[BUILD] GIX-51X tube preamp

[bruce0]

I checked the HV on the output of the HV board and it's 242V when I power the unit up and then decreases to about 233V after half a minute. The voltage should be around 245V I guess.
Should I change the resistors on the voltage-divider to get closer to the 245 Volts?

242V is fine, and if you have heater voltage (which you do) then you are heating an powering the tube correctly.

Your brief error with the cap should not have made a difference, that is just the voltage reference and you stabilized it incorrectly (although you did put 230 plus volts across the 50 volt cap for a bit, it is a ceramic and probably can take it, and if that was the problem then it would be short or open, and both of those would manifest as a stability problem so that is not the problem.)

Photo's would help but here are some thoughts:

There is a jumper labeled on the board HV jumper.  It needs to be connected somehow with a wire or resistor (I used a resistor but it works with wire too).  if not, then your lovely 242 volts are no where near the tubes!

So some test methods:

A thought on HV probing (this is what I do)
Plug the unit into a test power supply turned off. Select a good set of probes for your DVM (no loose wires, cracked insulation or funkyness allowed) ground the COM leg of your DVM with a clip to a good ground on the card and then tape with electrical tape over the entire end of the RED dvm probe exposing only the tip.  Set the DVM for 2000 volts and then turn on the PSU, don't connect any speakers or headphones (so you don't blow them or your ears), wear rubber shoes, no loose cloths, and with your left hand in your pocket and NOT leaning over the unit you use your right hand to carefully probe the HV source. (this avoids making you a ground, and avoids having as much current through your heart if you become one, you could screw up and get blown across the room but you have a better chance of not dying this way.)

What to test:

I would probe the bottom plate of the tube board and look for HV to the tube itself.  There should be.

There should be I think around 70 to 100 volts running to one side of the output cap, and pretty much nothing on the other side.

if you have that, then you have 1) a heated tube (because you saw it) 2) B+ to the tube (because you measured it) and 3) connectivity to the output cap (which is working fine).

If you have all that, then you have to kind of wonder about RV1 the output pot or the filter network (high pass filter on the switch) turn the switch off and that eliminates those filter caps (if you forgot to install them.. on the little daughter board. like I did the first time).

I suppose you could have a bad or incorrect output pot, you could check that (should be 50K ish across it and the center should vary resistance )



After that it is a bad tube but I am not willing to suggest that without some more data.  Here is the data that would help:

What does it "sound" like or what signal are you seeing? is it a clean signal? Noisy? Tell us more.

What is the input signal?  Are you using a mic or a test generator?  What is the impedance of the test generator and where is it connected?

Suggestion:  You can connect a test generator  or signal to the High impedance plug on the front of the unit... thus bypassing some circuitry (good for debugging). You should turn OFF the phantom power (good for protecting test generators) and turn mic line to LINE (though it doen't matter if you are using HiZ input plug).  The output POT should be adjusted to the middle position (just in case you have a reverse log pot in there or something, at least some signal will get through).

Show us a picture of the card.  It is worth 1000 words.  A nice big picture clear is good.  A good place to put them is Photobucket it is free, and then link them here which they make easy.

Inspect the back of all the cards for solder bridges and cold solder joints.  Do this with a strong magnifier (my eyes are bad I use a Loupe) slowly, left to right and right to left creeping across the face of the card, stopping at each joint.  have a sharpie with you to mark questionable joints.  If it doesn't take 20 minutes at least, you didn't do it.

Inspect the front of the card for missing components.  Really... it is quite common.  Any open holes in the card should make sense (like you know why nothing is in it).

Let us know more details of sounds, measurements, etc.

Good luck

Bruce

 

[Benny]

Hi Bruce.

Thanks for the detailed information. I'll post a photo of the board tomorrow and try to check the rest you've mentioned.

Benjamin

 

[gemini86]

Hi Benny,

Looks like bruce is on top of helping get you started. He's right about being cautious around tube voltages, your natural instinct is to brace yourself when trying to poke around, but make sure you don't. No part of your body should be touching any part of the preamp or the table you're working on.

First thing I do when I fire up a new piece of gear is check all running voltages. Print out a copy of the schematic, then start checking voltages and write them down next to the corresponding points in the circuit. This will make it easy to spot problems. Then if everything looks right, you can move on.

You can post your voltages here for us to help troubleshoot, just check at each pin on each tube. Looking at the tube pins from the bottom, pin one is the counter clockwise most, and then 2, 3, etc... they are also labeled V1 and V2, v1 being on top and V2 on bottom (it's weird I know, but the layout just worked better that way.)

Let's do that first, then we can move on to signal flow testing.

 

[mylesgm]

pulling parts together.  Would the Lundahl 1530s currently for sale in the BM suit this build?

 

[Benny]

Hey guys.

Today I found out that somehow my 51X psu puts out -34V on the -24V pin. Last time I checked the voltages was about half a year ago where all voltages were perfectly set. Misterious. I'm gonna fix that first before I investige further on the GIX problems. The -24V are used for the heater voltages, I think. So can a too high heater voltage result in a lack of gain in my preamp? I just turned power on for about a minute or so on every test. So not longer than 3 minutes with too high heater voltage.
Time for further testing...

 

[100WChris]

Hi!
I´m finishing my frontplate with frontdesign and i wanted to be sure about the Phase-Switch:
Which position is (up or down) the reverse-Psoition or 180°? i can´t really tell from the schematic and the fotos?
thanks
Chris

 

[Benny]

Hi.

The original frontplate says IN(in phase) in the upper switch position and OUT (out of phase) in the lower.
But I haven't checked if it's actually right.

 

[Marc Duchesne]

Here is my new addition of those pre's, sounds sweet.

 

[mulletchuck]

Where'd you get the knobs?

 

[Marc Duchesne]

Here...

http://www.tubesandmore.com/products/P-K900

regards,
-marc

 

[Marc Duchesne]

Hi people, is this normal to get some induced noise when touching the switches. Ground or something... I am also getting some random really small buzzing noise on the output signal.

Thanks for any help !
-marc

 

[mulletchuck]

Like what happens when you touch the strings on a stratocaster and how it mutes the buzz?

 

[Marc Duchesne]

Yes exactly... If I lower and raise volume it goes away. It is pretty small signal but annoying.

 

[Toure14]

Rodney I'm placing my order for tubes today and had a quick question. This is my first time working with tubes so is there anything I need to know? Or is it as simple as ordering 2 identical tubes? or do they need to be for input or output? certain size and style?

Again sorry for the newbish question but I want to make sure I get what I need thanks.
G

 

[gemini86]

Hi people, is this normal to get some induced noise when touching the switches. Ground or something... I am also getting some random really small buzzing noise on the output signal.

Thanks for any help !
-marc

Hey Marc,

No that's not normal. Check to make sure your chassis/L bracket has good continuity to ground. Place an ohm meter between one of the toggles and a good known ground point. If you're not in a chassis yet, this could go away when you get it all buttoned up.

Rodney I'm placing my order for tubes today and had a quick question. This is my first time working with tubes so is there anything I need to know? Or is it as simple as ordering 2 identical tubes? or do they need to be for input or output? certain size and style?

Again sorry for the newbish question but I want to make sure I get what I need thanks.
G

I recommend new tubes for this preamp, unless you want to hand select a couple really quiet ones. I've V1 is the only real concern here, since it handles most of the voltage gain. V2 always seems to be silent for me, no matter what crusty old tube I jam in there.

ECC82, E82CC, 12AU7, doesn't matter. I have some old GE's in mine now. Pulled out of an old Hammond B3.

Just don't be a fool and spend money on the gold pins.  :

 

[kvothe]

Hello everyone,

it has been some time since I built these, but I did join to learn and contribute. So here are some pictures.

The first pair I built with both Lundahl input and output transformers and a some very nice matched 60´s Tesla 802S tubes. Very clear, 3dimensional, a lot of gain. I have used these live for leadvocals and love them.

The second pair has Ed Anderson´s input transformer and matched 60´s RCA blackplates tubes. They have a little less gain, a more "up front" sound, better bass response, but occasionally I hear a frequency changing "whine". I think maybe it is a shielding issue, but mu-metal is on its way (I will copy what bruce0 did)

So far this project is great. Needs some time to fit everything, but greatly rewarding when they are up an running.
I have additional pairs of GE triple Mica, Lorenz and Amperex "Bugle Boys" and I will test those with the different transformer options when my microphone split is done.

The last pair I will do with OEPs and an all black frontplates I got from Frank at NRG.

You can find more pictures in my galleries.

 

[bruce0]

As I posted earlier, I measured the effect of the Mu-Metal shield and it is pretty darn small, 3dB in noise at most.  This is unlikely to be your whine.

When I had a whine it was a noisy tube.  If you don't have a spare tube then try swapping the two for each other.

 

[kvothe]

I will check for noisy tubes, but this "whine" occurs periodically, that is why I did not recognise in the initial test. Could it be something else?

 

[bruce0]

I had the intermittent whine, seemed to go away at some times (gain levels maybe) or maybe it was just the vibration of changing gain levels.  I would say the whine I heard was maybe 3k-8K hz never could measure it successfully but it was a high whine, and would sort of fade/flop in and out.  That sound familiar?

I fixed it by replacing the tube, but I don't remember whether it was the gain or buffer tube (probably gain).

 

[emrr]

sounds like a tube noise.  leave them on a few weeks and it may go away.