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Heathkit UA-1 Ground up rebuild

The .1 is a solen and I used .047 Sprague 716P in the .05 positions.

I've attached the UA-2 schematic with the mods noted. (I'm pretty sure this is correct) In the end I did not up all the multicap can values to 50uF. Only those in my first post here.

Read up on Dave's thread. He walked me through adapting it to the UA-2
 

Attachments

That took a while! Finally done. Did the suggested mods. Still need to drop the heater voltage a but, ordering resistors for that.

Also waiting for some new tubes, not sure about the el84’s that are in them now.

Sound very nice, overall very pleased with them.

Thank you for the suggestions and thoughts on how to approach these.

Now I just have to get rid of the low level hum combining out of both of them :( They both have the exact same noise issue, which makes sense as the were built exactly the same…

Gain has no effect on the volume of the hum/noise. Is the same regardless of a source being plugged in or not. It’s not a loud noise, but I can hear it about 2 feet from my speakers. Not looking forward to tracking it down, especially if I discover it’s a layout issue. It’s pretty tight in there, changing stuff will not be fun.

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It looks like the HV center tap is connected a pc board. Is that a ground break? Where is the first capacitor in the power supply. The CT should connect directly to the negative side of that cap. Not doing this can cause all sorts of hum issues as this is the highest ripple current section of the supply.
 
The centertap of the HV secondary is connected to a zener mosfet voltage reducer. The output of the board goes to the ground bus for all the filtered caps. Can’t see the bus as it’s on the bottom of the caps.
 
Have you tried a shorting plug in the RCA input? It's very near to a bunch of power supply stuff and may be picking up some hum right at the input. Shorting the input might reveal something.
 
Just tried shorting the input. The amp is actually quieter with the preamp plugged in than with no connection or the shorting plug in place.
 
Is the green wire from the mosfet reducer the main ground to the supply caps. If that is so, then that is not a good place to ground the volume control which looks like that is where it is connected. There is a lot of power supply ripple current in that ground leg. I might suggest that the volume pot ground should connect to the common right at the input tube. Just my ideas as I look at your picture.

If that green wire is the main common connection for the input and inverter stages I might suggest moving it to the common point at the capacitor that feeds those stages. It's about controlling where the common currents flow not just thinking of it as a common for voltages. The currents cause minute voltage drops in the wiring and this can be a cause for hum. I've had experiences where moving the connection point of the common wires, especially the HV ct, just a couple of inches to a better location made a measurable reduction of the noise voltage on the output terminals.
 
Thanks again.The short green wire from the pot to the center ground was a quick test. I realized last night when I was awake thinking about the hum, that I had not grounded the input wire - duh. However, when I checked the ground of the RCA in before I added the little green wire, the RCA ground was already connected to ground via the pot and chassis. I guess the metal of the Alps pot is part of the ground. The little green wire also made no difference. It was a quick test to see if it made any difference. It didn’t. I was going to connect the rca ground where the cathode on the 6AN8 is grounded. I’m wondering if I need some nylon washers to isolate the body of the pot from the chassis.


The ground bus on the filter caps is connected to the house ground where the line in is grounded to the chassis. I had also considered lifting that, or adding a 10 ohm resistor there.
 
Yes, try a cheater plug on the power cord to see if it helps. If yes, then just leave the chassis grounded and disconnect the signal common from chassis. You can use a 10 ohm resistor if you like. You can also do a full "ground break" type circuit with a resistor, capacitor and diodes. The diodes will limit the max voltage between the two "grounds" and conduct if it exceeds the forward drop of the diodes.

Ground breaker/isolation

1. The first question I have should be straightforward… When using a ground/loop breaker should the return connection go a chassis point near the safety earth chassis point or to the safety earth chassis point itself? I've read articles stipulating both ways. I was always under the...
www.diyaudio.com www.diyaudio.com
 
Thanks. I have a couple of the Glassware ground lift boards here that I could put into the circuit. But space is at a bit of a premium :) I was also considering using the damping switch, which is not used, as a ground lift switch. But I’m gonna do a quick test of lifting the ground w a cheater plug, or by some other means. Gotta cut the grass first.
 
Could the orientation of that choke be an issue? I have zero hum in mine.

Also, it can be difficult to find quiet 6AN8's. I went through many before finding ones that do not hum.
 
Thanks. Yeah, I had considered the tube being the culprit. But I have a few of them here and they are all the same from a noise standpoint. Having said that, it’s possible. Do you have a photo of your layout?

I had concerns about the orientation of the choke, but it’s mounted to aluminum and somewhat shielded?

How did you mount yours?

And it’s strange, as both amps make the same noise. So likely something I did on both. Question is, what mistakes did I make?

Used a ground lift plug, and didn’t make any difference. Having said that, if I remove the preamp RCA, it buzzes louder with the ground lift in place. Going to move the grounding point of the gain pot and see if that helps.
 
This is what I have access to at the moment. Send me a PM with your email and I can send you high res/detailed shots. All grounds go to the same star ground at the can cap.




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One down, one to go. I forgot that I ran a ground of the RCA to the ground bus. That, combined w the body of the gain pot being grounding at a second spot created a loop.

I re routed the A/C line away from the center of the amp. I disconnected the wire between the ground of the RCA and the ground bus, and I added a ground from the output of the pot to the closest ground point to the cathode of the 6AN8. 3mv of A/C on the speaker terminals. Dead silent! So happy.

Thank you for all the input that helped me find my way through the forest of noise :)

Per
 
Repeated the process on number two, and it’s silent now as well :)

Curious thing, when I touch the gain knobs, there is a quiet buzzing. Metal knobs, I guess I’m creating some kind of interference when I touch them.
 
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