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Old 22nd September 2012, 10:46 AM   #1
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Default Tubelab SE problem

A question for you, I have had the SE running all day so roughly 8hrs, voltage across R30 is 360/370 all of a sudden the sound fades down and the voltage across R30 is 414v, I should mention the R30 resistor could be a 120k about 2W not sure it was salvaged one. I turned off the amp for a few minutes and then on again it works for about 10 minutes then the same thing happens. Could this be the resistor failing? its not burning, actually nothing smells funny. or could it be the chips Q1 and 2 the solder joins look like they have been hot.

anyways look forward to you comment

cheers

Luke
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Old 22nd September 2012, 10:53 AM   #2
M Gregg is offline M Gregg  United Kingdom
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Hi,

If you could post a link to a schematic...

Then we can comment..

Is it this one?

http://www.tubelab.com/Simple_SE_schematic.htm


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M. Gregg
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Last edited by M Gregg; 22nd September 2012 at 10:57 AM.
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Old 22nd September 2012, 11:14 AM   #3
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My apoligies M Gregg, here is the Schematic, R30 is in the HT part of the power supply.

Luke
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Old 22nd September 2012, 11:21 AM   #4
M Gregg is offline M Gregg  United Kingdom
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Could be,

A bias problem..if the power tubes stop conducting the voltage drop across the R4 or choke will decrease due to less current drawn.

Have you checked the bias when it fails?

Ie what is the voltage across R18/R29 normal and fail condition?
Its on the HT (B+) so be carefull..I guess you are used to working on this amp?

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M. Gregg
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What is the sound of one hand clapping?

Last edited by M Gregg; 22nd September 2012 at 11:25 AM.
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Old 22nd September 2012, 11:26 AM   #5
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No I have not checked that. I will put the multimeters in tomorrow and see what is happening.

cheers
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Old 22nd September 2012, 11:28 AM   #6
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Actually thinking about it more, would both channels simultaneously fade out if it was the bias?
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Old 22nd September 2012, 11:30 AM   #7
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by magnetman View Post
No I have not checked that. I will put the multimeters in tomorrow and see what is happening.

cheers
Also,

Check the voltage across the bias trim pots and see if there is any change.

You need to check the valuse around the trim pots are to the drawing spec unless this amp has been running for some time ie (months) is this a first run?

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M. Gregg
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Old 22nd September 2012, 11:31 AM   #8
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by magnetman View Post
Actually thinking about it more, would both channels simultaneously fade out if it was the bias?
If somethng happens to the bias voltage yes its got to be common to both channels..so it could be the power supply..

I assume you have checked B+ is ok AT THE TUBES?

The voltage across R30 would seem to say you have B+..

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M. Gregg
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What is the sound of one hand clapping?

Last edited by M Gregg; 22nd September 2012 at 11:33 AM.
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Old 22nd September 2012, 11:33 AM   #9
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The amp has been on maybe a total of 20hrs, not long. The values are all correct i am sure of it, I have had a mate check as well.

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Old 22nd September 2012, 11:36 AM   #10
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B+ was checked, yes. however this was before i started running it for long periods. I should mention that all components bar the resistors are mounted on the underside of the board.

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