I thought each anode/plate only took half of the total current drawn on the cathode (as the anodes only conduct on the positive half of each AC cycle).
Although, I would add a significant margin to the calculated power, when picking the wattage of the series resistors.
As that exchange indicated, the design of the circuit does not assume additional resistors will be needed. (otherwise, arguably, there would be a spot for them on the board)
Instead, the design of the circuit assumes the DCR of the secondary will be sufficient to satisfy the ~100R of limiting resistance before each plate.
Hence, I say, since the transformer I picked doesn't provide the expected secondary resistance, it is not the best transformer for the job. Instead, I should have gone with the transformer (a Hammond) George had in mind when he designed the power supply of that circuit.
I am not saying Edcor makes bad transformers(, although the one I got buzzes and it should not.)
Edcor makes good transformers, but for whatever reasons, the DCR of their windings is much lower than that of transformers made before 1980.
My calculations said 65 ohms to be safe before each plate. PSUD2 also says that should keep the current under 3A through the 5AR4. I went with that and ordered them.
I tried pretending I got bad 5AR4s 2 times already. Now, I just want to put the problem to bed.
I agree that I have spent a colossal amount of time reading as many posts on the topic as I could, but that's only because I was very ignorant on that topic before then.
Thank you very much for your input!
I cannot say enough how much I appreciate you, @6A3sUMMER and others for your kind help!
That is quite possible. But with that PCB, the 2-resistor approach is actually easier and the heat dissipation will be spread across 2 aluminum bodies.
I have lived by this mantra my whole life. I found "but no simpler" to be most challenging.
StephenClose,
Given a center tapped secondary and a 5AR4:
With 60Hz power mains, a plate resistor gets heated 60 times per second (positive alternation from that secondary lead)
With 60Hz power mains, the Other plate resistor gets heated 60 times per second (positive alternation from the Other secondary lead)
With 60Hz power mains, a single cathode resistor gets heated 120 times per second (positive alternations from one secondary lead, And from the other secondary lead).
It looks to me like the power dissipated in a single cathode resistor is exactly 2 x the power dissipated in a single plate resistor (and there are 2 plate resistors).
I hope that clears up what I was saying.
Given a center tapped secondary and a 5AR4:
With 60Hz power mains, a plate resistor gets heated 60 times per second (positive alternation from that secondary lead)
With 60Hz power mains, the Other plate resistor gets heated 60 times per second (positive alternation from the Other secondary lead)
With 60Hz power mains, a single cathode resistor gets heated 120 times per second (positive alternations from one secondary lead, And from the other secondary lead).
It looks to me like the power dissipated in a single cathode resistor is exactly 2 x the power dissipated in a single plate resistor (and there are 2 plate resistors).
I hope that clears up what I was saying.
I built the first version and did loose a JJ 5AR4 rectifier that had low hours on it.
I also lost A JJ 5AR4 in my VTA ST-70. Both rectifiers had low hours on them before failing. I built both amps around the same time. The TAD brand I'm using in my ST-70 has been rock solid for about 10 years so far. The Psvane 5AR4 I put in the 300B amp is holding up well also.
I haven't compared schematics, but is the main difference just the regulator used?
I used a huge heat sink on that 5v regulator and haven't had an issue with it yet. I did buy an extra just incase.
I also lost A JJ 5AR4 in my VTA ST-70. Both rectifiers had low hours on them before failing. I built both amps around the same time. The TAD brand I'm using in my ST-70 has been rock solid for about 10 years so far. The Psvane 5AR4 I put in the 300B amp is holding up well also.
I haven't compared schematics, but is the main difference just the regulator used?
I used a huge heat sink on that 5v regulator and haven't had an issue with it yet. I did buy an extra just incase.
all the more reason to abandon the full wave center tapped psu..
the new mods I did for the ST70 is to use a single 0-330vac secondary winding instead of 360-0-360 of old,
not only is it easier to wind, but insulation required is easier too...
so with transformer utilization, you get 30% more power....
which you can then opt to lower the flux density for cooler running transformer...
the new mods I did for the ST70 is to use a single 0-330vac secondary winding instead of 360-0-360 of old,
not only is it easier to wind, but insulation required is easier too...
so with transformer utilization, you get 30% more power....
which you can then opt to lower the flux density for cooler running transformer...
I deleted previous post because George already explained why silicon diodes in series with the 5AR4 plates won't work on a TSE.
However, what about series resistors? I'd think one 10R 5W wirewound in series with each plate of the 5AR4 should help damp the turn-on surge.
Also, what about reducing the value of the reservoir capacitor (C4) from 47uF down to 33uF, or even 22uF? Reducing the value of the reservoir cap will reduce the length of time of the current surge at startup from charging up the reservoir cap, which should reduce stress on the 5AR4. Even a 10uF cap should work there, since most of the ripple reduction is performed by the choke and first filter cap.
No?
However, what about series resistors? I'd think one 10R 5W wirewound in series with each plate of the 5AR4 should help damp the turn-on surge.
Also, what about reducing the value of the reservoir capacitor (C4) from 47uF down to 33uF, or even 22uF? Reducing the value of the reservoir cap will reduce the length of time of the current surge at startup from charging up the reservoir cap, which should reduce stress on the 5AR4. Even a 10uF cap should work there, since most of the ripple reduction is performed by the choke and first filter cap.
No?
I installed the series resistors today. Went with some Dale 65 ohm 20W. It’s about 7 ohm greater than required, but a little more can’t hurt.
Checkout and biasing went fine. Now let’s see if this Tung Sol 5AR4 lasts more than 6 months. 😂
i have plenty of 10ufd/600vdc polypropylene caps that i use as first cap at rectifier cathode, then the choke and then a 470ufd'500vdc ecaps...
Trying to use a large capacitor right after the rectifier in order to get low ripple might not be the best way.
But if you have enough secondary voltage, a choke input filter has the following advantages:
Cooler Rectifier
Cooler Power Transformer
Better Voltage Regulation
It is getting too late here for me to think of more reasons, but there must be more.
And do not only concentrate on reducing 100Hz Ripple, and 120Hz ripple, you need to think about the output stage (especially for a Class A push pull amplifier).
After the choke, a filter capacitor, a series resisitor, and the output stage filter cap . . . Use lots of capacitance to power the output stage.
It has to filter transient loads of things like bass drums, thunder drums; and the constant power of a pipe organ's 32Hz note.
But if you have enough secondary voltage, a choke input filter has the following advantages:
Cooler Rectifier
Cooler Power Transformer
Better Voltage Regulation
It is getting too late here for me to think of more reasons, but there must be more.
And do not only concentrate on reducing 100Hz Ripple, and 120Hz ripple, you need to think about the output stage (especially for a Class A push pull amplifier).
After the choke, a filter capacitor, a series resisitor, and the output stage filter cap . . . Use lots of capacitance to power the output stage.
It has to filter transient loads of things like bass drums, thunder drums; and the constant power of a pipe organ's 32Hz note.
By how much did the B+ drop? I figure it will have gone down by about 10V DC.
Hopefully that's not too much voltage burned off across those resistors.
20W parts should run nice and cool.
Fingers crossed for a long life for your 5AR4.
If there's room for it, there are 22uF 600V Solen SCR caps that should work fine for C4. I see them going for about $15 each.
Mouser is selling Panasonic DC Link caps, which cost less than 'audiophile' branded caps like Audyn, Solen, etc. and are every bit as good, likely better. They are large, though, and they have four connector pins (which is a good thing, but make them harder to fit into existing builds).
https://www.mouser.com/c/passive-components/capacitors/?capacitance=22 uF&voltage rating dc=600 VDC&instock=y
The main problem with switching from a pi filter (CRC or CLC) to a choke input filter (LC) is the big drop in B+ voltage that results.
The pi-filter here is yielding about 450V B+ (I think). That's about 1.414 * Vac of secondary winding minus copper and iron losses. If figure the transformer secondary Vac is about 350Vac (700VCT).
Choke input filter would yield Vac of secondary * 0.9, which would come out as only 315V DC with this transformer. That's at the upper limits for 2A3, but 300B yields more audio output power with higher B+ voltage.
There is a grey area between a full-on pi filter and a choke input filter.
For each transformer + rectifier combination, there is a critical value of C4 that will get it to function as a full pi filter. Values of C4 lower than that critical value will yield lower B+ voltage, in that grey area between pi filter and choke input filter. Reduce the value of C4 to 0.22uF and you'll have a choke input filter (Vac*0.9). Usually it takes only about 2uF to 3uF to achieve the full B+ voltage of a pi filter. 10uF is usually more than enough to achieve full voltage, and 22uF should be well past the border into full pi filter territory. 47uF will suppress a little bit more ripple (AC noise riding on the B+ DC) but not very much. The reservoir cap (C4) doesn't have a huge influence on ripple filtering. It's the choke and the cap after that that do the filtering (ripple suppression).
Long story short, C4 could probably be as low a value as 4.7uF and you'd still get the full 450V B+. It's probably not necessary to have 47uF there. Remember that the C4 reservoir capacitor has to charge up during startup. While C4 is charging it will be drawing great big gulps of current right through the poor 5AR4 tube. Reducing the value of C4 will shorten the duration of that charging period, saving wear and tear on the 5AR4.
As in all things, it's going to be a balancing act, a compromise between this and that. Almost always, engineering is finding the balance between desirable features and undesirable side effects. A 300B would give the most power and lowest distortion if fed 500V on its plate and drawing 100mA of plate current. But it would burn out quickly, which defeats the purpose. One has to compromise and stay within the limits of the device you've chosen. So it is with the power transformer, the rectifier, and the reservoir cap. You may have to give up a little ripple reduction to reduce the charging time of the whole thing, so you don't overstress the rectifier. Everything is a compromise.
There is another issue that might be at work here.
The choke has very low DCR, correct?
It also has fairly high inductance. 10 Henries, correct?
If so, it could be that there is a large oscillation at power-up while the reservoir cap is charging, or the transformer-rectifier-reservoir-choke-filter cap system may be ringing (resonating) pretty badly. If that's the case, then it may be necessary to damp that oscillation with series resistance added to the choke, to effectively increase its DCR. Yes, that would drop volts, lowering the B+. It would also increase the output impedance of the power supply. But if the low impedance power supply is ringing (resonating) badly, then what's worse? Ringing and oscillation, or higher PSU Zout? Yup. Everything is a compromise.
The choke has very low DCR, correct?
It also has fairly high inductance. 10 Henries, correct?
If so, it could be that there is a large oscillation at power-up while the reservoir cap is charging, or the transformer-rectifier-reservoir-choke-filter cap system may be ringing (resonating) pretty badly. If that's the case, then it may be necessary to damp that oscillation with series resistance added to the choke, to effectively increase its DCR. Yes, that would drop volts, lowering the B+. It would also increase the output impedance of the power supply. But if the low impedance power supply is ringing (resonating) badly, then what's worse? Ringing and oscillation, or higher PSU Zout? Yup. Everything is a compromise.
Can you post your PSUD2 schematic and result - just for assurance 🙂
Also can you confirm that you are using the Tubelab SE board? Even when using that pcb, series resistors can be added to the anodes of the 5AR4, but would need the relevant traces cut, and some care taken.
10VDC
It’s fine. It won’t allow me to run the 300Bs at their full potential, maybe. But they’ll last longer.
Thank you. I appreciate the advice. That’ll be next, if rectifiers keep arcing.
For now, I am going to assume plate limiting resistance was the problem. The datasheet also says the 5AR4 should be able to handle 60uf at that voltage.
Correct.
I don’t know enough to speak to the odds of this, but I think a lot more than Edcor/modern PT people would be having that problem if those odds were high.
Yet, this is a great learning experience for me.
I am committed to keeping the TSE-II as close to its design as possible, but in the future, I will pay greater attention to power supplies and aim to make them independent from the main circuit.
I won’t be near that computer for a bit. I will post a screenshot later.
It’s a TSE-II board.
I made it work:
Little did I know this oversized Hammond steel enclosure would prove so useful.
Addendum: Very interesting additional finding…
After installing the new resistors and going through checkout, I noticed I had no B-.
After swearing for a few minutes and wondering how far the rabbit hole would go, I discovered R5 had failed open (check the schematic for its location).
Based on reading so many posts on TSE quirks, I recognized mystery #2 some builders encounter.
The odd thing about that one is how the resistor failed. It’s indeed completely open, but looks absolutely pristine. Hence I never suspected it would be a problem and had to take out the board out a second time to replace it.
After that I was able to complete checkout without issue.
After installing the new resistors and going through checkout, I noticed I had no B-.
After swearing for a few minutes and wondering how far the rabbit hole would go, I discovered R5 had failed open (check the schematic for its location).
Based on reading so many posts on TSE quirks, I recognized mystery #2 some builders encounter.
The odd thing about that one is how the resistor failed. It’s indeed completely open, but looks absolutely pristine. Hence I never suspected it would be a problem and had to take out the board out a second time to replace it.
After that I was able to complete checkout without issue.
It's really too bad none if the tube companies are producing GZ-37's. There are a lot of ST-70's, etc, that could benefit. I just can't bring myself to pay crazy money for a NOS GZ-37 rectifier.
Recently I've been using a 5V3A in my ST-70. It does have a 3 amp draw, where as the earlier 5V3 was higher. It has a 17v drop compared to the 5AR4 though, and isn't a soft start rectifier, but it doesn't have much in the way if voltage sag like the 5AR4. I was thinking of trying it in my 300B. I just need to check to see if the 5v winding can handle the extra current draw.
Recently I've been using a 5V3A in my ST-70. It does have a 3 amp draw, where as the earlier 5V3 was higher. It has a 17v drop compared to the 5AR4 though, and isn't a soft start rectifier, but it doesn't have much in the way if voltage sag like the 5AR4. I was thinking of trying it in my 300B. I just need to check to see if the 5v winding can handle the extra current draw.
What type of resistor are you using for R5? That's the first resistor in series with the rectifier diodes for the negative supply, correct?
If so, it could be that on startup there's a lot of voltage differential across R5, and it's burning out after a few power cycles. Try using a 2W or 3W resistor rated for much higher voltage than it will ever see. 1/2 watt metal film resistors have a max voltage rating of 300V or 350V. It could be that you need to use bigger wattage resistors for their larger voltage ratings.
This Vishay 270R 3W metal film resistor is rated for 750V.
https://www.newark.com/vishay/pr03000202700jac00/metal-film-resistor-270-ohm-3/dp/94C3772
That would remove this worry, right?