) to reply and I still almost never see the pictures... hard to say. I have an app in Firefox called Firebug (useful for web developers and coders which is part of my job) and it might be interfering with pictures loading here. It can do all sorts of strange things to some sites when I forget and leave it running. Don't know if that is 100% it, just a guess.