First time #183 Build

[Structo]

I second that motion.

The life you save may be your own.

[MusicMaker]

If you are getting a near full-brightness glow from the bulb that would indicate excessive current draw. The power tube voltages look good (these are without the bulb limiter, I assume),

Correct.

and you didn't report any smoke or smell, so it doesn't seem that there is anything seriously wrong. What happened when you turned on the main power? What happened when you switched from standby to play?

No smoke, no smell. Nothing 'happened' turning on main. Nothing 'happened' putting standby to play.

It made sound until bringing up the bias and then all hell breaks loose.

If I were to guess, it almost sounds as if the volume is maxed out when I bring bias up?

You don't show grid or cathode voltages for the preamp tubes. Were they zero or did you just not measure them?

Didn't measure them, it was late. see pic.

The loud buzz could be due to reversed PI outputs or OT primaries, either of which results in positive feedback to the PI. A quick test for that would be to disconnect the negative feedback from the impedance selector switch.

Pulled green -FB wire from 4ohm lug. With amp on limiter, the bulb glowed, but not as bright and when off standby it glowed bright and then dimmed.

See "OT primaries" pic and OT wires reversed pic - I might have them flipped! Followed Nik's diag!

Do you have a diagram of the OT you can post?

See pic




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Does it matter not having a guitar plugged in? Mine is, just curious.

[Structo]

The power tube grid should be corrected to show negative voltage, ie, -64v

That seems pretty hot to me.

[MusicMaker]

You are right it is minus 64.

-64

Just not used to thinking in the negative range.


What about the red and blue OT primaries? Would it matter that they were flipped?

[martin manning]

I was asking about the bulb behavior when the power and standby switches were flipped- bright flash, dimming, etc.

Looks like the OT primaries (red and blue leads) need to be reversed, or the PI outputs (the leads going to the 5k6 resistors on the output tube pin 5's) can be reversed instead. Just to be sure of how it is connected now, check for continuity (should measure zero ohms) as shown in the pic below.

From the voltage chart I don't see evidence of proper current flow through the preamp tubes, but let's try to get the power amp working and biased first.

[MusicMaker]

The presence of 400VDC on the speaker jack is not just a missing ground. The output transformer provides isolation from primary to secondary. Assuming the transformer does not have an internal primary to secondary short, there is a serious wiring error, and simply adding the missing speaker jack ground is going to cause a short from +400V to ground.

I do not mean to discourage the OP, but my advice is to take this build to someone with more experience to sort out the wiring errors - do it before you cause another injury to yourself (or someone else), and before serious damage is inflicted on the amp itself.

Who is in Colorado?

[martin manning]

It's not unusual to build up a high voltage on the OT secondary if it is not grounded, so I'm not worried about that. You have it grounded now, and if you had a short and the associated high current draw there is no way you would have 450V on the plates.

[MusicMaker]

It's not unusual to build up a high voltage on the OT secondary if it is not grounded, so I'm not worried about that. You have it grounded now, and if you had a short and the associated high current draw there is no way you would have 450V on the plates.

Thanks Martin.

That's comforting. I remember in school (years ago...) about such ground conditions.

I am very careful. I have one hand in my pocket when checking voltages.

Other than that missing ground wire, I've not been anywhere close to getting zapped. I think troubleshooting this is a good thing. I am learning and understanding and that's the point for me.

I get the sense we're not too far off.


What about the red/blue OT input wires being 'different' than the schematic? Should I flip them or leave them be?

[norburybrook]

if the OT primaries are the wrong way round you'll know immediately when you plug in and switch on .....trust me I've got it the wrong way 100% of the time on my last 3 builds ......the noise will certainly make you jump



Marcus

[martin manning]

I say flip them, but check to be sure you have zero ohms between the two points I marked in the picture so we know which side of the PI is driving which side of the power stage. Transformer wire color codes vary. Looking at the phasing dots on the OT spec sheet tells me the red and blue wires should be reversed. It sounds like it is behaving better with the NFB disconnected, and that is also consistent with them being swapped.

[MusicMaker]

I was asking about the bulb behavior when the power and standby switches were flipped- bright flash, dimming, etc.

Looks like the OT primaries (red and blue leads) need to be reversed, or the PI outputs (the leads going to the 5k6 resistors on the output tube pin 5's) can be reversed instead. Just to be sure of how it is connected now,
check for continuity (should measure zero ohms) as shown in the pic below.

From the voltage chart I don't see evidence of proper current flow through the preamp tubes, but let's try to get the power amp working and biased first.

Yes, there is continuity.


It might be simpler to reverse the PI outputs, as the OT red/blue are a bit short...


Shall I reverse them?

[MusicMaker]

if the OT primaries are the wrong way round you'll know immediately when you plug in and switch on .....trust me I've got it the wrong way 100% of the time on my last 3 builds ......the noise will certainly make you jump



Marcus

And yes, it's quite the noise indeed!

[martin manning]

It might be simpler to reverse the PI outputs, as the OT red/blue are a bit short... Shall I reverse them?

Yes, either way will work. Do that, reconnect the negative feedback and see if you can bring the bias up.

[MusicMaker]

SUCCESS!


It works!


I thought I was fairly careful wiring it.

The 'problem' was following Nik's layout - the color of the OT wires were a swapped vs the TAG schematic. (I wonder why he did that?? Hmmm). I think it would make the PI oscillate improperly. Might it even be additive to the signal therefore that huge noise??

Hopefully, anyone building one of these will follow the schematic and this thread and avoid that.



I've been jamming through it for the last 30 mins.

35mV drifted to 34mV, so I bumped it back up. I'm letting it burn in till after dinner, then I will measure Ik.


One thing I noticed is that the Volume control didn't seem to have any effect at all. The Main Volume did and it has a really sweet distortion too. No effect with the Treble, Mid, and Bass pots, nor did the switches up front do anything noticeable. Pedal works.

There's still the 10K and FET pots to tweak, so that might be the issue?


I made a diddle stick out of a plastic fruit kabob. It's about 8 inches long and perfect for inside. It may be 'amateurish', but I also cut a slot in the bias pot with my Dremel. Those tubes tend to get warm and it's a tight fit. Not sure a pair of needle nose in there is the wise choice!


Thank you Martin and everyone else who helped this project out!

[martin manning]

OK great!

Now get a complete set of voltages. Sounds like there is something still not right in the preamp.

With the reversed push-pull phasing what you have is a 100W oscillator, with no volume control!