New Build Stereomour II [resolved]

[amritdesai]

I've been waiting some time to build a stereomour unit and to say I was excited when the kit arrived is an understatement. I have a pair of Zu Audio Soul 99dB and plan to run them with this amp.

I decided to build my own base from walnut to match my speakers - I built the base frame at 6" and the top plate sits around an 1" below resting on some brackets. I'm not a huge fan of the swirly scratches on the plate so I bead blasted the surface. Overall, I like the contrast between the walnut, the silver plate and the various components that are visible on the plate.

 

[amritdesai]

I don't have much experience building tube amps but recently finished a Quickie just to familiarize myself with the Bottlehead instructions, etc. I recommend for beginners to start with a project like this prior to building the Stereomour II.

I found a good set of thin needle nosed pliers to make a huge difference in handling wire, wrapping it around posts and making mechanical connections. Also, invest in a good wire stripper and light!

The amp took me about a week to build and I found the instructions to be thorough and well organized - I was very impressed. The project came together without any issues.

 

[Natural Sound]

Very nice job so far. I'd kill to have a selection of hardwood like that! A photo with the finished wood / Stereomour II would be appreciated. And most importantly, it would be nice to hear your impressions of the sound.

 

[amritdesai]

While testing post 7 and 14 was reading ~ 252 VDC. This seemed a bit high... Also, at terminal 10 and 11 I was getting over 0-50KΩ closer to 65KΩ, although I think the variation in these readings were due to the position of the volume/balance controls as stated in the manual.

Because all of the other measurements were very close and within what I thought was an acceptable range I went ahead and tested the amp with speakers.
I was particularity impressed by how quiet the amp was. There is very little noise or interference - something I had to deal with more with the Quickie build.

I spent the afternoon blasting my favorite tunes really enjoying quality and character of the Stereomour II. With this amp the bass was noticeable louder and overall the sound was much more rich and textural.

More pics and impressions to follow.

 

[Paul Birkeland]

Please do remember that the rubber feet on the wood base provide an entry point for cool air, and are part of how the amplifier maintains a reasonable temperature.

I love those big black knobs!

 

[amritdesai]

Thanks Paul! I'll put the feet on.

I decided to check all my voltages again because I was getting ~258 VDC at terminal 7.

I made the mistake of measuring terminal 32 and heard a pop :-\ Afterwards, the two LED's closer to the front of the top plate are out. Terminals 11 reads -.462VDC, terminal 14 reads -.942VDC, 15 reads .024VDC, 17 reads -.14VDC, terminal 19 reads -.129 20VDC, reads -.03VDC, and so on. Essentially, the entire left channel is out.

Clearly something happened and I have no idea where to start. Super bummer 

Help would be greatly appreciated. 

 

[Paul Birkeland]

I think you may have blown the fuse.  258V at terminal 7 is a function of the 12AT7, which may just need to run for a little while.

 

[amritdesai]

Okay, I checked the fuse and it's fine.

All voltage measurements on the right channel are fine. The entire left channel is all over the place and the 2A3 tube is not heating. I hooked the amp up to a smaller set of speakers - the right channel is playing music the left channel is silent with not interference or noise.

Doing a resistance check now. Thanks again Paul for your help.

 

[amritdesai]

I performed a resistance check. Everything is fine. No visual damage that I can see...

 

[Paul Birkeland]

It's odd, the heating of the 2A3 is independent of the heating of the 12AT7, all of which are independent of the high voltage supply.

I would start by swapping tubes, just to see if anything might follow a tube. 

 

[amritdesai]

Paul,

I switched tubes around and same issue. Both 12AT tubes glow but only the right 2A3 tube glows (even after switching).

I really appreciate your help.

 

[Paul Birkeland]

What AC voltage do you get between pins 1 and 4 on the 4 pin socket of the offending side?

 

[amritdesai]

Paul,

D1: 0
D2: -1.34
D3: -.2
D4: 0

Thanks,

Amrit

 

[Paul Birkeland]

Put your red probe on D1 and your black prove on D4, then set your meter to AC volts and see what you get.

-PB

 

[amritdesai]

Getting 2.53v.

Amrit

 

[Paul Birkeland]

If you have 2.53V there, then the 2A3 will glow.  If the tube isn't glowing, try remeasuring that voltage with the 2A3 plugged in.

 

[amritdesai]

Sorry for the confusion. I am getting 3.35v without the tube and 2.53v with the tube in the socket. My common (black) probe on D4 and my red probe on D1 with my meter on AC.

Thanks.

 

[Paul Birkeland]

With that voltage there, the 2A3 should be glowing, and you should see ~60V DC from pins 1 and 4 to ground.  If you don't see the tube glowing, you will want to double check that both tubes don't glow in that socket, as well as that the socket is installed properly with the big pins facing the rear of the chassis (You mentioned that the initial voltage checks were OK, which would mean that the socket is where it needs to be).

 

[amritdesai]

Hi Paul,

Previously, when everything was working both tubes were glowing quite bright emitting a blue band. Now only the right tube is doing that. The tube on the offending side is glowing slightly but now no blue color and much darker than before. There is no audible music coming from that channel as well. Both sockets are in the correct orientation - big pins facing the rear of the chassis.

I measured pins 1 and 4 to ground and ~0V DC on the left channel. On the working right channel it measures ~60V DC like it should. Also, the two LEDs closest to the potentiometer are no longer illuminating like they were previously. Just to describe more thoroughly what happened - i put my probe precisely on terminal 32 while the unit was plugged in and turned on. There was a spark and a LOUD POP at that terminal.

I really appreciate you taking your time to help me! Thank you so much.

Amrit

 

[Paul Birkeland]

What DC voltage do you get at D2?