Amp build thread #2: Super Reverb

[clintj]

Originally Posted by moon

Is there a simple way to disconnect the vibrato and reverb parts of the circuit? That might help to isolate the problem.

If you're stuck you can often get some really good technical advice on these forums:

music electronics forum: debugging your build

diy audio

TDPRI

Not really. I've asked an online group of builders and gotten some good suggestions so far. Consensus is the power section has something going on, as it's more capable of drawing enough power to slow blow a fuse. A serious problem would have blown the fuse instantly. There's even a slight chance that short cycling the amp like I did caused enough inrush current to blow the fuse. Oh, the joys of troubleshooting.

I'll have to pick up a pack of fuses and systematically troubleshoot the amp next weekend.

[Dru Edwards]

Originally Posted by clintj

Switched the power switch on, got a good pilot light and heaters glowing. Cool.

Switched the standby switch to On for a couple of seconds, nothing weird happening (pops, smoke, hums, etc). Cool again. Power down.

Plug in a guitar, power up, switch Standby to On.... Hmm. No pilot light, no sound. Crap. Blew the fuse. Got to pull the chassis, bleed the filter caps down, and have a look around now.

It's only a minor delay, Clint. Great that you got the amp assembled so quickly. With your previous build, did it work from the first try?

[clintj]

Actually, yes. Much simpler circuit, though. At least the magic smoke stayed inside.

[moon]

Been making any progress - or just a bigger pile of blown fuses?

[clintj]

Originally Posted by moon

Been making any progress - or just a bigger pile of blown fuses?

Not yet. Been working; I'll pick it back up Saturday. I did confirm the wiring continuity by multimeter though, and found a missing B+ connection on the Vibrato circuit. Easy fix there. That shouldn't cause the fuse issue though. Next is a slow, controlled, step-by-step startup checking voltages as I go. I've gotten a couple of votes for the theory that the inrush current from turning on the Standby switch could have caused it. We'll find out this weekend.

[moon]

That sounds plausible. With a tube rectifier, allowing the tube to warm up on standby before switching on seems to increase the inrush current compared to no standby and a cold tube.

You've probably seen this already but the consensus seems to be that standby switches don't actually do anything useful in modern amps.

Inrush current seems to be a common weak point in tube amp designs. Don't know why thermistors aren't standard. Maybe they introduce some noise.

[muscmp]

Originally Posted by clintj

Not yet. Been working; I'll pick it back up Saturday. I did confirm the wiring continuity by multimeter though, and found a missing B+ connection on the Vibrato circuit. Easy fix there. That shouldn't cause the fuse issue though. Next is a slow, controlled, step-by-step startup checking voltages as I go. I've gotten a couple of votes for the theory that the inrush current from turning on the Standby switch could have caused it. We'll find out this weekend.

clintj: this won't help with your present problem but this video by uncle doug shows a current limiter that i've built using a 300 watt light bulb from walmart and and an ac outlet. it is my "fail safe" so that it shows that there is a short and it doesn't go thru me. he has other audio repair-related videos also.

also, have you thought about a variac or an isolation transformer?

https://www.youtube.com/watch?v=wRFRwOnLsZI

play music!

[clintj]

Ok, got some help from a Facebook builder's group. Turns out I had the bias rectifier diode backwards. Fender has the schematic symbol right, but added a helpful +/- symbol backwards. The layout is also labeled backwards, as are a fair number of pics of diode polarity markings. For the record, the stripe is on the positive end, it goes towards the bias AC supply, and the amp is much happier that way. Haven't blown a fuse yet, either, and I'm up to installing the preamp and power tubes now. To be continued tomorrow. I need a good night's sleep and a clear head to continue this safely. Even with insulated gloves, dealing with high voltage is mentally tiring.

[moon]

That's great . Tough one to track down if the layout is wrong.

[Steve DeRosa]

Originally Posted by clintj

...Fender has the schematic symbol right, but added a helpful +/- symbol backwards. The layout is also labeled backwards, as are a fair number of pics of diode polarity markings...

Not totally sure it isn't intentional on Fender's part - if you really know what you're doing you'll catch the error, and it keeps the "unauthorized" from messing around inside their amps; kinda makes it tough for guys like you, though...

[moon]

Mojotone provide schematics and layouts for the kit on their website. It's odd they haven't corrected it. You'd think they'd have had some customer feedback about that.

[clintj]

Originally Posted by moon

Mojotone provide schematics and layouts for the kit on their website. It's odd they haven't corrected it. You'd think they'd have had some customer feedback about that.

You know, this made sense last night but looking at it again this morning I'm all turned around again. Big picture is it is a circuit designed to rectify AC to DC, but end up with a negative instead of a positive voltage with respect to ground so the diode and cap should be reversed. Oy. Either way, I swapped the diode around and the bias voltage is right (currently about -55V) now.

[moon]

You've probably got this Fender schematic with voltages relative to ground already. If not, that might help to check if everything's working as it should. Presumably these were measured with the tubes installed.

Be careful measuring a live circuit

[clintj]

Moon, thanks. I was just looking for the proper point to check bias voltage.

Installed all tubes, plugged in the speakers, and powered up. Very faint white noise. Very good sign! Time to adjust bias and install the chassis into the cab after the filter caps bleed down. Exciting!

[clintj]

Weird again. Almost everything seems to work, but the amp is really quiet. I checked voltages against the schematic moon listed, and they check out ok. All pots have the effect they should, except the reverb one for some reason. Time to discharge the caps and follow wiring. Again.