Help with Gates tube mic preamp???

[rock soderstrom]

The hum is around -40dbu at full gain in both channels. I feel like I may just be missing something.

Let's start with the simplest things. Is your heater connected with one side to the central ground?
Does the hum vary with the gain knob?

 

[beatnik]

+1 on moving the PSU to an external box, or perhaps try a toroidal transformer, you can find them for tube preamps with both HT and filament winding

The wires going to the 6J7 grid cap should be shielded. The old amps even had an additional metal cover that clip on the tube to shield even the exposed connection

Have you checked the resistance between the 0V bus bar and chassis is very close to 0 Ohm ?
Personally I would rather connect the 0V bar to chassis on a bolted solder tag, instead of the xlr connector tab.
And also use a dedicated grounding lug for the mains earth bolted to the chassis with a locking nut, to avoid the safety earth become loose.

Also check the polarity of the filaments is the same on all tube sockets.

Have you actually measured is 60Hz and not perhaps 120Hz ? In that case the HT power supply might need additional filtering

 

[beezer4]

Let's start with the simplest things. Is your heater connected with one side to the central ground?
Does the hum vary with the gain knob?

One side of the heater is not connected to a central ground, but the center tap of that transformer is to ground.

The hum does vary with gain knob and increases as gain increases.

 

[beezer4]

+1 on moving the PSU to an external box, or perhaps try a toroidal transformer, you can find them for tube preamps with both HT and filament winding

The wires going to the 6J7 grid cap should be shielded. The old amps even had an additional metal cover that clip on the tube to shield even the exposed connection

Have you checked the resistance between the 0V bus bar and chassis is very close to 0 Ohm ?
Personally I would rather connect the 0V bar to chassis on a bolted solder tag, instead of the xlr connector tab.
And also use a dedicated grounding lug for the mains earth bolted to the chassis with a locking nut, to avoid the safety earth become loose.

Also check the polarity of the filaments is the same on all tube sockets.

Have you actually measured is 60Hz and not perhaps 120Hz ? In that case the HT power supply might need additional filtering

Measuring 60hz the strongest for sure... there is some 120hz in there as well but at a much lower level.

The wires going to the grid caps are definitely not shielded. That's a great call out. Thank you.

Polarity for filaments is the same on all tube sockets, checked that.

I'll check the resistance between the bus bar and chassis. Thanks for the feedback around connecting the 0V bar and earth. Those are both good points.

Moving the power supply external is definitely a good option as well.

Thanks for all this!

 

[lassoharp]

Measuring 60hz the strongest for sure... there is some 120hz in there as well but at a much lower level.

The wires going to the grid caps are definitely not shielded. That's a great call out. Thank you.

Polarity for filaments is the same on all tube sockets, checked that.

I'll check the resistance between the bus bar and chassis. Thanks for the feedback around connecting the 0V bar and earth. Those are both good points.

Moving the power supply external is definitely a good option as well.

Thanks for all this!

The grid cap wires have to be both shielded and grounded on one end

 

[beatnik]

Another thing, on these metal tubes pin1 is actually connected to the shield and this also has to be grounded for best noise reduction

 

[emrr]

Everything about this looks like PSU proximity.

 

[beezer4]

Everything about this looks like PSU proximity.

Thanks Doug! I'm going to try out the other shielding suggestions, but plan to move the PSU externally. In the past when building external PSUs for solid state gear, I typically used a 5 pin XLR and a Chauvet Pro DMX5P5FT 5-pin/5-conductor DMX Cable.

This will be my first external PSU with high DC B+ voltage. Do you have any suggestions for proper connectors. It sees most XLR connects aren't rated for that high of DC.

Thanks everyone for the help!

 

[emrr]

XLR are fine

 

[mattrad]

Is there a shield pin on those Chicago inputs? If so definitely make sure thats grounded! If you don't have a pinout you can check for continuity between pins & the transformer housing. I would double check that the secondary is grounded as well. PSU is suspect definitely, strange though, if proximity were the main issue I would guess that the channel closer to the PSU would hum significantly more.

 

[beezer4]

Is there a shield pin on those Chicago inputs? If so definitely make sure thats grounded! If you don't have a pinout you can check for continuity between pins & the transformer housing. I would double check that the secondary is grounded as well. PSU is suspect definitely, strange though, if proximity were the main issue I would guess that the channel closer to the PSU would hum significantly more.

No shield pin on the Chicago transformers. Secondary is grounded. The channel closest to the PSU does have a little more hum, but not signification.

Seems like PSU proximity is the driven force being the hum though. I’ll follow-up on this thread ones I make the recommended changes.

 

[emrr]

No shield pin on the Chicago transformers.

It's probably the mounting lug on the case rather than separate.

 

[beezer4]

UPDATE:
Moved the power supply externally and it made a huge difference. Re-checked all the grounds and added shielded cables for the 6J7s. Now we're in great shape noise wise. Working now to add a negative feedback control pot similar to the Coil pres.

Thanks everyone for the help!

 

[mdcmdcmdc]

Hi folks,

I'm really interested in building an SA70 or two, but new to tube circuits. I've built a few dozen guitar stompboxes, and can generally follow a schematic; but high-voltage stuff is a bit more intimidating than plugging a fuzz into a 9v power supply.

A few semi-related Qs to start: first, (with the caveat that every tube cicruit is a potentially dangerous/lethal project) is the SA70 a decent place for a beginner to start? Is there an appropriate off-the-shelf drop-in available for the PSU? Suggestions for appropriate transformers?

Thanks in advance! I'm sure I'll have a million more Qs if this project is a good fit.

 

[beatnik]

Plenty of info online about safety practices.

The SA70 is simple but the RCA BA-2A is even simpler. I would start with that one as a first tube preamp.

For buying transformers you could get better suggestions if you put your location in the user panel.
I know various sources in the EU but that wouldn't be so useful if you are based in USA for example.

 

[mdcmdcmdc]

Ah yes, good point - I’m in Canada. Thanks for the suggestion on the RCA - I’ll take a look at the schem.

 

[beatnik]

You could get transformers from Hammond, they are a Canadian company.

For the Gates SA70 you could use 850NA for input and 850LA for output for example, they should be very nice sounding transformers.

269EX could power two channels with decent headroom.
If you want the best noise performance put the power transformer and power supply in a separate box.

You can also use a toroidal and that could be placed in the same chassis as the amplifiers as it emits less magnetic field, but it needs to be of good quality.

The BA-2A output transformer is a different type (gapped), as you can see from the schematic it has DC current flowing through the primary. But you could modify the circuit adding a plate resistor and coupling capacitor on the output tube like the SA70 is configured.
Cinemag, Lundahl and Sowter make suitable transformers for the BA-2A output stage.