1 tube Red Plating on KG-250

shimniok

shimniok

Super Member
Thanks for your patience with a tube noob. I know this has been asked before and I tried to educate myself before posting. Wanted to check and see if I am on the right track in fixing a red plating problem.

I have a Knight KG-250 integrated amplifier and just got a set of new "Winged C" 6F3P output tubes for it (6BM8 = ECL82 = 6F3P). The amplifier was recapped and gone through by one of my local pals / AK members. I tested on dim bulb and it seemed to be ok. On line voltage, however, one output tube started glowing red and quickly grew crazy f'n hot. I turned it off within a few minutes. Didn't notice right away.

I'm new to tubes and I did some searching on here. -- do I have the following correct... ?

I gather red plating is due to too much current/voltage (ie, too much power). It can be caused by loss of bias voltage (which is supposed to be negative, but if it goes 0 due to broken wire, or goes positive, can allow too much current). Can be caused by short circuit in the tube, or by coupling capacitor gone bad (presumably allowing AC to reach bias or...?), or by HF oscillation. Is that right? Am I missing anything?

Some things I plan to try -- suggestions welcome...

Testing the tubes for shorts. I will *try* but unfortunately the docs for my tube tester are incorrect for this particular tube type.

Checking socket hookup/wiring against schematic & other channel

Tube swapping -- is it the amp or the tube that has the issue? Will this burn up a tube if I shut it off within a minute?

Checking bias voltage -- quickly and safely
 
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Try replacing the output tube coupling caps to all the output tubes. More often than not this will be the cause of redplating due to the bias going out of spec from the caps.

Then test, and go thru the rest fo the caps, test the tubes.

Larry
 
I'd swap the tubes between the channels and then turn the amp on in a very dark room. First sign of pink turn the switch off. What you find will narrow the possibilities and make the search for the problem much easier.

Post a schematic or link. It would be interesting to see how the Knight is set up compared to my Kenwood with 6BM8s.
 
Schematic I got from Mark is attached. Will try swapping tubes & let you know what happens.

I was able to test the tubes for shorts and all is ok there.

(I got mixed up; I can't test my fisher 400 tubes, *can* test the KG250 outputs).

All caps are brand new but maybe one is bad or something. :dunno:

Michael
 

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The problem stays with the one socket after swapping tubes around.

Also there's a weird sort of humming-buzz/buzzing-hum for a few seconds that immediately goes down in pitch, then disappears, shortly after turning the unit on. I suspect that is related. Maybe an oscillation I dunno.

When I say red plating what I see is that the visible grey plate starts to glow in the center after a few minutes of warmup as if it is getting ridiculously hot. About the same red color as a piece of metal heated up with a welding torch.

Michael
 
shimniok said:
The problem stays with the one socket after swapping tubes around.
Click to expand...

So now you can check the bias on that socket and figure out what is going on. Your schematic is a bit too fuzzy at higher resolutions to point to any specific resistors and caps.

About the same red color as a piece of metal heated up with a welding torch.
Click to expand...

Kinda the same thing only in this case it's electrons and not flame causing the heat.
 
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Mike, when I got that amp it had been fiddled with. It actually hadd loose/disconected wires inside. I did not trace out any circuits or anything. I simply replaced all the resistors and all the caps in the amp and ps circuit. I did nothing to the preamp circuit.

And, if you remeber, I tried running that amp with el86's(STUPID ME). Had one of those brain dead moments that lasted maybe a couple of hours.

So, possible a resistor went poof. Might check that.
Also, just check the side that works against the side that doesn't. See if you can see any difference in the value of resisotrs or hoe the components are hooked up.

Good luck!
 
BTW - For those who want to help. I did replace all of the caps in that unit about a year ago. I used brand new 716p orange drops. I would be very supprised if it were a cap. Most likely it is either wired wrong, or damaged from my incompetance.
 
I found a higher res pic of part of the schematic reproduced in Eagle -- see attachment. Should come up full res; may need to click again to 'zoom in'

I found several possible issues so far.

  • The 820K resistor R-19 is hooked up to pin 6 not pin 3 of V2. However...
  • The 820K resistor on the same tube for the other channel is also hooked to 6 not 3. But it's not red plating that I can see. :scratch2:
  • R16's color code says it's a 150 ohm, not 1.5K; have not measured to verify
  • R20 the 1M is hooked to pin 1 on V2 instead of pin 1 of V3. It's hooked to the same pin 1 that connects to the 270K resistor to the treble control. It should be hooked to the pin 1 that has R19 (820K) connected.
  • Blue and Brown audio transformer wires are swapped. Blue is hooked to V2 instead of V3 and Brown to V3 instead of V2.

Thoughts?

Michael
 

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Jcricket said:
Mike, when I got that amp it had been fiddled with. It actually hadd loose/disconected wires inside.
Click to expand...

Hey Mark thanks for posting. That is interesting info! I think that the fiddling that happened prior to your getting it is to blame. :D Don't be so hard on yourself. The cap / resistor replacement work looks great!

Jcricket said:
I did not trace out any circuits or anything. I simply replaced all the resistors and all the caps in the amp and ps circuit. I did nothing to the preamp circuit.

And, if you remeber, I tried running that amp with el86's(STUPID ME). Had one of those brain dead moments that lasted maybe a couple of hours.
Click to expand...

I ran the thing with the wrong tubes too and smoked something... probably a resistor. Oops. The only website I found listing the unit incorrectly states that it uses ECL86's. Doh!! (The schematic calls for 6BM8's)

Just checked and the other side of the amp also has the same issues with the 820K, 1M, and blue/brown audio transformer wire hookups.

Hopefully if I can just rewire and verify the resistor that I smoked, all will be well...

Michael
 
shimniok said:
I found a higher res pic of part of the schematic reproduced in Eagle -- see attachment. Should come up full res; may need to click again to 'zoom in'

I found several possible issues so far.

  • The 820K resistor R-19 is hooked up to pin 6 not pin 3 of V2. However...
  • The 820K resistor on the same tube for the other channel is also hooked to 6 not 3. But it's not red plating that I can see. :scratch2:
  • R16's color code says it's a 150 ohm, not 1.5K; have not measured to verify
  • R20 the 1M is hooked to pin 1 on V2 instead of pin 1 of V3. It's hooked to the same pin 1 that connects to the 270K resistor to the treble control. It should be hooked to the pin 1 that has R19 (820K) connected.
  • Blue and Brown audio transformer wires are swapped. Blue is hooked to V2 instead of V3 and Brown to V3 instead of V2.

Thoughts?

Michael
Click to expand...

THoughts? Well I think you have just found a whole mess of problems. If I remeber correctly, that ampo looked pretty good - like maybe it was assembled wrong from the get go and was rarely used - maybe shelved becasue they couldn't get it to work right,
You are definately onto something.

BTW - if you need parts I likely have everything on my bench. Just let me know.
 
Hmmm... that wouldn't surprise me. :scratch2:

So, I got the amplifier section rewired according to the schematic -- after goofing up a couple times. The one resistor measures 1500 after all.

Tried it out. Now I get virtually nothing out of one channel except a weird buzzy/squealy tone that changes pitch whenever I move the volume knob.

The other channel emits sound... not loudly but maybe that is normal, I don't know.

None of the tubes are red plating. They glow orange and two of them also have a blue glow.

Sounds like I should basically go through this thing stem to stern and verify all the wiring... so, hopefully I can locate a hi res schematic. I PMed the guy you got yours from, Mark. One of the online manual websites has one for $22... but you never quite know what you're going to get with those.

On the other hand, I do have the wiring instructions. I could cook up the schematic from that I suppose.

Michael
 
I just finished checking my emails, files and other piles of stuff. No luck. I thought I had a hard copy. Sorry!!
 
It works!! My first tube stereo. Too cool.

Turns out the weird buzzing has to do with the leads to the audio output transformers. More on that in another thread. With the leads in just the right positions both channels are silent.

Currently playing one of the most amazing music captures I've ever heard-- Muddy Waters, Folk Singer. Playing thru DCM TimePieces.

Man this sounds nice. More detail and low end than I expected out of this humble little amp... only listened for a few minutes so will have to leave full impressions for another thread.

Michael
 
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