The official G9 help thread

[Stagefright13]

I would think with my limited knowledge that you would need a higher ratio output transformer. Otherwise you would saturate the input tubes. For ass kicking gain I would change the output trannie. And keep the circuit as is.

But you may be surprised how good it is stock and leave it alone. However the 2 toroids create a HUGE field. So you may need to MU them from internal and external gear.

 

[Stagefright13]

OUCH! My G9 just fried! One of the rectifier diodes D19 just caught on fire. Melted the circuit board and everything. I replaced all 4 of them and it works perfect again. And the rectifiers don't even get warm now. Is it possible it just went south after 72 hours use?? Maybe the diode was bad? It's a 1 amp 4007. Do I need a higher amp one?

 

[gyraf]

1N4007 is what's needed - either it was bad from start, or was overloaded by a casual short...

Jakob E.

 

[crazytooguy]

Hello! I just completed my G9 and while I thought I had adequate heatsinking for the 12V regulator, I apparently did not and it gave up the ghost after half an hour. I'm going to mount the new 12V regulator to the chassis, but would it help to substitute a 78T12 for the 78S12, since the 78T12 has a 3A capacity? Seems like an easy extra measure of insurance. Thanks!

 

[gyraf]

The heaters in the G9 only draw 600mA of total current, so a 78s12 is plenty. They vrey rarely fail, because they have safe-area temperature shutdown - power off and wait, and they'll come around again. Easiest way too heatsink is to mount (isolated!!) on chassis baseplate.

Jakob E.

 

[steppenwolf]

I think you'll have 100mA extra on the heaters when using 5814 Tubes as they draw 175mA @ 12,6V per tube (700mA total), just a little higher than the ECC82...
relays and light(s) aren't mentioned, though...
With appropriate heat sinking should be no problem...

Greets,
Stefan

 

[crazytooguy]

My G9 is a bit of a modded one - I layed out my own PCBs, and added a relay on each channel for the Instrument input, high brightness LEDs for phantom and power indicators, as well as a VU meter circuit and small cooling fan. You wouldn't think those would add that much current draw, but I did manage to kill the 78S12 - it wouldn't come back on. I will heatsink its replacement to the chassis, though. Even though the heatsink I had for the regulator was fairly large, it got quite hot. I think it just wasn't enough.

 

[steppenwolf]

Hi!

I was thinking to use a VU meter, too.
My problem with this:
Where should I put it. Directly on the output?
That would be possible, but as the circuit uses an output pot, it can only measure the output but I could have distortion before the pot...
I would like to put an additional switch to be able to look at the level
1) Before the output pot and
2) directly on the output and calibrate those two settings with a resistor attached to the aforementioned switch.

Is this possible?
What is the impedance around the output pot? I don't want to drop the signal when loading it with the VU meter circuit.
That shouldn't be a problem with the 600Ohm output, right?

Thanks!
Stefan

 

[Stagefright13]

I just use my ears for sound. The VU may be nice to look at but your DAW or Tape deck will give you the level. Trust me VU is NOT something you need on this project. It is perfect the way it is. It's already as full featured as my commercial UA 610's less the ground lift. And of course that is an easy add.

Preamps as far as I am concerned don't need VU cause before they start clipping your next chain (Recording media) will be buried LONG before.

Especially A true tube pre such as this. I never saw a led meter or VU on a Marshall before. Use your ears and sidestep the extra stage. Just my opinion.

 

[steppenwolf]

Right, but the output pot is there to attenuate the signal in case you drive the previous stage hard so you won't overload the ad converter for example. But I would like to monitor, how much I'm overloading that stage...

 

[Stagefright13]

You can hear the overload trust me. Gentle overload has a nice smoothness to it. Great with an LDC. Smooths the brittleness of a cheap mic. Makes a good mic sound fantastic. Or just look at the knobs! Low input settings are very clean with moderate output settings. Just like a tube guitar amp. think of it that way. It is a tube amp for a mic. Use your ears and trust me you will enjoy it. But is not an easy project! At least for me I am not like the Super Pro's.

:green:

You speak great English from Germany!! My Family is from there. You are right the output is there to attenuate the signal to your DAW. It is there to allow the the input pot to be turned to the level of saturation or no saturation you desire. The output pot is there to give the desired output to your compressor or converters. And does a great job! Gyraf did his homework.

Your DAW or tape deck will see the levels of the output so no meter will be necessary. And I believe you need a meter buffer such as the one on the Purple 1176 so you won't add distortion. But I am an amateur experimenter so don't go by me.

 

[steppenwolf]

You can hear the overload trust me. Gentle overload has a nice smoothness to it. Great with an LDC. Smooths the brittleness of a cheap mic. Makes a good mic sound fantastic. Or just look at the knobs! Low input settings are very clean with moderate output settings. Just like a tube guitar amp. think of it that way. It is a tube amp for a mic. Use your ears and trust me you will enjoy it. But is not an easy project! At least for me I am not like the Super Pro's.

:grin: You mean, crank the gain and listen...

Of course you are right, but those VUs look so nice... :wink:

You speak great English from Germany!! My Family is from there. You are right the output is there to attenuate the signal to your DAW. It is there to allow the the input pot to be turned to the level of saturation or no saturation you desire. The output pot is there to give the desired output to your compressor or converters. And does a great job! Gyraf did his homework.

Thanks, I can't even think of how many hours I spent in english speaking forums. I think I have had a good education especially languages. Is it weird that I'm studying electronic engineering... :wink:
Where do your parents come from?

Have a nice day!
Stefan

 

[crazytooguy]

Yeah, the VU meters I added are basically eye candy. I did, though, calibrate them so that 0VU = +4 dBu so I have some idea of how much signal I'm hitting my A-D converter with. The circuit I installed has a selector switch to choose between VU indication and Peak Program Meter indication, which is more useful. Still, you don't really need it. I have to admit to liking dancing needles. :wink:

I take the signal for the meter circuitry off off the SRPP output stage, at the hot lead connected to the primary of the output transformer. It has a 47k resistor on it to prevent loading the output stage. The meter drive circuitry has an FET opamp buffer at the input, so there's no distortion of the signal it's monitoring. Stagefright's got it right, though: Set the gain by ear. That's what's great about the design; you can dial in controlled amounts of fat tubey goodness by gently overloading the stages. Or get nice shimmery clarity by not overloading them. Or go for that crunchy Lenny Kravitz "American Woman" vocal sound by cranking it. Best to set this by ear, though. Meter readings might be useful for a few times, but you'll stop paying attention to them after a few trys. Meters on a Marshall would be useless, but it'd look cool!

 

[Stagefright13]

My family is from Erlangen. Hope I spelled it right

The faceplate on my G9 is pretty busy I wouldn't even know where I could put a meter if I even wanted one.

 

[matta]

Ok, spent the last 2 hours reading all 20 pages of this thread... great to be able to afford some time for DIY again!

I'm in the process of building my G9 (abandoned the first one because of insufficient funds which became Godder's) and I have a couple questions.

For the 2 offboard filter caps C15-15 (100-220UF/350VDC min) I noticed that lot of guys are using these SERIOUS JJ Tesla ones, no doubt springing from Sklyar's G9 parts list... but it seems overkill...

Could one not just use a 'snap in' type, I'm looking at the Rubycon CE series, as shown below (ironically in the same value, series and voltage I'd be looking at) and try and find a mounting clip or mount them on perfboard? Anyone see a problem with that?

Th other thing is that with the power transformers, I have mine wound and was wondering if it is feasible to not just wind a single transformer with multiple taps rather than using 2 transformers, or is this a big no-no?

Thanks in advance!

Matt

 

[gyraf]

Could one not just use a 'snap in' type, I'm looking at the Rubycon CE series, as shown below (ironically in the same value, series and voltage I'd be looking at) and try and find a mounting clip or mount them on perfboard? Anyone see a problem with that?

No problem

Th other thing is that with the power transformers, I have mine wound and was wondering if it is feasible to not just wind a single transformer with multiple taps rather than using 2 transformers, or is this a big no-no?

No problem

 

[matta]

Thanks Jakob,

A man of few words, LOL!

On the transformer am I right in thinking the specs would be as follows:

Primary 1 = 230VAC
Primary 2 = 230VAC
Secondary 1 = 15+15VAC
Secondary 2 = 15+15VAC

Each Primary/Secondary pair being a MIN of 30VA, from the sounds of it it is better to go with 50VA.

Cheers

Matt

 

[Stagefright13]

One transformer is 230 to 15. The other is a step up to 220 volts. Here's a pic:

http://www.groupdiy.com/index.php?topic=16168

Obviously the pic is wired for 110 mains. But you get the idea. Only 1 transformer is connected to the mains.

 

[Stagefright13]

Oh and just for the record I found out why my rectifier diode fried. The power switch/iec/fuseholder was arcing. It must of had a defective switch. Sometimes when I turned it on the power light would flicker and I could hear a nasty high voltage sound. I hear arcing can increase voltage. New switch and all is fine.

I did notice after using this alot that the output pot on channel #2 will hum somewhat at over 80% volume. Didn't notice at first. But I have a bunch of mu metal and will try that around the transformers after my ebay tin snips arrive.

Probably going with higher rated transformers would be a good idea. Less load on them would probably tame the noise some. My 1176 has about 5 times the transformer needed lol. But Is quiet as a mouse.

But this is an exceptionally great sounding preamp. WELL worth the time and cost to build it. Thanks again Gyraf! It's without a doubt my favorite project I have built so far.

Oh and one more thing. Anyone that put a VU meter on their G9 could you show a pic? I can't even imagine where it would fit. I am obviously a big G9 fan and would like to see others creations!

 

[Godders]

I just built a second G9 and have a small concern. I used a different brand for the power transformers and they’re rated at 80VA (my original had 30VA and then 50VA because of low HT voltage). They are also stacked one on top of the other (they were mounted side by side in my first build).

When I power up, the transformers make a brief humming noise for about 2-3 seconds. After that all is well and the pre works fine.

I’m not overly concerned about it as I’ve left the unit powered up for around 24 hours without any problems but then I’ve built a few DIY units using torroidal transformers, including another G9 like I said and have never witnessed this before. So I’d like some reassurance that this isn’t abnormal and isn’t indicative of anything untoward or dangerous? :?

Cheers

Nick