Lost left channel after shunt regulator install

[blue360cuda]

Hey everyone,
I finished the stock StereomourII build last week and all resistance / voltage measurements checked fine. It sounds awesome - the bass did pick up after a few days of breaking in. 

So last night I put in the shunt regulator mod- all voltages were within spec but when I fired it up there's no sound out of the left channel. I swapped tubes, RCAs, sources - no change.  Rechecked resistance and voltage measurements:
Resistance's all good
Voltage also good but 2 were suspect -  T14 was 304 V and T15 is 21 V.

I checked the 41-45 but the manual didn't include reference ranges,  they are: 
41- O
42- 208
43- 0
44- 415
45- 208

I backtracked my work referencing the manual expecting to find some silly miss wire but everything looks good.  Solder joints are good. Not sure what I'm missing here . . .

Any other ideas to narrow down the problem further?

The only other change I made was originally I had a mundorf silver/oil 0.1uf installed and swapped that for the original kit 0.1uf cap to see if that would impact bass response.   Both caps measured good.

[Paul Birkeland]

I'd post some build photos.

21V at terminal 15 means the 2A3 on that side is drawing very, very little current.  Typically that would be a disturbance in the wiring leading from the power transformer through the resistors to pins 1 and 4 on that 2A3 socket.  You can use your meter to measure the AC voltage between pins 1 and 4 on that 2A3 socket (it should be close to 2.5V AC).

[blue360cuda]

Ok, think I found the culprit . . . disconnected the wires and removed the shunt and C4S board.   Found a loose solder joint on the 365 ohm resistor at C center to C3. Remembered when I did the bridge from C4/5 I accidentally did it C3 to C4 and had to un solder the connection. Must have forgot to reflow C3 in all the excitement.  Will rewire everything after the kids are asleep and hopefully that'll be it.

Do I need to worry about roasting any components or tubes?

[blue360cuda]

Well that did it!   She's singing again in stereo

[Paul Birkeland]

None of that will cause the 21V at terminal 15.  I suspect there's still a flaky joint around that 4 pin socket.

[blue360cuda]

This is what I got after reassembly.    D1 to D4 is 2.5ac    Maybe there was something else I unknowingly "fixed" during disassembly and reassembly?

1- 366 Vdc
2- 395
6- 62
7- 201
14- 161 --only one slightly out of spec (170-230) what do you make of this?
15- 61
19- 395
20- 367
21- 62
22- 62
24- 62
25- 62
27- 396
30- 405
31- 405
34- 396
36- 61
37- 61
39- 61
40- 61

[Paul Birkeland]

The 161 isn't concerning.  I'd say that there's probably another joint in there that may give you trouble down the road around that 2A3 socket.

[blue360cuda]

Thanks PB,  I'll take another look under the hood and reflow everything around that side for good measure.

BTW, I read on an old post the possibility of running Kaiju chokes / OT on the Stereomour II . . . is that a possible upgrade in the future?

[Paul Joppa]

...
BTW, I read on an old post the possibility of running Kaiju chokes / OT on the Stereomour II . . . is that a possible upgrade in the future?

The Stereomour II was designed to leave that possibility open, but so far the market is thought to be too small to justify the effort to make it an upgrade kit.

The design idea was implemented in the (now discontinued, for reasons unconnected to performance) MonAmour. The problem is that to work right, the plate current would have to increase, exceeding the rated limit for traditional 2A3s by 20%. It works fine with modern single-plate versions which have higher dissipation. The MonAmour had a switch to choose the current, making it easy to compare the effect, and the lower current was clearly inferior. MonAmour also used the 5670 instead of a 12AT7, with a different bias arrangement. Given these complicated caveats, it just doesn't seem practical.

[blue360cuda]

PJ - If someone wanted to go down the iron upgrade path in the future what would you recommend?  It seems like Isotango is one of the top options out there, would this be a good option without having to tweak too much of the original circuit? I see the OT2 is 4k and this is 2.5k
https://www.tubesusa.com/FC-30-2k5S.jpg

Another company I keep seeing pop up is Thermioniclab. They seem to be essentially made to order with lots of customization options. Has anyone had experience with these guys?

[Paul Birkeland]

The Tango stuff is generally a downgrade from our iron, especially compared to what's in the Kaiju, and you'd have a tough time getting them to fit.  I've used Thermonic Labs stuff a couple of times and so far so good, but he doesn't use keeper lams, so they can come apart a bit in shipping. 

Lundahl can make a 70H/50mA plate choke that you could use with the stock output transformers we provide (along with an ~8uF parallel feed cap), but again, getting this stuff to fit is precarious, and you're basically just buying deeper low frequency extension. 

Sowter does have an appropriate parallel feed transformer in their catalog, but recent orders after Brian's retirement don't leave me feeling super confident about recommending them right now.

[Paul Joppa]

PB beat me to the punch, so some of this is redundant  :^)

I would not recommend any transformer with a design impedance below 4K, for an unmodified Stereomour.

The linked transformer is far too large to fit on the chassis plate, and weighs ~9 pounds each, yet it only achieves 20 henries inductance, the same as the stock PC-3. The Kaiju's PC-5 provides 60 henries in MonAmour, or 40 henries in Kaiju.

Running a modern single-plate 2A3 at 18 watts plate dissipation, as in MonAmour, would require only a change in the cathode bias resistors.  You night be able to  change the four power supply resistors  and change the bias resistor to get good operation with the Kaiju iron at 15 watts dissipation.

[blue360cuda]

Thanks Paul and Paul !

So you're really not impressed with the Tango OTs and Chokes?   I have zero experience with them but "everyone" seems to think they're one of the best.  I guess it's hard to improve on something bespoke designed for your amps like the original irons.   

The Stereomour II just keeps getting better and better     I've been playing my streamer through it as background music every evening for the past week to hopefully break things in a bit.  Just got notified my Mour Quiet  for the S2 will be here tomorrow so that's exciting!   Also pulled the trigger on one of those goldpoint 47 step attenuators for the SEX.  What can I say, the tinkering bug has got me this winter.   

Side bar -  do you guys have any good resources / books you'd recommend to learn more of the circuit design theory and other amp-related electronical wizardry?  Being a bio/chem guy this stuff is not exactly in my wheelhouse but its very interesting and has been a fun hobby to get into!

Appreciate it!

[Paul Birkeland]

I have built amps with Tango transformers before, as sometimes people send me what they really want for a custom build and I just go with it.  My impression is that these designs haven't changed much in 35 years, and they aren't competitive with what's currently on the market. 

You could check out the old Valve magazines:
https://forum.bottlehead.com/index.php?topic=1642.0

A lot of people recommend the Morgan Jones books, and the Valve Wizard has some good content as well:
https://valvewizard.co.uk/

Morgan Jones

[blue360cuda]

PB-  At the risk of beating the horse to death,  what specifically do you not like about the Iso tangos?  In some aspects things have improved for sure with audio but it seems in other ways the old 50s-70s tech and manufacturing was superior. Take this as me asking you to school me, not start an argument. I'm honestly curious.   

Thanks for the links too!  I'll check them out