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?
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?
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.
Yes, that's exactly where I have it.
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.
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?
Mazuki said:
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.
Mazuki said:
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.
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.
The glow in a 300B tube is fairly dim and may not be visible in a bright environment. Turn off the room lights and you should see 4 faintly glowing vertical lines when viewed from the top of the tube. If you have 5 volts and the heat sink gets pretty warm, they should be OK. If you have no visible glow in a dark room though, something is wrong. Check the continuity between the two fat pins on the tubes (out of the amp) with an ohmeter. My Shuguang 300B-98's measure about 1/2 an ohm, but I am using a $3 meter.
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