Some G9 questions

[madreza]

thanks Jakob

what I've heard for the moment sounds real great !

I'm gonna test it this evening on some vocals !!!

:grin: :wink:

Cool

 

[tweety]

Hello all,

i´m new here! I have done my G9, but now i had some 100HZ
hum on all 2 channels. :sad:
I have checked all my wires but i don´t find the problem.
The hum is very low, but if you turn the Gain to 11 and the
Output CW then you can here it with the Mic signal.

I think this is a ground thing...but i don´t know...

Grettings
Tweety

 

[funkalicious]

i had the same problem. try to increase the ht-filtercaps. this solved the problem for me.

cheers
dietmar

 

[tweety]

Sorry for the delay,

i´ve bought two 330uF/385V caps and now it works great!!!

THANXXS
Tweety

 

[gyraf]

Thanks, Dietmar!

:thumb:

Jakob E.

 

[netcastle]

Okay, I am resurecting this thread.

My problem is that I am getting no useable sound at all out of the unit on either channel. My question is, when I crank up the gain to its highest settings (on one channel and then the other, tested separately) I hear oscillations on both channels. Does this mean that the unit is amplifying, and no signal is getting to amplification stage?

Tim

 

[gyraf]

Tim,

Try tracing the signal through the circuit. That's the only way that makes sense, if you want to know where your signal disappears.. Fix oscillation later, if that persists after you've corrected the errors..

Jakob E.

 

[netcastle]

I am using open circuit 1/4 jacks. Not switched. I noticed that the schematic shows the signal path flowing through this jack whether it is from the xlr or instrument. I can see where this would create an open circuit for the xlr and so I will change the jack. The only question I have is shouldn't the instument input work regardless of whether it is switched or not?

Tim

 

[Soeren_DK]

? :shock:

After a long time away from my unit I decided to fix it. I got this hum on both channels and a little crackel. I put on new 0.5 mm2 wires from the mainboard to the controlsboards. I fixed all wires from the toirod and place them safe on the board. I set up to wires from my ground/0V point to the output shield and it helped a lot. But the the little hum and a little crackel still is there.

I did some research and found out that:

- Channel A without the tubes in channel B there was nothing on A.
- Channel B without the tubes in channel A there was a little crackel but no hum. The crackel is probably because of the toiods. they a close to the channel B.

But if I put all the tubes in the unit the hum start.. Can it be C14 and C15 that are to small (100u - 350V)???

HELP


www.mentol.dk/pics/before.jpg
www.mentol.dk/pics/after.jpg

 

[gyraf]

The HT is regulated, so this is probably not the problem (unless you have too low an AC voltage for the HT)

Could be the heater regulator? Remember good heatsink for that one.

Check ripple voltage on your supply lines - that will show where the problem is..

Jakob E.

 

[Soeren_DK]

Hi Jakob.

I had the regulater in sight. I think i will ordre a new one. But how can i check the ripple voltage on the supply lines? With a multimeter or with a scope and what is the supply lines?

 

[pmroz]

.
Soeren... this is a guess.

Regulated power should give flat (DC) volts.
Ripple voltage will read as sine wave.
So... i think... but don't know for sure...
If you read the supply lines on AC setting of DMM, maybe it will show the ripple in the signal.
Or scope on AC setting will show sine wave amplitude (ripple)

You should get confirmation on this...
I have a little knowledge, therefore i am dangerous.
Also never built a tube anything... if there are other safety issues for this, please somebody pipe in..

 

[netcastle]

It works. Could have been anything the first time around. I completely disassembled and reassembled. I suspect it was the switching jack (or lack thereof, duh) that caused my initial problem. Now, there is just one immediate kink that I would like to describe that I haven't really seen discussed.

Early on, before I had a solid signal path I noticed oscilations when I turned the gain past 8 on the right channel and 7 on the left when using the microphone input. After reworking the the unit I no longer have audible oscilations, but both sides, respectively, still misbehave at these gain stages when set to microphone. My routing mixer shows a solid spike its output level meter (meaning the level doesn't change) as if the G9 unit is generating a constant DC voltage at output (I haven't tested for this). There is still an audible mic signal, but it is greatly diminished vs what the signal is at lower gain settings. I do not have these problems when using the instrument input.

Any ideas?

Tim

 

[gyraf]

Tim,

that could well be inaudible-frequency oscillation. Look at the oscillation-fixing posts..

Jakob E.

 

[WJS]

be aware that some multi meters will read dc as half of an ac wave form. mine, for example gave me a headache when I was building this power supply for a pre amp. I was using my variac to bring up the b+ and had the meter showing what exactly the b+ was. I didn't get very far because the meter reading was 245 on caps rated for 250. this was scarry because the variac was at 1/2 line voltage. solution: meter was set on AC mode not DC. when I put it on dc it cut in half, right to where it should be. and I do know the AC side of the meter works, just doesn't like DC.

I wouldn't expect there to be dc on the output of the g9 as the last tube stage is cap coupled to a transformer.... unless something is royaly messed up I dont see how you could get dc on the output.

my guess is something is giving you too much gain, or a signal wire is too close to an output something

looking for oscilations seems like the way to go

 

[netcastle]

That has been my quest. I have tried shielding the 5 wires from the control PCB to the main PCB, but that didn't work. It didn't even alter the outcome slightly and considering the fact that the instrument inputs work fine at the higher gain settings I assume that these wires are not a part of oscillation equation.

Just on a side note, what is the 68pf (c6) cap for?

Tim

 

[WJS]

I believe it is a high frequency bypass cap. I'm not the g9 expert... but thats my guess

have you tried putting a multi meter accross the output? I wouldn't expect the mixer to show a reading of signal present with a dc only input.

what exactly is the problem you are experienceing? at low gain you get a noisey output? when using one input or both? i'm not sure i understand exactly. i'd love to help

i'm sure shielding the wire is part of the equation, just not the part thats out of wack for your particular project.

 

[netcastle]

No, I haven't tried a multimeter. The pre sounds great on both channels except when the gain is set above 8 clicks on the left side and above 7 clicks on the right side. When I set the gain to these respective levels, the mic signal diminises greatly (moreso with every click) I don't hear audible oscillations, but the mixer that I route through shows a solid level reading at peak (in other words the meter is stuck at peak). There are no audible cues to accompany this except for the diminised mic signal and a bit more pink noise. When I use the instrument jacks I do not have these problems (gain works all the way up the dial).

Like I said, this thing sounds beautiful, it just doesn't like to drive a mic signal at these higher gain settings. Wierd, huh? I'l read on.

Tim

BTW - I used Radioshack ceramics for C6. I'm wondering if a low quality cap could have anything to do with it.

 

[Minguta]

[quote author="netcastle"]Like I said, this thing sounds beautiful, it just doesn't like to drive a mic signal at these higher gain settings. Wierd, huh? I'l read on.[/quote]
Same my problem. But the problem is wrong wired front Panel Jacks for me.
Check Tubes too..

 

[netcastle]

Minguta,

Was it exactly the same problem that I described in my previous post? If not, could you describe your problem in a little more detail? Is your problem fixed? I was very careful about how I hooked up the PCB's, but I will check again.

Tim