Humming cathodyne splitter

[crholguin]

Hi,

I'm hoping someone can offer some insight into a stubborn problem I'm having with a hybrid amp I'm building. The preamp and phase inverter are a copy of a 5E3 Deluxe's. The cathodyne splitter feeds a fixed bias, 2x6L6 power section, which is from a Bandmaster AB763, a pretty common blackface topography.

I'm getting a 120Hz hum that is always present, even when the volume controls are at zero and there's no cord or guitar plugged in. The hum doesn't change no matter what: plug in a guitar, turn up the volume pots, whatever: it always stays the same.

Probing with a capacitor feeding another amp, I've isolated the hum to the splitter. It's there even with no lead connected to the grid, and it's there even with the standby switch off, so I suspected AC from the heaters was leaking, perhaps as a result of a bad socket, but it persists despite using a different socket.

I've now checked and re-checked all the components of the splitter, their values, and the wiring. The voltages to the splitter's plate and cathode are within 10v of what Leo calls out for a 5E8A Twin, which also uses a cathodyne. So, everything checks out, but the hum remains.

I'm at a loss to figure out why it's humming, especially when the preamp is nice and quiet. The heater leads are identical to all the tube sockets, so why would the PI be the only one humming when it's configured to unity gain anyway? I would think the preamp tubes would be picking up AC hum like gangbusters were that the problem.

Apart from the quiescent hum, the amp sounds wonderful for jazz, blues, and rock. It grabs one vibe I've been looking for for years: BB King's uber cool, slightly microphonic, tone from "Live at Cook County Jail."

I don't know. Maybe a bit of extra hum is par for the course with this type of splitter, but I'd sure like to ace it. Any help will be greatly appreciated.

TIA,

Carlos.

[Arthur B.]

If it is from the heaters, then raising the heater reference should fix it. You mentioned it's not due to the power supply but it doesn't hurt to do this mod to remove it as a a possible cause:

http://www.tubecad.com/april99/page6.html

[Amp Kat]

Being that the hum is also present at quiescent would seem you likely have a grounding problem. Heaters usually buzz when picked up as coupling more than hum. It is also posible that a wire may need to be moved that may be too close together. Did you install a master volume pot and is the biasing scheme the bandmaster circuit ?

[Enzo]

Pull the splitter tube. Does it still hum? If so, it is not the splitter. Check the bias supply for ripple. Any ripple there will be on the power tube grids, and if it is there, it will also be on the other side of the coupling caps over at the splitter socket.

If it only hums with the splitter in the socket, then ground off the input to its grid. We don't want to assume it is not coming into the splitter from outside. if grounding the grid kills the hum, it was coming from elsewhere.

[crholguin]

Originally Posted by Amp Kat

Being that the hum is also present at quiescent would seem you likely have a grounding problem. Heaters usually buzz when picked up as coupling more than hum. It is also posible that a wire may need to be moved that may be too close together. Did you install a master volume pot and is the biasing scheme the bandmaster circuit ?

Thanks for the reply. Biasing is stock Bandmaster. No MV.

Carlos.

[crholguin]

Originally Posted by Enzo

Pull the splitter tube. Does it still hum? If so, it is not the splitter. Check the bias supply for ripple. Any ripple there will be on the power tube grids, and if it is there, it will also be on the other side of the coupling caps over at the splitter socket.

If it only hums with the splitter in the socket, then ground off the input to its grid. We don't want to assume it is not coming into the splitter from outside. if grounding the grid kills the hum, it was coming from elsewhere.

Thanks for the reply. Hum goes when the splitter tube is pulled. Unfortunately, I don't have a scope, but I tried changing out the bias supply filter cap; it didn't help. I'll try grounding the grid and report back.

Carlos.

[crholguin]

Grounding the splitter's grid didn't make any difference. Arrrgh!

Carlos.

[Ray Ivers]

Carlos,

Grounding the splitter's grid didn't make any difference. Arrrgh!

No - this is good! If grounding the splitter's grid made no difference, but pulling the splitter tube did - and the hum remains with the amp in standby (for the few seconds while the hiss fades away, I assume - ruling out the B+ supply), and the hum is 120Hz (ruling out the heaters - you're sure of this frequency?) then IMO the hum is almost certainly coming from the ground point of the splitter's cathode resistor; try moving this ground point around and see what happens.

Ray

[crholguin]

Alas, the hum goes when I put the amp into standby. The hum sounds about Bb, 5th string, 1st fret, on the guitar. I'm pretty sure that's 120Hz. I'll try moving around the cathode ground point to see if that makes any difference. Right now, I have it grounded to a buss wire that grounds the pots and input jacks, per Hoffman's grounding scheme. I'll try moving it to the mains ground point to see what happens. Thanks for all the help. Much appreciated. Carlos.

[Ray Ivers]

Carlos,

Alas, the hum goes when I put the amp into standby.

Oh, OK - then the problem could be an abnormal amount of ripple at the B+ supply node that feeds the PI; an AC-voltage reading on B+ should reveal this very quickly.

Ray

[crholguin]

Sorry, I was mixed up a bit: With my cap-on-a-chopstick probe, yes, I can hear the hum in the splitter even with the amp in standby, but nothing in the speaker <duh>.

I tried moving the splitter's cathode ground to various places, but that had only a slight effect on the hum.

I did a cap job on this amp about 4-5 years ago, there's no hum in the preamp tubes, and supplying the splitter's B+ from different nodes makes no difference. My multimeter couldn't settle on how much AC is present on the B+ to the splitter's plate; best I can tell, it's about .2-.4 mV.

I keep thinking I have something wired wrong, though I've checked that again and again. Or maybe the hum I'm getting is normal for a power section sans negative feedback.

I have a friend coming over this weekend who's an experienced amp builder. If there is a problem, maybe he can track it down. If not, then I'll just live with the hum. I had the amp out to rehearsal. It sounds really, really nice, and the hum isn't even noticable. I'll just have to use something else for recording.

Thanks to all of you for the kind assistance. You are scholars and gentlemen.

Carlos.

[Roy Blankenship]

Hello. I realize I am two years late to the game here, but if any of you guys are still on the board, I wanted to relate to you that I experienced a similar problem in attempting to build a custom high-power 5E3 circuit recently. The hum was coming from excessive ripple in the bias supply. A similar circuit is employed in the tweed Vibrolux (fixed bias with a cathodyne splitter), but they incorporated feedback to shut it up. Good luck!

Roy Blankenship

[crholguin]

Nope, not too late: two years later, and this baby is still a hummin', whenever it's not gathering dust, that is.

I tried running NFB through a 56k, as called out by the Vibrolux schemo. It helped, but there's still quite a bit of hum. Then I tried removing the 25uf cathode bypass cap, like the Vibro, and the thing went into oscillation.

I dunno. I think maybe the only cure is to replace the cathodyne with split tail and be done with it. Not sure how much of the 5E3 vibe is in that splitter, though. We'll see. Thanks for the help.

[Roy Blankenship]

You are on the right track. It is interesting that the Vibrolux circuit (which I have built) is quiet, but the 5E3 is not, even with the feedback. You could try a two stage bias supply (like a 4-10 Bassman), or increasing the C of the bias cap. When I went to 200uf in my circuit WITHOUT the feedback fix, it was quieter, but this is too much for a single stage bias supply, it interferes with the time constant of the circuit. Good luck!

[JP GREGOIRE]

An easy fix: Adding an extra filter cap from the bias pot wiper to ground will fix your problem! The reason why the cathodyne is humming using the same bias supply from the previous long tail pair inverter remains a mystery to me. Probably because the long-tail is more balanced and it cancels the residual ripple better that the cathodyne. Anyways, I won't go into a psychosis about cathodyne inverters but doing the extra filter trick will cure your hum problem. Adding extra capacitance to the 25uF that's already in there will not be enough to cut the residual ripple entirely. I tried up to 150uf and there was less hum but still audible. Adding an extra filter to the bias pot wiper is the key here!
Regards,
JP