Re: Split; PT's and heater noise.

[HotBluePlates]

Sorry Doug, I split this section off. Here's where it started out guys;

Ken Moon said;

Doug,

I know I'm new here, but please allow me to pitch in:

As for the Hammonds, I've found that some of their 2XX series of PTs run very hot, and they can be sucseptible to heater noise being coupled to the B+. On a couple of amps with the 276X PT, I've had to use a separate heater tranny to get the noise down.

The 3XX series is better built, runs cooler, and seems to better shielded from heater noise.

Although Edcor takes forever, you may be able to get them to make batches of trannies for you, and since they're all built to order, you can even customize stuff like OT secondaries.

RJ Guitars over at Ampgarage got them to make some 15-watt OTs with extra secondaries - I'd suggest PMing him about his experience with Edcor.

Edcor seems to be well built and perfrom well, so I'm sticking to 1) Heyboer (OT and PT), 2) Hammond 3XX series (PT), and Edcor PT and OT). The only ClassicTone tranny I'll buy is the paper-bobbin Deluxe OT.

Here's the schematic and tranny layout of the amp where I first saw the problem:
...
I saw a 120Hz ripple start to appear on the scope at just over half power into a resistive load. You can hear the hum start to develop pretty easily if you crank it through an attenuator.

The 120mA choke before B+ 1 is rated too low. 120mA of current gets drawn, then the choke saturates and stops filtering. It might even couple hum into nearby parts.

If the fuse attached to it is 250mA, then I'd be inclined to use a choke rated at least 250mA (but those get to be big hunks of iron).

[SILVERGUN]

Here's the schematic and tranny layout of the amp where I first saw the problem:

I saw a 120Hz ripple start to appear on the scope at just over half power into a resistive load. You can hear the hum start to develop pretty easily if you crank it through an attenuator.

The heaters are elevated 70V in this design (with 120V AC mains input).

It went away completely when I substituted a small shielded Hammond 6.3VAC/4ACT filament tranny, so that's what I've done on all my amps since.

My newest 2x6V6 amp is using a 372FX PT, and it is cool and quiet.

Thanks for posting this Ken,
I'm assuming you tried everything, but I have to ask,,,did you disconnect the heater elevation tap to see if the hum was reduced?

oops, HBP beat me to it

But since I was afraid to be wrong I froze up....

So HBP, would it be more likely that the hum comes through the elevation tap or just physically coupled to the output tubes due to their proximity to the choke?

[SILVERGUN]

I guess the answer has to be the tap because it went away when he switched to a separate fil. xfmr

[Willabe]

Elevating the heaters (if anything) should make them quitter not noisier.

(He doesn't say if when he added a separate heater transformer if he still elevated the heaters.)


                  Brad      

[SILVERGUN]

Elevating the heaters (if anything) should make them quitter not noisier.

(He doesn't say if when he added a separate heater transformer if he still elevated the heaters.)

That's why I was asking if he tried it without the tap....you wouldn't think that the one move you make to make the heaters quiter would actually make them hum because of core saturation in the choke....and the tap being right there

[EL34]

Sorry guys, can you post the amp trouble shooting on another post?

Edit; Sorry Doug, I split the topic off. Brad

[Ken Moon]

Sorry, Doug!

I stepped all over your thread.

 

I will take my whuppins like a man

 

[Willabe]

Here's 1 more post from the other thread;

Ken Moon said;
"Here's the schematic and tranny layout of the amp where I first saw the problem: (Edit; see schematic below, Brad)

I saw a 120Hz ripple start to appear on the scope at just over half power into a resistive load. You can hear the hum start to develop pretty easily if you crank it through an attenuator.

The heaters are elevated 70V in this design (with 120V AC mains input).

It went away completely when I substituted a small shielded Hammond 6.3VAC/4ACT filament tranny, so that's what I've done on all my amps since.

My newest 2x6V6 amp is using a 372FX PT, and it is cool and quiet."

[HotBluePlates]

So HBP, would it be more likely that the hum comes through the elevation tap or just physically coupled to the output tubes due to their proximity to the choke?

The heaters are elevated 70V in this design (with 120V AC mains input).

It went away completely when I substituted a small shielded Hammond 6.3VAC/4ACT filament tranny, so that's what I've done on all my amps since.

I missed the part when he said the hum went away with a separate filament transformer. So maybe the original PT was at fault. Still, something seems obviously wrong with a 120mA choke & a 250mA fuse.  Granted, the choke's wire won't melt at 120mA (or probably even 250mA), but when it saturates due to excess current the inductance falls to zero and you get no filtering.

There may have been a problem with the original PT. I have a 2xx-series Hammond in an amp that's dead-clean & quiet up to the full 25w clean output power. So it seems they can't all be bad; I'd use any Hammond without reservation. That said, if it has a fault, I'd return it for a replacement or refund...

[tubenit]

I've used 5 Hammond 2XX trannies.  Quiet. No issues.  Never hot and just warm.  Nothing but good experiences.  All the amps were cathode biased and less then 30 watts.

With respect, Tubenit

[kagliostro]

I don't remember were I've read it (but I'm near sure is in one of Merlin's books)

To avoid interaction between B+ noise and Heater the council is to connect a 100-200n cap to heaters directly on V1 pins

K

[Ken Moon]

Good gravy, Batman - it seems to be the first choke is not good enough. And there may be a problem with it's physical position, too.

On my first Talon build, I used a MM JTM45 choke, which was physically larger than the Webers I've used since. The second choke was a Heyboer, but it's not important.


I switched to Weber chokes because I went to a shorter 17" chassis.

 
They seemed to have the right specs (>7H, min 75 ma). After doing some more reading, I found that in a CLCLC filter, the first choke is often spec'd at twice the average current, so maybe I should have went to a 150mA spec on the choke. The inductance can be lower than 7H, because the choke's inductance drops under a heavy load. A 3H first choke has been used successfully in this PS. Around 100 ohms DC resistance maintains the desired B+ voltage.

This morning I tacked in another MM choke (I had one left, good I didn't sell it), and the problem went away.

I could see on the scope that with the little choke, a 120Hz ripple starts to appear on the HT rail at about 1/2 volume. This could be (?) the first choke not being able to hold up to the heavy current spikes, where the MM JTM45 did a better job. This power supply was simmed and prototyped by a group of very sharp amp builders, and it turns out I'm the only one who's seen the problem (I know of 4 others who have built the same amp). AND, I'm the only one who used this particular Weber 9H/120mA choke. It may be coupling to the signal path through the heater wires or elevation components. I've heard of a tremolo effect on some amps at full volume, and I'm wondering if this is what I saw/heard.

On the smaller chassis, I also had the GZ34 in a different place, and the newer layout does put the heater elevation wire directly under the first choke (separated only by the chassis itself). Maybe that contributed to the problem?



BTW, the 276X is still hot. I can keep my hand on it, but just barely. I think that's a quirk with this particular tranny, as I've read complaints about their heat before.

Thanks, guys! I thought I had left that amp in the rear view mirror, but now I'm digging playing it again