Filament hum in original Heyboer PT...

Express, Liverpool, Rocket, Dirty Little Monster, etc.

Moderators: pompeiisneaks, Colossal

Post Reply
User avatar
rooster
Posts: 1616
Joined: Wed Apr 02, 2008 8:50 am
Location: Pacific NW

Filament hum in original Heyboer PT...

Post by rooster »

In my original Express build, I am suddenly hearing filament hum from the original 'Tonesluts' PT. I can't help but wonder how many other 'early' builds suffer from this? I have a newer one that Phil made for me that is so quiet on the bench that I had to check the leads to see if it was actually plugged in. This is what I want, always.

Q: Should I send it back to Heyboer? Might they do something for me?
Most people stall out when fixing a mistake that they've made. Why?
User avatar
rooster
Posts: 1616
Joined: Wed Apr 02, 2008 8:50 am
Location: Pacific NW

Re: Filament hum in original Heyboer PT...

Post by rooster »

And here's my follow up to swapping the PT this afternoon.

My newer Heyboer PT is slightly smaller front to back (of chassis), but the same width, so I had to add a couple of holes to the chassis to mount this. This is custom made for me so it may not be what you are using. One difference is that I had the filament tap reduced to 5A, not the stock Tonesluts @ 7A. It delivers 400VDC on the plates and my power tubes are NOS Mullard XF2s biased to 27mAs. (The old PT delivered 398VDC and was biased to 28mAs, BTW.)

OK, as to the difference? I am amazed to report that the amp is noticeably different. It is tighter and with much better definition, and yet still breaks up like before. Also, before, as you turned the tone controls the reaction of the T/M/B pots was somewhat 'smoothed over' or blurred. Now there is a noticeable change in the pot's rotation, particularly in that area around what would be the #6 position on a Fender amp knob (PEC pots). Which is actually a good thing, IMO. It actually has a character somewhat similar to a '59 Tweed Twin circuit I built and love. The Express has a lot more gain, no doubt, but at lower levels there is a similarity now, with the Express having more midrange. Oh, and no PT hum as you initiate the main power on switch. Again, I think this was a filament related thing as I had another one do this, only that one was more pronounced. With this PT, I actually debated swapping it because it wasn't perfect but wasn't super stinky either, just somewhere in between. Since I had a spare PT on hand that I was going to use for another build someday, I just made the move.

Anyway, conclusions. I should have done this a long time ago! I cannot explain the clarity that this swap has made to the amp's character. I played the amp for 3 hours straight today, totally inspired and loving it. As to the science of what happens when your PT produces 'noise' that should not exist in the power supply, I will leave that to others. Honestly, this experience is causing me to rethink the importance of the PT. BIG TIME.
Most people stall out when fixing a mistake that they've made. Why?
Post Reply