My First p2p Project!! HELP NEEDED!!

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g9builder

Well-known member
Joined
Dec 17, 2009
Messages
87
Location
Midlands UK
Hi All,

Just wanted to share with you all my first attempt at a p2p project - a dual slowblow micpre.
http://s1209.photobucket.com/albums/cc394/g9builder/

I am a newbee to this game having before this project only built 2 G9's. They went pretty well and any problems I had were answered by great folks here. Many thanks!

I wanted to try and build one of these pres (based on the Don Hicks Slow Blow) and as there were no pcb's availiable I thought I would throw myself in at the deep end and try point to point.  ;D Well six months later and plenty of head scrathing but I finally made it. I do however have a small problem and am seeking the help once again of the good folks here to help me fix it?!!

As you will see from the pics its not the neatest thing inside the box and when I do my next project there are definately things I will do differently. I am also yetto implement the phase switch on the unit. I will do this as and when I decide to add output iron.

Anyhow my problem is this - noise!!

If I use the DI it works great and is almost noise free - (as good if not better noise wise than my G9's just a faint noise in the background but totally useable)

BUT - When I use the Mic input i get noise and pretty bad. Its not just mains hum either. it makes no difference If I select line/mic or 48v either and the noise increases with the gain knob. The pre still works and passes audio but just unusable as the noise is too great.

Therefore i obviously have a problem with the input stage because the only things in the circuit before the Di input is basically the transformer and the resistors for the 48v.?

I find it stange that it can be so quiet on the di but go through a transformer before it and its a noisey as hell. Same on both channels.

Like I said i'm a newbee and so maybe I'm missing something ? Any suggestions before I spend the next 6 months trying to eliminate the noise.?

Just a quick thankyou also to all at this forum  - there is no way in hell I would have been able to undertake this project without this facility here to search and find answers!! many thanks to everyone. Oh and big thumbs up to grand mater Audio for the front panel -looks great. Just a shame about the noise... ???

Cheers

ian
 
Show us a schematic.

What tests have you done?

Ask a specific question or two (about what you've tested, about the circuit, etc. instead of general questions).

Your build actually looks very tidy!  Wow, huge caps.  Congrats! 
 
did you use shielded wire for your volume pot ?
and proper grounding... you need to twist some of your cables too..
google p2p LA2A, you get idea of wiring in general for tube circuits ...
 
Is the noise hiss,  RFI EMI type, or just 60/120 Hum?

Just some thoughts - Possible that input iron is picking up hum from the PT.  I assume you used DC for the heaters.  Not sure how that translates in terms of elevation voltage but for AC heaters the slow blow version I built up on the bench required elevating the heater voltage with respect to ground.  I suspect that isn't the source though since you say the HiZ instrument input works fine.

One other general thing is the perf board.  Notorious for shaky joints so possibly a loose ground tie or other.

Build looks great BTW! - good luck. 
 
  Nice looking build !  I have been collecting bits and pieces of info on the SlowBlow Pre , as most who have built them rave about them.    I'm a newbee too, so I dont have  a clue to help you , other than making sure the screening cans are soldered to the trans ground tab and ground, ( I think there are pix of this in the G9 help thread )

  I am sorting out a regulated B+ supply for a 4 X NewYork Dave One Bottle P2P , coming along well, and when I'm finished, want to start on a 2 channel SlowBlow ..  watching with interest  ;D

Great build and info link kazper - Thanks

 
 
Hi and thanks for the replies.

Since posting I am redoing the layout to try and reduce/eliminate the noise.

I am getting hum and noise on both the DI and mic input but much worse on the mic input.
I have changed to shielded wire from transformer secondaries to diI input and onto grid of tube and this improved things. I also rotated the transformer which made a difference but I am still not noise free.

I have some 60hz noise and 100hz noise. The 60hz noise is almost gone now I have moved the transformer around a bit but the other noise/hiss is still there and increases with the gain knob. Also when I turn the unit off the noise dies away slowly - ?

I think most of it has to do with my wiring so I am redoing this trying to keep the length of connections as short as possible.

Are there any genral tips anyone can give about wiring tube projects p2p.?

The input transformers have the shielding cans on so I hope these are not the problem.
I'm determined to get this thing noise free as I have spent soo long on it.

many thanks

Ian
 
g9builder said:
Are there any genral tips anyone can give about wiring tube projects p2p.?

The input transformers have the shielding cans on so I hope these are not the problem.
I'm determined to get this thing noise free as I have spent soo long on it.

many thanks

Ian

Hi,

mic input transformer need to be shielded, just don't forget to ground the shield pin on your OEP. By looking to your preamp i would say you should move your terminal strips to the left and closer to tubes. HT wires should stay away from low level wire, use shielded wire for connections to and from pot, connect shield only at the source, etc. Really important thing is that all grounds meet at central ground point-star ground. Use fat wire for all ground connections and gound buss (ground wire that runs on the bottom of terminal strips).
There are more things about it, hopefully others will correct me if necessary and add more.

Good luck!
 
Thanks Tubeuser for all of your help  ;D

I've started redoing the tube connections on my new layout keeping all wires away from Ht
 
Hi G9builder,

What everyone has said is correct and you have made a good job of the project.
You have made it like Fender used to make their amps but they spent a long time getting the layout right before putting it into production.  You don't really need any boards.

This approach is not what I would call P2P, it is more like this:-
wchj7r.jpg

Here are my tips for achieving a noise free layout.

1.  Fit a big busbar from the mains inlet earth to the valve spigots, input tube last in line.
2.  Connect the power supply and cap earths nearest the mains input.
3.  input/output sockets connected to chassis via pin 1.
4.  Chassis only connected to earth at mains input earth.
5.  All wiring to input grid must be screened if longer than a few inches.
6.  The input transformer must be well shielded and earthed (think guitar pick-up)
7.  The heaters must always be at a higher voltage than the cathodes to avoid diode leakage.
8.  The tubes using the least current farthest from earth, the most the nearest.
9.  Wire components directly to the tube terminals using shortest lengths or wire (think aerials)
10.  All balanced wires to be twisted for common mode rejection.
I've probably forgotten something but I've covered the main points I think.
best
Davep
 
Thanks Davep!

i don't thionk i'm quite up to that kind of layout!! the one I have done has taken me 6 months  so far!!

I'll work down your list and check to see waht i can improve on!

Kind Regards

Ian
 
Ian,
dont sell yourself short, I wouldn't have suggested it if I'd thought it was beyond you.

The amp part which is equivalent your amp section took about 3 evenings, thats all.

I do it like this, first get a spare schematic then everytime you solder in a part you go over it on the schematic with a red pen.  When the whole thing is continously red then its finished!  That way nothing is forgotten and you can leave it to come back to and know just how far you got.

Since I followed this discipline every project has worked first time with no smoke!
best
DaveP
 
Also when I turn the unit off the noise dies away slowly - ?

ok.  This is a good basic troubleshooting observation and it differentiates between tubes on and tubes off issues.  Hook the unit to a power amp.  Turn it on - note whether you immediately hear the 60/120 hum or whether it only starts when the tubes start conducting (~9+ sec).  If it starts immediately you are likely looking at PT radiation pickup - later only - ground loop,  poor connection, or other.  It can be a combination of both.

I also rotated the transformer which made a difference but I am still not noise free.

You want to be very thorough and check all degrees of rotation between PT and each input transformer.  It can be a bit tedious but one method is to connect to a power amp and use a speaker for monitoring the hum level.  This allows you to keep your eyes on the circuit instead of having to watch a scope screen.  Try rotating input xform for lowest hum - hold it there - then rotate PT for best possible null.  Hopefully you 'll find something acceptable but you may be in a corner.  Distance and the amount of magnetic shielding on the input xfor and inherent radiation strength of the PT do set practical limits on what can be achieved without changing one or the other.

If the hum and noise is being caused by PT field into the input xformers then cleaning up the wiring will probably have no effect. 

As Dave mentioned double check that the input xformer screen shields are connected to ground. 
 
Thanks Lassoharp,

Yes input trannies are grounded.

Noise appears slowly so I think it must be a problem with the wiring. I'm redoing it and following the great input the kind people here are giving.

I will post back here once I have one channel wired back up with results and picks.

Many thanks all

Ian


 
DaveP said:
Ian,
dont sell yourself short, I wouldn't have suggested it if I'd thought it was beyond you.

The amp part which is equivalent your amp section took about 3 evenings, thats all.

I do it like this, first get a spare schematic then everytime you solder in a part you go over it on the schematic with a red pen.  When the whole thing is continously red then its finished!  That way nothing is forgotten and you can leave it to come back to and know just how far you got.

Since I followed this discipline every project has worked first time with no smoke!
best
DaveP

Hi Dave,

i like your layout a lot; short connections, good grounding, very simple, it's built on small space and connections will hold for long time. BUT, i don't think layout like this is a good practice for beginner because if it's not soldered well, shorts can happen, components fall off..specially when there are more controls, 2ch, etc.
I think Ian have good discipline, but by talking to him i got a feeling he would have troubles making layout like you did.

It looks like he followed Don Hick's layout which is on solder terminals:
Ian's
http://s1209.photobucket.com/albums/cc394/g9builder/?action=view&current=slowblowinside.jpg
Hick's
http://www.hicksorganservice.net/misc/pretubeclose.jpg

I think properly finishing (with some help from here) what he started would be a good lesson for him, and he wouldn't need to buy new components. And as addictive as diy is, i'm sure he can build something similar to yours next time, maybe 1ch of American preamp from 40's & 50's into small chasis and with only a few controls like on/off, pad and "volume".

Bye!
 
Just wanted to update you and thank you all for the help given (especially TubeUser who is a pot full of knowledge) :)

I now have the slowblow up and running (almost noise free) It is silent up until the gain pot is at and above half way. The thing has so much gain though that when DI'ing my bass last night I only need the gain set at 20% to get a good level into protools so completely noise free!!

I think i might need to implement a pad on the thing if I want to use this on any loud sources. I will do a  search on this.

To get to this stage I redid the heater wires twisting tightly and re-routing them. I re routed the hT wires keeping them as short as possible. I also redid the connections to the tubes from components again keeping the wires as short as possible and used shielded cable for the gain pots and DI connections.

I also found the best spot for the power transformer (by turning it around until noise free) and clamped it there.

A great sounding pre and a lot of fun to build,  - I also learn't a lot in the process too.

I do think though for someone like me who is still a real newbie that building off a pcb is easier than point to point at this momment in time.

Next project a compressor - LA2A or G1176. Mainly for trackng bass/vox and guitars - any sugestions?

Cheers

Ian :)

 
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