Interstage curious

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Nice.

A 'feature' of grid cap tubes like 6J7 is the ability to switch grid caps.  I have a grid cap lead wired to a jack with a ground clip.  I can pull the grid cap lead coming from an input transformer, attach the DI grid cap and ground, use a preamp as a DI. 
 
EmRR said:
Nice.

A 'feature' of grid cap tubes like 6J7 is the ability to switch grid caps.  I have a grid cap lead wired to a jack with a ground clip.  I can pull the grid cap lead coming from an input transformer, attach the DI grid cap and ground, use a preamp as a DI.

I thought for a minute about doing something involving a switch to select the instrument input (sort of like Jakob did with the G9).
I've already got two other tube preamps with DI inputs, so I'm not exactly married to the idea of having one here.

A was a little curious about why I didn't get away with it here, seeing as it's the same input stage as the last project.  Maybe having such a ridiculous amount of gain just means the input stage is all the more sensitive to things like that.

I'll kick it around over the weekend and see if I can make it work, if only to say I tried.
 
EmRR said:
A 'feature' of grid cap tubes like 6J7 is the ability to switch grid caps.  I have a grid cap lead wired to a jack with a ground clip.  I can pull the grid cap lead coming from an input transformer, attach the DI grid cap and ground, use a preamp as a DI.

That is a really cool trick, Doug!  Might have to solder up one of those myself.
 
I have a feeling the hum that snuck its way back in is coming from the phantom supply.

I slapped together a little perf board with the decoupling cap, LED, and resistors to keep everything tidy.  Then ran the 48VDC to the primary of the input transformer.  Grounded it correctly.  The perf board is parked near the front panel (where I was planning to put it), far from the input stages, but within a few inches of the output stage tube.  When I moved it toward the back of the chassis, the hum faded away.

I guess I'll be relocating that.  Or shielding it.  Was hoping to have the switch and LED on the front panel.
 
I don't believe this thing!

The hum is back!  Maybe it never left in the first place.  Anyway, the thing hums like crazy regardless of gain settings.  I went back to just the first two stages connected to the output transformer, and it went back to being unbelievably quiet; so we're back to chasing down some sort of ground loop that only shows up when the PP stage is added.

And this is without the phantom supply hooked up, or any other bells and whistles.

One possibility...
The additional gain is simply so much that even the slightest supply noise from the previous stages is amplified.  I tried adding additional decoupling before the IS transformer stage, and I added decoupling to the PP stage.  Still hums.

Another possibility....
These particular transformers (especially the output) just don't like being hooked up this way?  Maybe the circuit is a sound concept, but requires hardware specific to this purpose?

Ground loop?
I've tried every grounding scheme I can imagine.  Clearly, something goes horribly wrong when connecting the PP to the IS transformer.  All else is quiet.  Two stages into the output is quiet.  Input directly to IS primary is quiet.

I even went so far as to move the thing into my studio to test it.  Turned off everything else in the room.  No HVAC noise or other background noise to interfere with listening.  Clean power.  Still hums.



 
Fluorescent Lights?

Modern high frequency fluorescents radiate all kinds of hash, I have to take noise measurements with an old school incandescent bulb for lighting in my workshop.

DaveP
 
DaveP said:
Fluorescent Lights?

Modern high frequency fluorescents radiate all kinds of hash, I have to take noise measurements with an old school incandescent bulb for lighting in my workshop.

DaveP


None whatsoever.  I even unplugged my freezer for a few minutes, and that's not even on the same branch.
 
One week later, and I have made HUGE progress!!

I finally found the hum loop in this gadget!  It was staring me in the face the whole time.  I knew I had to be dealing with a ground loop that was somehow only rearing its ugly head when both "halves" of the circuit were running.  Each stage by itself was clean.  So, what was happening only when both were in the works?  After going back and re-reading everything I had about grounding, I kept being reminded of a very basic (novice-level basic, at that...) rule.  A ground loop is caused when there is more than one path to ground in a circuit.

So I sat there staring at this thing, looking at where there could be two paths to ground for the audio circuit.  There it was, right in front of me the whole time.  Right next to the power supply, I have a terminal strip where the various supply rails come out.  Makes it easier to change things during the design phase.  Anyway, two terminals are for the B+ supply and the 0V ground.  There were two wires going to 0V.  One came from the board for the first two tube stages.  The other came from the PP stage.  All this time I had treated them as two separate circuits with an interstage transformer between them, almost as some sort of "demilitarized zone."

I guess I also figured that, ultimately, they both ended up at 0V anyway.  I kept thinking of it as a star ground.  I forget which document I read that got me thinking in terms of a bus ground (think ground bar on a point-to-point rig...).  So, I walked my way from one end to the other, and instead of connecting the PP ground to the 0V terminal, I connected it to the same ground run as the rest of the stages.

The big, nasty hum disappeared.

I could still hear a slight buzz, so I set about shuffling a few things.  Discovered that by rotating the supply transformers a little I could get rid of that.  All that was left was some hiss.  Believe it or not, actually replacing the first tube with an OLD one got rid of that!

Added phantom power, and the noise stayed away.

Added an instrument input, and the noise stayed away, until....

When I plugged in an instrument, it started buzzing again.  Tried a few tweaks to that input in the way of grounding, shielding, etc., but just can't seem to get rid of the buzz when something is plugged into the instrument input.  Not a deal-breaker.  I was happy to not bother with that feature anyway.
 
CurtZHP said:
When I plugged in an instrument, it started buzzing again.  Tried a few tweaks to that input in the way of grounding, shielding, etc., but just can't seem to get rid of the buzz when something is plugged into the instrument input.  Not a deal-breaker.  I was happy to not bother with that feature anyway.

What is the 'instrument' that is being plugged in ?
 
Newmarket said:
What is the 'instrument' that is being plugged in ?

Bass.

Tried a different cable.  Haven't yet tried a different instrument.  But it's quiet on anything else I plug it into.
 
OK, we're going to blame the hum in the instrument input on the instrument in this case.  I tried it again.  As long as my hands are on the strings, no hum.  If I turn down the tone control on the instrument, no hum.  And the hum actually almost goes away when I unplug the instrument.  And the mic input continues to be clean as a whistle.

By the way, the hum aside, my bass sounds really good through this thing!

I think I can finally finish it!

 
I'm never going to finish this boat anchor....

Whatever I did to earn this much penance, it must have been a doozy!

Got everything hum-free and laid out nice and neat.  Replaced a few old nasty components that were left with some brand new ones.  Fresh wiring job, keeping it clean and following what worked during testing.

Connected it to the same mixer input I've been using for most of this testing.  Immediately, without even turning the gain up, I can see meter deflection on the mixer.  Significant meter deflection.  Hear nothing in the headphones except for a slight hiss.  Hooked up the scope and see a nasty looking waveform.  If I'm reading the scope right  ---  it's set for 0.1V/div, 50uS/div  ---  this signal is 300mV at a frequency of 20kHz!  See picture....

I started picking my way through the circuit to see where the fun starts.  Once again, we're back to the PP stage misbehaving.  Tried swapping tubes.  No good.
Put back the old components.  No good.
Checked and rechecked wiring.  No change.
Redid some wiring, just for grins.  No change.
Pulled tube for first two stages.  No change.
Completely disconnected first two stages.  No change.
Checked supply rails.  They couldn't get any flatter.
Sacrificed a live chicken to this thing...  No, not really; but thought about it.

Additional oddities....
The nastiness also shows up when connecting the scope to the primary of the IS transformer.  But, when I disconnect the PP stage and look at either side of the IS, it's flat.
Making minor adjustments to the trimpot on the PP cathode network, causes the suspect waveform to shrink somewhat, but also brings the level of this noise up on the mixer meters.  Of course, this also screws with the current balancing act that needs to be done on the output transformer primary.

 

Attachments

  • scope.jpg
    scope.jpg
    209.6 KB · Views: 9
Went back over this thread, because something told me we had been here before.

Sure enough, I did have a problem with a 20kHz oscillation previously.  The problem seems to have been a funky tube socket.

Gonna clean this one and see if it helps, but might have to replace it.
 
So, after cleaning a few things up, and a few more tests to make sure everything is well behaved....

Gentlemen (and any ladies that happen to wander in here...), I give you The Fe-3!

It works!  Can't wait to record with it!

 

Attachments

  • 0218200923.jpg
    0218200923.jpg
    650.9 KB · Views: 19
Back
Top