Final checkout problems with Tubelab SE - diyAudio
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Old 19th June 2009, 07:50 AM   #1
Mazuki is offline Mazuki  United States
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Default Final checkout problems with Tubelab SE

I've just about completed a 300B Tubelab SE. I'm using a Hammond 272JX power trans. My B+ with all the tubes plugged in is around 365V.

The 5842s bias fine and have a plate voltage of around 175.

I biased the 300Bs to about 65mA as well.

The only thing that is off is the bias supply - it's stuck at -250V. Furthermore I tried playing some music and can only hear some faint sound at full volume.

Does anyone know what's causing the problem with the bias supply. It that why I'm not hearing sound?

As a side note, I bought these Gui Guang tubes on eBay for cheap, they have nothing glowing inside at all (filament is running at 5V). Are they defective?
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Old 19th June 2009, 02:13 PM   #2
rknize is offline rknize  United States
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Where are you measuring the bias supply? -250V is about right at R6/R7. The filaments in the 300Bs should be glowing orange. Did you actually measure the voltage while the amp was on? What are you doing for a heat sink on the filament regulator? The "stock" regulator is not enough for the requirements of the 300B and regulator will shut down after a few minutes.
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Old 19th June 2009, 04:30 PM   #3
Mazuki is offline Mazuki  United States
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Quote:
Where are you measuring the bias supply? -250V is about right at R6/R7
Yes, that's exactly where I have it.

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The filaments in the 300Bs should be glowing orange.
There's no glow at all, but the filaments are running at 5V, I've got a DMM clamped on pins 1 and 4. I saw George's note about the silkscreen error so the tubes are connected correctly.

Quote:
What are you doing for a heat sink on the filament regulator?
I'm using a fairly beefy one that I found saved from my computer modding days. It's the size of a small toothpaste tube.



I did notice an error though. I didn't realize the 272JX had a 5V CT so the CT wire was connected to mains ground, while the amp ground was floating. What are the implications of that?
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Old 19th June 2009, 04:35 PM   #4
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Did you follow through the checkout step by step? It lets you check the filament voltage on the first step. What does yours show?
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Old 19th June 2009, 04:41 PM   #5
Mazuki is offline Mazuki  United States
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Quote:
Did you follow through the checkout step by step? It lets you check the filament voltage on the first step. What does yours show?
Yes, it's showing 4.98 volts
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Old 19th June 2009, 05:06 PM   #6
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Quote:
Originally posted by Mazuki
I didn't realize the 272JX had a 5V CT so the CT wire was connected to mains ground, while the amp ground was floating. What are the implications of that?
Do you mean T1-6 shown here? For 300B, PAD 1 is disconnected so it's floating anyway.
What do you mean amp ground was floating, not connected to ground anywhere?
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Old 19th June 2009, 05:08 PM   #7
rknize is offline rknize  United States
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Quote:
Originally posted by Mazuki
I did notice an error though. I didn't realize the 272JX had a 5V CT so the CT wire was connected to mains ground, while the amp ground was floating. What are the implications of that?
Hmmm...the 5V for the rectifier? That would ground the output of the rectifier tube which would push the whole amp to -300VDC. You are lucky that you didn't have the amp grounded to main as well, as that would have arc-ed over the rectifier tube. I would fix that ASAP...leave the 5V CT floating.
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Old 19th June 2009, 05:11 PM   #8
rknize is offline rknize  United States
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Quote:
Originally posted by Mazuki
There's no glow at all, but the filaments are running at 5V, I've got a DMM clamped on pins 1 and 4. I saw George's note about the silkscreen error so the tubes are connected correctly.

I'm using a fairly beefy one that I found saved from my computer modding days. It's the size of a small toothpaste tube.
That should be fine. The heat sink should get very warm after a few minutes if the tubes are drawing current. If the heatsink is cold, the tubes are not drawing. You can check the filaments for continuity. When looking at the top of the board from the front, the 2 bigger cathode/heater pins on each socket are on the right.
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Old 19th June 2009, 05:32 PM   #9
Mazuki is offline Mazuki  United States
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The heat sink should get very warm after a few minutes if the tubes are drawing current
Yes, the heatsink gets fairly warm after a bit.

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You can check the filaments for continuity
I'll do that. Otherwise there's no other explanation for the tube not glowing right?
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Old 19th June 2009, 05:43 PM   #10
rknize is offline rknize  United States
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Not really...if you are measure 5V at the tube and it's on the right pins (the 2 big ones), you should be good. If you have a good multimeter that can measure AC current on its 10A setting, put it in series with the 6.3V supply to the PCB. That way you can at least see if they are drawing the expected amount of current. You are using Shuguang 300B-98? Mine glow just fine, but I suppose it's conceivable that there exists a 300B whose filaments don't glow. I find this hard to believe, though, especially given that the output sounds really weak. It sounds like they are not getting enough current.
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