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 Post subject: Of all the stupid things...
PostPosted: Dec Tue 04, 2018 11:41 pm 
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Joined: Jul Sat 28, 2012 9:32 pm
Posts: 162
I didn't see a category for stupid stings, so I'm taking a chance I can post it here. My buddy bought himself a nice Sherwood S 5500 II amp got it today. So, he plugs it up and runs it for 20 minutes with no speakers attached. Then, while running, he proceeds to begin hooking up speakers, hooking one leg to the 8 ohm connection and the other to the 4 ohm connection. Immediately one of the 7868 output tubes flared up and was GONE... history... as in kaput. Instead of my risking my friendship with this good buddy rubbing his nose in it, I'll leave it to you guys to opine as to what he has left of an amplifier now, as he had to go to work and has to spend 8 hours on the job wondering what he will come home to.


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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 12:21 am 
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Location: Ohio 45177
Unless the Asians or Russians make that tube, and they may, he could be in for a nice set of new tubes for as much or more than the amp, I suppose.


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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 12:25 am 
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He's already waiting on a quad set of NOS Soviet tubes. It's the output Xformer I'm worried about.


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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 1:02 am 
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I'm not so sure that hooking up one of those speakers, the way your friend said he did, would have caused that tube to fail. What about the other tube on that channel? Those tube amps with output audio transformers were built like tanks and it would take a lot more then just hooking up an 8 ohm speaker between the 8 ohm and 4 ohm connections to do all that damage. That's just 4 ohms, no different then hooking up a speaker between the 4 ohm and ground connections. Many speakers say that their 8 ohms but if you actually ohm them out with an ohm meter, more likely they'll show around 6 ohms. I get the feeling that there's more to this story then what was said by your friend or what he understands about the situation. It could be that with all of the physical jostling around while the amp was on may have stressed out an old and tired tube to the point that it just gave out right then and there.

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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 4:16 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5160
Location: Rochester NY USA
Running the amp without speakers is unlikely to cause a problem. Driving it into clipping without a load would likely arc over a transformer or tube socket. A tube with grid leakage might take a while to run away - it gets worse as it heats up. A red-plating tube would take a few minutes to overheat an output transformer, though the fuse would probably go first.


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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 4:21 am 
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Location: Central PA 16801
there is either more to this story or the tube had a bad case of grid emission or a catastrophic short.

:arrow: :arrow: *some* of the new production 7868 tubes have fatter pins than original USA made 7868 tubes.

I never experienced this, but I was told about it in someone's horror story within my circle.

when these soviet fatter pinned tubes are placed into the sockets, they will stretch the metal contacts.

have him make double darn sure that the pin size of the soviet tubes match the diameter of the original tubes.

again, I was told this and never experienced it since I've only ever used USA 7868 tubes.

read this post:

http://www.antiqueradios.com/forums/vie ... w=previous

I absolute loved the sound of every amplifier that I ever repaired/restored/heard that used 7868 tubes, but they are very expensive, driven hard, usually weak/need to be replaced, and are very prone to grid emission. I had a few NOS tubes red plate right out of the box. the problem followed the tube in several perfectly restored amplifiers.

steve

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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 4:29 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 10907
Location: Powell River BC Canada
The Sherwood 5500 uses a rather complex output transformer
secondary feedback system that might have gone into oscillation
by not hooking it up as instructions say.

I seem to remember that amplifiers with an 'idiot' speaker phasing
switch and feedback loops like this one gave trouble back in the day.

Anyway, here is is the schem of the output.

Tubes blown ? I'd rather worry about a flashed output transformer .
That used to happen a lot, and while not killing the channel, forever thereafter
they never quite sounded the same.

Attachment:
Sherwood S5000 speakerfeedback  circut.JPG
Sherwood S5000 speakerfeedback circut.JPG [ 58.15 KiB | Viewed 1290 times ]

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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 4:34 am 
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Location: Texas. USA
Indiana Radios wrote:
I'm not so sure that hooking up one of those speakers, the way your friend said he did, would have caused that tube to fail. What about the other tube on that channel? Those tube amps with output audio transformers were built like tanks and it would take a lot more then just hooking up an 8 ohm speaker between the 8 ohm and 4 ohm connections to do all that damage. That's just 4 ohms, no different then hooking up a speaker between the 4 ohm and ground connections. Many speakers say that their 8 ohms but if you actually ohm them out with an ohm meter, more likely they'll show around 6 ohms. I get the feeling that there's more to this story then what was said by your friend or what he understands about the situation. It could be that with all of the physical jostling around while the amp was on may have stressed out an old and tired tube to the point that it just gave out right then and there.
8 Ohm speakers measure "around 6 ohms" because that's the RESISTANCE whereas the 8 Ohm rating is IMPEDANCE (at frequency). They're not the same thing.

While the impedance ratio should be about the same wiring them between the 8 Ohm and 4 Ohm taps they're not electrically the same because of where the feedback tap is taken off which, of course, is (usually) referenced to ground. Exactly what the results would be I can't say but it surely won't work as intended. Seeing as how the amp in question self destructed I'd guess some catastrophic instability ensued.


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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 5:28 am 
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Joined: Jul Sat 28, 2012 9:32 pm
Posts: 162
I trie posting a photo of the speaker terminals because they really are confusing. Upon trying thay it all came back to me. Posting photos on this site is hopeless.


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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 5:58 am 
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Joined: Feb Wed 04, 2015 12:26 am
Posts: 811
I own two of these amplifiers and it's actually quite easy to make a mistake with speaker connections if youre not familiar with them.

Your friend needs to give this to someone that knows what they are doing if they didn't destory the O/T.


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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 6:12 am 
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Joined: Jul Sat 28, 2012 9:32 pm
Posts: 162
I have every confidence it will be destroyed. He's a good auto mechanic but approacheseverything else like a bull in a china shop. I've been teaching him electronics for 20 years but have yet to even convey the concept of voltage.


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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 9:57 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5563
Location: 253 Blanche St. Plymouth, MI USA
henry finley wrote:
I have every confidence it will be destroyed. He's a good auto mechanic but approacheseverything else like a bull in a china shop. I've been teaching him electronics for 20 years but have yet to even convey the concept of voltage.


I always teach voltage as if it was height. A 6' person is say 6 volts "above ground" or POSITIVE. His feet are zero, his knees are 2' above ground. Put him in a hole 6' deep and his feet are NEGATIVE 6volts with RESPECT to GROUND. His knees are what... minus 4 volts, his head is now ZERO volts. He is still 6 ft tall, but can be measured many ways. This is the concept of BIAS voltage too.
Mark Oppat


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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 10:01 am 
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Joined: Aug Tue 24, 2010 8:56 pm
Posts: 5557
Location: Northeast Florida
I just want to know why he ran it for 20 minutes with no speakers connected? What was the point of that?

The next time you talk to him, you should tell him that tube amps should never be operated without a speaker load connected

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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 4:34 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4206
Location: Boston, MA USA
I had an S5000II from 1968 to 1982 and it did this repeatedly. It first suffered a "combustion event" while in the hands of its original owner, a friend of my parents. He gave it to me, and I rebuilt the output section and used it for several years. But I always had to be watchful for another event -- a loud click and a low hum. Kill the power immediately and it would be OK, and then find which 7868 suffered the G2 to G1 short. This happened four or five times over the years. Once I didn't catch it in time and another combustion event occurred. This time it was into the dumpster.

I think it is a combination of 7868 design, weird angle of installation in the S5000II, and possibly excessive G2 dissipation. It's not a good design although it's an excellently sounding amplifier when working.

-David


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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 4:59 pm 
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Posts: 14270
Location: Fernandina Beach, FL
Tom Bavis wrote:
Running the amp without speakers is unlikely to cause a problem. Driving it into clipping without a load would likely arc over a transformer or tube socket. A tube with grid leakage might take a while to run away - it gets worse as it heats up. A red-plating tube would take a few minutes to overheat an output transformer, though the fuse would probably go first.

Tom,

"Yes, yes, yes and yes". I have repaired a few amplifiers that had arced output tube sockets. This forum is what told my why and then it became clear. Lucky for me, they were point to point amplifiers not printed circuits.

Just too many variables here...

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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 7:10 pm 
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Joined: May Sat 30, 2015 11:49 pm
Posts: 614
Location: Keystone Heights, FL, USA
Posting pictures...

1. Find your picture and resize it to no more than 800p on the longest side; file size no larger than 256 kb.
2. Scroll down to the "upload attachment" window.
3. Click browse.
4. Find your picture and click on the file name.
5. Click "Add the file".
Attachment:
Screen Shot 2018-12-05 at 12.59.01 PM.png
Screen Shot 2018-12-05 at 12.59.01 PM.png [ 42.73 KiB | Viewed 1113 times ]

6. Put your curser where you want the picture to appear.
7. Click "Place inline".
Attachment:
Screen Shot 2018-12-05 at 1.05.25 PM.png
Screen Shot 2018-12-05 at 1.05.25 PM.png [ 65.34 KiB | Viewed 1113 times ]

8. Done deal.


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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Wed 05, 2018 9:17 pm 
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Joined: Jul Sat 28, 2012 9:32 pm
Posts: 162
I have already scaled the photo I want to post. It's 600 pixels on the longest size and the file is 92.7kb and it is a jpeg. Like I said--I learned years ago that trying to post a picture on this site is hopeless. It's too big, so you size it EXACTLY like they say, then it's too small. Or some othe stupid reason. None of it has ever worked or ever will work.


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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Thu 06, 2018 1:10 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 10907
Location: Powell River BC Canada
One reason I know of that will stop a picture from being posted, even if it less the 800K
and one dimension is less than required, is to have the name contain anything but letters.

BiGpicture OK
BiG/Picture Nope



Attachment:
BiGPicture.JPG
BiGPicture.JPG [ 10.61 KiB | Viewed 1051 times ]


Attachments:
BiGPicture.JPG
BiGPicture.JPG [ 10.61 KiB | Viewed 1053 times ]

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de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca
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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Sat 08, 2018 2:27 pm 
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Joined: Mar Tue 30, 2010 2:24 am
Posts: 8190
Location: Lehighton, PA.
henry finley wrote:
I have already scaled the photo I want to post. It's 600 pixels on the longest size and the file is 92.7kb and it is a jpeg. Like I said--I learned years ago that trying to post a picture on this site is hopeless. It's too big, so you size it EXACTLY like they say, then it's too small. Or some othe stupid reason. None of it has ever worked or ever will work.

Rename the extension jpg.

Larry

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 Post subject: Re: Of all the stupid things...
PostPosted: Dec Sun 09, 2018 4:03 am 
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Posts: 8007
Location: Ohio 45177
It sounds like a cool amp to have, but if they have a tendency to blow up and eat expensive tubes, I will stick to my Fisher.


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