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I corrected the mistakes I made,
pin 5 on both trannies looks unconnected!  :eek:,
pin8 on the bottom 1538 looks unconnected.

And now I have 51dB total gain.
Signal to noise 45dB
THD+n (a-weighted) 43dB

Does this seem about right ?
 
So I got some help at the end finishing this up, I had the DI jack wired wrong and a couple of missing ground jumpers or connections.

I think that the maximum gain is quite low though, I need to test it properly, as when turned up full the noise is very high, mainly hiss, but there is also some AC hum. What kind of gain should I be seeing on the output?

So hard to be completely objective about it, but I think it sounds amazing apart from these issues...!

DOJYPE.jpg


Thanks Jakob and to all who have written in this thread..

Gareth
 
Re 60 or 50HZ hum (or 100 or 120 hz)....

Forgive me if this is redundant.

Suggestions:

Between the transformer and the barrier strip, and the transformer and the mains: Twist each pair associated with the transformer (not all together, just the pair for each coil).  They will radiate less because the radiation will cancel more.

The big Caps are reservoir caps for smoothing, so the power from should avoid being run through AC fields.  One approach is to route them around the AC lines.  Another approach is to twist them (as you have done with the AC lines).  Twisting them will reduce the amount of noise they pick up because they will both pick up the same amount of noise and it "can" cancel.  Doing both is best.  You could also move the caps closer to the PSU.  (from the picture it looks like the leads going to the caps are separated and actually touching the ac PSU input lines).


Other stuff I could not see... 1) How is your DI wiring routed.  This is a very sensitive part of the circuit, maybe don't run it too near the case.


General rules of thumb:

Be careful about routing wires near inputs because inputs are amplified (and so is any noise).  This is ESPECIALLY true of the di input.

Twist anything that radiates
Twist anything that can pick up noise
Cross radiators and things that pick up noise at a 90 degree angle.
Keep radiators and things that pick up noise as far apart as you can (distance is nature's mu-metal).

(A gilding the lilly suggestion... The mains transformer tends to pollute the case with noise, so you can sometimes reduce noise by lifting the PCB a bit higher (3/8 inch standoffs instead of 1/4) if the tops of the tubes clear.  But this can only matter (and not very much)  if you are managing radiation in the box well to start with. It is primarily effective in getting the DI and the input transformers a little further from the case. Lifting the transformer would probably work too.)

Figure out what the frequency of noises is, and use that to guide you.  Low frequency constant noise is generally called hum and related to mains power frequency and multiples thereof (in the USA 60 120 240hz, etc)

High frequency noise is generally called hiss and is not related to power but usually related to high impedance inputs (like the DI input) picking stuff up (stuff includes switching power supply noise , fluorescent lamps, dimmers, radio).

When testing your preamp terminate the input with something that looks like a mic (150 ohm resistor for instance).  Unterminated it is not a good test.  Furthermore any noise test should be done with the lid on (because the metal box shields noise from fluorescent lamps and switching PSU's etc.).



djfatum a few posts before this one put some pictures up.  djfatum's wire routing appears excellent.  Look how far he was able to separate the AC and DC power. Look at the AC mains routing, it is very nicely done.  note how the input leads lean away from the PSU.  Note how the AC leads are short as practical, they run over the board (but not over the transformers) to be shorter.  The only improvements I could see might be to move the board further from the transformer and move the PSU caps closer to the board, but there may be reasons that he/she couldn't do that. djfatm did a really nice job. Here is a link to the picture:
11096678_636264866504427_4726492808626214371_n.jpg
 
It's been a while, but I have now completed my G9 and it works beautifully. I've documented the build here:

https://lensprojects.wordpress.com/2015/07/11/gyraf-gix-tube-preamplifier-build/

The main twist that differs from the stock design is the power supply. I used three smaller 12V transformers,  without wiring them back-to-back, so lower-power transformers could be used. Check the link for details.
 

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Hey all. I recently ordered a PCB from pcbgrinder and I'm rustling up some parts so I can build one of these preamp units. I'm very excited to get started on my first high-voltage or tube build and I plan to move very slowly to avoid catastrophe.

I wonder if anyone can recommend some pots for the input and output? I did a quick search through this thread but didn't see quite the advice I was looking for. What manufacturers/models have worked well for you? I've only worked with cheap Alpha pots. I see prices can vary a lot by manufacturer and I'm not sure if there's much benefit to paying more.

Thanks.
 
Another concern:

User deepdark helpfully posted this mouser cart, including the necessary resistors, caps and some other parts:
http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=0bb030cf12

Gyraf's parts list says all resistor values are 1/4w, 5% unless otherwise noted, but in deepdark's cart a lot of the resistors are 1/2W instead. Is this just for peace of mind or is there a good argument for using 1/2 resistors?
 
Thanks, gyraf. And thanks for opening up your design for us.

I'm trying to select appropriate transformers and I have a couple candidates in mind. Can someone confirm that these will be safe?
(Here in Canada we have ~120V/60Hz mains. From a convenient outlet in my living room I meausred 118.4vAC)

http://www.mouser.com/ProductDetail/Hammond-Manufacturing/1182J15/?qs=sGAEpiMZZMvwUzoUXIIvySPCJQuQgm7bHVHWGhyFRdy1apTpPsxxxQ%3d%3d
&
http://www.mouser.com/ProductDetail/Hammond/1182K12/?qs=qPaGMJUC%252bSuS8n1h4NMLTQ%3d%3d

Also, I want to be sure that these tube sockets will work with the PCB:

http://www.thetubestore.com/Parts-Accessories/9-Pin-PC-Mount-Sockets/9-Pin-Belton-PC-Mount
 
JingleDjango said:
Another concern:

User deepdark helpfully posted this mouser cart, including the necessary resistors, caps and some other parts:
http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=0bb030cf12

Gyraf's parts list says all resistor values are 1/4w, 5% unless otherwise noted, but in deepdark's cart a lot of the resistors are 1/2W instead. Is this just for peace of mind or is there a good argument for using 1/2 resistors?

Hi there

I just used 1/2W because it's always what I used, don't know why. Given the price difference, why not :)
 
JingleDjango said:
Thanks, gyraf. And thanks for opening up your design for us.

I'm trying to select appropriate transformers and I have a couple candidates in mind. Can someone confirm that these will be safe?
(Here in Canada we have ~120V/60Hz mains. From a convenient outlet in my living room I meausred 118.4vAC)

http://www.mouser.com/ProductDetail/Hammond-Manufacturing/1182J15/?qs=sGAEpiMZZMvwUzoUXIIvySPCJQuQgm7bHVHWGhyFRdy1apTpPsxxxQ%3d%3d
&
http://www.mouser.com/ProductDetail/Hammond/1182K12/?qs=qPaGMJUC%252bSuS8n1h4NMLTQ%3d%3d

Also, I want to be sure that these tube sockets will work with the PCB:

http://www.thetubestore.com/Parts-Accessories/9-Pin-PC-Mount-Sockets/9-Pin-Belton-PC-Mount

I used Triad transformer from Digikey. 50VA each. One is 120-240/15V and the other is 120-240/12V. Here are the models: 237-1333-ND and 237-1332-ND

Hope this help
 
Thanks, deepdark. I'm trying to consolidate as many parts as I can in the one mouser order to avoid paying a fortune in shipping. Would it be wise to get transformers rated for more VA? Would this keep heat down?
 
JingleDjango said:
Thanks, deepdark. I'm trying to consolidate as many parts as I can in the one mouser order to avoid paying a fortune in shipping. Would it be wise to get transformers rated for more VA? Would this keep heat down?

Take a look for the same triad over Mouser, they probably have them too. Yes it's not a bad thing to get a little higher rated transformer. No need to go crazy with some over rated things like 100VA but 50VA should be pl;enty enought
 
Just placed my order with Mouser. Thank you both for your help. I think it'll probably be another 6 months until I can afford the rest of the components and a case.  Transformers alone are bloody expensive.
 
I decided to try the OEP transformers instead. The Lundahls are much more expensive and if I'm being honest with myself, I can't afford them. The OEP transformers are available from Newark, who ship within Canada and who also carry the Lorlin switches needed for the front panel.

http://canada.newark.com/oep-oxford-electrical-products/a262a2e/transformer-audio-1-1-2-2-150ct/dp/02M8356
http://canada.newark.com/oep-oxford-electrical-products/a262a3e/transformer-audio-1-1-6-45-6-45/dp/24M0686
http://canada.newark.com/oep-oxford-electrical-products/a262can/transformer-enclosure/dp/25M7635
 
I went with OEP at input and Edcor output. You can honnestly go with whatever you want, just respect the impedance. Cinemag have had an awesome suggestion for input transformer. can't remember, though. CMLI-10/10 or something like that. Anyway, you can just stick with the OEP :) they will do the job nicely. I'm in Québec so I know that brand like Lundahl, Sowter, etc. cost a real fortune in shipping, exchange rate, etc. CAD vs GBP is about 1.85$ if not worse. It's completly insane.
 
In another thread about the OEP shielding cans, Jakob suggests using a 50w soldering gun to solder the bottom plate to the can. I'm using a Hakko 936 station which has a variable temperature up to 480ºC.  What temperature corresponds to a 50w gun? I suppose I should use a bulky tip.
 
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