DC on Power Transformer Secondary

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guavatone

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May 21, 2005
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I'm Getting 156VDC on the secondary before a bridge rectifier using the 260V tap  using antek AS-05T280.

http://www.antekinc.com/as-05t280-50va-280v-transformer/

and 4 HER208G diodes here:
https://www.mouser.com/datasheet/2/258/HER201G-HER208G(DO-15)-479248.pdf

I think this is causing the 12.6V winding to have rectifier induced heater hum described in Merlin B's book "Designing Power Supplies for Tube amplifiers" page 194.

The Scope image below doesn't appear as extreme as the one in Merlin's book. but I am having a hell of a time getting AC Heaters quiet.  Elevating them to 68V via the CT got me to -60-55 dBu.  He says bridge rectifiers are more prone to this rather than the "2-phase" 2 diode with CT traditional tube supplies. before elevating the Hum Buzz and general HF hash was horrible. I tried Humdinger and balance resistors to pin 9 on the input valve.  But Elevation seemed the best. The noise his highest at 60 Hz with some funky HF hash and buzz below 80-90 dBu.

The B+ is going from 47R to 68uF to 1K1 to 68uF to 1K1 to 44uF and feeding a McCurdy AU300 type PP preamp PCB.  The ripple seems tame and pulling around 16 mA going to 12u7 at 285-290 VDC  via a Cinemag 9600T.

Anyone have any thought's?  Merlin? Is this particular to Toroids?

Scope of 12.6 VAC
Heaters%205.8.18.JPG
 
The "average DC" on a winding feeding a FWB with one side grounded WILL be half the DC output. Both sides are flapping around from ground to peak AC, result, half-voltage.

This, so far, is Not A Problem.

What are you doing that is different from everybody else?
 
Thanks for the reply PRR.  Possibly some moderately sacrilegious stuff.  I'm running 12.6 (real world 13V) AC across the top edge of a PCB with both on top of each other of opposite sides of the board then coming down about 1/4" to pins 4/5 with pin 9 unconnected .  But there is one trace that do pass by Pin 9's solder joint on the first stage (cathode trace).  I might try clipping the pin 9 PCB pins from the valves.

The use of the 9600T tertiary is different from the original circuit.  I'm conservatively biasing the 12au7 with a 1K shared cathode resistor. 

But what's got my head scratching was what originally sounded the amount of 120Hz Rectifier buzz pretty high before heater elevation.

here's the original circuit:
AU300sc2.png
 
guavatone said:
But there is one trace that do pass by Pin 9's solder joint on the first stage (cathode trace).  I might try clipping the pin 9 PCB pins from the valves.
What do you mean "pass by"? Is it in contact? If yes, you are shorting one of the heaters, which makes one half of the dual triode to emit too much because of the elevated temperature, and the other side is inactive because 0V on the heater. So the whole circuit is out of balance, which may explain poor Power Supply rejection.
 
Thanks abbey. No just a few millimeters away.  I imagine that would sound like crap.  Actually I'm very happy with the audio it's just the noise floor that's bothering me.
 
The -60dBu, is is measured at the output of the preamp? If it is, what is the gain setting for this measurement? What, if anything, is connected to the preamp input?

Cheers

Ian
 
It’s got 50 dBu of gain. Measurements are from a senheiser 421 in the studio where I can record into DAW and view the spectrum. And compare any modifications. I also check with an AP portable one with 150 Ohm termination.  The AP Just tells me the non-frequency specific reading of -53-58 dBu without signal.

The studio is in the basement and the AP is on the 1st floor 3 feet away from my workbench. I try my best to take accurate notes, but something strange, with the humdinger, it seemed to get quieter when being fairly off balance. I thought that was strange, because I’ve generally been fine with the Fender 100R resistor artificial tap to ground.
 
guavatone said:
I also check with an AP portable one with 150 Ohm termination.  The AP Just tells me the non-frequency specific reading of -53-58 dBu without signal.
Have you done the measurement with limited bandwidth? It should be set to 22-22kHz.

something strange, with the humdinger, it seemed to get quieter when being fairly off balance.
That is no surprize. Tubes are not 100% perfectly balanced.

I thought that was strange, because I’ve generally been fine with the Fender 100R resistor artificial tap to ground.
Noise level in a guitar amp is at least an order of magnitude above that of a mic pre.
 
guavatone said:
It’s got 50 dBu of gain. Measurements are from a senheiser 421 in the studio where I can record into DAW and view the spectrum. And compare any modifications. I also check with an AP portable one with 150 Ohm termination.  The AP Just tells me the non-frequency specific reading of -53-58 dBu without signal.

The studio is in the basement and the AP is on the 1st floor 3 feet away from my workbench. I try my best to take accurate notes, but something strange, with the humdinger, it seemed to get quieter when being fairly off balance. I thought that was strange, because I’ve generally been fine with the Fender 100R resistor artificial tap to ground.

So if you have the input terminated with 150ohms, gain of 50dBu, and a -55dBu noise floor then your EIN is -105dBu.  Not perfect, but better than many tube preamps.
 
Thanks Ian. I’m curious If an EI transformer and/or half wave CT would quiet things down another 10-15 dBu.

I’d like to back up for a minute.  Is DC  on the AC side of a FWB normal?
 
Thanks CJ

Here is the page from Merlin's Book.  His scope shots show Input grid over AC Heaters, so I tried to do the same.

https://www.dropbox.com/s/zfennht0nh3cipl/Merlin%20p194.JPG?dl=0

Here are my scope shots:
V1 Grid Pin 2
V1Gpin%202.JPG

V1 Grid pin 8
V1Gpin%208.JPG

V3 Grid pin 8
V3G2.JPG
 
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