Jump to content

Strange Fade-Out Problem


artto
 Share

Recommended Posts

One of my SET amps (integrated stereo 300B) has developed a strange problem. After it’s been on for about 1.5 hours the right channel starts to fade, then goes out all together. Sometimes it will fade back in briefly and then go dead again. Leave the amp off over night, next day same problem at aproximately1.5 hours.

 

Upon more detailed “physical” inspection this morning I noticed the right channel 300B’s base seems a little loose. I can twist it slightly with little effort.

 

The amp is about 4 months old, purchased new. Not more than a couple hundred hours on it.

 

Any idea what the probable cause might be? The 300B?, A bad driver tube (6SN7)? Or something internal?

Link to comment
Share on other sites

If the sound actually Fades in and out Volume wise then try to observe if any of the Filaments of the right channel Tubes are out.  I’ve had bad solder connections in filament circuit as well as bad tube socket connections and have even seen some bad solder connections on the cheaper china 6SN7 tube pins themselves.

 

I actually suggest swapping tubes from Left to Right channels so that if the problem follows the tube swap then you will find the intermittent tube if one of them is your problem. If problem stays with right channel then inspect (retention) tube socket pins otherwise suspect bad solder connection in filament circuit if they are going out intermittently as next likely culprit or somewhere else in the circuit is possible if the filaments aren’t the issue.

 

 

miketn

  • Like 1
Link to comment
Share on other sites

Yeah, it's under test right now. I swapped the L&R input tubes first. They're mid 50's Sylvania 6SN7 (fwiw JAN chrome dome bad boys). The originals were $hit.

 

The filaments were all lit when this happens.

 

So far they've been on for 2 hours today and no problem yet. 

  • Like 2
Link to comment
Share on other sites

Randy, it's a Boyuurange A50.

 

I bought several of these cheap Chinese SET amps to experiment with since I'm having so many supply chain issues for parts to get my older tube gear back up to par. Since I can "read" (schematic) but not "write" (design), I can find my way around to do some mods on these cheap amps. I'll probably get a pair of Cary CAD805 eventually. And I have a pair of Decware SE84 on order.

 

This is all about a little, somewhat unconventional experiment I'm conducting, so I don't want to let on as to what it's about. The inexpensive Chinese made amps are just part of the comparison and more readily available at the moment.

Link to comment
Share on other sites

I ran the amp for 6 hours yesterday with no problems.

 

However, that was after swapping the L&R input tubes, and removing/reinstalling the 300B's. I really don't like the way the tube sockets hold the 300B pins, especially the right channel. The 300B tubes aren't held very tight. When I'm changing amps/speaker connections it's easy to bump them and see the resulting tilt from my arm having slightly brushed against them.

 

At the moment I think the 300B pin sockets might be the problem. BUT, I would think that I would see something else, like Mike mentioned, with the filaments not being lit.

  • Like 1
Link to comment
Share on other sites

It's not unusual for a large tube like the 300B to tilt to one side or the other - and still be perfectly operational.  I'm looking at mine right now, and one is not perfectly perpendicular to the chassis either.  It was working just fine last night. The four pins don't have to be very tight; they need to make contact with the socket pins consistently, though.  Considering the physics of this, things expand in the presence of heat, which in this particular instance regarding the output tube socket, would probably just help improve the contact with is associated pins on the tube.  By the way, I think you mentioned the socket base being slightly loose, and you're able to gently move it side to side.  That's actually not that big of a problem, unless it got so aggressively twisted that the leads coming out of the base of the glass envelope itself become fatigued and break.  The socket base can be fixed, and there are a couple of way of doing that.

 

Removing and reinstalling the 300Bs may have cleared/cleaned oxidation or some other contaminant that was preventing adequate electrical contact, and so you found that you were able to listen without problems for several hours.

 

It's helpful that you can navigate around a schematic.  I know you've been here awhile atto, as have I under my old avatar; although I don't remember if you have some technical ability in terms of using a multi-meter.  This may be a case where tube socket re-tensioning might help, though.  If you do know how to use a meter, measure residual charge in reservoir capacitors to confirm they've bled down.  If you have the schematic, also check if there is a power supply bleeder resistor on the output of the power supply to common (ground).

 

CAUTION!  POTENTIAL SEVERE ELECTRICAL HAZARD  (Please understand this is not intended as an insult to your abilities, experience, or knowledge).  I can't count how many times over the past twenty years I've been associated with this forum that otherwise well-intentioned suggestions of working with high voltage equipment was expressed as if doing so was as harmless as playing with that old Fisher-Price farm toy, where the barn doors go 'moo,' like a cow, when you open them!  Clip one of the meter probes, and with one hand in your pocket (as I think Curious George (smartly!) mentioned awhile back), use the other hand to take the measurement.  If there is still any voltage present, LEAVE IT ALONE.  Let someone else work on it who knows how to bleed that voltage away.  A guy in Germany once sent me a large OTL amp to repair that uses a total of 22 tubes (I built two of those amps myself), and, since it came all the way from Europe, I was certain it would be fine to work on it, and that any voltages that were in the huge PSU caps would be gone.  I just did the usual quick check (some of you know what that entails, but I'm not going to describe it here), and POW!!! , accompanied by a bright blue-white flash. My workroom is upstairs, and my wife, who was all the way downstairs at the time, called up and asked if I was ok.  Assuming things is not the best practice (with anything), and can represent a potentially fatal mistake in the presence of high voltages - and especially high current.

 

With the amp turned off and DISCONNECTED FROM THE WALL SOCKET, and if you've confirmed that storage charges are no longer present with your meter set to DC, then (and only then!), and depending on the type of four pin sockets the amp uses, slightly adjust the contacts of the socket pins (gently) so that they grip the pins on the tubes just a little more tightly. It would be good at this time to re-tension all sockets, the signal tubes usually being a bit different from the 4-pin outputs.  I use Dental tools and picks for those.  You can also give them a short burst of De-Oxit spray (or other good electrical contact cleaner), following the directions for use on the can.

 

(I apologize for all the safety verbiage here.  I taught both college and public school art for over 30 years, and safety precautions were always important.  When you have a college-age junior (a Business major) ask you if his head would get burned if he stuck it in a 2,000 F ceramics kiln, one learns to not make assumptions).  Or the high school senior in a linear perspective drawing class who rose his 18-inch-ruler-holding hand and asked, 'How long is one inch?'  I looked at him for a moment, and could tell he was actually sincere.  'Ok, look at the end of the ruler opposite the 18" end on the right. Do you see that?  Great. Now follow the markings until you come to the number 1.  That's 1".  He was a great fella, and I thought very well of him - called his parents to tell them so, too.  One just never knows what someone else might or might not know.

 

There are a few other things that I know of that can cause this kind of problem.  Does the amp use a vacuum tube diode rectifier or SS diodes?  are there coupling capacitors between the input and driver stage and final stage (going to the 300B)?  What components are in front of the amp?  Are you using a line stage preamp ahead of it? The system is a chain, and in order to fix a problem you have to isolate the failing link.

 

edit:  Despite my otherwise being an absolute bimbo-fool with modern technology, I can type fast.  Lots of practice, obviously!

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...