![[-]](https://theultimatetone.com/images/collapse.png "[-]")
06-24-2024, 08:33 PM
(This post was last modified: 06-24-2024, 08:35 PM by physics.
Edit Reason: Note that bias supply should be fine, and I don't understand why.
)
I guess I expected there to be the same cost-cutting measures across the whole product line, so it threw me for a loop when the 50W model had it but not the 20W or 100W models.
What are the design guidelines for choosing these resistors? Since my last message I've been working out that detail in parallel to getting the power supply rebuilt after switching to an eyelet card (the original salvaged turret board was too delicate, could barely handle heat). So far I've checked that the average power dissipated by the voltage doubler diodes due to ripple is within margins, and it looks like ambient temp can get up to 51 C -- 66 C before the 1n4007 diodes get above 60% -- 70% of max junction temp, so that should be fine I figure. The bias supply should be fine too since it's much lighter duty, but I haven't explicitly checked that.
I'm currently hung up on the diode surge current ratings right now. For the voltage doubler things look fine, I have two half-wave rectifiers stacked on top of each other and each one has a 100uF cap fed by a 10ohm 5W resistor (per original schematics) which means assuming 265Vpeak from the transformer Ipeak is 26.5A at turn on, which is less than the 1n4007's 30A peak surge into a resistive load. The charging waveform should be just a quarter sine instead of the half sine the peak current is given at too, so that's some extra margin. The bias supply is a different story though...
For the bias supply it's just a bridge rectifier of 1n4007's feeding a 100uF cap through a 1ohm 5W ohm resistor. Initially I thought I'd be fine with this since it's basically the same as the B+ for The Standard in TUT5 (1n4007's feeding 88uF through 1 ohm 5W resistor, with no powerscaling )and my peak VAC is a fraction of what The Standard's B+ supply takes. Should add that I recently noticed that the cap bank in The Standard is actually fed through 2 x 1ohm 5W resistors plus another 1n4007, but the B+ input voltage is more than 2x that of the bias supply I'm putting together so that just makes them a little more comparable. For the bias supply I'm putting into this amp, I plan to run a 120VAC to 6VCT transformer in reverse off the heater winding, meaning I'll get about 90Vpeak out of it. At turn on then, peak current draw is 90A! Now, average current is less than that, but I know average current alone doesn't account for differences in waveform shape or the change in voltage drop with forward current. Due to that, I'm still not entirely sure how to work from the peak current waveform in the datasheet back to whatever waveform I have and make sure it's in spec. A rough method might be to pick the lowest voltage drop across the diode for the half cycle and multiply by the average current to get a lower bound on max average surge power.
After seeing that there'd be a giant spike at turn-on, I figured I'd double-check what the B+ supply looks like for The Standard at turn-on, since if that's more demanding and is fine design-wise, then my bias supply shouldn't be an issue. Anyhow, the 272JX transformer in the TUT5's The Standard is 600Vrms center-tapped, so when applied across a 2-ohm load at turn-on that's a peak of over 300A! Again average current is less over that conduction cycle, roughly 12.7A, but like I mentioned before I don't think I can go off average current based on what I think I understand so far. Obviously my little 90V bias supply should be fine if The Standard's B+ supply is fine, but I don't understand why it's fine.
I could always just chuck in some 1n5408 diodes into the bias supply instead if I didn't want to mess with either the cap size or RC constant (right now it's just barely under 1/10th of the B+'s RC constant), but it seems a bit ridiculous to have such beefy diodes in the bias supply while having 1n4007's in the voltage doubler B+ supply. Plus, I want to actually figure out what's going on here so that I'm better at electronics instead of just chucking bigger parts at circuits when I don't understand them, as valid an approach as that may be.
So, what am I missing? If anyone can direct me to some good books/resources on this particular aspect of power supply design, that'd be much appreciated too.
Thanks for any input!
What are the design guidelines for choosing these resistors? Since my last message I've been working out that detail in parallel to getting the power supply rebuilt after switching to an eyelet card (the original salvaged turret board was too delicate, could barely handle heat). So far I've checked that the average power dissipated by the voltage doubler diodes due to ripple is within margins, and it looks like ambient temp can get up to 51 C -- 66 C before the 1n4007 diodes get above 60% -- 70% of max junction temp, so that should be fine I figure. The bias supply should be fine too since it's much lighter duty, but I haven't explicitly checked that.
I'm currently hung up on the diode surge current ratings right now. For the voltage doubler things look fine, I have two half-wave rectifiers stacked on top of each other and each one has a 100uF cap fed by a 10ohm 5W resistor (per original schematics) which means assuming 265Vpeak from the transformer Ipeak is 26.5A at turn on, which is less than the 1n4007's 30A peak surge into a resistive load. The charging waveform should be just a quarter sine instead of the half sine the peak current is given at too, so that's some extra margin. The bias supply is a different story though...
For the bias supply it's just a bridge rectifier of 1n4007's feeding a 100uF cap through a 1ohm 5W ohm resistor. Initially I thought I'd be fine with this since it's basically the same as the B+ for The Standard in TUT5 (1n4007's feeding 88uF through 1 ohm 5W resistor, with no powerscaling )and my peak VAC is a fraction of what The Standard's B+ supply takes. Should add that I recently noticed that the cap bank in The Standard is actually fed through 2 x 1ohm 5W resistors plus another 1n4007, but the B+ input voltage is more than 2x that of the bias supply I'm putting together so that just makes them a little more comparable. For the bias supply I'm putting into this amp, I plan to run a 120VAC to 6VCT transformer in reverse off the heater winding, meaning I'll get about 90Vpeak out of it. At turn on then, peak current draw is 90A! Now, average current is less than that, but I know average current alone doesn't account for differences in waveform shape or the change in voltage drop with forward current. Due to that, I'm still not entirely sure how to work from the peak current waveform in the datasheet back to whatever waveform I have and make sure it's in spec. A rough method might be to pick the lowest voltage drop across the diode for the half cycle and multiply by the average current to get a lower bound on max average surge power.
After seeing that there'd be a giant spike at turn-on, I figured I'd double-check what the B+ supply looks like for The Standard at turn-on, since if that's more demanding and is fine design-wise, then my bias supply shouldn't be an issue. Anyhow, the 272JX transformer in the TUT5's The Standard is 600Vrms center-tapped, so when applied across a 2-ohm load at turn-on that's a peak of over 300A! Again average current is less over that conduction cycle, roughly 12.7A, but like I mentioned before I don't think I can go off average current based on what I think I understand so far. Obviously my little 90V bias supply should be fine if The Standard's B+ supply is fine, but I don't understand why it's fine.
I could always just chuck in some 1n5408 diodes into the bias supply instead if I didn't want to mess with either the cap size or RC constant (right now it's just barely under 1/10th of the B+'s RC constant), but it seems a bit ridiculous to have such beefy diodes in the bias supply while having 1n4007's in the voltage doubler B+ supply. Plus, I want to actually figure out what's going on here so that I'm better at electronics instead of just chucking bigger parts at circuits when I don't understand them, as valid an approach as that may be.
So, what am I missing? If anyone can direct me to some good books/resources on this particular aspect of power supply design, that'd be much appreciated too.
Thanks for any input!
