Transformer hum?

[Tom Phillips]

Still haven't changed the heater wire routing but what I did was disconnecting the heater wires and there was still the same hum from the speakers. Put 'em wires back on and disconnected the secondary HT wires. Still hum. Did the same with the bias supply wire. Still hum.???

Understood and your first post said that the Hum was present even when the amp was in standby mode. That's why I thought that the cause was magnetic coupling from the PT to the OT.

Since you are disconnecting so many wires anyway how about trying the following experiment.
1) Disconnect all the PT secondaries and insulate as required to prevent shorts.
2) Turn on the mains power and verify that the same hum is still there. (If not,report back. If yes then
3) Physically remove the PT from the chassis.
4) Connect a line cord to the Primary. Insulate as required. Including a fuse would be a good idea. You could mount the PT to a small board. (i.e. piece of wood) Now you have an PT independent of the rest of the amp with no connection to the secondaries.
5) Power the PT on by plugging in the line cord connected to the PT. Move it around with respect to the OT. You can now move it far far away from the amp or right next to the OT. From far far away there should e no hum from the speaker. Report back on the hum situation.

Be safe,
Tom

[txstrat]

Tom,

I'll try what you're suggesting. If it doesn't change anything I can even try another PT. I've ordered one for my next PP 6L6 build but could use it for testing purposes in this amp. Should be arriving these days.

Thanks for your safety advice

Matt

[Tom Phillips]

OK Matt. It will be interesting to hear the results of this experiment. It may not be a direct solution to the problem but it will give us information to help determine the cause of the hum.

It is actually more common to have hum induced in the OT, and therefore heard in the speaker, than people realize. However, the level is usually so low that the sound is not noticed.
Tom

[km6xz]

If it is in standby and the hum is present in all conditions do a transformer leakage test to find whether the there is current flowing to the chassis from corrupted insulation in the core. You can due a test with a current loop if you have one or just a sensitive AC ammeter, maybe your low current range of your metered variac.

[txstrat]

I've dismantled the PT, moved it about two inches away from the chassis and the hum was gone - totally silence. Flipped the std by switch to play mode and there was the amount of hum as it was before moving the PT.
Hum didn't change until the volume pot was at about two or three o'clock (this is better than before), then the hum got worse accompanied by a slight amount of buzz.
I somehow believe the PT might not meant to be built into a 6SL7 amp. I'm using exactly the same PT in a marshall clone with three 12AX7 and it's not as humming compared to this B15 copy. Maybe due to the hum cancelling heater structure of the 12AX7.

[Tom Phillips]

You are addressing two different things. Let’s take them one at a time.
The test I described was going to be without the secondaries hooked up but that’s OK. You have provided good information.

The fist issue is the hum that you heard through the speaker when the amp was in standby mode. This hum was present even with all the tubes pulled. You are saying that doesn’t happen when the PT is removed from the chassis. Does it come back if you touch the frame of the PT to the chassis? What happens when you connect the PT frame to the chassis with a test lead? If the “standby mode” hum is dependent on the PT touching the chassis then I believe that the PT has a current leakage problem as mentioned by km6xz. This probably means that there is a problem with the specific PT you are using.

...I somehow believe the PT might not meant to be built into a 6SL7 amp. I'm using exactly the same PT in a marshall clone with three 12AX7 and it's not as humming compared to this B15 copy....

I don’t think so. It’s probably just a defective transformer.

The second issue is the hum you hear when the amp is in play mode. You say that this hum also changes with control settings. This is a more common problem and is most likely caused by wiring dress, ground loops, problem tubes, heater circuit, excessive ripple on a power supply line etc. All the stuff that you see in lots of threads here. Since your amp is a custom build and it never has worked properly (yet) you just need to troubleshoot after you get a good PT.

That’s my best assessment anyway.

[txstrat]

The test I described was going to be without the secondaries hooked up...

Thanks for so much information.
I didn't mention it: I hooked up the PT to the primary only - no secondaries connected. The hum started the closer I put it the PT to the chassis. I can't say if the hum changed when I touched the chassis - just didn't watch for it.

Then I reconnected all secondaries.
Same thing: the closer the PT to the chassis the louder the hum.
Hum stops when the PT is 4 inches away. All this so far with the std by switch in "off" mode.
With the std by switch in play mode, the hum starts again, no matter which distance in between PT and chassis. This was of course with the tubes in.
Think I have to wait for the new transformer...

[tubeswell]

Hiya Matt - Since moving the PT appears to have helped with the coupling in power-up mode, then you could have more than one source of hum. Have you tried making a DC heater supply for those pre-amp tubes? (I believe octal tubes were originally made when heater supplies were taken from a 6V battery - but I could be mistaken about that)

[Tom Phillips]

Matt,

Based on your latest information it appears that just a little more spacing between the transformers will solve the "standby mode" hum problem. From your photos it looks like you could just rotate the OT down 90 degrees toward the end of the chassis and re-mount with new angle brackets. The re-orientation may help the situation as well as the re-location.

Then you need to do the wiring, grounding etc troubleshooting.
Your mains frequency is 50 Hz. Right?
Therefore, it the "operate mode" hum is 50 Hz then work on the heater circuit to correct.
If it is 100Hz then work on the other things I mentioned in post #21.

Regards,
Tom

[txstrat]

Thank you guys.
I've thought about DC heater too but wanted to rule out other issues before.
I've changed the grounding scheme several times from single buss to star to two point grounding until I ended up with a "sort of" Fender grounding (this has been my quietest grounding ever since). Single buss over the pots for the preamp and related filters as well as one grounding point at a transformer bolt for the power supply grounds.
I have already rotated the PT as I did so with the OT to no avail. It ONLY quits humming whit a distance in between chassis and PT.
The PT was made by a local supplier which is NO supplier for musical transformers in the first place. Maybe the PT is lacking something a musical transformer needs (i.e. shielding or something)?

[Tom Phillips]

... Maybe the PT is lacking something a musical transformer needs (i.e. shielding or something)?

Could be. There are many design considerations for a PT. Some economy designs radiate a stronger magnetic field. Some transformers can be placed right next to each other with no ill effect. See the attached picture of a classic 60W McIntosh power amp for example.

[km6xz]

The source of the transformer and still humming when removed from the chassis creates a whole new set of possibilities. The "remote" hum, with no electrical connection between the PT and chassis leaves magnetic coupling or witchcraft;>) The fact that the transformer was intended for purposes unknown I am willing to bet it is a control system transformer, not an electrical device transformer. There is a difference in how they are would and how the core is laminated. Call the supplier and ask what the transformer is usually used for. I will guess it is a control transformer and was never intended to be near sensitive circuits. He the supplier says Yes, it is used in industrial controls, you have two options, build a separate chassis with magnetic shielding or get a proper transformer. Also you owe me a beer if the answer is "yes" :>) since we are almost neighbors....some 2 hours away.

[Tom Phillips]

Good point. The photos back in post #8 do have that "control transformer" type look.

[txstrat]

I'm almost certain this is the culprit. My supplier wound that PT in the early 90s after my needs (voltages and currents) but I believe they hadn't wound any transformer for musical equipment. I also think I can't get any information about it anymore. The fact it didn't hum in it's former build might have been the big chassis (marshall clone with a great wide chassis made of 3mm iron - heavy amp to carry around).

[cminor9]

Jeez, I'd put some cans around those xformers. That might help with electromagnetically shielding them, but more importantly, it'll help prevent anyone from sticking their fingers on those bare contacts on the PT's secondary!