Stereomour Oops

[Drugolf]

I purchased a nice used original Stereomour and upon receiving it I made the bad mistake of missaligning the 2A3 tubes into the incorrect sockets.  It appears I fried a couple 1.6k resistors.  Smoke and all!   Yikes.  While I am dealing with that, are there any recommended upgrades to other parts - caps etc?

thanks!

[Paul Joppa]

Original or Stereomour II?

[Drugolf]

Original or Stereomour II?

Original.

The large cooked resistor there is a Xicon 10W 1K6 (1.6kohms).  I am thinking I should go ahead a change out the small Carbon Composite resistor there too.  It is red, red, grey, gold so is that a 2.2K?  The smaller 5w 4.7k wire wound could also be replaced while at it. 

Stay with the same wire wound style?

[Paul Birkeland]

The tube and the 1.6K resistor need to be replaced.  The carbon composition resistor does not. 

I would strongly recommend purchasing a DVM, then measuring the DC resistance of each plate choke.  This is done by measuring the resistance between where the blue wire attaches to each 4 pin socket and the black wire that attaches to the power supply PC board.  If you have cooked the 1.6K resistor, it's also possible that you've burned up a plate choke in the process.

The 220uF/250V capacitor on the power supply PC board on the side where this happened should probably also be replaced, as it will see more than 250V under these conditions, but on the Stereomours I have worked on with this issue, that cap hasn't been damaged.

[Drugolf]

The tube and the 1.6K resistor need to be replaced.  The carbon composition resistor does not. 

I would strongly recommend purchasing a DVM, then measuring the DC resistance of each plate choke.  This is done by measuring the resistance between where the blue wire attaches to each 4 pin socket and the black wire that attaches to the power supply PC board.  If you have cooked the 1.6K resistor, it's also possible that you've burned up a plate choke in the process.

The 220uF/250V capacitor on the power supply PC board on the side where this happened should probably also be replaced, as it will see more than 250V under these conditions, but on the Stereomours I have worked on with this issue, that cap hasn't been damaged.

Excellent! thanks Paul.

When you say tube, are you saying the 2A3 or the 12ax7?  (the 12ax7 tests fine.  I don't believe my Eico 667 cant test 2a3 socket tubes.)

Thanks for the detailed description on the other things to look into.  I currently don't have a manual for this so I am kinda blind.  Last night I emailed a request to add a manual to my cart that currently contains a Bottlehead power cord.   

As far as testing the DC resistence of each plate choke, what readings am I looking for?  Also, to be clear, leads go to Pin 2(blue) and Pin 1 (black wire that extends to the Power supply board)?  See photo.
 
Sorry to bug you on this.

[Tom-s]

In my EICO 666 i test 2A3 on following settings:
Edit: if you use an adapter of flying leads to test?

2A3   
Heater 2.5   
Grid 55
Plate 70
Levers 24511111      
V 2   
S 1   
Leaks 2,3   
Merit 2

Edit: these are in my personal settings list. There's no testing option in the original Eico scrolls for 2A3's.

[Paul Birkeland]

The 12AT7 will be fine, but when you plugged in the 2A3 improperly, you placed the 2.5V 2A3 filament between a 400V DC supply and a 1.6K resistor.  Even if the tube still appears to work, I wouldn't use it.

For the plate chokes, you have one channel that hasn't experienced this issue, so you can use that as your known good channel.  The plate choke in the other channel probably won't show exactly the same DC resistance, but it needs to be close.  If there's any doubt, there are some other simple experiments to do once you have the amp up and running again.

[Drugolf]

The 12AT7 will be fine, but when you plugged in the 2A3 improperly, you placed the 2.5V 2A3 filament between a 400V DC supply and a 1.6K resistor.  Even if the tube still appears to work, I wouldn't use it.

For the plate chokes, you have one channel that hasn't experienced this issue, so you can use that as your known good channel.  The plate choke in the other channel probably won't show exactly the same DC resistance, but it needs to be close.  If there's any doubt, there are some other simple experiments to do once you have the amp up and running again.

Both 2a3 were installed incorrectly.   
Testing across pins 2 (blue wire) and 1 (black wire) I am getting the same reading for both at about 22.2
Interestingly, I pulled one of the burnt looking 1.6k resistors and it tested at 1.6k.

[Drugolf]

In my EICO 666 i test 2A3 on following settings:
Edit: if you use an adapter of flying leads to test?

2A3   
Heater 2.5   
Grid 55
Plate 70
Levers 24511111      
V 2   
S 1   
Leaks 2,3   
Merit 2

Edit: these are in my personal settings list. There's no testing option in the original Eico scrolls for 2A3's.

Thanks.

I don't have an adapter unfortunately.

[Paul Birkeland]

Oh dear, so did you blow the 1.6K resistors in both channels?  I would toss both 2A3s and start over with a fresh pair.

22.2 ohms from the blue wire to the black wire is a destroyed plate choke.  Be sure you are actually getting 22 ohms by measuring one of the 22 ohm 5W resistors in the circuit (they will measure 22 ohms with no 2A3 installed).  The blue to black resistance measurement should be around 500 ohms.  It is worth it to heat the solder joint on the power supply PC board and slip the black choke wires out, then remeasure, just to be sure.

[Drugolf]

Oh dear, so did you blow the 1.6K resistors in both channels?  I would toss both 2A3s and start over with a fresh pair.

22.2 ohms from the blue wire to the black wire is a destroyed plate choke.  Be sure you are actually getting 22 ohms by measuring one of the 22 ohm 5W resistors in the circuit (they will measure 22 ohms with no 2A3 installed).  The blue to black resistance measurement should be around 500 ohms.  It is worth it to heat the solder joint on the power supply PC board and slip the black choke wires out, then remeasure, just to be sure.

The 22.2 was actually mohms so I broke out my laptop and DATs measurement software to use instead of my DVM for resistance measuring.  It gives me 1380ohm black to blue pins.  No tube installed.
Measuring the 22ohm resistors without a tube in the socket I get 13.3 ohms on each one.  My DVM gives me the same.

I wouldn't know which black choke wires you mean on the power supply board.  Frankly, I'm not sure where/what the choke is on this.  Here is a photos of my entire underside.

[Paul Birkeland]

The HV+ pads on each side of the power supply PC board have a black choke wire attached to them.

You can trace that wire from where it passes through the top panel, and it will terminate at the PC board.  It will also be the only stranded wire connected to the PC board at the HV+ pads.

[Paul Joppa]

As for upgrades, the only change to the amplifying circuit was to increase the 12AT7 current to 5.5mA (150 ohm R1 on the C4S board) and replace the 431 cathode bias with a 365-ohm resistor, unbypassed. This puts the 12AT7 in a slightly more linear operating region; it may or may not be audible. We made a few other changes, to make it easier to build and accommodate the SR driver, DC filaments, and the BeePre-style attenuator upgrade packages - these are not applicable to the original Stereomour.

[Drugolf]

As for upgrades, the only change to the amplifying circuit was to increase the 12AT7 current to 5.5mA (150 ohm R1 on the C4S board) and replace the 431 cathode bias with a 365-ohm resistor, unbypassed. This puts the 12AT7 in a slightly more linear operating region; it may or may not be audible. We made a few other changes, to make it easier to build and accommodate the SR driver, DC filaments, and the BeePre-style attenuator upgrade packages - these are not applicable to the original Stereomour.

Any value in upgrading the Solens?

[Paul Birkeland]

One thing at a time!  I would get the amp fixed first with some fresh 2A3s, then explore other options.

If you try to do it all at once, there is the potential to create new issues in the process.