Paramour hum

[barriep]

Hi:
I've been attempting to resolve a hum issue with one of my Paramours. For reference, it's the RHS one. You can walk into the room and hear the hum. The LHS side is really quite quiet - you have to put your ear right up the woofers to hear it. I did check using an online tone generator on my laptop in the room - it's 60 cycle hum.

(For completeness, and in case it helps - these are Paramour I from the spring of 2006. They were "upgraded" twice - the first time with the Edcor output transformers and the MQ BCP-16 plate choke and the Clarity caps. The caps were subsequently replaced. The last upgrade was to the MQ BH-5 output transformer.)

I've read tens (100s?) of postings on this forum and in the old AA one looking for suggestions/hints. As well, I've done a pile of reading on the interwebs w.r.t. hum.

In doing all the troubleshooting things I can think of, I've reflowed solder on all the terminal strips, both the tube sockets, and the C4S PCB. I've moved tubes from side-to-side, replaced 12AT7s, but not 2A3s. Nothing really helped.

I've visually compared the LHS construction with the RHS one. I didn't find anything obvious.

I'm attaching a copy of my voltage worksheet. Most of the voltages I measured seem to be okay to me. But the RHS CS4 output is significantly different from the suggested 155VDC. True the LHS is high, but it's only on the order of 12%. But the problem RHS is pushing 30%. Irrespective of the hum issue, is that a problem? Do I need to fix this?

As a last resort, I poked at the wiring with an orange stick in case there was a wire dressing/routing issue. This was with the input shorted and a speaker connected. I didn't find much until I physically moved the 2A3 cathode resistor and the 50ohm hum pot. Doing that, the hum almost completely went away. I did reflow the solder for the cathode resistor and the hum pot/tube socket.

Based on the Bench article, and the apparent current practice at Bottlehead, my thought is to replace the cement resistors in both sides with a different wire-wound one (not cement). As well, I would replace the hum pot with a 10ohm one and two non-inductive 20ohm wire-wound ones. Other than the space under the hood, is there any reason to not make this change?

The last thing that I've discovered is that I get a fair amount of hum when connecting the Paramour ICs to the powered-down line amp (it's an EFP3). Is this to be expected?

I'm not trying to get to no hum. I understand clearly that with AC "heat" there will always be some residual there. But it would be really nice to get the RHS hum down to the same level as the LHS.

Thank you for any assistance.
Regards,
Barrie.

[Paul Birkeland]

Have you tried adjusting the hum pot?

[Paul Joppa]

I agree with PB - it sounds like it might be a solder failure or a dirty hum pot. I say that because all the voltages look good, including the 2A3 bias voltages, which means  the 1200 ohm bias resistors are just fine.

I might have guessed one of the 2A3s was warping internally, but you said you'd swapped them between channels with no change, so that's ruled out.

[barriep]

Hi:

Paul Birkeland wrote:

Have you tried adjusting the hum pot?

Yes sir - numerous times, especially with moving the 2A3s side-to-side. The RHS pot seems to be "scratchier" mechanically, like the wiper moves the wire of the element as the shaft is turned. The LHS pot seems smoother. Perhaps it is dirt - I will try some contact cleaner on the pots. In any event, it seems that I can null out the filament hum audibly but there's still some greater background hum left.

I did find a set of new Sovtek 2A3s that I probably bought as spares a long time ago. I will take out the EH ones I'm using right now and try the new Sovteks and see what the hum level is with them.

I had mentioned the increased hum level when the interconnects are connected. It happens whether the EFP3 is turned on or off, with the volume controls at zero or not, and regardless of the input switch position (i.e. what's connected upstream). Is there anything reasonably that I can try or investigate to help with this?

Again, thank you for your help.
Regards,
Barrie.

[Paul Birkeland]

With the interconnects disconnected, is the noise performance of both amps the same? (there may be some extra static that wouldn't otherwise be there with nothing plugged into the amp, but it should be about the same in each amp)

[barriep]

Hi:

I'm just getting back to this - life gets busy at times.

To answer the question - not really. The right side hum is significantly louder than the left side. And I need to clarify that. This is true with the interconnects connected to the preamp regardless of whether it's turned on or not. For the right side, it's like the interconnect acts like something is open circuit or causes a ground loop.

With their inputs shorted, the Paramours have much less hum.

However I've done some more troubleshooting.

I swapped out the Paramours with a solid-state amp. To my surprise (chagrin?) the right-side hum was still there.

So I left the SS preamp in place, and swapped the Paramours back in. The excessive right-side hum went away. The background hum is now about the same on both sides.

The over all issue appears to be the EFP3. I'm attaching the voltages from it. There looks to be an issue there in the voltage regulator. Since I don't know what the regulator voltages should be, that's only a guess. And I'm unsure of how to proceed to resolve this.

I don't know if this would cause a hum issue, but it seems to me that I need to address the improper voltage.

Thank you again for your help.
Regards,
Barrie.

[Paul Birkeland]

Does the center board of your EFP3 have MJE350 or MJE5731A transistors? 

I fix a bunch of these every year and throw in the bigger MJE5731A transistors to nudge up the thermal margin on that center board.  You can do a flat rate repair on the preamp and I'll fix it for you, or we can work something out where I make you a board and mail it to you.  I generally prefer to perform the repair so I can do any debugging that's necessary.  Often times while I am in there people also request replacement of the Sweetest Whispers, so you end up with more of an overhaul than a repair.

[barriep]

Hi Paul:

I'd be interested in talking with you further on your suggestion(s). What's the best email (or another means) to take this off-line?

Thank you.
Regards,
Barrie.

[Paul Birkeland]

pb(at)bottlehead(dot)com