Tubelab SE initial testing problems. Help - diyAudio
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Old 27th May 2012, 07:22 AM   #1
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Default Tubelab SE initial testing problems. Help

Hey guys. Bit of an unfortunate first post. I'm building a tubelab se 300b. Getting through initial testing is proving difficult.

Backstory: used a conductive material as board spacers for the capacitors. Caused 2amp fuse to blow. Figured out the problem. Removed the material and resoldered the caps. Found melted foam on the base of c6.
Replaced fuse with 3amp (only one available). Started noticing a hum from the transformer. (rknize edcor) Not sure if that was there before the first fuse blowing. Continued checkout.

From memory
B+ around 440 from memory
Filament voltages correct at 5v
B- 270 ish
Output grid voltage seemed fine. Both negative and adjustable.

The next problem came when installing the 5842s. Once b+ raised up, heard a crack and heard burning in the right channel q1 area, my meter connection to the coupling caps was a bit sketchy (kept disconnecting with movement), and the voltage was set pretty high before I turned the pot back to 175v. Turned off. Back on again. Adjusted driver tube plate voltages to 175v seemed ok. Started smelling burning again, noticed r34 smoking just before blowing the 3amp fuse.

My thoughts are I've somehow damaged the MOSFET q1 on the right channel? I'm replacing r34. But what would cause this to burn?

Bit stumped. If it is q1, what could cause that to blow. Got two more on order bit don't want to just replace it if something is causing it to blow. Have I destroyed something else with my initial mistake? All voltages seem good though?

Any insight would be amazing. I'm also going to test to see if I still get negative output tube grid voltages. Really I'm wondering why r34 would blow with all tubes minus the 300b?

Cheers,
Luke
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Old 27th May 2012, 03:20 PM   #2
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Quote:
Originally Posted by thommo007 View Post
my meter connection to the coupling caps was a bit sketchy (kept disconnecting with movement),
I've had a similar problem before. Link
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Old 27th May 2012, 03:57 PM   #3
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I see... So my problem may be with U2 instead of q1? Do you think the same cause could affect q1? My limited knowledge of circuits would place q1 as the culprit for burning r34. Don't exactly understand how this could be though :? I am just hoping it had nothing to do with my earlier stuff up, meaning i somehow destroyed the power supply. Thanks for the reply btw! . This is very frustrating after so much work. Waiting for parts from the us is the worst :/ I'll put a resistor and fuse back in tomorrow and go through initial setup again.
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Old 28th May 2012, 04:51 PM   #4
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Ok, something's wrong dammit. Blowing 2A fuses instantly. No tubes. Decided to check FRED diodes due to another thorough forum search. I'm measuring resistance both ways when out of the board. I'll try initial checkout again in 6 days when theyarrive :/ it just doesn't make sense.
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Old 29th May 2012, 02:19 PM   #5
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One thought for troubleshooting......

If you unsolder the power transformer leads from the PCB you can check resistance from each transformer connection pad on the PCB to ground; this may help diagnose what part of the circuit is shorted/blowing fuses.

You can also check that the various transformer secondaries are outputting the correct VAC; they should be a bit high since the transformer will be unloaded with the secondaries unsoldered.
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Old 29th May 2012, 07:10 PM   #6
jrobson is offline jrobson  South Africa
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Hi

Faulty FRED diodes certainly blows fuses! Could be your rectifier tube blowing the FRED's as well.... This happend to me and I installed a SS rectifier, no problems since then.
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Old 29th May 2012, 08:11 PM   #7
jrenkin is offline jrenkin  United States
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Oscillation from the 5842? Make sure your grid blockers and grid leaks are in place and well valued.
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Old 30th May 2012, 11:16 AM   #8
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Thanks guys. Lots of things to analyse now. I will disconnect the transformer if the FREDs don't fix the problem. I initially ordered the FREDs as per the parts list and apparantly they havent been as reliable as of late. I ordered the stealth fairchilds to replace with. Hopefully it was just a coincidence that they went, as I believe they have nothing to do with the r34 problem? Which in itself is strange because I've upped the grid stoppers and they are carbon comps as per George's update.

I also did some ohm readings on the pin of the mosfets. And they were inconsistent with one another, indicating my suspicion of the right channel failure is correct. Possible my right channel 5842 went into oscillation when its plate voltage was set sky high?

Hopefully my rectifier tube is ok and the FREDs were just duds. This will be a pain if I have to look into ss rectification which I haven't the first idea about.

I'll keep thinking and see what happens when the parts arrive.

Cheers
Luke
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Old 1st June 2012, 07:56 PM   #9
jrobson is offline jrobson  South Africa
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I use an ebay one, but if you want slow start: Kitparts - A provider of parts and kits
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Old 9th June 2012, 05:34 AM   #10
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Thanks for the info man. Apparently my rectifier tube is ok. Received the mosfets and Freds yesterday. Used a non chinese source for the toshiba part and went with the fairchild stealth fred diodes, which actually work. I can add another recommendation against the diode on the parts list. Bit of a pain desoldering the heat sinks. Went through checkout. And , it works!! Best feeling. Then it shut off after a minute or so. Until I realised i shouldn't have been lazy and not put thermal grease on both sides of the filament supply chip insulators. Did that and it is comfortable for well over an hour (and still going) inside its small aluminium enclosure. Thought I had a problem with one channel but it turns out my brand new fostex woofer is poling So once I get that sorted I'll be a happy chappy . It occasionally blows the 2A Ceramic fuse on startup. Will try a slow blow or possibly a 3A. Thanks all for the help! I'll post some pictures when I get the chance. Btw. Sounds amazing!!!!! Unbelievable.
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