G-PULTEC problems, please help!

[frazzman]

Just thought I'd share a lesson learnt today when troubleshooting one of my very noisy gpultecs. I had two built identically, both sounded great but one had a horrid hum/buzz even in bypass or with power off I'd still get noise on my meters. So after many hours of troubleshooting and checking grounds etc. The lesson learnt (and this could apply to any build is, if you're using a 'flat packed' style enclosure - make sure when you assemble it that all sides, front, back, top and bottom all have continuity with each other! You should be able to measure ohms from your earth connection to every single chassis screw hole!! After filing off all the powdercoat around each screw hole, I could then measure continuity between earth and every piece of the enclosure, and funnily enough the unit is dead quiet now. Zero noise in bypass. Seems pretty obvious in the end but an easy oversight when you know you've tightened every screw as tight as you can...but it still was not enough in this situation.  Ymmv

 

[frazzman]

Hrmmm... I'm still having issues with noise on one of my builds. I managed to minimise it a lot by ensuring proper grounding and continuity between all pieces of the chassis. I'm still finding that I have a slight buzz on the high boost, its more apparent at 2.1khz and gets progressively quieter at each step up. My ground is now spot on so I am wondering if maybe cheap inductors are to blame? When I disconnect the filter PCB theres no noise so I know that it has something to do with something on the filter PCB.... are average inductors a vulnerability when it comes to noise in the high boost? I have no noise at all in the low boost or in bypass. Thanks!

 

[gyraf]

You have magnetic interference from power transformer to the HF boost inductor.

Options:

- Rotate and/or move power transformer until noise goes away
- Move inductor away from power transformer (on extension cabling)
- Magnetically shield inductor (e.g. in an OEP can)
- Magnetically shield power transformer - box it in some soft iron or mount a shield between transformer and inductor

Jakob E.

 

[AntoineQ]

Hi everyone,

I've noticed a difference between GPultec schematics and PCB layout. In schemo, pin 7 of the input transformer goes to filter section and pin 10 to ground, while pin 7 goes to ground and pin 10 to filter in PCB.
Doesnt it give a phase inverted signal to the filter?

Antoine.

 

[gyraf]

yes, and then the amplifier reverses again. The pcb layout is tested and found good - if you have polarity problems, you just swap pin2+3 at output XLR...

Jakob E.

 

[atticmike]

I don't get it why nobody experiences the same hpf as I do. Does it have to do with the aurora's impedance?

 

[imloggedin]

What power transformer have people been using?

 

[atticmike]

What power transformer have people been using?

Found out that it's rather a build issue, I've opened up a separate thread for that, any input's hugely appreciated.

http://www.groupdiy.com/index.php?topic=52634.0

 

[gyraf]

Naa, that is for input transformer, not power transformer...

As for power transformers, generic 12V or 15V 30VA toroid transformers are known to work.

Look earlier in this thread for multiple options.

Jakob E.

 

[atticmike]

@Jakob

Do you refuse to answer because I've opened up a separate thread for my issue?

 

[gyraf]

Hi Mike,

Certainly not -  I simply don't know (or have opinion about) the LL88 ASC transformer...

Jakob E.

 

[atticmike]

Hi Mike,

Certainly not -  I simply don't know (or have opinion about) the LL88 ASC transformer...

Jakob E.

The issue is not about the LL88 transformer. It's about a malfunction of the powerboard and I don't know where it's coming from / how to fix it.

Could you please check the topic out?

 

[frazzman]

Troubleshooting a build for my friend.... we have an issue somewhere on the amplifier pcb. I've tried his filter pcbs and xlr wiring looms on one of my builds and they work fine.

As for the issue here - not getting any output really, well infact with headphones turned up to max I can hear a very very quiet signal, barely registering on the meters. All components appear correct, as are orientations. Tube emits a healthy glow

Were using a don audio toroid,  as I have done in the past so no room for trafo wiring errors.
No input tx

Tried swapping tubes, no luck...

A bit stumped on where to go next, my experience in the past with these is that they are a pretty straightforward build.. any ideas or suggestions ? Thanks in advance

 

[frazzman]

Some further testing shows that voltage across 3k3 resistor is too low at around 100v. Working unit shows around 250v. What I can't work out is why it's so low. Component wise there isn't much different between my working build and my buddies non working build. The only component difference i could observe was different bridge rectifiers and he's used higher voltage caps - I.e. 35v when unit calls for 16v , but surely that wouldn't make a difference ?

 

[gyraf]

If you have a drop of 100V over a 3K3 resistor, you're drawing some 30mA - which is more than you should. I think the normal range is somewhere around 5mA.

Check that you have some 1.5-2.5V at the lowest cathode, and some 100-120V at the upper cathode.

Swap tube for a known-working one?

(35V capacitors are recommended for heater supply - 16V is on the low side)

Jakob E.

 

[frazzman]

If you have a drop of 100V over a 3K3 resistor, you're drawing some 30mA - which is more than you should. I think the normal range is somewhere around 5mA.

Check that you have some 1.5-2.5V at the lowest cathode, and some 100-120V at the upper cathode.

Swap tube for a known-working one?

(35V capacitors are recommended for heater supply - 16V is on the low side)

Jakob E.

Hi Jakob,

Thanks for the reply. I've swapped the tube from my working build into the non working one but it doesn't help the situation.

For the polarised caps, this build has:
470uF/50V
10uF/35V
4.7uF/250V
4700uF/25V

..Which all looks good to me?

I'm just going through making measurements and comparing to the working unit.

Am I supposed to be measuring relative to gnd?

For example - when I measure either side of the 3k3 resistor + gnd I get 50.4VAC which is way too low.

Should I go through and measure voltage at each of the tube socket pins relative to ground and compare to working unit?

Measurements on the screw terminals indicate the right voltage coming in from the toroid PS secondaries - we have 262V across the 2x HV terminals and 10.4V across the other (i'm using Don's toroid).

I can see that obviously I am loosing some voltage along the way but I can't work out why...

The wiring is all solid, i've checked for shorts and there are no other differences to the working build so I suspect a bad component somewhere...

 

[frazzman]

On second check... scratch what I said in my previous post about the measurements being off between the working and non working units.

I stupidly forgot that I didn't have the PCB linked to ground so my measurements referenced to chassis ground where off when making comparisons between the two units.

Once I had the non working build pcb tied to gnd (as per the working unit) - I was able to replicate pretty much the exact same voltages between each unit (referenced to ground on both the working and non working units).

The voltages are more or less the same on each of the tube socket pins and across 3k3 resistor.

So that leads me to think that voltages are not the source of my low/virtually non existent makeup gain. With head phones on full blast, I can hear the EQ working correctly. What could be resulting in my lack of gain issue?

Bear in mind that I swap to the working unit with the exact some configuration, everything works beautifully (so this rules out a audio interface/levels i/o issue).

It's safe to say this ones got me beat tonight but I'm really eager to resolve it

Thanks again

 

[gyraf]

If you have approximate correct voltages, the amplifier is working.

Check that you have signal coming in on the first grid of the tube.

Check connection from amplifier output (cathode of upper tube half), through 4u7 output capacitor, and through transformer primary (continuity check the trafo)

Check continuity on your output - you should have some 15 Ohms between XLR(m)pins 2+3 - this is your transformer secondary.

Jakob E.

 

[firfe]

I finished my g-pultec but have a problem. The HF controls are not working properly. LF works fine but the HF changes in frequency , boost and cut are barely noticeable.

My voltages are not right.

HT is 324V not 250
6.3V is 6.14V
The 220v coming off the transformer is 259V
The 9v from the transformer is 10.6V

Anyone think I only have a bad transformer? I'm using don-audio g-pultec transformer.

hi
i've the same problem with the don audio trafo.
i've change the 3K3 with a 18K10w and it's looks ok ...
but i've a buzz ....
maybe try with another trafo.
what was the issu ?

thank you in advance and sorry for my english

 

[gyraf]

Pultec depends on low drive impedance - i.e. the preceding stage must be able to drive it. Many consumer hifi thingys will not, and symptom is lower hf boost available. This can also come from wrong input transformer, if it has too high DC resistance.

As for hum/buzz, see earlier solutions

Jakob E.