New Hammond "Fender"pt hums

[worth]

I just replaced a Mojo/heyboer PT, ( voltages too high for my taste ) in my AB763 Super Reverb build with a Hammond/Fender 290 DX. There were no noise issues with the Heyboer... now there is an annoying low-level hum. ALL grounds are good , and in their proper place.  I've tried both grounding the heater center tap , and using a virtual center tap , ( not at the same time of course ). I also used the shield wire on the
PT. I changed NOTHING but the PT. Both transformers had the Weber end-bell replacements to convert them to vertical/stand up mount. My question is this.. would contact to the grounded chassis through the end bells cause hum in this hammond PT ?

[eleventeen]

Weird. Just to clarify; are you saying the transformer electronically works and successfully powers the amp BUT there is a hum coming through the speakers during operation; OR, are you saying the transformer itself imparts an audible, mechanical hum into the sheet metal of the chassis via hysteresis or other unknown voodoo?

Do you have the older "death cap" swiched grounding scheme or is your AC line ground hard-wired to chassis ground? If it's hard wired, maybe you need to bolster that connection and/or move it closer to the power transformer.

I think my troubleshooting procedure would be something like:

1: Pull the rectifier tube, hum still there? If so, it would seem like there's an internal short in the tranny. Find it.
2: Pull the PI tube. Hum still there with no output stage drive?
3: Pull the output tubes. Hum still there with no output stage at all?

1a: Pull the rectifier tube, disconnect the HV CT from ground and measure AC volts between chassis and the HV CT.
1b: Pull rectifier tube, let the amp sit powered off for quite a while (unless it's a build or mod monster you've done that possesses a good bleeder resistor) and check for a short between the 5v winding and the HV winding, which you can do from the rectifier socket.

[worth]

The Hammond is humming through the speakers. There is no death cap.. this is a recent build of mine, which was noise-free with the other transformer. I'm thinking that maybe by using other end-bells ,the PT isn't insulated from the chassis , and maybe needs to be ? There's only one way to find out.. but I thiught I'd get some opinions before I take the PT out and put the plastic insulators,( between the PT bolts and end-bells ) back on .

[eleventeen]

Well, I'm sure not gonna tell you I know or have seen everything, but I've never heard of a transformer that did NOT want its frame bolted to the chassis with authority, both mechanically and electrically.

I'm sure you've read plenty of things that reco as a "best practice" in builds orienting the fields of chokes, OTs, and PTs in ways where one cannot (or should not) induce a field into another. Does this line of thought make sense?

Any chance of having enough lead length on your tranny to try rotating it 90 degrees as a test? (clockwise or c-clockwise as viewed from the top...not as viewed from the side) 

Was the tranny originally a "through-chassis" type, eg; like a stock Super Reverb PT and now you've changed to an "upright" style like a stock Super/Twin Reverb >>OT<<? In other words, is there a big rectangular cutout in the chassis or are there only two relatively small holes?

Still seems pretty weird and I imagine, frustrating.

Would it be too bizarre to contemplate placing a bent piece of sheet metal between the PT and the rest of the amp to see if the shielding factor changed anything? Yeah, I know it seems weird but if you have something like that laying around (could be a bent-up pie plate for pity's sake) you could just throw it in there while the amp was working and see if it made a difference. Has to be grounded, of course.

[FYL]

Both transformers had the Weber end-bell replacements to convert them to vertical/stand up mount. My question is this.. would contact to the grounded chassis through the end bells cause hum in this hammond PT ?

The screws holding the end-bells thru the lams should be fully isolated. If not they can touch the lams and short the eddy currents. The core should be properly earthed.

[worth]

The Hammond is now insulated from the chassis.. same hum. Not loud , but it's there. Also, the PT is rated at 325-0-325 @200ma.... the HV secondary at the rectifier reads 352 V. Usually Hammonds are right on the money.

[eleventeen]

You done any of the "tube pulls" I suggested upthread?

[FYL]

The Hammond is now insulated from the chassis.. same hum.

Do the screws touch the laminations ? Use isolation washers and bushings if necessary.

Is the core orientation correct ? The PT could radiate. Try to move and/or rotate the transformer, check hum level.

Also, the PT is rated at 325-0-325 @200ma.... the HV secondary at the rectifier reads 352 V. Usually Hammonds are right on the money.

Is your reading for unloaded or loaded ops?

[worth]

Voltage Readings are from loaded conditions.. no eleventeen.. I have not yet done what you've put forward. I resist believing , ( for now ) that a new Hammond PT has issues. NO FYL.. no screws touch the laminations, ( same as your pics ).. and of course rule# 1 has been observed.. transformer laminations don't face each other.

[plexi50]

I had a 65 Deluxe reverb reissue 3 months ago and the amp hummed with the guitar unplugged and no volume

I pulled the PI tube and it was still present. It turned out that this PT did not like the power tubes that were in it

I swaped out the power tubes for some old 6V6 Sylvanias and the hum completley dissapeared

There was some sort of power transformer magnetic field or osscillation occuring between those tubes and the PT

I wouldnt have believed it if i had not swapped power tubes in and out a few times just for fun to confirm the difference

A before and after kinda thing

[phsyconoodler]

"the HV secondary at the rectifier reads 352 V"

  That voltage reading tells me that you may have a bad tube(s) and/or they are drawing too much current to pull the voltage down that low.Check the bias and the first filter cap ground.Something may be causing the extra current draw which would cause hum.

[FYL]

Voltage Readings are from loaded conditions..

You should get between 380 and 400V. With 352V only either something drws too much current (partial short, wiring error, etc.) or the PT is defective - you can easily check it: amp unplugged and caps drained, measure winding resistance (primary and secondary), then calculate the source Z, plug the amp and measure unloaded voltage, calculate loaded voltage using for instance Duncan's PSUIID.

 

[eleventeen]

Does this hum occur the instant you turn on the amp (Both stdy and power. at the same time, yeah, I know you're not supposed to do that) or does it take "tube-warm-up-time" = ~~15 seconds to occur?

I'm thinking maybe you have a borderline electrolytic that's leaky enough to drag down your highest-high B+, and a node or two down from there is the culprit. Not leaky enough to produce a dead short which would probably cook one of the resistors separating the filter caps. But leaky. Possibly, try running the amp for 2 minutes, shutting it off, and then CAREFULLY using your finger check the temperatures of the intra-node resistors. I say carefully because if you don't have a bleeder on there, there could still be some good-sized (well, not good for you) volts on some of those caps. You already know that.

[worth]

I'm sorry.. when I said "at the rectifier" I meant that the reading on the HV secondary was 352V AC at the rectifier.. not DC.

[phsyconoodler]

Having used the Hammond 290DX many times,the voltage i get are consistently around 415-420 with properly biased tubes.that voltage reading makes me think 'draw'.

Ok,what is the DC voltage at pin 8 of the rectifier tube and the choke.What about a bad rectifier tube?

[worth]

352V AC  ... not DC. The DC voltage at pin 8 on the rectifier reads 444V DC , 42 ma. bias. That's what I'm saying... the voltages are about 25 volts higher than the Hammond rating.

[phsyconoodler]

One wired thing here; Hammond lists that PT as 650v CT.That should only give about 422v with a GZ34 rectifier tube.A Blackface Super Reverb is about 450v on the plates.Your voltage is not far off but not according to the Hammond specs.
  You should send it back and get another one.I did have a bad Hammond PT recently.An upgrade Princeton Reverb PTIt shorted to ground and shocked the hell out of me.I sent it back and they immediately sent another,no charge.But I did not get the 'upgrade' version.They described it as having thinner laminates or something along those lines,but the specs are the same between the regular and the 'upgrade'. I suspect they have had other issues with the 'upgrades'.
  If yours is one of those,that may be the issue.

[worth]

I've just contacted Antique Electronics about the problem.. We'll see.

[phsyconoodler]

I am not too impressed lately with Hammond.If you look at the specs of the larger PT's for guitar amp replacement,the Fender specs don't wash.650CT for a Super is not enough.And the problem I had with almost getting killed makes me think twice about the Line.
  I have had Zero problems with the 'Classic Tube' transformers however,and will continue using them.
  Me thinks we need to take another look at Mr Hoffman for some reliable transformers.Yeah,we save a few bucks here and there,but I bet Doug has not had any come back for problems like this.

[FYL]

But I did not get the 'upgrade' version.They described it as having thinner laminates or something along those lines,but the specs are the same between the regular and the 'upgrade'. I suspect they have had other issues with the 'upgrades'.

The upgraded models use M6 lams - thinner, better permeability, lower losses, std versions use M19 - good quality industrial lams, more than adequate for 60 Hz ops but sometimes marginal at 50 Hz. The carcass and everything else are the same, hence identical static specs.

[eleventeen]

"The upgraded models use M6 lams.......

Arggggghhhh, is there now a whole new dimension of fetishism we have to become obsessed over...?

"Dude, what permeability laminations are you running in that thing? What about the remanance? You melt and pour the steel yourself?" 

[The_Gaz]

 

[FYL]

"Dude, what permeability laminations are you running in that thing? What about the remanance? You melt and pour the steel yourself?"  

Well, you've never seen audiophile-approved iron using cobalt/permalloy lams and silver windings...

[eleventeen]

No, I'm still waiting for the neodymium lams that, built into a Vibro-Champ, can stop a Skilsaw at 25 feet.

[FYL]

These are an option on Master Built Custom Shop™ models fitted with the Vintage Road Worn Relic Talent Boost Switch®.
 

[billcreller]

I don't have a switch in me, and have no talent, but I'm a road-worn relic