Hi All -- hoping for some help in what was supposed to be a simple Power Transformer replacement for my TSE-II. I desoldered C4, and replaced some screw binding posts at 6.3V heater connection. (I'm measuring b+ across R30).
I previously had an Edcor XPWR-008 (300-0-300) with a Triad C17-X Choke (40ohm), and a 47uf C4, and got around 360 B+. I replaced with an XPWR-131 (330-0-330) and Triad C14-X choke (150ohm), and my B+ numbers are all quite a bit higher then what PSUD (which has usually been pretty good for me) predicted: with a 8.2uf C4, I am still getting 415V B+, and with 10uf it's around 425V, and the new transformer is buzzing.
Here's the thing: to troubleshoot, I hooked the old PT back up, and now the B+ is 390V (and that's with the new 150ohm choke in place).
I can figure out how this is possible, unless I fried a component when desoldering? I have been checking things more or less at random, and wondering if anyone had any ideas if there is a particular resistor or cap that if fried would result in this jump? The only thing behind R30 is the choke, which I am getting 150ohm on, so I can't really figure how the B+ is suddenly higher. Would a dead D7 cause this? I would have put a lot of heat on that pad. (Not quite sure how to check components in place though).
The amp will bias up and play music fine (though I haven't wanted to run it too long with the high B+). I'm also wondering if I've simply done something to screw my measurements up, and the B+ is in fact lower? I've tried different multimeters. . .
Any suggestions would be appreciated -- thanks!
EDIT: After some further testing, I noticed two strange things. When measuring the HV secondary on the PT when the amp is turned on, it loops every 5-6 seconds between 700VAC, falling to ~685VAC over a few seconds, and then climbing back to 700VAC. And once the amp is powered off, the B+ drops to 1-2VDC after 2-3 seconds. I remember it taking longer in the past.
I previously had an Edcor XPWR-008 (300-0-300) with a Triad C17-X Choke (40ohm), and a 47uf C4, and got around 360 B+. I replaced with an XPWR-131 (330-0-330) and Triad C14-X choke (150ohm), and my B+ numbers are all quite a bit higher then what PSUD (which has usually been pretty good for me) predicted: with a 8.2uf C4, I am still getting 415V B+, and with 10uf it's around 425V, and the new transformer is buzzing.
Here's the thing: to troubleshoot, I hooked the old PT back up, and now the B+ is 390V (and that's with the new 150ohm choke in place).
I can figure out how this is possible, unless I fried a component when desoldering? I have been checking things more or less at random, and wondering if anyone had any ideas if there is a particular resistor or cap that if fried would result in this jump? The only thing behind R30 is the choke, which I am getting 150ohm on, so I can't really figure how the B+ is suddenly higher. Would a dead D7 cause this? I would have put a lot of heat on that pad. (Not quite sure how to check components in place though).
The amp will bias up and play music fine (though I haven't wanted to run it too long with the high B+). I'm also wondering if I've simply done something to screw my measurements up, and the B+ is in fact lower? I've tried different multimeters. . .
Any suggestions would be appreciated -- thanks!
EDIT: After some further testing, I noticed two strange things. When measuring the HV secondary on the PT when the amp is turned on, it loops every 5-6 seconds between 700VAC, falling to ~685VAC over a few seconds, and then climbing back to 700VAC. And once the amp is powered off, the B+ drops to 1-2VDC after 2-3 seconds. I remember it taking longer in the past.
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Some further tests: I put the old choke back in as well. So back to exactly the same parts as the original build, B+ is now apx 30V higher. Also, my old transformer is measuring 640VAC across the HV secondary while the amp is running, when I would expect closer to 600.
Thoughts:
1) this amp has been running untouched for a couple of years, so I suppose it's possible something has drifted / failed to result in this higher B+. If that's the case, would there be a component that would result in these transformers putting out more VAC on the taps than they should? It seems unlikely that both transformers are out of spec -- could a short on the board cause this?
2) I have always used the 20K Resistors for R14 & R25 (I didn't see that note on the old TSE page until recently that recommends 36K for higher B+ voltages). Would running at 360V B+ for years with these values have caused an issue?
3) Are there any other component failures that would cause the amp to still work, but run higher B+ like this?
Finally: When I mentioned the HV secondary being so high to Edcor they asked that I measure the current being drawn from the various taps while the amp is in use, does anyone know how to do this? Is there a way to do it at a single point for each tap, or would I need to measure individual components and add it all up?
Apologies for the bump, but I'm really stumped 🙂
Thoughts:
1) this amp has been running untouched for a couple of years, so I suppose it's possible something has drifted / failed to result in this higher B+. If that's the case, would there be a component that would result in these transformers putting out more VAC on the taps than they should? It seems unlikely that both transformers are out of spec -- could a short on the board cause this?
2) I have always used the 20K Resistors for R14 & R25 (I didn't see that note on the old TSE page until recently that recommends 36K for higher B+ voltages). Would running at 360V B+ for years with these values have caused an issue?
3) Are there any other component failures that would cause the amp to still work, but run higher B+ like this?
Finally: When I mentioned the HV secondary being so high to Edcor they asked that I measure the current being drawn from the various taps while the amp is in use, does anyone know how to do this? Is there a way to do it at a single point for each tap, or would I need to measure individual components and add it all up?
Apologies for the bump, but I'm really stumped 🙂
Assuming the power supply is CLC then I think you can measure the voltage drop across the choke and infer the current from that. You know the resistances of the two chokes. Regarding the fluctuation in voltage, have you measured your AC supply over the same time?
One other question......when is the last time you checked the accuracy of your voltmeter?
I will specifically check against a known voltage to be sure, but I have tried a couple different meters and replaced all the batteries, and the heater voltages check. Will double check to be sure though. . .
Alright, starting to get somewhere: measuring the voltage drop across the choke (8.7V, 150ohm choke) gives me 60ma of current, which makes no sense because I biased the 300b at ~65ma each. SO I need to setup some more meters (wasn't measuring the tubes and the choke at the same time), and see where I am measuring wrong.
The high B+ would be explained by the tubes drawing only 60ma total current. Is it possible that a faulty tube would cause me to read a plate current value at the shunt resistors (R18/R29) that is incorrect in terms what the plate is actually drawing?
The high B+ would be explained by the tubes drawing only 60ma total current. Is it possible that a faulty tube would cause me to read a plate current value at the shunt resistors (R18/R29) that is incorrect in terms what the plate is actually drawing?
Something is definitely messed up: pic below of 3 of the meters (which I assume can't all be wrong) showing 8.8V on the choke, and ~68mv across R18/R29.
The 425 B+ makes me think the tubes are not actually drawing that current, but rather < than 60ma or so as indicated by the 8.8 on the coke. PSUDII also predicts about 420 B+ with 60ma total output. I don't want to mess with it too much with the tubes in, but I suppose I should check quickly to see if it plays music.
I think I mentioned above but the other strange thing is the B+ drops to 1V in like 2 seconds after being switched off -- I would have expected more time for it to decay (that said, I am not trusting anything at this point and staying far away from the amp for 5 min at least after power off).
And apologies for all the bumps, but it seems you can't edit after 30 mins. . .
The 425 B+ makes me think the tubes are not actually drawing that current, but rather < than 60ma or so as indicated by the 8.8 on the coke. PSUDII also predicts about 420 B+ with 60ma total output. I don't want to mess with it too much with the tubes in, but I suppose I should check quickly to see if it plays music.
I think I mentioned above but the other strange thing is the B+ drops to 1V in like 2 seconds after being switched off -- I would have expected more time for it to decay (that said, I am not trusting anything at this point and staying far away from the amp for 5 min at least after power off).
And apologies for all the bumps, but it seems you can't edit after 30 mins. . .
Alright, just played music for 5 mins and everything sounded fine. My new theory is that the voltage drop on the choke is only measuring roughly half the current, as it's ignoring the B- side, and the tubes are in fact working properly and have the plate current I am measuring. Is that possible?
Which leaves me with: if the tubes bias and it sounds fine, are there any failing / failed component that anyone is aware of that would bump the B+ up like I am experiencing? I can add a resistor in series with the choke to drop it and be done with it, but it'd be great to have some assurance that something isn't amiss, as the B+ is quite a bit higher then would be expected, and is higher using my old PT as well (compared to a couple years ago when it was last measured).
With my (pretty limited) understanding, the choke is the only component behind R30 (where I am measuring B+) that could make it higher if failed (the caps if failed would make it lower, no?). Is there anything downstream of R30 that if failed would raise B+? I don't think so but wanted to make sure.
Which leaves me with: if the tubes bias and it sounds fine, are there any failing / failed component that anyone is aware of that would bump the B+ up like I am experiencing? I can add a resistor in series with the choke to drop it and be done with it, but it'd be great to have some assurance that something isn't amiss, as the B+ is quite a bit higher then would be expected, and is higher using my old PT as well (compared to a couple years ago when it was last measured).
With my (pretty limited) understanding, the choke is the only component behind R30 (where I am measuring B+) that could make it higher if failed (the caps if failed would make it lower, no?). Is there anything downstream of R30 that if failed would raise B+? I don't think so but wanted to make sure.
I dont know what rectifiers you are using but 300 * 1.414 is 424 volts DC peak before choke if solid state rectifiers.
Maybe the old transformer was a little under rated and sagged more ?
Maybe the old transformer was a little under rated and sagged more ?
When you stated that it played music for 5 minutes, was the music loud and clean, or was there some distortion if you turned it up?
8.7 volts across a 150 ohm choke is indeed 58 mA. The circuitry on the board itself consumes 20 to 25 mA depending on the B+ voltage. Most of that is burned in the mosfets. Each driver tube consumes about 10 mA.
Do you have 10 ohm resistors placed for R18 and R 29? If your 68 mV readings equal 6.8 mA for each output tube. You should read about 650 mV across the 10 ohm resistors to get 65 mA per tube.
8.7 volts across a 150 ohm choke is indeed 58 mA. The circuitry on the board itself consumes 20 to 25 mA depending on the B+ voltage. Most of that is burned in the mosfets. Each driver tube consumes about 10 mA.
Do you have 10 ohm resistors placed for R18 and R 29? If your 68 mV readings equal 6.8 mA for each output tube. You should read about 650 mV across the 10 ohm resistors to get 65 mA per tube.
I didn't turn it up at all (son was sleeping), BUT: you're right George -- what a boneheaded move on my part with the bias! In the past, I had always set my meter to measure current, and placed the probes across R18/R29, and measured ma that way. In my troubleshooting, it occurred to me to try measuring voltage drop across those resistors, as that's what the checkout pages are doing on the tubelab site. But I used the 200m setting on the meter, so you're right I'm off by an order of magnitude! This would also explain the high B+. . . Geez. . .
One last sanity check, as I am scared to turn the bias up so "high." The 8.7 volts across the choke makes sense with my super low bias (i.e. all the board current should show on the choke?). If so, then 58ma would roughly = ~25ma for the board + 24ma for the two 5842s + 12ma for the 300bs.
So, I'm going to double check that R18/R29 are 10ohm, and then set the meter to 2000mv instead of 200mv, and look for 680mv, not 68mv. And just curious: if I do something wrong and bias waay to high, are the 300bs instantly dead? Or should I be okay as long as it's brief? I'm not sure of the board / PT is able to put out enough current to fry them instantly.
Really appreaciate all the help here everyone!
Huzzah! All is right! I biased the tube properly to ~68ma, and B+ voltage dropped to 370, exactly where PSUDII predicts it should. measured 26V across the choke for 173ma total current, which is about would is should be.
Sorry for the big time waster, everyone, but I really appreciate the help, as always. I learned a lot, and hopefully someone else finds this thread useful 🙂
Two final things that aren't critical:
1) I have always measured bias my using probes across R18/R29, and setting the meter to 200ma setting and measuring current. Is it possible I have been measuring wrong the whole time -- I would have been been placing the multimeter's shunt resistor in parallel with R18/R29, right? If so, would actual current have been higher or lower then I was measuring?
2) I only saw the note originally on the TSE page that R14 and R25 should be 36K with B+ over 350V. Mine have been 20K the whole time, as on the BOM for the TSE-II. Would it be important to change these to 36K asap, or is this not an issue with the TSE-II?
Thanks again, all!
Sorry for the big time waster, everyone, but I really appreciate the help, as always. I learned a lot, and hopefully someone else finds this thread useful 🙂
Two final things that aren't critical:
1) I have always measured bias my using probes across R18/R29, and setting the meter to 200ma setting and measuring current. Is it possible I have been measuring wrong the whole time -- I would have been been placing the multimeter's shunt resistor in parallel with R18/R29, right? If so, would actual current have been higher or lower then I was measuring?
2) I only saw the note originally on the TSE page that R14 and R25 should be 36K with B+ over 350V. Mine have been 20K the whole time, as on the BOM for the TSE-II. Would it be important to change these to 36K asap, or is this not an issue with the TSE-II?
Thanks again, all!
1) Yes your actual current would be a bit higher than what you were measuring on the 200 mA scale since some current is going through the 10 ohm resistor. How much higher depends on the internal resistance of the meter.
2) The heat buildup mitigation that took place when the original TSE became the TSE-II fixed the hot parts issue at higher voltages, so R14 and R25 can be 20K which provides better performance. Leave those resistors alone.