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*my* Tubelab SE build...

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#43
Doc Jr 8156 said:
Hello guys,
Excuse me but i'm ordering the boards tonight and I have 2 questions. First, Where is the parts list for the Tubelab SE? Second, I will use a choke for my Simple SE, what is the value? Thanks and pardon me. Godspeed.

John Revilla
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Is this for a Simple SE? Then you should use: http://www.tubelab.com/AssemblyManualSimpleSE/PartsLists_SSE.htm

The build I'm doing is for Tubelab's SE, not the same as the Simple SE.
 
#45
From the tubelab checkout page:

Put a meter from the grid of the output tubes to ground. One meter for each tube. Power up the board. Each meter should read a negative voltage. With ONE HAND BEHIND YOUR BACK carefully adjust one of the bias pots (R12 or R23). The negative voltage for the corresponding output tube should change. Set it to the most negative voltage, and then adjust the bias pot for the other channel. Set them both to the most negative voltage. Power OFF the board. Do NOT touch the board for 5 minutes after the power is disconnected.
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Ok, slight confusion here. It doesn't tell me to put the output tubes in their sockets for this part of the checkout... it would seem to me they should be, but if I follow the directions exactly - no tubes in for the adjustments.

So tubes in or out?
 
#47
BB King is singing in my garage...

Checkout is done! Not so bad really... But there is this smell of newly heated tubes/plastic - have to keep an eye on that. And a fire extinguisher nearby 😉

At first I was worried that there was no output on the speakers, untill I set the bias for the 300b's to 60ma or so. After about 5 minutes all sound stopped. Since there was still B+ I knew this was the thermal shutdown of the CCS kicking in... So obviously I need to take care of that asap.

Here is a pic of the 300b's and one of them with a blue glow
 

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#50
Here's a shot of the setup... yeah, it's not exactly the neatest working environment - but I know where everything is and is going. Both tubes are set to 59ma, and B+ is at 395V.

I'm not sure I'll even need the big motor run cap, but I have it in there just in case (not wired at the moment).

Right now I'm driving some very old 8ohm KLH indestructo speakers... I need to crank up the volume to 3/4's just to get decent volume out of them. I generally only use them for bring up since they are very inefficient and have no response below 80hz. But they sound pretty good in the garage...
 

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#52
The other tubes are pretty standard for the SE - 5842 for the gain stage, and 5AR4 for the rectifier.

I did have some trouble getting one of the 5842's in the socket though, and based on another thread, the Chinese ceramic sockets are a bit iffy... Flexing the PCB too much always worries me.

So far the current through the 300b's has remained steady. 60ma...
 
#57
oldmanStrat said:
Turned the amp on again this morning to double check the bias again... and that smell of overheated plastic was pretty strong.

Is this common - or might I have a problem? I still need to change R6...
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I've had that before and it was coming from R14 and R25 which was turning brown (burnt) due to current overload issues I had with output tubes I was using at the time. I had to replace those resistors. I'm not sure if it's the same case with yours but check the resistors visually.
 
#58
oldmanStrat said:
Turned the amp on again this morning to double check the bias again... and that smell of overheated plastic was pretty strong.

Is this common - or might I have a problem? I still need to change R6...
Click to expand...

R6 (even the 7W version) stank for a week or two at least. Now I only smell it when I am up close.

The other thing that runs really hot are the two diodes feeding the bottom half of the heater circuit: D4,D5. These are not Schottky diodes and so they have a higher forward drop than D1 and run really hot. They are only 3A diodes, so they are being pushed really hard. When I had the original breadboard apart, the area of the PCB around these was noticeably burnt. Some of the smell is likely coming from these. I should have left more clearance under these. I may switch them to a dual Schotty package like D1.
 
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