Low Level Hum - WA-P1 PreAmp: What to do??

Wharfcreek

Wharfcreek

Jack of all trades, master of none!!
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About a year ago I decided to make a 'mono' system, do so on a budget, and do it with 'separate' power & Pre amps. Long story short, I ended up converting an old EA-3 to UA-2 specs, and it turned out beautifully. I also included an 'umbilical' out the back that powered the pre-amp. Next I did a complete rebuild of an old WA-P1, but 'deleted' the phono section. This essentially left me with a 1-tube (12AU7) preamp gain stage with what I believe can be considered an 'active' tone control section, as the bass and treble controls are between the two triode sections of the 12AU. I used all new parts and followed the schematic meticulously! The power amp alone is dead quiet, no hum, and sounds great! The pre-amp, on the other hand, is OK, but has a low-level hum that appears when the tube warms up and begins conducting. It's worse as the bass control is turned up, but seems unaffected by the treble control. I can turn the power amp 'on' with a separate switch built into the unit, or with the power switch in the pre-amp that runs thru the umbilical. So, with the power amp 'on', there is no hum from inter-connect RCA cables. Seems all a function of the pre-amp itself once operational.

So, my question to the group here is just what might be suggested to alleviate this issue? I have tried a few different tubes, thinking the noise might be the tube itself. I'm also thinking about it being some kind of 'filament' noise...... but if so, not sure how to progress with that. The pre-amp itself has no 'hum' balance' type control, nor does the power amp that I recall building into it. SO...... just wondering; what about maybe a pair of 100 ohm resistors off either side of the filament wires to ground? Or, any other thoughts? I'd like to tame this hum issue, so I appreciate any suggestions anyone might have. Many thanks!! WC
 
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You could try powering the filament from a bench DC power supply to rule out any AC related issues, then work your way backwards
 
It would help to post a schematic. Does the filament wiring for the preamp include a grounded center tap?

Jack
 
The EA-3 schematic has a balance pot for grounding and adjusting the filaments. The UA-2 does not. Did you remove it when you converted the EA-3? If it's still in place you might try lifting the ground at the amp and installing a balance pot in the preamp instead.

I would also look at the umbilical. If the filament leads are not twisted they can induce hum in the DC leads.
 
Another thing to look at is a possible ground loop, or lack of proper grounding. Where is the circuit grounded to chassis? Or is it? Is the amp chassis grounded to AC ground? Is the preamp? What's the ground relationship between the amp and the preamp? You could try grounding the two chassis to each other. Or use a ground isolation circuit in the amp to maintain safety ground but isolate the circuit from the chassis.
 
All good thoughts & suggestions! For the record the umbilical does carry an electrical ground between the source power amp and the slave preamp. Though aware of this, I did try adding yet another grounding wire via the good'ole alligator clip jumper method. No help! And, yes, I did remove the hum balance pot when converting the EA-3 to a UA-2. So, I've got to come up w/ a plan there. I also thought about the concept of running a resistor off the OPT cathode to one side of the filament string. And now the suggestion of trying an accessory outboard filament supply is also an option.

The WA-P1 has no 'internal' PS, and takes both HV and Filament voltage from the power amp. BUT, I could conceivably disconnect the 12AU filament from the power amp supply and try a wall-wart type source to light it up? I plan to get into this later tonight, so I'll have more feedback on it then.

Many thanks!! WC
 
Just make a derived CT for the filaments with 2 resistors. The UA-1 schematic shows two 1k resistors making a CT and mounted at the octal plug that feeds the preamp. You either need the hum pot or a pair of resistors. You might even run the center connection of the resistor to one of the EL84 cathodes for a bit of DC elevation.
 
I was thinking about that! I think I gotta check what I did when I built the power amp, but I was kinda hoping I could fix this at the preamp end. I was thinking about just putting the two resistors off the filament lines in the preamp, but then I'd lose the option to get the DC off the OPT cathodes. Not sure how important it is to fix it one place or the other (power vs pre amp) as long as I get the noise eliminated. I sincerely doubt the combo of power & pre will ever get broken up
 
Here's a pic of the little thing! Kinda cute, ain't it..... lolimage.jpgimage.jpgimage.jpg
 
Wharfcreek said:
I did try adding yet another grounding wire via the good'ole alligator clip jumper method.
Click to expand...
You can't add another ground strap between units without removing the original one in the OEM connector. All that will do is possibly add another ground loop.

I would try cleaning ALL sockets, tube pins, and connectors. Make sure everything is a tight fit between connectors. Do this before any surgery on the unit. You'd be surprised how this sometimes works and saves you a lot of messing around.

A link to the Holy Jim McShane post that all tube techs should have. :thumbsup:

"Tube Problems" That Aren't Tube Problems
 
Well, seems that I've uncovered at least one issue that I'll need to address before going any further. But to back up a bit, I found that I DID leave the hum pot in place in the EA amp. As part of pulling covers and wiggling tubes all in preparation for adjusting that hum pot, I lost my output from the amp. I first suspected the 6AN8, but after substituting a new one, I found that the old 'waffer' tube socket had rather lost it's 'clamping'. I re-tensioned it, but I think I'm just going to replace it altogether. But, as part of sorting all that out, I also noted that the 6AN8 was (is) badly microphonic, AND, I've got some kind of weird sensitivity in one of the two 6BQ5s. While one is quiet, I can approach the other and as my hand gets within a few inches of it, the hum level starts to increase. So, after sitting pretty much idle for the past several months, it's now developed a few issues to 'fix'.......any and all of which might add up to fixing my hum problem. BTW, the hum pot also seems to have issues, as it has intermittent effect as it sweeps through it's travel. I may have to remove it and see what I can do to restore it to full functionality. OR, replace it with something suitable. Either way, just another task on this amp, that has now become more work than I originally thought it would be. And all in the 'power amp' section as well!! I guess we'll see what happens when I get all this stuff done!!
 
Wharfcreek said:
Well, seems that I've uncovered at least one issue that I'll need to address before going any further. But to back up a bit, I found that I DID leave the hum pot in place in the EA amp. As part of pulling covers and wiggling tubes all in preparation for adjusting that hum pot, I lost my output from the amp. I first suspected the 6AN8, but after substituting a new one, I found that the old 'waffer' tube socket had rather lost it's 'clamping'. I re-tensioned it, but I think I'm just going to replace it altogether. But, as part of sorting all that out, I also noted that the 6AN8 was (is) badly microphonic, AND, I've got some kind of weird sensitivity in one of the two 6BQ5s. While one is quiet, I can approach the other and as my hand gets within a few inches of it, the hum level starts to increase. So, after sitting pretty much idle for the past several months, it's now developed a few issues to 'fix'.......any and all of which might add up to fixing my hum problem. BTW, the hum pot also seems to have issues, as it has intermittent effect as it sweeps through it's travel. I may have to remove it and see what I can do to restore it to full functionality. OR, replace it with something suitable. Either way, just another task on this amp, that has now become more work than I originally thought it would be. And all in the 'power amp' section as well!! I guess we'll see what happens when I get all this stuff done!!
Click to expand...
I'd try the contract cleaner first. Then, sort through the components one by one that don't respond to a good cleaning.

Best to be systematic from the input section to the output section.

Good luck!

:thumbsup:
 
I've applied a good bit of DeOxit Gold. However the problem in the socket is the socket itself. These waffer sockets use a form of stamped & crimped metal to fabricate the individual contact points on each of the 9 pins. Over time, the 'wedge' portion, the point where each pin protrudes down into the socket and makes electrical contact, it can not only corrode or suffer from oxidation, but the metal wedge tabs can fatigue, fracture at the bend points, and breake off. Thats where I am with this socket. In attempting to retension it I can tell that the metal is weak and failing to retain any 'spring' action to retain good pin contact. Fix:?... Replace the socket! Its shot!!
 
I dislike wafer sockets. Not overjoyed at changing them in general but as soon as one gives me problems, out it goes. I've had to replace more because they were broken than worn out though. I managed to destroy one in an amp of mine about a year ago when changing a tube. Plugged it in and half of it just broke off. Gave me an excuse to replace both wafers and the mismatched socket with matching new ones.
 
Yea, had many a similar experience myself, particularly on old console amps where these waffer sockets seem highly popular! Clearly a cost-savings factor on those. Amid some garage cleanup today, but hope to tackle the amp tonight!!
 
Wharfcreek said:
Yea, had many a similar experience myself, particularly on old console amps where these waffer sockets seem highly popular! Clearly a cost-savings factor on those. Amid some garage cleanup today, but hope to tackle the amp tonight!!
Click to expand...
Little did they know people would be using and restoring their products nearly a century later.

;)
 
^^ yea thats the real problem. They were cheap and adequate at the time, now they are just cheap :) I've had "real" sockets that were just worn out too though so its not unique to wafers.

Its not a new thing either, I own some pre-war stuff with wafer style octal sockets. Pretty sure I have some pre-octals too but I'd have to dig it out to confirm.
 
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