5E3 Build - B+ Issue

[Morgan]

Hi guys,

I'm having an issue with a 5E3 build and could use some help troubleshooting.

It's a layout I made based off a Hoffman layout; stock values. I added a Mini-Watt from Skipz Circuits; it's similar to a VVR. I tend use the Mini-Watt in tweed style amps because I like how it takes the place of the power switch. This is the third Mini-Watt I've installed into a 5E3.

When I installed the Mini-Watt, I was probably working too late and mistakenly reversed the source and drain connections on the mosfet. During 1st power up, everything went fine (voltage-wise) with checks on the heater rail, rectifier, and pre amp tubes. When I got to the power tubes, that's when I started having problems.

I was running the amp off a light bulb limiter. When I installed the power tubes to check B+, I got smoke coming from the Mini-Watt assembly. I traced it to a 10k resistor on the board that was glowing bright red. At this point, I don't remember if I had the power supply limited, or if I gave it full voltage. I think that everything went fine when it was limited, and the resistor didn't smoke until I gave it full voltage.

I've taken the Mini-Watt out of the circuit to see if the rest of the amp is okay. What I have now, is when I power up the amp with the limiter, everything goes fine as the tubes warm up, until B+ at the first filter cap hits about 450 vdc, then the bulb starts to glow and B+ drops fairly rapidly.

I've checked wiring visually a few times, traced out connections with a DMM, checked all the grounds, made sure all other components are not grounded, verified filter caps are not burned open, and tried a known good rectifier. I'm a bit at a loss at this point.

Any ideas/suggestions would be greatly appreciated !

[xtian]

What I have now, is when I power up the amp with the limiter, everything goes fine as the tubes warm up, until B+ at the first filter cap hits about 450 vdc, then the bulb starts to glow and B+ drops fairly rapidly.

Just "glow" or "glow BRIGHTLY"? This sounds like my experience with a bulb limiter, when things are functioning normally. Heaters will warm up, and when amp starts drawing current, bulb begins to glow, and B+ sags.

Anyway, wait for smarter dudes to confirm or deny.

[Morgan]

Just "glow" or "glow BRIGHTLY"?

Good point! As the B+ peaks around 450, the bulb begins to glow brightly - plenty bright enough to indicate that something is drawing way too much current. It's not that dim glow that indicates normal operation.

Anyway, wait for smarter dudes to confirm or deny.

That's why I'm posting here!

[Firestorm]

Can you tell what part of the tube is glowing? I will bet it is the screen. You see that in Fender Deluxes ( especially BF) run "over" voltage.

[Morgan]

Can you tell what part of the tube is glowing? I will bet it is the screen. You see that in Fender Deluxes ( especially BF) run "over" voltage.

Did not notice any of the tubes acting weird. No red plating; just the normal heater glow. Should I look closer at one of them in particular?

[ChrisM]

Pull all tubes, does the light bulb glow bright (large current draw)?

Now put in the recto tube, does the light bulb glow bright (large current draw)?

Continue with power tubes and preamp. My guess is you have an issue somewhere in the power supply or output stage. Try a new set of output tubes and recto tube.

[Morgan]

Hi Chris!

That's the procedure I used at first start up and there were no issues. I didn't go back to that though, after the screw up, so I'll go back and check again.

[Morgan]

Pull all tubes, does the light bulb glow bright (large current draw)?

Negative - no glow

Now put in the recto tube, does the light bulb glow bright (large current draw)?

Negative - no glow. B+ peaks and holds steady at 477vdc.

Continue with power tubes and preamp. My guess is you have an issue somewhere in the power supply or output stage. Try a new set of output tubes and recto tube.

No glow with the pre amp tubes.

Glows with the power tubes installed. B+ rises as the recto heats up, peaks at 450, then steadily decreases. Bulb starts to glow around 300 volts, and B+ seems to want to stabilize around 245 volts with a very brightly glowing light bulb.

Already tried a couple of different 5Y3 tubes (known good) with the same results, but didn't think about the power tubes and didn't see your note to mention them until now. I'll try another set and report back...

[Cygnus X1]

I don't know about the rest of you but while I like the safety factor of the light bulb limiter...it has mislead me too many times to trust at full power up.
Good for certain things, sure.
I still use it for first time startups and questionable older electronics.

But when the rubber hits the road I go for it and usually my area of concern turns out to be just fine.

[Morgan]

Try a new set of output tubes...

Swapped in a known good set of output tubes and still have the same symptoms.

Ugh...this may be heading toward OT ugliness...

Another thing to consider, the Mini-Watt installation puts a 25µf/500 volt & .047µf/600v filter cap pair in parallel to ground on the B+ rail, a 330k 1/2 watt bleeder resistor across the first filter cap, and adds diodes from the OT primaries to ground. I checked out the caps and they appear fine. I did not desolder & check the diodes. I doubt anything there is an issue, just putting it out there.

[Morgan]

I don't know about the rest of you but while I like the safety factor of the light bulb limiter...it has mislead me too many times to trust at full power up.
Good for certain things, sure.
I still use it for first time startups and questionable older electronics.

But when the rubber hits the road I go for it and usually my area of concern turns out to be just fine.

I hear you and I like where you're going with that! Maybe I can flip the switch and the damn thing will be be fine! I may be spooked because I know I screwed something up with the mosfet leads and the smoking/burning resistor. But I've built a whole bunch of these amps in the past and haven't had this limiter issue along with the weird B+ sag before, so I'm fairly convinced something is off and sucking up current.

[ChrisM]

Remove that Mini-Watt thing. I don't mean to be rude but it looks and sounds so hack. All the parts are cheap looking and undersized.

There is a good chance the diodes could have gone and are shorting.

That 1/2W 330k resistor is barley big enough for the job. It should be 1W or more. Remove the mini watt for now and get the amp going.

[Morgan]

The mini-watt has been out of the circuit - the symptoms I'm describing are without the mini-watt. I took it out as soon as I cooked that resistor. But I'll check out the diodes and ditch the bleeder - thanks!

BTW - the mini-watt is great for what it is; a small VVR-type circuit to reduce B+ for volume attenuation. I'm sure there's improvements that could be made, but there's nothing really wrong with it. I've gigged with one fairly heavily for a good 3 years or so and have installed them in a couple other amps. The fet is robust and the resistors look small but they are just 1-watt metal oxide resistors in a smaller form factor so the size of thing can be reduced - which is why I use that instead of a VVR; it fits perfectly where the power switch on a 5E3 goes. The pic on the website is a bit misleading.

[Firestorm]

I'm a little confused as to what the problem is. You turn the amp on and it pulls current. Aside from toasting the mini-watt thingy (which is now out of circuit) your problem is what?

[Firestorm]

Okay, I finally read the whole thread (probably should do that before I post things). Light bulb will glow on power-up as the caps charge. Then it will dim (because the caps are already charged). If it stays bright, you have a short. But if you want to test things again and again, you must drain the filter caps each time or they will make the light bulb lie to you. The limiter only sees current draw for the whole power supply.

I'll bet you don't have a problem (except for destroying your mosfet). The caps won't have noticed that; resistors might, but unless they're shunts, probably not.