Bottlehead Forum
General Category => General Discussion => Topic started by: Drugolf on December 14, 2019, 07:00:35 PM
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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!
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Original or Stereomour II?
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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?
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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.
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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.
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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.
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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.
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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. :P
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.
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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.
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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.
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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.
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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.
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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.
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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?
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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.
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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.
Understood, but since ordering stuff.......
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You may find that other than something like a Solen Silver, most of the fancy caps just won't fit in the space allotted very well.
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I replaced the two burnt looking resistors.
Is there a way to isolate and properly test the choke?
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See post #4 (from PB) in this thread. :^)
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See post #4 (from PB) in this thread. :^)
Ah yes, I forgot that was what that test was for. Shall do.
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I de-soldered the blue and black wires (the one that extends to HV+ on teh PSB) that extend from the choke and measure resistence. Both are at 586 ohm.
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That's very good news!
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That's very good news!
Agreed!
I went ahead and pulled one of the 220 uF caps on the PSB and it measured 202uF. See the following screenshot of the test. I don't know how to determine if those various numbers for ESR disapation factor etc are good or bad.
I then also did all the initial measurements of the terminal strip locations per the build instructions manual and all were as they should be.
Then installed the 12AX7 tube and powered it up with Dimm bulb connected and it seemed fine. Using a 60 watt bulb it was very dimly lit. 200v bulb nothing lit. 12AX7 appeared to light up as it should.
Powered up again with the 2A3 tubes installed. Didn't go well. The 12ax7 got way too bright and I also heard a crack/pop noise. Nothing visually apparent though.
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Did you install the 2A3's that were installed in the wrong way before?
Or did you install new ones?
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Did you install the 2A3's that were installed in the wrong way before?
Or did you install new ones?
The old ones. Waiting for new ones to arrive. So yes, I wonder if it's just the tubes.
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You mention a 12AX7, that would not be the correct tube. I believe the Stereomour uses a 12AT7...John
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The 12ax7 got way too bright and I also heard a crack/pop noise. Nothing visually apparent though.
Can you be more specific about what you mean about the tubes being "way too bright"?
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Can you be more specific about what you mean about the tubes being "way too bright"?
I think one of the plates was aglow.
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You mention a 12AX7, that would not be the correct tube. I believe the Stereomour uses a 12AT7...John
Sorry, brain fart. It's an AT
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A glowing 12AT7 plate is quite unusual. Can I see some photos of the patched up amp?
Are you 100% certain that the plate doesn't glow without the 2A3s in the sockets?
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In removing one of the caps on the PT-6 board I damaged the trace pretty bad. It will either need to be repaired or the board replaced. I emailed Eileen to see about the availability of a replacement board.
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Those might have ended up with me, I'll have a look in my stash.
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Thanks Paul. Eileen responded too that you would be the one likely to know if there are any still available at this point. She is sending me a up grade power cord I purchased so I thought it made sense to see if you still had a board to include in the shipment.
BTW, I tested the 2A3 tubes on a Hickok 800A tester. no shorts and passed mutual conductance wit numbers that reflect new and the same.
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I did find a small stack of Stereomour I power supply boards, but I don't think I'll be at the office for a week or so. If you contact me at pb(at)bottlehead(dot)com, we can see if the USPS letter sorting machines can handle a PC board in a paper envelope ;)
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Perfect! Thanks Paul.