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goofycarp 2nd December 2012 10:07 PM

Tubelab SE capacitor question
Just starting a Tubelab SE dedicated 300b amp and have come across some least to me...information concerning the C2 capacitor. On one parts list I have come across a "note" for C2, "Omit for 5V tubes (300b) N.B. + side to ground silk screen correct orientation schematic incorrect orientation" I don't quite understand the note, that there is no capacitor used at C2? and if not what does one do there at C2? Sorry for the neophyte querry and clarification is appreciated. thanks !!

corporal clegg 12th December 2012 06:30 AM

Hi goofy,
And I thought I qas goofy when I was examining my Tubelab SE board..I for some odd reason am intrigued by traces on pc boards and I found 3 caps labled with the wrong polarity on the board...C2, C6, & C7. Use your multimeter and put your ground lead on the "GND-2" pad on the board and then check each capacitor for correct negative polarity. Now look at the schematic and there are a couple of descrepancies there as well, namely in the power supply section directly below the title "Tubelab SE Amplifier Board", namely C6 & C7. Notice that the polarity on these also show the positive side going to ground. I tried emailing George with a question about this but didn't receive a reply. Now if you print out a copy of the silkscreening of this board it is exactly the same as the actual board, so it appears to be correct. Also, if you look at a photo of his fully populated board paying specific attention to the capacitors the polarity is just as it is on the board and on the schematic. I am dumbfounded as to how you can reverse polarity on 3 capacitors and not have it explode when you fire it up...<no pun intended> I myself am new to this hobby and I have nothing but praise for my opinion he is gifted. I'm sure there is an explanation about why the polarity is reversed...possibly for the negative bias circuit that the caps are on...just as C2 is on the 5842 F- circuit path.
I'm learning something new every time I pick up a schematic or a new pc board and I have faith in what George has done here. My suggestion would be to follow his instructions, and make absolutely sure that you read all of his safety info. His links are rather difficult to access because of the new server format and he hasn't bwen able to update yet, but if you put your cursor over the blank title tabs they'll show up on the bottom of your info bar. Also, I can't find anywhere where he says to omit C6 cap on the Tubelab SE parts list...the other board he sells is the SSE, and maybe thats the parts list you have....which would cause mass confusion. I've found the easiest way to navigate Tubelab is to go to the wayback machine.(internet archives) and then go to january or so of 2010 and his entire website is much easier to navigate before his internet host screwed up his format. Give it a try...and the parts lists and instructions are all current too! Hope this helps.

corporal clegg 12th December 2012 07:18 AM

Sorry, I meant to say C2 cap and I said C6 about being omitted

cotdt 12th December 2012 07:38 AM

I've built a TSE for 300B. You can install C2 as I did.

The PCB board capacitor orientations are correct. It's the pdf schematics that have the errors, like missing resistors. If you follow the PCB you'll be fine. Be aware that you'll need some good heatsinking though, especially for the filament supply regulator.

cotdt 12th December 2012 09:21 PM

C2 gets shorted anyway when used in 5V mode, but is nice to have onboard for switching to 2.5V.

Those capacitors have reverse orientation because ground is the positive voltage in those. You are making -150V bias voltage supply.

corporal clegg 12th December 2012 10:26 PM

So my hunch was "partially" correct about the polarity being reversed for the minus F path for the filaments. I didn't come by that assumption by my knowledge of reading schematics....just by the amount of hair I pulled out :P Thanks so much for clearing this up for me!! I was going to follow the instructions anyway after thinking about how safety minded George is...I just know he wouldn't let something like that slip by...thats why I went with his board in the first place....I trust and believe what he says :)

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