New build, some problems.

[bluegate]

I have started my second Express build and it works, no high pitch squeal or hiss and sounds great when testing on livingroom volume.

This build I used better materials including Toneslut trannies.

But I have two problems.
1. The PT was warm when I stopped playing (2 hours) on livingroom volume. OT was cool.
2 There is a light hum coming thru the speakers while the amp is still on standby.

Suggestions anyone?

Thanks.

[fishy]

Whats your idea of warm?
I have yet to play any tube amp for two hours and the PT not be warm.

[bluegate]

I could still hold the PT with my hand, but the OT was cool in comparison to the PT.

[paulster]

Warm is fine. It's supplying a fair bit of current just to keep the heaters running, so you will probably find it warm after half an hour on standby without even playing the amp.

The hum in standby mode is magnetic coupling between the PT and the OT. Don't even bother checking your grounds, etc. because you've got no HT and therefore no current flowing in standby mode so it can't be coming from your amp's circuitry.

You might find that the couple of millimetres you can slide the trannies on their mounting brackets will improve things a tiny amount if you can increase the distance between them (i.e. move the OT towards the rear of the chassis as far as it will slide) and get the PT centralised as much as possible on the OT.

[tweedeluxe]

Unless it's too hot to touch for more than a couple seconds, don't worry about it.

[Blind Lemon]

Remember also you're in a quiet environment when you hear the hum and your listening for it.

BL

[Blind Lemon]

Hey that was my 100th post.........and such words of wisdom

BL

[rooster]

Agreed then this sounds normal. If you can keep your hand on it after an hour, that's a good indicator.

As to hum, well this is a hard one to call. ?? Overall I would say that any of the Hammond PTs that I have, or Drakes, or Schumacher [sp?], or Freedmans are probably quieter than the TS in standby mode. But here it is silence vs a slight hum/grind. As somebody said above, it could be tranny position. Maybe it is. I kinda think its the filament supply because that's really the only thing running in the standby mode, something I talked to Chris about. I think its 7+ amps? For a 5 tube amp I think this is way overkill. I think the filament supply on a SR is 4 amps maybe, running 8 tubes for example. But, hey, its working, so no complaints.

FWIW, I did pick up a PT from TS about a year ago and it had a large hum in standby mode - and - it got HOT after 15 minutes of run time. Hot to the point of I couldn't keep my hand on it for more than 10 seconds or so. And it would run all day, too, hot didn't seem to bother it.

It was my first Express build and I was very much in the dark, too. For all I knew everybody was running a PT that you could cook an egg on, you know?

After hanging with it for a month, I began to consider that this hum was affecting the audio. And the running temp of the tranny was just too strange - not like anything I had run into before. So I contacted Chris and he swapped me out, a gentleman who backs up his product of course.

So it happens, but Chris assured me that this was a bad batch from Heyboar. The replacement is fine BTW, and clearly out performs the first one. I mean, a nite and day difference really. Hope this helps.

[bluegate]

Thanks to all.

Later today I hope to test the induction with a piece of sheet metal between the two trannies.
If that doesn't work, I'll desolder wire by wire to find out what causes this hum.

As for the warm PT, I got similar reactions on a dutch forum, heatercurrent supply.
I'll just keep my eye on it, and test it on high volume to see if it gets worse.

[paulster]

If you want to test the magnetic coupling theory then just pull the power tubes.

That will guarantee that it's not getting amplified through the amp somehow and is purely the interaction between the transformers. You will have reduced the current draw on the PT significantly which may have an effect, but it should still tell you whether this is the source of the hum.

If you look at the magnetic field pattern from a transformer then it's a figure 8 top to bottom if the bell covers are oriented left to right.

Ken picked the absolute worst position for the OT in this particular regard, putting it right in the strongest part of the PT's field. That's exactly why most designers orient the transformers the same way but put the OT to the side of the PT where it sits in the quietest point.

[bluegate]

I used a chassis with other dimensions so my trannies are not mounted the same way
as an original Express.
My PT is on the far left where the powerswitch and standbyswitch are, and my OT
is rotated 90 degrees and placed in front of the powertubes.
There is a 1 inch gap between the PT and OT.

Currentdraw would also explain why the PT is getting warm, but what causes this.
I rechecked every wire and the voltages are near the expected voltage chart.

Tonight I loosened the bolts of the OT and placed it further away from the PT.
I rewired the fillament wires.
No time to test though, hopefully I can fire her up this saturday.

[bluegate]

Just tested the mods, no tubes in the express.
Still hum thru the speakers on standby.

I also placed a bottomplate of a big effectsbox between the PT and OT, no difference
no matter how I place/hold the metal bottomplate

really weird problem

[paulster]

You're unlikely to get magnetic shielding of any value from holding a piece of metal between the two transformers so it's not really a good test.

It is almost certainly magnetic coupling between the transformers, which doesn't surprise me. The wreck transformers seem to have pretty strong magnetic flux, which is probably because they are so over-specced for the application.

This is exactly why I like the headphone trick for placing OTs, so you don't get a nasty surprise post-installation.

If it's practical I'd recommend disconnecting the OT, pulling the leads through the chassis and then seeing where you can practically move it, whilst the secondary is connected to either a speaker or to a pair of headphones (you don't need anything connected to the primary). You can then see if you can find a quieter spot that you can still get the wires to and doesn't ruin your chassis.

Paul

[bluegate]

Thanks Paulster, you were right!

I desoldered the OT and attached a speaker to the OT (primary not connected)
There was still the hum I had.
So I moved the OT with speaker attached over the chassis and it got softer.
Most quiet was in a straight line with the PT, but due to my shallow width chassis this was
impossible.
Had to compromise with a slight hum as a result, but much quieter than before.
So thanks for pointing that out.

I also connected my second PPIMV, to switch ontop of the first PPIMV.
Also did this on my first Express clone and I liked it very much.
Its easier to achieve crunchy chords at manageble levels.
I use a Type 2 thats always on, and a Type 3 that is footswitchable.

Next thing I want to try is a footswitchable Fast/Gradual switch.
Now I need to find info on what the Fast/Gradual switch actually does in the circuit.

[rooster]

paulster - Wow, this PT hum testing method is rather fasinating to me. What I don't understand is - if the OT is not hooked up - and the hum is generating from the PT in standby mode - why do you need to attach a speaker or headphones to the OT? I mean, if the placement of the OT is causing the hum in the PT in standby mode, why couldn't you just move the raw OT around on the chassis and listen to the PT?

OK, what am I missing? Is there audio thru the OT that is in addition to the hum in the PT - and somehow they coincide? Eh? Please explain as you say you use this method quite a bit for transformer layout in your builds. This is new to me. Thanks.