SE-5a no output

[PhilDawson8270]

Hello everybody.

First, I apologise for having to introduce myself and seek advise at the same time!

This is the first time I've worked with tubes and audio electronics, and hit a wall with my amplifier.

I'm using the Ampmaker SE-5a kit, here is a link to the schematic.
http://www.ampmaker.com/images/ak01kit/ak01sc2.jpg


I went through the initial and final testing as shown on the build guide here.
SE-5a Initial testing

and

SE-5a Final testing


All tests seem ok, so I went ahead and powered it up, I get the hum from my speaker as expected, however, when plugging in my guitar, I get no output from the guitar, just the hum.

I do have a scope I can use.

What was unusual to me was that when plugging an instrument lead into speaker output, I get continuity between the point and the sleeve, this measures at 2ohm, is this normal?

Any advice on where to look?

Thanks.

[MWJB]

You shouldn't expect hum from the speaker.

You say that you have done the voltage tests as described in Barry's "Final Testing" section...humour us & list the voltages found. Do you hear any pops or crackles when probing the plates of the EL84 pin 7) or 12AX7 (pins 1 & 6)?

OT secondary dc resistance is typically very low, so your 2ohm reading isn't any help, may be normal (I'd expect a little less, but the primary of the OT should be more like 300ohms).

A scope isn't going to be much use to you at this stage.

Show us some shots of the build, links to the schematic are no use to us, if the amp was built to the schematic, then it would work perfectly.

[PhilDawson8270]

For further information.

Scoping the pre amp tube, V1 Pin2 I can see the signal from the guitar, however. When I scoped the input to the Power Amp R8 I had no signal from the guitar, so testing further back in the circuit, I do not see the signal on V1 Pin7, this goes to my Gain Pot (VR1)

Further to this, I see no signal on V1 Pin 6 either, which goes to my tone controls. These are all testing on the underneath of the preamp tube socket, not on the turret board or components.

This to me indicates a faulty tube? Can anybody advise me on this, as I am an Electrical Engineer, with electronic background (though mainly logic circuits), and as a working maintenance engineer, this to me indicates that my tests are of the tube itself and nothing more?

[PhilDawson8270]

You shouldn't expect hum from the speaker.

Words can be so very easily misleading when describing sounds.

It is not so much of a hum, than the background buzz you get from a speaker and amp when no signal is being provided.

To quote Barry,
"You should hear a very low background hum from the speaker; this is normal, and the hum confirms that the wiring from T2 to the speaker is correct (Note: this hum may be hard to hear in a noisy room)"

This is what I believe I am hearing, I will go through all my tests again and post the results.

[MWJB]

What do you see at 12AX7 pin 1? Pin 2 just tells us that you apparently wired the input jack correctly, there are no electronics between this point & your guitar. Confirm dc voltages (no signal) read at V1 pins 1 & 6, V1 pins 3 & 8.

[PhilDawson8270]

Here is a photo of the internals of my amp.

[PhilDawson8270]

Slightly embarrassing.


Upon testing V1, 1

For some reason it looked peculiar (terminated underneath turret board), so I looked at it through inspection lamp, and it appears to have broken off during flipping the chassis upside down testing and flipping back etc.

I have reconnected it, and the amp sings like a beauty now!!

Thanks for assistance in where to start looking MWJB.

However, for the benefit of anybody reading this with similar problems, let me state.

I am an Electrical Engineer who has done a LOT of electronic projects (though not audio), and also a head technician on high tech machinery, with which I do a lot of electronic repairs.

Now, I trusted my wiring 100%, and was adamant that I wouldn't make a bad joint. But, it came back to bite me! The amp construction was otherwise perfect, so for those of you, no matter how experienced you are, always check your simple wiring!

[MWJB]

Good news.

I don't like having the hook up wire terminating under the board, I know some folks think it neater, but it makes troubleshooting more difficult.

[PhilDawson8270]

I like it being neat, however. I think in future builds, my turret board wiring will be on top, if this was, the build would have worked first time.

But, we don't learn without the stupid mistakes

[Enzo]

Hi Phil, welcome to the forum, and glad your story ended well.

Good you can tell the sad tale on yourself. I have been soldering for over 55 years now, and while I like to think I am pretty good at it, mistakes still happen. I know better than to think I can;t make mistakes anymore.