Ampmaker P1800

Fatfingers

MtB said:

Ha, snap. This is the exact same thing that happened to me on my build. I eventually worked it out, but I fed pack to Barry that I felt that this particular point was where you need photos rather than a diagram.

I have photos of the wiring of the inputs on mine. PM me with your email address if you want me to sent some photos.

Hi. I'm glad it's not just me that couldn't 'see' that diagram. It just didn't make sense to me at all. Everything else was straightforward.

Yes - please do email me pics - that'd be a great help

Jalapeno

Love these pictures diaries - most excellent !  =D>

MtB

I've sent you 3 piccies. 

MtB

So in the diagram above the bottom of the two jacks is the "hot" input.

Misterg

Fatfingers said:

Here's what I've done...

That looks right to me. (Both logically and to the diagram - to make the diagram match yours, swap the position of the two sockets on the diagram without changing any wiring - you then need your black jumper wire instead of carrying the link across between the sockets.)

MtB

^^  Exactly.

Fatfingers

Hmm. This is what MtB sent me. As far as I can tell he's wired it the same as me.

Fatfingers

The lefthand socket in MtB's picture is the hot input.

Misterg

I think the input wiring is OK - the 'hot' input should go to pin 2 of V1 (which it does, AFAICT).

Recap on the symptoms:

'Low' input works as expected....
'Hot' input squeals...

Does it squeal even with the volume on the guitar down?

Daft questions:
You haven't got plugs in both jacks at the same time, have you?
You're not stood over the amp with the guitar, are you?

Fatfingers

Yes - they both work as expected, except that the hot input squeals. And, when there are no jacks plugged into the amp it's quite noisy. I think the problem is that with no jacks plugged in, the hot input is live, instead of being grounded. With a jack in the normal socket the hot input is grounded, but with a jack in the hot socket (and no jack in the normal), the normal isn't grounded, hence the squeal. 

I think I just have an input switching problem. 

MtB

You'll have to wire yours up exactly as mine is as there is only one link to the volume pot they MUST cascade (one through the other - tho' I don't see how)

Misterg

The normal input is never grounded - The output from the 'hot' input is just passed to the normal input (after being amplified by V1a), unless there is a jack into the normal input which (should) just disconnect the feed from V1a and feed the signal from the tip of the normal jack instead. (As I'm sure you know, the 'hot' input jack should ground itself when there is nothing plugged in).

Worth checking that you've got good ground connections (lead between the jacks and the chassis and the two(?) solder tags that are under the turret board stand-offs: one near the input and one near the power supply).

I don't know how 'touchy' the circuit is to layout, but self oscillation is a possibility. Make sure to keep the blue wire from the back half of the volume pot well away from anything connected to the front half (I notice that it comes very close to the connection from the front half of the volume pot to the normal input - try looping the back connection around the back of the pot instead of over the top, maybe...?). Also, keep that connection to the low input well away from anything to do with the high input (and as short as feasible).

FWIW: I still think your input wiring is OK.

Edit - except that I think you have built it 'opposite hand' from the instructions and @MtB. Electrically it's the same, but by pointing the lugs of the jacks towards the volume pot, you have moved the relevant connecting wires closer together. (May or may not be relevant :-/ )

MtB

Checking the schematic, that IS how they work - one cascades to the other.

thermionic

Misterg said:

Worth checking that you've got good ground connections (lead between the jacks and the chassis and the two(?) solder tags that are under the turret board stand-offs: one near the input and one near the power supply).

Yes. I was getting oscillation (squealing and motorboating) when I built my N5X. Intermittent... Barry suggested I tap parts of the circuit to narrow it down and it was a toggle switch. I unscrewed the switch, thinking I'd damaged it with excess heat (it's a tiny switch) and was about to replace it when I noticed the ground wire was just pushed into a turret and I'd neglected to give it the once over with the iron.

Fatfingers

I'm away at the moment so can't check, but I'm pretty sure both inputs should be grounded when no jack is plugged in. In fact I remember the ground continuity checks including both jack tips, and that one of mine failed this test. Thinking about it, I'm almost sure that's what the problem is. I think when when I plug into the hot input the normal input is still live, so the guitar is effectively connected to both. And as the hot input cascades into the normal, oscillation is the result.

Fatfingers

Turns out I'd actually wired it up correctly in the first place (although I still reckon that diagram is hard to make sense of).

The squealing was down to a microphonic preamp valve.

The next problem is how loud it is. I'll save that for a new post.

Fatfingers

Okay. Here is the old Selmer cab (once part of a Selmer Lead 30 combo) with the Tolex removed from the top (with a hairdryer) and holes cut for the amp front panel and a cooling vent.

Here's the amp chassis trial fitted after I reglued the Tolex.

Fatfingers

I'm using a pair of Vintage 30s because they're 100db speakers and the amp wasn't quite as loud as I'd have liked when I played it through less efficient speakers. Ideally I'd have a liked Celestion Blue and a GL30 to go with it but the cost of those Alnico speakers put me off for the time being.

Here it is so far. I need to strip the Tolex from the rear panel so I can cut it into to two parts – a strip[ at the top with the amp chassis bolted to it and another one for the bottom, leaving a large gap in-between a la Blusebreaker. I also need to fit a grille behind the vent slot.

WezV

good work, that's going to look nice

DartmoorHedgehog

I like the look of that.  You need to make a "Selmershall" logo for it now.