Wonderland/JM100 build

[norburybrook]

Had a bit of time today to do some more building and here's where I'm at;

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I recon another day should see me finished.

ooooo the anticipation


Marcus

[telentubes]

"ooooo the anticipation" is right! Looking forward to your first impressions.

[norburybrook]

I've finished wiring....

I decided to use up some of my left over wire collection, which on hindsight might not have been the greatest idea on an amp with 5 pre amp valves. It takes up quite a lot of real-estate over the solid core, thin stuff I've used on my other Dumble builds. Anyway, I'm sure it's only cosmetic but I would like to build a really neat amp one day.

Questions about the reverb; I don't have a 5751 so would a 12 AY7 be OK? I know it's a lower gain but I presume that means I'd just have to dial in more reverb, which might not be a bad thing.

or should I stick in a 12AX7?

Which is the reverb in and which is out on the layout? I think the input to the reverb is the white RCA plug and the out is red, is that correct? going from V3(send) to V2(Return)

As I've already checked out the power section is there any point in me not just putting the valves in a firing straight up?...obviously my 100% record of OT primaries being the wrong way round is still running and I'll be ready for the howl

here's some pics.

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[martin manning]

Marcus, can you link or post the layout you are working to? Reverb send is the green wire from the reverb transformer.

Re the tube 12AX7 is closer to 5751 than 12AY7.

[norburybrook]

Wonderland 100w jan2017.jpg

Wonderland schematic.jpg

I've found a 12 AT7 from the donor peavey amp reverb so I'm going to give that a try just putting some phono plugs on the reverb tank cables.

M

[tubedogsmith]

Fwiw, be careful you don't have any of your PT wire contacting the chassis coming up over the sides. I've repaired a couple amps built like that and it's a big, scary burned up mess if the wire wears through. You mat already have that covered and I just can't see it. If so, never mind.

[norburybrook]

Ah...here we go again

right, so I put the valves in and plugged in to the light bulb limiter. All seemed fine, dim glow from the bulb. Plugged in without the light bulb, amp was humming and very low signal when I plugged a guitar in.

I started checking plate voltages from V1. they were 145vDc rising slightly as I went through V2-V4. When I got to V5 the P.I. one plate was 250vDc and the other was about 350vDc I think but within a millisecond there was a crackle and the amp went dead as it blew the mains fuse. I put in a new fuse and it blew instantly(I did have standby on play when I flicked the power switch)

I've taken the valves out and put it back on the light bulb limiter and the bulb is glowing brightly and the my fender jewel light isn't illuminating now.


So..I presume I have a short somewhere or heaven forbid I've blown up the transformer!!!!

any one care to get me started on trouble shooting?


Happy Easter


Marcus

[martin manning]

Well, first make sure you don't have any shorts to ground where the PT wires pass by the chassis cut-out edge as tubedogsmith warned above.

[norburybrook]

It's all clean and clear there the cabling is all intact.

Is there an easy way to check my mains transformer is OK?



Marcus

[martin manning]

I'd be surprised if you have any damage, but you could disconnect the secondaries and measure voltage on each winding. Then you could measure resistance to ground at each place where the secondaries were connected and see if there is any low-resistance path.

[amplifiednation]

Check all your B+ connections and make sure there is no accidental continuity to ground. Check standby switch leads too, it's easy to leave a lead long and forget about it. Also tube pins.

Your transformer is prob fine.

[norburybrook]

OK, if the transformer is likely oK, before I start disconnecting things, where should I start tracing the short? I have no 6v power light coming on now and as I said the light bulb limiter is glowing brightly.

Is it safe to continue probing voltages etc with the light bulb limiter glowing brightly?


Marcus

[martin manning]

You can check voltage at each secondary connection and look for one that is very low. Be aware that even with the limiter in you are running the limited current through the shorted path and something might get overheated. Is the 6V bulb socket isolated from ground? Has the bulb gone open?

If you disconnect all the secondaries and the limiter goes dim, then you can reconnect them one winding at a time and find the offender.

[norburybrook]

it's just blown the fuse when turning from stand by to play even on the limiter.

how can I check if the bulb has gone open? (I've put another bulb in since it originally blew the fuse)

I'm going to run out of fuses very soon so need to find it quick

The bulb itself is isolated from ground but the actual socket/holder isn't.

it's strange that I tested the whole power section and it was fine at the time.



Marcus

[martin manning]

it's just blown the fuse when turning from stand by to play even on the limiter.

Suggests some problem with the HT.

how can I check if the bulb has gone open? (I've put another bulb in since it originally blew the fuse)

Measure its resistance.

The bulb itself is isolated from ground but the actual socket/holder isn't.

Aslong as the terminal lugs are isolated it's good.

it's strange that I tested the whole power section and it was fine at the time.

Something you've done since, then.