Triad C7-X in Stereomour II

[Paul Joppa]

The 10 ohms will pretty much eliminate any effect of that stage. Look up PSUD on the Duncan Amps site; it's an excellent learning tool, practically a video game for this community!

[chard]

Thanks Paul, for the information.

[Jamier]

Would a Hammond 156L (135 ohm / 5H / 400V /  75ma) be a good
choice for replacement of the last 130 ohm resistor? Or the first
130?

[Paul Joppa]

Looks good to me.

Try this: install the choke(s) without connecting them to anything at first. Read the AC voltage between the two choke wires - this will be the induced hum from the power transformer magnetic field. If it's very small then you can put the choke in place of the last 130 ohm resistor. If it's large then you'll want it in place of the first 130 ohm resistor. I would guess that 10mVrms is "small". Of course it's not terribly difficult to try it both ways, instead of taking a guess from some dude on the internet (me, in this case).   :^)

[mcandmar]

I love how you come out with these brilliant ideas, yet they sound so obvious when you say them

[amritdesai]

I started this thread so I figured I'd give the Hammonds a shot.

I mounted the Hammond 156L inductors to the amp base with their coil axis perpendicular to the power transformers. I measure the AC voltage between the choke wires without connecting anything with a reading of around 6mVrms for both inductors. I tried moving them around and changing orientation but the reading was pretty consistent. I soldered the chokes in place of the last 130ohm resistors.

All resistance checks were within range - some of the voltage readings were higher. I performed these checks with the stock tubes.

Terminals

1. 370 VDC
2. 407 VDC
19.407 VDC
20. 370 VDC

I tested the amp on a spare set of speakers and everything sounded fine. I'm not sure if there is any audible difference or benefit with the inductors. Previous to making this change the amp was sounding wonderful - very open, sweet and textural.

After a day of listening I'm finding much of the brightness and sparkle has been replaced with a more neutral tone - overall a little less dynamic. The image though has tightened up quite a bit. Likely, things will open up more over time.

[Jamier]

Are you still using the spare speakers or have you gone back to the
Zu's? If you did go back to the Zu's and you feel that the tonal
quality is less desireable with the 156Ls could it be that the 5H of
this particular choke is not enough (?). Maybe Paul knows.

[amritdesai]

I've got everything hooked up on the main system and it sounds fantastic - I don't think the amplifier has ever sounded so balanced - the imaging is insane!  I would use the term neutral but not in a bad way. The tonal quality isn't less desirable, just different. I was expecting more sparkle and to be honest this change has had the opposite effect - perhaps in a good way.

Replacing the resistors with the the Hammond inductors has not changed the sound as dramatically as tube rolling or capacitors but it seems like it has removed the harshness.  I can listen at higher volumes with less fatigue. There is a richness to the sound and added depth/improved soundstage - these are my initial impressions.

Also, the amp is dead quiet now - previously there was a slight hum.

[Jamier]

That's great! When I researched possible PS chokes the Hammond
156Ls were the only ones I could find that met all the criteria.
I was thinking that the inductance might be a little low though.
Glad to hear they seem to be an improvement.

[amritdesai]

Jamier,

Let me know if you install the chokes - I'm still undecided about the sound.

A

[Jamier]

I just received the chokes today.I don't know when I will get them in
as I just completed the amp and I haven't even voltage tested yet.
I will follow up though and let you know what I think.Others will probably
be adding 156Ls also, so I'm sure their thoughts will also be posted.
I don't know if chokes are like Caps with respect to "burn in", but give
them some time(?).

[Jamier]

Look at the site aikenamps.com. This is where I got the idea that
5H might be on the low side for inductance. I was hoping PJ might
weigh in on this subject. There is a section on that site that explains
the function of chokes in power supplies, both choke regulated and
capacitor regulated. In Cap regulated 5H appears to be on the low
side but I do not have enough experience to know myself.I think it
is important to remember that while the CX7 has a dramatic effect in
the Crack circuit, it is different than the Stereomour II (OTL vs OT
etc). Also the CX7 has 10H inductance which might be a factor in it's
effect. Tweeking these circuits is a complicated endeavor especially
for those of us with limited experience.

[Paul Joppa]

... I was hoping PJ might
weigh in on this subject. ...

I did. Twice, on this page (didn't check the first page). As you said, it can be complex; that's why I recommended the PSUD software. Be sure to look at the start-up transient in addition to the steady-state performance.

[amritdesai]

Thanks Paul. Trying to get PSUD running on bootcamp has been more difficult than it should be. I've been having issues with the installation asking for a second disc - i'll try on my PC at the office.

I've also noticed an excess amount of heat coming from the amp - certainly more with the chokes installed. I think I am going to revert to stock to compare again sonically the differences/see if the heat is an actual issue.

A

[Paul Joppa]

There should be no difference at all in heat generated. Something is wrong. Might be time to check the voltages?