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Old 28th July 2014, 12:12 AM   #1
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Default SSE very quiet

Hi,

I've just finished building a Tubelab SSE and have a problem - it's mega quiet. I'm using Transcendar TT-312-OTs (SE, 5k, UL tap for 8 ohm speakers), Tung-sol KT120 power tubes, a Mullard CV4024 driver and a Sovtek rectifier. The amp is mounted on an aluminium plate. For now I'm driving a Mission 750LE speaker for testing (6 ohms not 8 but that shouldn't make a vast difference should it?). For a test input I'm just using a laptop playing mp3s. If I turn the volume pot all the way up I can hear music extremely faintly at the speaker (as in, I have to put my ear right up to the speaker to hear it) but there seems to be a huge gain problem somewhere. I checked the components very carefully and the sound is the same on both channels. I've tried both solid state and valve rectification and it doesn't make a difference.

As I understand it the KT120s are drop-in replacements for KT88s but can handle more power, and the CV4024 is equivalent to a 12AT7 so hopefully the tubes I've chosen are ok. I'd be very grateful for help troubleshooting this, as I'm stumped.
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Old 28th July 2014, 12:58 AM   #2
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Pictures would help, and how did you mount the output binding posts - they're not shorted to your chassis plate are they?
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Old 28th July 2014, 01:49 AM   #3
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I would have applied a sine wave and followed it through the amp with a scope.
The DC conditions should be checked first.
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Old 28th July 2014, 06:03 AM   #4
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OK, I'll post some pictures tonight. Output binding posts are mounted through plastic collars and measure >20M to ground and to each other with the OPT secondary disconnected. I don't have a scope, just a multimeter, but I suppose I could use the laptop to generate a sine wave and measure RMS values.
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Old 28th July 2014, 09:13 AM   #5
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A couple more questions in preparation for more fault finding tonight:

1) Should the DC resistance of the OPT secondary be 8 ohms, or is that just the AC impedance?
2) Can the tubes I'm using be operated upside-down? (This will make fault finding a lot easier but I gather there are some tubes where the orientation matters).
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Old 28th July 2014, 12:39 PM   #6
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Originally Posted by adrianbaugh View Post
2) Can the tubes I'm using be operated upside-down? (This will make fault finding a lot easier but I gather there are some tubes where the orientation matters).
The Tung-Sol KT120 data sheet I have says any orientation is ok....
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Old 28th July 2014, 07:15 PM   #7
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OK so I'm home now and have taken some photos and made some further measurements. First the photos:
http://www.adrianbaugh.org.uk/amp1.jpg http://www.adrianbaugh.org.uk/amp2.jpg

I guess the first thing is, can anyone see anything obviously wrong? (The disconnected wires near the front are from the volume pot, I chopped up a 3.5mm jack cable to make a very simple input from my laptop which is what is connected at the moment.)

And now the measurements:
I'm using a 440Hz sine wave generated on my laptop as input, measured at 1.0Vac using my multimeter.
Heater voltage: 6.50Vac. (The rectifier tube is unplugged for the moment, I'm using the diodes; the heater voltage to the rectifier socket looks okay at 5.3Vac.
B+ to ground measures 470Vdc.
I checked the OPTs too just in case they were dodgy: primary resistance was 410 ohms; secondary resistance was 1.3 ohms.
In the preamp, between the grid and anode measures 8.6V.

I'd be very grateful if anyone can point out a fault or suggest other measurements I could make to try to find the fault.

Not sure what else would be useful measurements
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Old 28th July 2014, 09:00 PM   #8
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I've taken the board out to check each component in turn, so here's another photo of the topside of the board: http://www.adrianbaugh.org.uk/amp3.jpg

I've swung through every resistor on the board and all the values are correct. All the diodes test correct and even though I've mounted the FREDs on the reverse of the board the dot on the package does line up correctly with the dot on the silkscreen layer. There is no visible sign of damage to any of the electrolytics and I've observed the NTC thermistor behaving correctly. All tube sockets line up correctly with the lug on the silkscreen.

I don't know how to test correct operation of the CCS ICs: when installing them I did handle them a fair bit before noticing my wrist strap wasn't plugged into the antistatic mat - are they particularly static-sensitive, and is there a way of checking rather than just replacing them on the offchance?

Next I need to check the tubes themselves. Are there measurements I can take with them disconnected that will indicate whether they are working? The KT120s are brand new so they should be okay (especially as the fault isn't channel-specific); the CV4024 is old stock and a potential point of failure common to both channels. I don't have a spare, but I could get my hands on a 12AU7 or a 12AX7 - albeit at reduced quality, would one of those suffice to test whether the fault disappears by swapping out the driver tube?

Thanks for all your help so far.
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Old 28th July 2014, 10:59 PM   #9
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I think it might be the CCS ICs: there is a faint smell of hot silicon (no smoke though) and the TO220 tab and heatsink are not at B+ as the semiconductors page of the assembly manual says they should be. I'll change them and see if that fixes things.

By the way, while I'm buying more parts, can anyone suggest a good (preferably UK based) source of 47uF 500V electrolytics? I couldn't find one before so there is a 68uF in there at the moment which is too big really.
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Old 29th July 2014, 06:12 PM   #10
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Originally Posted by adrianbaugh View Post
By the way, while I'm buying more parts, can anyone suggest a good (preferably UK based) source of 47uF 500V electrolytics? I couldn't find one before so there is a 68uF in there at the moment which is too big really.
Bought my parts at mouser and hificollective.co.uk

Michael
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