Headphone trick: Wow! How do I eliminate xfrmr crosstalk?

[xtian]

@ViperDoc was asking me about the headphone trick. For evaluation, I give you this Marshall Origin 50. Look at how the PT and OT are arranged. Sure enough, this amp hums as soon as you flip the power switch on...even with all tubes pulled out.

IMG_0429 copy.jpg

[ChopSauce]

May the kind of metal enclosures for transformers as these used for HiFi (& Ampeg) help?

I started to realize that the design of the amp just isn't that good. Everything is locally grounded to the chassis, including the input jacks, volume pots, heaters at each socket, etc.

Out of curiosity (I'll soon figure that by myself:) anyone knows how critical is this local grounding issue for what doesn't concern the above mentionned?
(that is one local ground per cathode, but no loops at all, mostly)

[ViperDoc]

I just tried this for the first time on some Tweed Super transformers and it's quite informative! Thanks to xtian for the hot tip. And yes, my workspace is a tornado, but I'm cleaning/preparing a space for a new storage cabinet and just pushed stuff aside to test this.



I taped off all my PT secondaries and wired my PT primary to an IEC socket. I made some sordid-(but useful)-looking jumpers with wire and old gator clips and clip-connected a headphone jack to the OT common and one OT secondary tap. I also tried this on that fat choke in the picture. I plugged in the power cord and it started humming away. As I moved the OT and choke away from the PT, the hum diminished. I noticed that the choke's position was silent when tilted 90 degrees in a laydown position; the OT incidentally sounded silent when axis-tilted at 45 degrees. I suppose that could suggest I'd be better off with a laydown power transformer, which might complicate my initial plans, but perhaps it's worth taking into account. I am not aware of how to test the proximity of the OT and choke in relation to the PT, but it didn't seem to make a difference in this test.

Is the OT position at the opposite end a problem for the preamp input or long leads back to the power tubes/reservoir cap???

[sluckey]

Is the OT position at the opposite end a problem for the preamp input or long leads back to the power tubes/reservoir cap???

This layout is very quiet...

[pdf64]

This layout is very quiet...

Are the choke and reverb transformer inside the chassis? http://www.prowessamplifiers.com/schema ... ceptre.pdf

My experience is that layout arrangement can work ok, provided there's some sort of screening between the input and output sections.
As max system gain increases, more care and mitigation may be required to maintain stability.

[sluckey]

Are the choke and reverb transformer inside the chassis?

yes

[TUBEDUDE]

I just tried this for the first time on some Tweed Super transformers and it's quite informative! Thanks to xtian for the hot tip. And yes, my workspace is a tornado, but I'm cleaning/preparing a space for a new storage cabinet and just pushed stuff aside to test this.



I taped off all my PT secondaries and wired my PT primary to an IEC socket. I made some sordid-(but useful)-looking jumpers with wire and old gator clips and clip-connected a headphone jack to the OT common and one OT secondary tap. I also tried this on that fat choke in the picture. I plugged in the power cord and it started humming away. As I moved the OT and choke away from the PT, the hum diminished. I noticed that the choke's position was silent when tilted 90 degrees in a laydown position; the OT incidentally sounded silent when axis-tilted at 45 degrees. I suppose that could suggest I'd be better off with a laydown power transformer, which might complicate my initial plans, but perhaps it's worth taking into account. I am not aware of how to test the proximity of the OT and choke in relation to the PT, but it didn't seem to make a difference in this test.

Is the OT position at the opposite end a problem for the preamp input or long leads back to the power tubes/reservoir cap???

I've had to mount the O.T. to the chassis on a section of angle aluminum to get it in the Z axis for silence. That might be easier for you than changing the power xfmr orientation.

Also, for those of you doing the "headphone trick", lay all iron on the chassis. Your choke and verb xfmr will shift the flux lines and you will have to realign positions to find the new null.

[pdf64]

yes

Thanks! As it’s handling the whole HT current, ie including power valve anodes, I was expecting something the size of the OT. Whereas that looks pretty much the same size as a typical Fender choke, which doesn’t have to accommodate power valve anode current.
At high power outputs I suspect that choke will saturate and lose its inductance and so filtering efficacy. It looks like Sundholm was prepared to tolerate a degree of HT ripple modulating the signal (ghosting) under those conditions.

[Leo_Gnardo]

At high power outputs I suspect that choke will saturate and lose its inductance and so filtering efficacy. It looks like Sundholm was prepared to tolerate a degree of HT ripple modulating the signal (ghosting) under those conditions.

That thumb-size choke is a feature (?) of the Dynaco Mark III circuit that Sunn copied into its larger amps. Mark III were intended as hi fi amps, where Dyna could well ignore what happens with that choke when the amp is driven past its clean limits. Apparently Sunn carried on without a further thought (or engineering improvement) as long as the amps didn't burst into flames no matter how hard they were used. By Mountain, for instance, often seen with a big Sunn back line, and a rip roaring sound that set the standard for rock and roll at the time.

I'm sure Sunns, and any copies or other amps emulating them, could be well improved by placing a more robust filter choke in the power supply.

[ViperDoc]

I've had to mount the O.T. to the chassis on a section of angle aluminum to get it in the Z axis for silence. That might be easier for you than changing the power xfmr orientation.

Maybe just a bit like this?





One concern I have is whether the thin, ductile choke/OT frame ears will stretch/twist/tear over time from the cantilevered mounting? I've got a few ideas...

[sluckey]

The size of that Sunn choke is misleading. Here's the specs for the replacement from Triode...

Replacement for chokes in Dynaco ST70, Mk2, Mk3 and Mk4. 1 1/2 inches (40 mm) tall on 2 3/8 inch (60 mm) mounting hole centers. 55 ohms, 1.5 henries, 10% tolerance, 2500 volt DC hipot tested, 200 ma DC. The maximum ripple voltage 35V @ 120 Hz.

[xtian]

One concern I have is whether the thin, ductile choke/OT frame ears will stretch/twist/tear over time from the cantilevered mounting? I've got a few ideas...

That doesn't look bad. Remember there are lots of OTs hanging from the "rear" panel of Fender Hot Rods in this orientation. I'd add fender washers between screw heads and OT to help spread the strain.

[TUBEDUDE]

I've had to mount the O.T. to the chassis on a section of angle aluminum to get it in the Z axis for silence. That might be easier for you than changing the power xfmr orientation.

Maybe just a bit like this?





One concern I have is whether the thin, ductile choke/OT frame ears will stretch/twist/tear over time from the cantilevered mounting? I've got a few ideas...

Yep, just like that. Don't worry about the ears. That's steel, large washers and that's solid. Now drill your entry holes for the leads, grommet them, twist your wires, heatshrink them in groups for added abrasion resistance, stuff them through the holes and solder.