Clean channel distorting, unsure of cause

[Ravenant211]

Hi everyone,

New guy here and I am hoping you might be able to direct me down the right path on what to look for in my tube amp that is having trouble.

I am getting undesirable unwanted distortion at all levels on my amp. It's by far the most noticeable on the clean channel, but it also seems to affect the character of the distortion channels as well.

It's kind of a spongy/raspy distortion that is present in all the notes. If I turn my guitar up to full volume, I can hear the full note get through but the break up is still there, but sounds pushed to the background. As I lower my guitar volume the note slowly gets more and more buried in the distortion, so it's almost like there is a certain noise floor of distortion that the notes have to get over.

Some other oddities, if I hit a note or a chord and let it just ring out, it sounds like it is "pulsing" like you had a very slow tremolo turned on. It could just be me but it also sounds like when the chord is the loudest (i.e. right after you strum) the pulsing is a little faster and as the notes decay the pulsing gets a little slower and deeper, almost like it's cutting out in the dips. Other than that it also sounds almost like their is a gate on the amp now, especially when I play with the guitar volume turned down a little. The sustain is pretty bad and it feels like the notes are being squashed and gating somehow.

The amp usually has a very clean sounding clean channel. It can be a little on the warm side but never breaks up at the volumes I use on the clean when it's operating normally, even on a chord strummed with astrong attack.

I tried switching out the power amp section of tubes with no change, then I tried the rectifier tube with no change (was planning to change all the tubes soon anyway). I kinda expected it to be the pre-amp section but I also switched out the Phase Inverter tube prior to changing out the V1 through V4 tubes,checked it, and it was still distorting. and so when I changed all the rest of the tubes out (including the reverb tube) it started operating normally. I played around for about 20-30 minutes to make sure and I was satisfied it was just old tubes. I turned it on today, just one day after, and now it has the exact same issue as before the tubes were switched out. I'm afraid that there is something more serious internally wrong that is damaging some or all of the preamp tubes. It was clean and normal for one day after I replaced the V1 through V4 so I am guessing it's something to do with that section of the amp.

I am very cautious with my tube amps, I always allow it to warm up in standby and cool down before turning the power off. It hasn't been moved more than 6 inches in the past year, just enough to get my arm behind it to replace the tubes. The amp is a modded B-52 ST100A. It's been modded to run cooler as they tend to overheat the way the factory sets them up and mine was going through tubes like crazy originally. It's also had a biasing circuit added and some of the capacitors etc that deal with that section switched out to a higher quality. After the mods it has been working great for close to 2 years now with one full tube set switch out in that time.

Any ideas about what could be happening? Sorry about the long post, I know details are important when troubleshooting these things and wanted to provide as much as I could. I can replace or repair whatever needs to be done, I just don't know enough about tube amps to have a realistic idea about what could be going wrong here. Haven't had much luck searching the net for a solution either. Lots about microphonic tube issues, but can't find anything about an issue like mine. Any help is appreciated and thanks for your patience!

Also I can probably record the amp if my description isn't sufficient enough to get a good idea of what is going on and post an mp3 or something up on here.

[Reeltarded]

You changed to known good tubes and the problem persists after a short period.

I would first suspect tube sockets in the 1st and PI positions of being dirty. Next, I would look to the other preamp tube, then I would consider a bad solder joint in those positions, an then I would consider it's the circuit related to whichever tube you find the trouble on, like cathode grounding, or even the gain control for that section.

Clean the tube sockets well. Try it. If it starts acting up again, pull then reseat each tube one at a time, trying the amp after each reseat. This will help focus on the problem area. Need toisolate the trouble no matter what it turns out to be.

And welcome to the funestest place on the whole interweb!

[Ravenant211]

Thanks for that! I will try the steps you suggest and see if I can isolate the issue. It would be great if it was just dirty tube sockets, though it does concern me that it was sounding great and then messed up again.

I'll try it out and see what I can find... I'll post back if I get a solution... and I'll post back with more questions if not

Thanks again!

[Ravenant211]

I got to try the initial cleaning of the sockets today but no luck there. There might have been a very slight improvement, but I believe it might just have been a psychological wish to have my labors reap a reward

I'm hoping to get the time on Tuesday to take the chassis out and clean the solder joints and inspect everything in the amp for crud, broken joints or cracks in the board etc. I'm hoping to spend some time tonight or tomorrow playing musical chairs with the tubes, switching them around to see if I can pinpoint which socket(s) are causing the problem as well. I'll report back if I can find anything definite.

[selloutrr]

do you have a scope?

[ampdoc1]

You might try hitting the components 1 by 1 with a freon spray. This can quickly turn up minor problems. A cheap substitute is the cans of "air" they sell for computers. The propellant is a freon type, so turn the can upside down to release just that.

a'doc

[Ravenant211]

Hey guys,

Just reporting in. I got to take out the chassis today and start looking over the components inside. Didn't get as much time as I would have liked though. It was pretty clean in there as far as dirt and oxidation goes. Very little of any grit at all and components seemed solid and of good quality over all (mostly from the mods I pad to have done I believe, Orange Drops replaced old components etc.).

I couldn't see any cracks on the board or obviously bad traces after fairly close inspection with a magnifying glass. I did notice that there are more than a few solder joints that appear to be in poor shape or that were just plain done badly to begin with though.

I don't know if they are bad enough to absolutely be causing my issues, but there are a few on both the preamp and the power amp sections where the tubes mount to the board. At least 3 joints appear to be cold joints. Didn't get the time to try redoing those today. Hoping to have a chance to wick and redo those tomorrow.

I will let everyone know how it goes after I fix those and test it. If that doesn't work there is nothing visually obvious on the inside that I am aware of that could be wrong. I have a multimeter, but no oscilloscope or anything like that so if there is any testing I could try with a meter that might reveal very hard to see issues I would be interested in knowing a wise course of action to take there. I'll report back and if re-soldering doesn't change anything then maybe you guys might have some idea of a direction to take for me?

Thanks again for the help and the leads so far, I'll keep you posted!

[selloutrr]

Can you split the amp into two parts Preamp / power amp and narrow down which side of the amp is generating the issue?

Not a whole lot you can do with a meter for this type of fix other then test impedance and continuity. A tube tester, scope, dummy load, and signal generator are most likely needed if nothing comes up in a visual inspection.

You can also try chopsticking the components as well as freezing them as mentioned above.

While you have the amp open clean the pots and tighten any bolts, screws, and sockets.

[Ravenant211]

Well, looks like I'm not going to get the time to spend working on the amp today like I had hoped. It might be the weekend before I get a chance to set down with it now.

I believe that it's the preamp section, but I guess I can't be 100% on that. When I changed the tubes I changed them on two different days. I had a set of power amp tubes already and a spare 12AX7 and 12AT7. I switched out the power amp tubes the PI and the reverb tube on day one and there was zero improvement.

The next day I went out and bought a new set of the 12AX7s for V1 - V4, plugged those in and it was clean and worked great for a total of one day (played it for about an hour before turning it off). The next day I turned it on and the distortion was back. Because of this I am thinking it's most likely the preamp section, though I guess it's possible a problem elsewhere in the amp is causing the preamp section to die/burn up tubes on me?

I'll also see if I can find a friend who has some other tools so I can try a little more in depth. I can set up a load for the amp and I might have a family member who has an oscilloscope, I might even be able to rig up a signal generator using an output from my PC (I have a noise generating software if that will suffice?)

I've thought often about getting a tube tester and may go ahead and buy one, though I'll have to order online. Definitely will not find one around here, much too rural to carry locally. Any recommendations on a decent but not excessively expensive tube tester? At this point it's hard for me to test the preamp theory because it's possible the problem has blown one or more tubes. I only have one more brand new 12AX7 and locally they cost about $20 a tube. I figure it's better to spend a little money on getting a tester and knowing which tubes I have are actually good. Blowing $80-$100 on a set of 12AX7s every time I hear distortion isn't very cost effective.

Thanks again for all the time on this! I would also appreciate it if you could tell me how best to put a scope to use for working on the amp? I am pretty sure I can get a hold of one.

[ampgeek]

Consider buying/borrowing an el-cheapo solid state "listening amplifier" (or any other known working amplifier for that matter) and building the stethoscope shown at the bottom of the page here:

http://www.el34world.com/Hoffman/tools.htm

Assuming you are comfortable poking around in a hot amp, you will be able to very quickly identify the problem area(s).

Just about any amp can be used as the listening amplifier as long as it can be kept clean. The 1 meg pot on the stethoscope is a must IMHO. The later amp stages will easily overdrive the listening amp's input so having local attenuation is a very good thing!!

You will also need to build a dummy load for the amp being tested but that, too, is an easy and inexpensive undertaking compared to multiple tube purchases, tube tester procurement and other potentially expensive test gear.

I have a mint TV7A/U tube tester but would give it up in a heartbeat if I had to chose between it and the stethoscope. Nothing compares, again IMHO, to being able to "test" a tube in true battle conditions.

Good luck!
Dave O.

[selloutrr]

if you are burning up tubes I'd look at what is causing your voltages to be to hot. use your meter to test the voltages going into the tubes.

I can't recommend a good and cheap tube tester. They sort of come two ways, Cheap or good. With that said you can find tester that are affordable. What you want to look for in a tube tester is actual operating voltages. so you know the tube will operate properly in your circuit.

how to scope a tube amp.
It's easier to read then type on a phone.
http://search.yahoo.com/search;_ylt=A0o ... tb-hptb5-s

[Ravenant211]

Thanks for the info guys,

Perhaps I misspoke when I said that the amp was burning up tubes. Not a good thing to do when you're asking for technical advice huh?

No tubes are burning out in an obvious way. I'm not even sure the tubes ARE burning out at all. I only have one spare preamp tube though at the moment and the little bit of moving around that I did of the pres in the amp didn't seem to help the distortion issue. I still don't know if the issue is causing damage to the tubes, or if it's internal to the amp and the tubes I currently have are still fine. I do know they were working properly when I bought them as I had a clean signal for at least one day. The tubes aren't a cheap brand and I would expect should withstand adverse conditions fairly well. Still without a way to test them, that is another area that I am in the dark about.

I see what you mean selloutrr, I did a search of tube testers yesterday and today and to be honest I don't know if they are within my price range. I couldn't even find any new testers anywhere I looked. I was always steered toward eBay in the end no matter what link I followed where they fell into two camps. Testers that are really old that could be purchased for cheap, but the seller didn't know anything about them and could only verify that they power on but have no idea whether they function properly OR really old testers being sold by people who know what they have and how to use them, can verify that they work and are selling them for $250 and up.

The verified working ones I can't really afford and the cheaper ones are too risky since the sellers don't know anything about them and are selling as is. Looks like I'd have about $100 on average on an unknown anyway by time the auction is done, not to mention having to wait 3-7 days just to find out what you have to pay and whether you win or not. I have 2 tube amps but I don't think I can justify spending so much on a tester that I will hardly ever use. I don't scrounge tubes normally, I keep a brand new backup set of preamps and that is about it. Otherwise I buy as I need them. I didn't realize that tube testers were so hard to find. I thought there might be a company or two who sold a "musicians" tester that had the common tubes to amps. Nothing fancy but in the $150-$200 range max. If I spend even $200, between some other gear I have for trade in, I will already be getting close to the range of being able to buy a new tube amp at my local store. Especially if I end up having to replace components in this amp on top of buying a tester. All in all its looking like a route I can't really go down at this point.

ampgeek, cool concept! It looks like it would be a fun project even besides being an efficient, effective tool. How much would you say that you would have in that setup grand total? I'm asking because it's going to come down to a money factor for me I believe. I do have some sentimental value attached to this amp between the mods that were done to it and the fact that I built a new cabinet for it, but I am limited on my cash supply. I can't really afford to sink too much money into anything that is of limited use to me. Building that stethoscope would be fun, but if it is somewhat costly, honestly I'll rarely ever use it as I don't switch amps out a lot and don't trouble shoot others amps. If it's fairly cheap then what the heck but if it's pricey I might be better off saving the money for a trade in, in the event I can't get this one up and working.

I am going to fix those cold joints tomorrow or Saturday and then I'll check it out. If that doesn't work I'll see what testing equipment I can get a hold of an trouble shoot further. I might have to start considering the likely hood of getting a new amp if it goes that far though... There is no one reliable around here to work on the amp if it ends up being over my head and the last time I tried shipping an amp UPS busted it to pieces for me very nicely

Thanks for the continuing input. I'll let you know how things stand after the soldering adventure!

[ampgeek]

Yep! It is a really nifty tool for sure. I must have spent $50 in pieces/parts and waaaay to many hours chasing a harsh distortion in a Dumblesque build a few years back. Whipped one up and went right to the problem in less than 30 seconds. Changed out a funky/noisy plate load resistor and...done!

About $20 at the Radio Shack should get you there with the scope as shown below. That will largely depend on what the minimum quantities of wire you can procure and whether or not you have a soldering iron and have/can borrow the listening amp.

The amp load resistor might be another $10 if you can pick up the resistor locally to avoid shipping charges.

What is the output power and/or output tube compliment for the amp in question? I am not at all familiar with the B-52 line of amps.

Dave O.

[Ravenant211]

Hey ampgeek.

The B-52 is a 100watt amp. 4 6L6s are used in the power amp section. Don't know if it would make one bit of difference for what you need to know on gauging things but it also has a 5AR4 tube for the rectifier stage and has a Class A/AB/Solid State switch.

I'll have a family member of mine who is much more experienced with building circuits and kits of that nature from scratch take a look at the info from your previous post. No worries getting a hold of a soldering iron, I have 2

The price seems pretty good... might be able to round up a fair bit of wire as well without having to pay for most of it. The listening amp is the catch on cost maybe. Does that amp have to be modified in any way? I have a solid state kinda crappy Kustom head I could use if it doesn't need to be modded or won't damage it. I plan to trade it in on some other gear in the near future. I only have it as a backup amp for when I was still touring around with my band. Both of us guitarists had a cheap solid state head just in case our tube amps got damaged or banged around too much. The amp is pretty crappy but I could probably get $200 or so on trade in so it has some worth to me to not get damaged.

If all the listening amp needs to do is send/receive a signal of some sort I also have a small recording set up and could generate/send almost any signal I needed from that as well.

[ampgeek]

Nope. Nothing special required of the listening amp. No chance of damaging it if the stethoscope is built/implemented properly.

Again, I urge the inclusion of a volume pot on the stethoscope to, among other things, avoid inadvertently blasting the input of the listening amp. It isn't required...but...it makes the signal tracing process a lot easier.

The solid state Kustom head would make a fine choice for the listening amp.

Also, another known to work tube amp can make a nice tube checker if you are questioning tube viability.

Cheers,
Dave O.