• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Stereo EL34 Baby Huey Build Log - an "Engineer's Baby Huey"

Finished the chassis, connected everything and turned it on for the first time yesterday. No smoke and no fires! I followed tristanc's power up procedure and there was no problem adjusting the amplifier.

I have also built the previous incarnation of the EL34 Baby Huey. This version is in my opinion easier to build. Very convenient with only one PCB and there are a lot less wires to keep track of.

Connected the amplifier up today and have listened to some music. It sounds very good and smooth as expected, and it's very quiet. More quiet than my previous Baby Huey for some reason.

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Very nice chassis, what did you use? Thank you for the answer about the obsolete MOSFET.
OK, so as I mentioned above, this BH replaced Pete's DCPP in my living room set-up. So both got used daily, for hours on end, for all TV, movies and music etc. I'm a heathen and listen to almost everything via AppleTV - the vinyl and turntable have been relegated to the loft until I have a bigger house.

I was happy with the DCPP, but getting valves was difficult here in the UK (I was going through them...) and I was looking to future proof. I'll fire it back up when I need another sound system, but it's under the bed currently awaiting that day...
How often were you going through tubes on your DCPP? How often were you using it (sounds daily)?

I built a DCPP around this time last year, love it, haven’t had to replace any tubes yet. My amp probably only runs 10-15 hours a week.
How often were you going through tubes on your DCPP? How often were you using it (sounds daily)?
I think I went through 2 or 3 power tubes over 3 years The same with preamp tubes. This was daily use, 3 or 4 hours a day I'd say. The only ones I could find were used / old and in random quantities, so I had to buy a few (as and when they came up on eBay) and then try my best to match them.
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I ended up using STF5N62K3 from Mouser. Seems to work OK. It does get quite hot, but I don't think it should be a problem.
Something failed yesterday. I heard first a slight hum from the speakers and then a strange "click" sound. The amplifier is now silent. The resistor R208 is burnt. I suspect that the STF5N62K3 transistor has failed. I'll remove and test it after work today and also test the tubes. I'll report back.
R208 is rated at 600 mW. Is that enough in this position? The schematic mentions:

Max V across the 22r resistor is V_Z - V_GS = 10 - 4 = 6
Max current is 6/22 = 270mA

Which gives 1.6 W. What do you think @tristanc ?

Eash. I'll try and check Merlin's power supply book tonight on why I chose a standard resistor there. Assuming 90mA per channel, P_diss >= 0.7W in R028 at idle. Yeah, pushing it... Perhaps I made a mistake and 2+W is needed.

You can always jumper it out. Pete didn't use one:

If it failed open the MOSFET may be fine? Perhaps the valves have settled and the bias needs adjusting - causing an increase in current etc.
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Is it R208 or R028? Where is the schematic? Is it available outside GitHub?

@tristanc, Jumper out where? Pete didn’t use what? I don’t understand.

However, the acquiescent current may be 90 ma for the amps, but that does not include bleeder currrents in the power supply, and at full power the current will be more.
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Joined 2004
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Is there a “GitHub Guide for Dummies” somewhere available so that I can learn how to use it. GitHub is unintuitive to me. How can one look at the schematic? I have marveled at @tristanc’s neat file organization before, but have not actually tried to open and look at the files
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Yeah - jumper out R208 and replace the FET. From memory (and I'll try and check tonight) the function of that resistor was a current limiter (as I wrote on the schematic) and not a "fuse" but I'd also trust Pete in leaving the 'nice to have' out.

I hope this hasn't propagated further - I can't think anything else would be impacted. And I'll also, given safety as a priority, look to replace my R208 with a higher wattage part and maybe lifted off the board to allow air circulation.
Actually - surely the FET shouldn't have failed under any likely conditions it would experience. Perhaps check there wasn't any flux bridge or stray solder etc? I left as much clearance as I could around the pins and the keep-out areas were set for the voltages experienced. It's definitely encapsulated?

Just thinking through the failure mode. Resistor gets hot (but current should still be within FET spec?), resistance goes down, current should stay same as demand is still there, resistor burns through and fails open. Would the FET fail due to thermals? Sorry, just trying to work out what could have happened.
OK. My tube tester is inoperative at the moment because the computer that runs it is stuck in a Windows update ... But at least I tested the tubes for shorts. Couldn't find anything obvious on the PCB, like bits of solder and other things that could have caused a short.

Anyway, I put a jumper in place of the resistor R208 and installed a new MOSFET. Turned on and adjusted everything like before. I think possibly that the MOSFET doesn't get as hot this time. It's about 30–35 degrees C (measured with an IR thermometer). Didn't measure it the last time, but my impression is that it was much hotter. Playing music now, and another difference is that there is a bit more 100 Hz noise for some reason. Not noticeable from a listening position. Is it possible that the resistor had a function in suppressing this noise? I wonder if the MOSFET might have been faulty. Must be quite unusual?

At least it still sounds just as beautiful! Which is the most important thing.
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