Hello,
I am wondering why peak current is such a problem in the fil circuits. That is, at power on, there is a surge as the filter caps charge and the heaters start to do their things. But, this is a very brief surge, and once things are warm and working, things are well within their design parameters. Why then, would this initial surge of peak current cause a transformer to get hot when it is so brief in duratuion?
I am fairly confident the heat is being caused by the current demands of the fils and not the caps... I have two identical circuits, one feeding the 6J5 and the other feeding the 300B (identical with exception that the voltage is set differently with the trimmer pot). The 6J5 is running cool and happy, with the 300B being hot. Both transformers are rated for 4A. Strange?
I will check for the not-so-obvious wiring mistakes this evening, and maybe measure the current being drawn from the trans with my meter. Ican also try adding some surge blocking resistance if needed and lowering the capacitance... If anyone can think of something else, I am open ears.
Thanks for aiding in the continual education!
Much Appreciation,
Bryan
... UGH, what a way to start the week on a rainy Monday morning!
I am wondering why peak current is such a problem in the fil circuits. That is, at power on, there is a surge as the filter caps charge and the heaters start to do their things. But, this is a very brief surge, and once things are warm and working, things are well within their design parameters. Why then, would this initial surge of peak current cause a transformer to get hot when it is so brief in duratuion?
I am fairly confident the heat is being caused by the current demands of the fils and not the caps... I have two identical circuits, one feeding the 6J5 and the other feeding the 300B (identical with exception that the voltage is set differently with the trimmer pot). The 6J5 is running cool and happy, with the 300B being hot. Both transformers are rated for 4A. Strange?
I will check for the not-so-obvious wiring mistakes this evening, and maybe measure the current being drawn from the trans with my meter. Ican also try adding some surge blocking resistance if needed and lowering the capacitance... If anyone can think of something else, I am open ears.
Thanks for aiding in the continual education!
Much Appreciation,
Bryan
I'll post back on the specs of the caps when I get home. I cant remember the exact type off the top of my head.
I will also try addins some ohms in front of the input cap. Hopefully it will smooth things out a little. If that doesn't work, the capacitance will be decreased. Hopefully with a little help from y'all I will get thing running within reason.
Safety!!!
Thanks,
bryan
I will also try addins some ohms in front of the input cap. Hopefully it will smooth things out a little. If that doesn't work, the capacitance will be decreased. Hopefully with a little help from y'all I will get thing running within reason.
Safety!!!
Thanks,
bryan
I've modded the circuits in PSUD, and sure enough the current at the transformer was above spec. I've added some resistance (which in PSUD brought the transformer current w/i spec) and I will see what the temp difference is...
I also noted that changing the input cap value had much less of an effect on the current draw at the transformer compared to the addition of resistance. Hopefully this will equate to cooler running on the actual circuit!
I also noted that changing the input cap value had much less of an effect on the current draw at the transformer compared to the addition of resistance. Hopefully this will equate to cooler running on the actual circuit!
Progress Report 
Things are good. I've tested all the filaments and all is good. I've built up a simple bias circuit (schottky - CRC - voltage dividers with pot ) and the negative rails are stable and working. I've brought up both HV rails and they are working just fine. The B+ on the 845 is somewhere over 1020VDC, as this is where my DVM goes into overload. I suspect I am near 1075VDC. I will get an accurate measurement from the string of bleeder resistors in the near future, but the less poking around I do there the better!
I have what is probably a very simple question. I will be using fixed bias on both the 300B and the 845. I have a nice Simpson 0-150DC mA meter which I want to use to test both tubes currents. So, what is the best way to employ one meter, and two circuits.
Is there make-before-break SPDT toggles that I can use to switch the ammeters in and out of the path to ground? I think this would be the easiest way to do this. Two switches, one for each tube. Suggestions?
Once I have this done, I will start the specific (R's and C's) of each stage and do some more testing, and then pull out the sig gen. Getting close, but still a ways away........
I will try to post a few pics of the bread-board in the near future. It is looking quite tidy compared to past projects.
Thanks Guys,
Bryan
Things are good. I've tested all the filaments and all is good. I've built up a simple bias circuit (schottky - CRC - voltage dividers with pot ) and the negative rails are stable and working. I've brought up both HV rails and they are working just fine. The B+ on the 845 is somewhere over 1020VDC, as this is where my DVM goes into overload. I suspect I am near 1075VDC. I will get an accurate measurement from the string of bleeder resistors in the near future, but the less poking around I do there the better!
I have what is probably a very simple question. I will be using fixed bias on both the 300B and the 845. I have a nice Simpson 0-150DC mA meter which I want to use to test both tubes currents. So, what is the best way to employ one meter, and two circuits.
Is there make-before-break SPDT toggles that I can use to switch the ammeters in and out of the path to ground? I think this would be the easiest way to do this. Two switches, one for each tube. Suggestions?
Once I have this done, I will start the specific (R's and C's) of each stage and do some more testing, and then pull out the sig gen. Getting close, but still a ways away........
I will try to post a few pics of the bread-board in the near future. It is looking quite tidy compared to past projects.
Thanks Guys,
Bryan
Hi Bryan,
Glad to hear things are going well
Not necessary to be "shorting". Remove the shunt from the meter(s) and use a low value cathode resistor in each valve equal to the shunt resistor. Then switch between them.
You could also use a different value shunt in the 300B cathodes so that the meter shows a reasonable deflection. On my OnRyoku amp I used this scheme.
BTW The degeneration caused by resistors this low is negligable.
___________
I got to thinking about the heater transformer problem. I don't think it's worth pussy-footing with an "almost adequate" transformer. Some weeks down the line you may get fed up with that "hot smell" in your lounge after long listening sessions. The design isn't set in stone yet, just get a bigger one, or parallel another.
Glad to hear things are going well
Not necessary to be "shorting". Remove the shunt from the meter(s) and use a low value cathode resistor in each valve equal to the shunt resistor. Then switch between them.
You could also use a different value shunt in the 300B cathodes so that the meter shows a reasonable deflection. On my OnRyoku amp I used this scheme.
BTW The degeneration caused by resistors this low is negligable.
___________
I got to thinking about the heater transformer problem. I don't think it's worth pussy-footing with an "almost adequate" transformer. Some weeks down the line you may get fed up with that "hot smell" in your lounge after long listening sessions. The design isn't set in stone yet, just get a bigger one, or parallel another.
John,
Thanks for the reply.
I went ahead and bought a beefier filament transformer, and also tweaked the resistor and trimmer values on the reg so that I can burn less total voltage through the reg. The original transformer was an open bobin hammond, rather cheap. I purchased an enclosed type, with a little higher wattage rating, and it is running warm to the touch, but not hot! I have no worries here.
Hmm, the shunt resistor on the ammerter is internal, so I don't think I can just pop it out. I should say, these meters are VERY nice. Retro style, easy on the eyes. If only they were illuminated.
Here are the specs. I have the 0-150mA version, model # 01800 on the link.
So, if I use a switch, and have an equivalent resistance of ~10 ohms in the alternate path to ground from the cathode all would be well? I guess I just thought that if I switched the live circuit all hell would break loose when the connection was broken and then re-made resulting in bumps and thumps at the speakers....
Thanks for the reply.
I went ahead and bought a beefier filament transformer, and also tweaked the resistor and trimmer values on the reg so that I can burn less total voltage through the reg. The original transformer was an open bobin hammond, rather cheap. I purchased an enclosed type, with a little higher wattage rating, and it is running warm to the touch, but not hot! I have no worries here.
Hmm, the shunt resistor on the ammerter is internal, so I don't think I can just pop it out. I should say, these meters are VERY nice. Retro style, easy on the eyes. If only they were illuminated.
Here are the specs. I have the 0-150mA version, model # 01800 on the link.
So, if I use a switch, and have an equivalent resistance of ~10 ohms in the alternate path to ground from the cathode all would be well? I guess I just thought that if I switched the live circuit all hell would break loose when the connection was broken and then re-made resulting in bumps and thumps at the speakers....
Bryan,
I think the internal shunt will be just behind the terminals. It looks like you will have to take off the terminal nuts and pop off the face - be really careful not to touch the needle or finger any of the internals, as it will show.
The scheme I am suggesting is a shunt resistor in each cathode (4). That way, each cathode never switches away from its ground path.
When you choose a cathode resistor, make sure it's a type that can't go open circut, or bypass it with a diode or 2 to protect the meter under valve fault condition. The value will be close to 10 ohms, as you say. Probably it's best to buy 10 ohm resitors and add some "fudge" components to tweak.
I think the internal shunt will be just behind the terminals. It looks like you will have to take off the terminal nuts and pop off the face - be really careful not to touch the needle or finger any of the internals, as it will show.
The scheme I am suggesting is a shunt resistor in each cathode (4). That way, each cathode never switches away from its ground path.
When you choose a cathode resistor, make sure it's a type that can't go open circut, or bypass it with a diode or 2 to protect the meter under valve fault condition. The value will be close to 10 ohms, as you say. Probably it's best to buy 10 ohm resitors and add some "fudge" components to tweak.
Bryan,
I can see you are worried about opening the thing
Why not give it a gentle urging and see if it comes apart? If it's not easy, we'll think up another method. 
Warning, semiconductors might be needed.....
Most probably. The meter manufacturer usually makes just one movement and adds internal shunts to offer a range.
I can see you are worried about opening the thing
Why not give it a gentle urging and see if it comes apart? If it's not easy, we'll think up another method. Warning, semiconductors might be needed.....OK, with a little coaxing, she gave up the goods
There are two resistors inside the case of the meter. Onoe is the shunt, and connects directly between the input and the putput of the meter, while a second resistor goes from the positive terminal to the movement via am insulated lead.
I assume I should just pull the shunt resistor and close her back up?
Also, with respect to the switch diagram below, can I simply use the salvaged resistor from the meter in place of R1, or will there need to be some change of resistance. It seems to me that it should be a direct drop in???
Photos coming tonight.
Bryan
There are two resistors inside the case of the meter. Onoe is the shunt, and connects directly between the input and the putput of the meter, while a second resistor goes from the positive terminal to the movement via am insulated lead.
I assume I should just pull the shunt resistor and close her back up?
Also, with respect to the switch diagram below, can I simply use the salvaged resistor from the meter in place of R1, or will there need to be some change of resistance. It seems to me that it should be a direct drop in???
Photos coming tonight.
Bryan
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