6021 submini preamp help

[goldstache]

Built up a 4 stage preamp with 6021 subminis.  On my v1B plate I have 65VAC!!!!??????
All test points test fine except that one.  Before my plate resistor I have healthy B+ VDC, at conjunction of plate resistor, anode and coupling cap is where the VACshows up!  What the heck is happening there?  Bad tube??????
I audio tested all other stages and they test fine! 
Any ideas?  I've never scene this kind of mu before.

[HotBluePlates]

You're gonna have to post a schematic with some voltages. How could we know if 65v is normal or not otherwise?

[goldstache]

I ditched the SMPS and went with a transformer based PS that is built to requirement, and works great with other tube projects I have around the studio. 
The resistor drops to all the plates have 20uF caps for filtering, not in schemo.  The part values have changed a bit (caps, resistors), but not drastically.
V1B is where the large AC is showing up 65VAC.  Where the plate resistor, cap, and anode connect. 


Any thoughts on why the AC is so high here.  Im missing something here.  All Connections are quadruple checked and again all other stages are as figured.


Thanks

[goldstache]

Also, this is New York Dave's Mila mic pre.  adapted a bit.  A study more or less. 

[HotBluePlates]

I ditched the SMPS and went with a transformer based PS that is built to requirement, and works great with other tube projects I have around the studio.  The resistor drops to all the plates have 20uF caps for filtering, not in schemo. 

So you start with 280vdc for the B+ like in the drawing?

There are no dropping resistors shown in the schematic, which would separate your 20uF filter caps. Any way to show those? We'd also need plate and cathode d.c. voltages for each triode.

V1B is where the large AC is showing up 65VAC.  Where the plate resistor, cap, and anode connect. 

Any thoughts on why the AC is so high here.

Am I reading this correctly? 65v a.c.? Is this at idle or with a signal applied? If signal is being applied, what kind & how big?

[sluckey]

I think it's a bogus reading. Or something is oscillating. What frequency is this 65vac?

[PRR]

What is the _DC_ voltage?

What does it sound like? (Is it "working" at all?)

How does it change when you turn the gain control?

When you move a "lead" pencil tip around V1 and its connections?

Is it tight inside shielding, or spread-out breadboard?

If the grid of V1 is not tied-down (by mike, guitar, or jack-ground) it is liable to pick-up 1V of buzz and radio from thin air, which will be many volts at its plate.

[DummyLoad]

never mind...


did you try disconnecting the NFB loop?



--pete

[PRR]

> why so many gain stages for a mic pre?

This does not seem to be NYDave's original drawing. The NFB connection needs another resistor, which is not shown. The 12V into a 12V regulator aint right.

And the pot between the last stage and the OT strongly suspects it is an "effect", not a clean amp. You wind it up until it comes back bent, then pot-down so it does not smoke the next piece of equipment.

ALSO: http://groupdiy.com/index.php?topic=43598.0

[goldstache]

Thanks for all the insight.  It turned out that on my Vero Board there was a bit of solder bridging my heater and cathode of V1b.  That's where my amplified AC came from. 


Voila quiet and operational.
RE: "master Volume" - I intended this project to get color and "effect".  Wanted a pushed, dirty preamp for DI and mic applications.  The unit sounds "good" and voltages and current are as plotted.  However, I need to experiment with the negative feedback a bit for to amp use. 


I am a newb and I appreciate everyone's time, I learn so much from each project I tackle.


I tried and tried a simple 555timer based SMPS for B+ with a 15VDC supply into the 12volt regulator for series wired heaters. I got all kinds of ultrasonic and HF noise and oscillations even when the powersupply was separately enclosed.  So I ditched it and used the transformer based supply I had around.


If ya have any thoughts I would love to hear them.  And Im still learning to plot the curves and do the math, so any pointers toward tube design would be greatly appreciated!  Have a great Weekend!