Hi Marcel,
Thanks for the reply! I looked at the datasheet for the LR8N3 voltage regulator, and found this phrase (bottom of the first page): "To maintain stability, a bypass capacitor of 1 muF or larger and a minimum DC output current of 500 muA are required."
Is that the minimum load current requirement you mention -- 500 muA, or 0.0005 A? If so, to get the required resistance to place instead of the amplifier circuit, would I use Ohm's law? I'm shooting for 250 volts, requiring a current of 0.0005 Amperes:
250 v = 0.0005 A * R --> R = 500,000 ohms. Does that sound right?
[Sorry to pick nits, but since in Ohm's law current is inversely related to resistance, higher resistance leads to lower current. I would have thought I'd want to be just below the calculated resistance to make sure there's at least 500 muA.]
Also, please forgive my ignorance, but how would I connect the resistors? One side to B+, the other side to ground?
Thanks so much for your help! In case its useful, I've attached a schematic of the B+ supply I've built. I did not wire my heater supply this way, as this power supply was created for a phono preamp using 12ax7's, and I'm using 6dj8's. My heater supply is, at this point, just a direct connection of the 6.3 windings to the heater pins.
Dan
Attachments
Yes, that's correct. By the way, the current drawn by the external feedback network also counts, so 500 kohm is already quite conservative.
Thanks!
Again, sorry for picking nits, but what exactly is the "external feedback network"? I'm thinking I'd undo the B+ connection from the power supply to the amp circuit (also not connect the 6.3v heater leads), and then just connect the B+ to a 500k ohm resistor and connect that to ground. What is the 'external feedback network' there?
Thanks!
Dan
Again, sorry for picking nits, but what exactly is the "external feedback network"? I'm thinking I'd undo the B+ connection from the power supply to the amp circuit (also not connect the 6.3v heater leads), and then just connect the B+ to a 500k ohm resistor and connect that to ground. What is the 'external feedback network' there?
Thanks!
Dan
Just a quick update...
I built a dim bulb tester (silly not to have had one of those from the start) and did some investigating. One problem I discovered is that I mistakenly grounded the center tap of the transformer. Evidently, this is bad for the B+ circuit in the attached power supply (I'm only building the B+; the heaters supply is just the 6.3v secondary and a humdinger for 6dj8 tubes). I corrected the mistake, and the dim bulb tester no longer glows when powered up.
But my B+ voltage is off. I'm getting B+ of about 50v on each channel (49.3 and 51.1, specifically). The HV winding on the transformer gives 272vac, which sounds reasonable given the secondary is 250vct. The voltage on the positive terminal of the big 100muF capacitor is 373vdc. Then everything gets weird...
The voltages on the Vin pins of the regulators are 373vdc, the voltages on the Vout pins of the regulators are 52vdc and 49vdc, and the voltages on the ADJ pins of the regulators are 51vdc and 49vdc. I guess the voltages on the Vout and ADJ pins could actually be the same, just measured slightly differently due to tolerances in the DMM.
I'm not sure what to make of this or where to start looking for problems or mistakes. I'll go over the circuit carefully this evening to make sure there's nothing obvious, but I'd be very grateful if anyone has suggestions or insight.
Thanks for your time and help!
Dan
I built a dim bulb tester (silly not to have had one of those from the start) and did some investigating. One problem I discovered is that I mistakenly grounded the center tap of the transformer. Evidently, this is bad for the B+ circuit in the attached power supply (I'm only building the B+; the heaters supply is just the 6.3v secondary and a humdinger for 6dj8 tubes). I corrected the mistake, and the dim bulb tester no longer glows when powered up.
But my B+ voltage is off. I'm getting B+ of about 50v on each channel (49.3 and 51.1, specifically). The HV winding on the transformer gives 272vac, which sounds reasonable given the secondary is 250vct. The voltage on the positive terminal of the big 100muF capacitor is 373vdc. Then everything gets weird...
The voltages on the Vin pins of the regulators are 373vdc, the voltages on the Vout pins of the regulators are 52vdc and 49vdc, and the voltages on the ADJ pins of the regulators are 51vdc and 49vdc. I guess the voltages on the Vout and ADJ pins could actually be the same, just measured slightly differently due to tolerances in the DMM.
I'm not sure what to make of this or where to start looking for problems or mistakes. I'll go over the circuit carefully this evening to make sure there's nothing obvious, but I'd be very grateful if anyone has suggestions or insight.
Thanks for your time and help!
Dan
Attachments
Do you get the weird voltages when you test the supply standalone, loaded by the amplifier or both? I know from others on this forum that it can be difficult to get LR8's to start up properly, especially with a large capacitive load. They tend to go into thermal fold-back protection before their load capacitance is fully charged.
It works!!!
I read something in the spec sheet about the LR8 going into some sort of shut down mode if it was forded to pass too much current, or something like that. So on a whim I tried plugging in just one of the two tubes. When I turned it on the B+ went to about 245 and stayed right there. I powered down, waited a bit, switched the tube to the other socket, and same thing. So I thought maybe the issue is that the 6922 tubes i'm using draw more current than the LR8 device can pass.
I had two extra LR8's and the associated caps and resistors, so I built another regulator circuit, and now each half of each tube gets its own regulated B+ (I'm using 4 of the LR8 regulators). Once I got everything connected and plugged in, the voltages looked exactly right and stable. I hooked it up to the stereo really quickly and it definitely plays music. I didn't listen for more than half a minute because I didn't secure all of the bits in the power supply very well in my rush to see if it would work. So I'll spend some time getting it all 'up to code' before listening any more to it. But I'm really thrilled it's working!
Thank you all for your help, and especially Marcel. I really appreciate you taking the time to help me understand what's wrong and what to do to fix it. Thanks!
Dan
I read something in the spec sheet about the LR8 going into some sort of shut down mode if it was forded to pass too much current, or something like that. So on a whim I tried plugging in just one of the two tubes. When I turned it on the B+ went to about 245 and stayed right there. I powered down, waited a bit, switched the tube to the other socket, and same thing. So I thought maybe the issue is that the 6922 tubes i'm using draw more current than the LR8 device can pass.
I had two extra LR8's and the associated caps and resistors, so I built another regulator circuit, and now each half of each tube gets its own regulated B+ (I'm using 4 of the LR8 regulators). Once I got everything connected and plugged in, the voltages looked exactly right and stable. I hooked it up to the stereo really quickly and it definitely plays music. I didn't listen for more than half a minute because I didn't secure all of the bits in the power supply very well in my rush to see if it would work. So I'll spend some time getting it all 'up to code' before listening any more to it. But I'm really thrilled it's working!
Thank you all for your help, and especially Marcel. I really appreciate you taking the time to help me understand what's wrong and what to do to fix it. Thanks!
Dan
I did! Hopefully you can follow what I did, what worked, what went wrong, and how I fixed it. If I had to do it over again, I'd just build one of RJM's suggested power supplies. I didn't know what I was doing and just figured I could substitute one 250v B+ power supply for another... But it's working and sounds very good.
There's some hum in my unit. I did a crap job w/ the heater wiring. I planned out the entire phono preamp layout, and only then realized "oh, I have to connect wires w/ AC to the heaters...". I read, after the fact, the way to do it is to start out by planning how you'll wire the heaters, and then plan the phono preamp layout. Makes sense! I'm planning on 'elevating' the heaters by 35v dc or so. Hopefully soon.
I have some no-name 6922 tubes, can't remember where I got them. The phono stage sounds good with those. Solid, somehow.
I have some Amperex 6922's made in India in the BEL factory that I got from Brent Jesse --- Those sound great! Very spacious, airy, great cymbals in jazz, wonderful midrange, nice bass if not super prominent (but in a good way), etc.
I had some orange logo Amperex 6922's that were microphonic as a ... They shrieked! Could barely tell if they sounded nice or not due to all of the squealing. If you tapped them w/ a chopstick they went nuts. Even just normal levels of volume on the speakers sent them into a fit. I ordered these from Brent Jesse first, and then he was super accommodating on the exchange. I'm told his prices are high, but his customer service is very much worth it for me.
Hope that helps. Good luck!
Dan
There are a few on page 1. That's basically what the current version is. Only real change is that there are 2 boards after the CRC filter in the power supply, one for each channel (and each board has 2 regulators, so each of the 4 triodes gets its own regulated b+). That and the power transformer is now outside of the chassis.
Dan
Dan