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Old 15th May 2011, 04:44 PM   #1
JTCamp is offline JTCamp  United States
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Default K12-G tube amp problem

Hello everyone,

I am new here but have referenced here many times for help and browse for the pure enjoyment.

I have the amp listed above and it is about 1 1/2 years old. I am having a problem that I would like some advice on.

Last night I turned the amp on for the first time in about a week. I noticed that the left channel was louder than the right. This has happened before and it was the volume pot. I removed the pot and tried the amp with no volume control...the problem still existed. I then removed the amp from the receiver and used the receiver to drive the speakers. Normally the amp powers the speakers and get it's input from the receiver's preamp output. Using the receiver only the problem went away. I then swapped the left and right channel tubes and the problem moved over to the right channel. Does this sound like a tube problem? What can I do to confirm?

I was frustrated and wanted to make sure I was hearing correctly so I used a low frequency bass track to better hear any differences between the channels. After a few minutes the left channel died. By the way I had switched the tubes back to their original configuration. What happened?

I went to bed and this morning, to my surprise, the was working again on both channels. However, the amp is distorting at a much lower volume than ever before. It isn't that powerful but normally it will drive the speakers more clearly up to a higher volume. Is this tube damage?

For what it is worth I performed most of the modifications here:
S5 Electronics K-12M Tube Amp

I also installed Hammond PT 1609 output transformers.

What do you'll think? Thank you very much.

John
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Old 15th May 2011, 05:56 PM   #2
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Hi John - Welcome to the Forum


Quote:
I then swapped the left and right channel tubes and the problem moved over to the right channel. Does this sound like a tube problem? What can I do to confirm?
As the problem moved with the tubes looks like you have already confirmed it. Whether the problem has originated within the tube is another question. My worry would be the possibility of damage to your nice 1609's if you continue to run this amp as is.

Have you noticed any unusual red hot spots on the metal plates inside the tube?

I would want to know if the tubes weren't conducting excessive current.

Did you do the ccs mod and if so can it be easily exchanged for the original cathode resistor in order to rule out a faulty ccs?

I'm afraid I'm raising more potential problems than solutions here.....


Bill

Last edited by Soonerorlater; 15th May 2011 at 06:02 PM.
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Old 15th May 2011, 06:03 PM   #3
JTCamp is offline JTCamp  United States
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Bill

I did not do the CCS modification and I have not noticed any hot spots on the tubes. How would I go about determining the current at the tubes or testing the tubes for that matter? Among my tools I do own a nice Fluke multimeter, but unfortunately no tube test or scope.

If new tubes are in order, would there be any gain to going with a different type or am I showing my ignorance of tube circuitry and their matched parts?

John
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Old 15th May 2011, 06:47 PM   #4
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The fluke will do just fine for checking the bias voltages on the tubes.

I have the schematic somewhere and will get back to you when I've refreshed my memory.

Someone more knowledgable ( and there are many .... ) may chip in before that.

IIRC your tubes uses 10 or 11 volt heaters so you dont have a lot of choice here.

Incidently I built the K502 kit which is very similar to yours and I also have put in the 1609's - big improvement. It uses the 11bm8 tube.
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Old 15th May 2011, 07:35 PM   #5
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Hi John,

Looks like your amp uses the 10gv8 tube - is that correct?

If so then I'm probably looking at the right schematic - I would post it but I'm not sure if that would be in breach of copywright.

Looks like the two pushpull output pentode sections in each channel have their cathodes tied together with a single 150 ohm resistor ( R8 and R17 ). Measuring the voltage across this resistor will give us the current through it ( from Ohms' law ). Unfortunately this will only give us the total current supplied to both pentodes and not their individual current used. We will though be able to compare the current usage of one channel with the other and if it's around the expected value. Will need to look up the specs for your tube and get back to you. No music should be playing while these measurements are taken.
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Old 15th May 2011, 07:56 PM   #6
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Bill

You are correct, they are 10gv8 tubes. I measure across R8(right) and get 16.86 Vdc and across R17(left) and get 16.96 Vdc.

The odd thing is that earlier this morning it was playing again, both channels that is, as opposed to one. The "power" doesn't seem to be there though. Poor bass response and distortion at what seems like a lower volume than I recall previously.

Thanks again, this is great.

John
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Old 15th May 2011, 10:39 PM   #7
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10GV8 Bias point for the K-12G


Having read the above thread I'm persuaded the 10gv8's are being driven too hard. Perhaps (in the short term) investing in a set of new valves would cure your problem. If the problem still persists you can start to look elsewhere. In the longer term I would want to reduce the cathode current, perhaps with a ccs. I wonder how hard other K12G owners are driving their 10gv8's

Brgds Bill
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Old 15th May 2011, 11:24 PM   #8
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Quote:
The odd thing is that earlier this morning it was playing again, both channels that is, as opposed to one.
The amp has had chance to cool down overnight after the tough workout you gave it with the high energy low frequency bass track. May indicate a temperature dependent fault though.
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Old 16th May 2011, 04:13 AM   #9
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Thank you for the thoughts, I might just be replacing some tubes.

So, if I understand correctly, at idle these tubes are at 112 mA on the right and 113 mA on the left? But this is through both correct?

Sounds like the threads regarding this tube indicate an ideal amperage of around 50mA? I wonder how others are driving theirs as well.

The CCS idea isn't something I'm familiar with, other than looking it up to answer your question about whether I had performed the modification.

John
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Old 16th May 2011, 10:39 AM   #10
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Quote:
So, if I understand correctly, at idle these tubes are at 112 mA on the right and 113 mA on the left?
That's what the maths tell me also......


Quote:
But this is through both correct?
Yes, and even if we make the assumption of the current being shared equally between the pentode sections that still means around a 10% increase over the " ideal " 50mA.

Perhaps somebody more knowledgable could have another look at the 10gv8 load lines to see if 50 mA at 210vdc (my assumption of your B+) is indeed what you should be aiming for.

Constant Current Source (ccs) - In some amps the cathode resistance can be adjusted so that as the tube ages it can be kept within its operating range. A way of getting around this adjustment is to use a ccs. There are arguments for and against the use of a ccs in the output stage but IMHO they are convenient. Just to hear the difference a ccs would make I replaced my K502 cathode resistors ( also R8 and R18 ) with a simple ccs using the LM317 chip and a single resistor. The resistor is used to set the desired current level of the ccs. Here are some links.

YH's Lazy Man Constant Current Source


http://www.freewebs.com/valvewizard/ccs.html

Last edited by Soonerorlater; 16th May 2011 at 10:52 AM.
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