[santa]

Hi,
I finished my scratch built Cornet last weekend and I must say that I am very pleased with the results.
This is the first tube RIAA pre I have built and it was interesting to experience the microphonic issues firsthand.
 I find that my Telefunken 12AX7's are pretty noisy. Kind of like tape hiss with the bass turned up a bit.
Currently running a Mullard in V1, Amperex in V2 and a clear top RCA in V3. This combo sounds good and microphonic issues are minimized.

I built the original version with the 30 K V3 cathode resistor as I had the parts. What sonic improvements could I expect when I add the transistor?

How can I post a pic?

[GRD]

I tried the resistor on the CF initially and then changed to the CCS (transistor).  The CCS seemed to "lock down" the sound, or make it sound more "solid", or clear up some haze.  I thought it was a definite improvement. 

[Bill Epstein]

How can I post a pic?

The far left icon in the bottom row is 'insert image'. Click on it and upload right from yopur hard drive.

I'd be very interested to see a scratch-built Cornet. Do add the CCS, so much better than just a resistor in so many different circuits.

[santa]

I need to tweak the series resistor in the heaters so I'll add it when I do that. I have a bottom cover of perforated stainless I'll be installing at that time. There is plenty of clearance between energized components and the cover. I may install some better coupling caps as well.
The tubes in the pics have been swapped for the ones in the first post.
The pre seems to have a bit less hum than my previous phono stage which was an APT Holman JFET type.
I place it alongside the turntable and dress the cables down each side.
I need to hook up with a woodworker as I don't have the tooling to make a really nice case.
For this one I just wanted a simple old school look. My wife really likes the jewel type indicator.
Here it is

[analog97]

Wow!!  You are an ambitious DIY guy.  Loking at the guts, you have confirmed why I bought the pcb from JH for my Cornet2.  That's too big a hurdle for me.  The hum issue you describe is complex and can come from many places.  I will let those more expert than me provide guidance.  Good luck and super kudos for your scratch build.   

[Bill Epstein]

a simple old school look

Old school, indeed, love that loong, loong, ground buss. Congrats!

[amandarae]

Very Nice!

Bro, about that "hum", maybe this can help

http://www.raleighaudio.com/Audio%20Component%20Grounding%20and%20Interconnection.pdf

I understood the article as follows, in summary:

"The signal reference and the power reference should be connected together for each circuit stage only.  If you try to make a connection from the main PSU filter caps to any gain stages, the signal current from the PSU which is the highest at the first filter cap will modulate any gain stages connected to it.  Remember, the ground reference does not act like a "vacuum cleaner".  Also, the shield(ground) and the signal should be treated as a pair."

As you can see, long ground buss is not always the way to go.  Of course it is different if you employ two-point ground technique especially if the RCA's are not isolated from the chassis ground, then a ground buss is more convenient to use being aware that there is always "hierarchy" of connections using ground bus, unlike stars where you can connect any star to the main ground point irregardless of order.

[santa]

The hum is really not that bad at all. My speakers are 97db and I confess that I have been playing a bit louder than usual.
I suppose I need to start with measurements and establish a baseline.

Should I leave the input jacks open for this?

Thanks for the link amandarae, I'll spend some time taking that in.
I had read about something like this in the past but it was a tough read as it was translated from another language.For this build I tried to keep all the PS noise at one end and the gain stages at the other.
Here's how I implemented my grounding:

From the line plug, house ground to the buss, TT ground to buss via green post, CT to buss, top plate to buss. first ps cap is grounded here.
moving past the rectifier,  ps caps to buss,  output grounds to buss, gain stage ground returns to buss, ps caps to buss and input grounds to buss.

The RCA plugs do not touch the top plate.

Any glaring errors here?

[amandarae]

The hum is really not that bad at all. My speakers are 97db and I confess that I have been playing a bit louder than usual.
I suppose I need to start with measurements and establish a baseline.

Should I leave the input jacks open for this?

Thanks for the link amandarae, I'll spend some time taking that in.
I had read about something like this in the past but it was a tough read as it was translated from another language.For this build I tried to keep all the PS noise at one end and the gain stages at the other.
Here's how I implemented my grounding:

From the line plug, house ground to the buss, TT ground to buss via green post, CT to buss, top plate to buss. first ps cap is grounded here.
moving past the rectifier,  ps caps to buss,  output grounds to buss, gain stage ground returns to buss, ps caps to buss and input grounds to buss.

The RCA plugs do not touch the top plate.

Any glaring errors here?

Nah!  Looks great!  Mine is Altec 604-8G's (100dB) and if I have a hum or buzz, it will be heard at the listening position.

Anyways, my grounding scheme in one of the preamp I built is somewhat similar to yours. The way I do it is this, assuming that we use the long buss that you have on the chassis.

Input ground from IEC(coming from the wall supply), to one and only one bolt via a ground tab directly touching the chassis and close to the 1st power supply cap.  On the bolt, just above the main ground ground lug, I connect the ground tabs of the HV CT and rectifier heater CT.  I  then connect a terminal strip.  The ground of the terminal strip is where the first cap ground is connected.  If there's a 2nd PSU cap or a third PSU cap, then I use one ungrounded hole of the terminal strip, connect the two caps ground together, and have a wire from that connection to the first PSU cap.  I know, why not connect it directly to the first power supply cap right?  I read that the reason behind was: "...because we want the higher magnitude current spike of the first PSU cap away from the other two caps".   Like what you said, "to keep away all the PSU noise out of the gain stages".  The ground lug for the TT connects to chassis only.

  On one slot of the terminal strip, not connected to ground, I connect the buss then extends it to the front close to the input, then ground it there through the chassis.

  From there, I will take the negative phase of the input RCA, the cathode ground of the first tube and the decoupling cap for the power of this stage only if any, in that order and connect it to the ground buss.  Any successive stages follows the same as you move along the buss towards the end that is not connected to ground.  Or, you can connect the output stage last tube(CF in our case) cathode ground, decoupling power supply cap ground of this stage, and any shunt resistor ground after the output coupling cap back to the chassis ground or to the ground where the 2nd and third PSU caps are joined.  I connect all the heater grounds for the tubes to the same point or to the chassis.

As you can see, there's no exclusive method in what grounding scheme to use.  In the end, what works in your lay out that does not produce anomalies would be best but keeping in mind to respect the circuit stages order as you move from input to output.

 YMMV of course.

Regards