DIY build - help!

Haych

So my high power Princeton build is finished.  I've powered it up and there are no sparks, bangs, smoke or other signs of bad things happening.

However, when I take it off standby things clearly aren't right.  First of all there is a loud hiss - not massively intrusive but it sounds like the amp is turned up all the way, when in actual fact all the dials are zeroed.

Whenever I touch any of the dials it gives off an intermittent crackling and there's also what sounds like low frequency feedback through the speakers.

I have briefly plugged in a guitar and turned up the volume a little and it does amplify the signal so it kinda works but would be nice to get rid of the low frequency feedback issue if nothing else.

I am for the moment a little bit afraid of the amp so it never stays on very long while it's not working right.

All valves I've used are good but I've tried a different and known good set and the same issues persist.

Not being an amp tech, or knowing what to check, I'm clueless as to how to diagnose the fault let alone fix it and while I'm not completely stupid I don't really want to kill myself - well, it's not the way I'd choose to go should I want to check out early, let's put it that way.

I appreciate that it's going to be very hard for anyone to help properly diagnose the fault without the amp in front of them but if anyone could suggest what the symptoms might indicate then that would be great.

If anyone is a glutton for punishment and does know what they are doing then I'd also be willing to ship it off to any tFB member who might want to have a go at fixing it and if there's a fee involved then I'm happy to talk about that too.

It was a fun project, I probably should have done a few things differently, but it would be nice to have it working and useable.

Thanks in advance.

Modulus_Amps

Photos?

bare basic, you need a multimeter to check voltages through the amp and check bias on the power valves

Does it have reverb and trem?

Haych

I’ll try to post some pics later if that’ll help. 

It does have Reverb and trem but the Reverb tank isn’t connected yet. 

I’ve got a tube amp doctor thingy for checking bias although I haven’t got that far yet. Too scared I’ll blow it up if I touch it when it’s powered on lol. 

I have a multimeter too, just don’t know what to do with it. 

ICBM

Haych said:

I’ll try to post some pics later if that’ll help. 

It does have Reverb and trem but the Reverb tank isn’t connected yet. 

I’ve got a tube amp doctor thingy for checking bias although I haven’t got that far yet. Too scared I’ll blow it up if I touch it when it’s powered on lol. 

I have a multimeter too, just don’t know what to do with it. 

If the tank isn't connected, remove the 12AT7 reverb driver valve - it's essentially a small power amp and with no load it can blow either the valve or if you're unlucky, the reverb transformer. Noise from the reverb driver can sometimes leak into the audio path as well, so it will eliminate that as a cause.

Does the amp have power stage negative feedback? Hiss is often a symptom of having none. Feedback is often a symptom of it being connected backwards (as positive feedback), but that will usually not just be a hum, it will be a high-pitched whistle.

Haych

Ok will remove the reverb driver and give it another try, thanks @ICBM. ;

I couldn’t tell you if the power stage has negative feedback or not - I’m not au fait enough with amps to know stuff like that. 

Wish I was, mind. 

ICBM

Haych said:

I couldn’t tell you if the power stage has negative feedback or not - I’m not au fait enough with amps to know stuff like that.

Is there a wire coming from the speaker jack that goes back to the circuit board and connects to a resistor? If so, it has negative feedback.

Haych

From memory, no. I don’t think there is. 

The schematic is here:

p90fool

Yes you do have one, it's R37, 2.7k at the top of the diagram.

Haych

p90fool said:

Yes you do have one, it's R37, 2.7k at the top of the diagram.

Ah, there we go then. Yes it does lol. 

ICBM

Assuming you did connect it! That’s the first thing I would check if there’s excessive hiss.

Haych

Couple of pics as promised. If there’s anything specific anyone needs to see then shout. Not the neatest job in the world but everything is solid. 

ICBM

Is the blue wire on the lower speaker jack in the pic the one that runs under the eyelet board to the feedback resistor? If so that’s the negative feedback wire and is connected to the wrong terminal on the speaker jack - it should be to the same one as the other blue wire.

RiftAmps

ICBM said:

Is the blue wire on the lower speaker jack in the pic the one that runs under the eyelet board to the feedback resistor? If so that’s the negative feedback wire and is connected to the wrong terminal on the speaker jack - it should be to the same one as the other blue wire.

Agreed, looks like you’ve grounded your NFB

Haych

Bugger! How did I manage to make such a cock up?

Thanks guys, will relocate that end of the wire and see what difference it makes. 

Stay tuned!

Haych

Righto!

So, negative feedback wire relocated and things are much quieter, although once I'd removed the reverb driver it fixed most of the noise anyway, and that low level feedback hasn't been heard since.

Still got some very loud intermittent cracking/popping, though.  Sounds kinda like the amp is turned up loud with a loose guitar input jack.

I can induce the crackling/popping by fiddling with the volume knob (not tried turning the other dials) or by tapping on the chassis!!

Not left it turned on for long so haven't tried a guitar through for a while.

p90fool

Be careful, a noisy chassis could be a grounding issue.

Haych

p90fool said:

Be careful, a noisy chassis could be a grounding issue.

Thanks @p90fool, I'll take that advice.  I have just checked resistance between the earth pin on the IEC and the far end of the chassis and it's reading 0.4 of an ohm.

I have no idea if that means anything at all for safety purposes though!

longjaw

It's a brave (or foolhardy) man who builds an amp without fully understanding the safety aspects involved.

Haych

longjaw said:

It's a brave (or foolhardy) man who builds an amp without fully understanding the safety aspects involved.

Helpful, thanks.

p90fool

longjaw said:

It's a brave (or foolhardy) man who builds an amp without fully understanding the safety aspects involved.

Most of the amps you buy in shops are built by semi-skilled workers, parrot-fashion. 

Diagnosis is a very different area though. 

Kalimna

I am paying this thread a lot of attention, as I will be picking up a kit shortly from @Modulus_Amps . I have built pedals from veroboard layouts, and know the difficulty in diagnosing problems there - can only imagine an amp being tricksier. I would be truly amazed if my own kit build proceeds without a hitch, and Im sure I'll be asking questions too.

Looking forward to seeing a completed amp.

Adam