I’d guess that the dual cap cans are pretty much a standard size. You can always fit smaller caps in a bigger enclosure, but not the reverse, so it pays the manufacturer to standardise on one or two enclosure sizes for this type of multiple cap.
I’d guess that the dual cap cans are pretty much a standard size. You can always fit smaller caps in a bigger enclosure, but not the reverse, so it pays the manufacturer to standardise on one or two enclosure sizes for this type of multiple cap.
Originally Posted by Simon Barden
My thoughts exactly, but it's a real bonus that I don't have to order a couple of new chassis mounting brackets, I can re-use the existing ones which are still perfectly good.
My understanding is that the can itself is tied/connected to the - terminal of each of the capacitors in the can, since the two can caps will be wired in series, that means one of them will have it's - terminal floating/sitting at half the total + HT voltage, I'm not sure what the voltage-rating of the plastic shrink-film covering each of the can caps is, so I'm going to use some heatshrink tubing to ensure that each can cap is fully insulated from the chassis.
Last thing I want is for the chassis to be sitting at something like 200 or 300V DC, when it is supposed to be at ground potential.
The worst electric shock I've ever experienced in my life was from a Wimshurst machine that was being demonstrated in science class when I was in high school, must have been something like 10,000 volts but didn't it ever make me jump, so glad I lived to tell the tale.
Last edited by DrNomis_44; 03-07-2023 at 04:13 PM.
I don’t know if the can contains 2x 500v capacitors or 2 x 250v capacitors to give a 500v rating.
I’ve just looked at a schematic for a DIY SLO 50 , and that shows 2 x 300v capacitors, with a DC reading of 498v coming off them.
2 x 500v caps internally is fine, but 2 x 250v isn’t.
Any idea? I’ve had a quick look around and haven’t found an answer yet.
Well, my understanding is that in a dual 200uF/500V can cap, the two 200uF/500V caps share a common - terminal, so, that basically means that each of the two 200uF caps both have a maximum working voltage of 500V, if you wire them in parallel by joining the two separate + terminals together, you effectively create a single 400uF/500V capacitor, and if you wire two separate 400uF/500V capacitors in series you effectively get a capacitor that's half the capacitance value, but double the voltage-rating of a single 400uF/500V cap, or a single 200uF/1,000V cap, this is something Marshall used to do in their JTM45 amps and Super Lead Plexi amps, Fender also used to do that too in their Twin Reverb amps, it's also common practice to solder-in a bleeder resistor in parallel with each can cap to equalize the voltage as well as bleed-off the charge when the amp is powered-down.
Here's a diagram I drew showing how the two 200uF/500V caps are connected internally in the can:
So, if you connect two 400uF/500V caps in series and connect the -terminal to circuit ground, and the remaining + terminal to the +HT supply like this with the two 220k/1W voltage equalizing resistors, each capacitor should have about half the +HT voltage on them:
I've just been looking through Evatco's range of power transformers, and I think I have found the ideal power transformer for my amp project, the power transformer I was going to use has an HT secondary rated for 375-0-50-375 V AC @ 201mA, my main concern is that if I use this transformer some of the supply filter caps in the power supply may be over-voltaged, the new power transformer I found has an HT secondary rated for 350-0-50-350 V AC @ 201mA, 350V AC is only 10 volts less than what's specified in the schematics I'm working from, they specify a power transformer with an HT secondary rated to put out 360-0-50-360 V AC @ 200mA, so, I think the new power transformer I found is a good compromise, here's a link to it, I think the heater secondaries should be adequate enough to power all the valve heaters, as well as the footswitching circuitry:
https://www.evatco.com.au/373bx-power-transformer
Last edited by DrNomis_44; 03-07-2023 at 07:29 PM.
Yes, if they are internally connected like that and you feed one multi-cap into the other (with the balancing resistor connection in the middle), then that's fine.
I was looking at a circuit digram and seeing one capacitor above another, had the mental picture of that's how the two capacitors were arranged in the common can. But its the side by side caps that are the common can, with one can connected to another/feeding into the other.
Yes, you then get about 250v across each capacitor, so a 500v rating is perfectly safe (providing the resistors don't loose their ground connection or a cap doesn't go short circuit - but you can't allow for every possible failure scenario).
With the way you've drawn it, it makes much more sense to me.
Thanks.
No worries, I have to admit that it took me a while to get my head around it too.Yes, if they are internally connected like that and you feed one multi-cap into the other (with the balancing resistor connection in the middle), then that's fine.
I was looking at a circuit digram and seeing one capacitor above another, had the mental picture of that's how the two capacitors were arranged in the common can. But its the side by side caps that are the common can, with one can connected to another/feeding into the other.
Yes, you then get about 250v across each capacitor, so a 500v rating is perfectly safe (providing the resistors don't loose their ground connection or a cap doesn't go short circuit - but you can't allow for every possible failure scenario).
With the way you've drawn it, it makes much more sense to me.
Thanks.
The thing I'm most concerned about is putting too much voltage on the single 47uF cap and the two 10uF caps in the power supply circuit in my build, I'm using a black F&T 47uF/500V Axial and two blue Sprague Atom 10uF/500V Axial caps in my build, if I go with the transformer with the 375-0-50-375 V AC @201mA secondary the main +HT voltage is definitely going to be higher than 498V DC, I'm estimating probably something like 560V DC, which could over-voltage some caps in the power supply.
I'm using two more black F&T 47uF/500V caps in the -50V Bias supply, which is probably overkill for the job, but at least it means that the -V Bias supply should be rock-solid reliable, which I think is important because you don't want your expensive power valves going horribly into red-plating because the -V Bias supply failed completely.
Last edited by DrNomis_44; 03-07-2023 at 10:00 PM.
Not only the caps, but also the valves! The maximum plate voltage for a 6L6GC in pentode mode is 500V. The SLO 50's design 495V is already pushing the valves to the limits, and you need to allow for supply voltage variations. I'd definitely go with the 350-0-50-350 transformer. It's fractons of a decibel less output volume and its safer.
I elected to go with a set of two matched JJ KT-77 power valves in my amp build, on the Evatco website they say that the KT-77 is a drop-in replacement for the EL-34, I compared the pin connections on the EL-34 datasheet with the ones on the KT-77 datasheet and they are virtually identical except for pin 1, which is unused in the KT-77 but is connected to the suppressor grid in the EL-34, whereas in the KT-77 the suppressor is internally connected to the cathode, in the JJ KT-77 datasheet it says that the maximum Anode/Plate voltage is 800V, for a JJ EL34 it says the maximum Anode/Plate voltage is 800V, or I may be totally wrong about that, but you're absolutely right, I'm no expert but I would say 500V in each case is a more realistic limit, at higher voltages there's more of a risk of arcing between adjacent pins on a valve socket, something that caused my Marshall MA100C amp to blow the HT fuse all the time, turned-out it was because the factory-installed power transformer was rated for 230V AC mains supply, not our Australian 240V AC mains supply, so the resulting +HT supply was higher than it was supposed to be, at least one of the factory installed Marshall EL34 valves suffered an arc between two pins, one other EL34 valve red-plated, and I think the remaining two were okay, cost me quite a bit to get it fixed so the amp wouldn't blow HT fuses all the time, think it was something like Au$800.00 or so, and I paid Au$1,300.00 or so when I bought the amp brand-new.
Last edited by DrNomis_44; 03-07-2023 at 11:17 PM.
EU/UK mains voltages are a nominal 230v, but that's with a ±10% tolerance, so 207v to 253v AC. This split the difference between the UKs 240v and Europe's 220v and pushed the big switch from linear to switched mode power supplies that could accept a wide range of voltage inputs.
But whilst new build offices and factories etc in the UK have transformers for the buildings set to 230v, a lot of the UK is still at a nominal 240v, so there shouldn't be any problems with 230v transformers running at 240v or there would be a lot of irate British customers!
Sounds like there might have been some other issues with the design of that amp as it's not something I've come across here. Possibly the factory buyer ordering some below-spec valve sockets because they were considerably cheaper without understanding the implications.
Very high plate voltages mean low current draw and apparently a lot more crossover distortion, so the plate voltage is normally kept lower, in the 400v region (unless you really want more power out of the valve at the expense of fidelity).
I've never actually tried JJ KT-77 valves before, so it'll be very interesting to hear what they sound like in practice, Evatco described them as sounding like a cross between an EL 34 and a 6L6, the original Soldano SLO 100 used 5881 power valves going by what I have read, EL 34 valves have that British crunch whereas the 6L6 has more of an American crunch going by what my ears tell me, in the Soldano SLO-100 schematics I'm working with, the screen grid resistors are 500 Ohms, in my build I changed them to 1k in value, I also made a simple mod to the power amp circuit to include an output power switch so the amp could be switched from the full 50W output to 25W output (Pentode/Triode mode switch).
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