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Old 8th October 2011, 02:01 AM   #11
rknize is offline rknize  United States
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The power handling and DC current are two different things, though. Power is largely limited by the core size while the DC current is a combination of that and the core gap width. But you do make a good point that one has to be mindful of that when selecting a transformer.

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Old 8th October 2011, 02:50 AM   #12
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Even if the 6K7VG can keep the B+ up to 450VDC under that kind of draw (optimistic)
I have an SSE that uses EH KT88's, Transcendar OPT's and is powered by a 6K7VG. I usually run it with the tube rectifier and the B+ is about 440 volts if I am remembering right. I made some measurements and found that I get the lowest distortion a n best sounding bass when I run the tubes at 100 mA each. With the solid state rectifiers enabled the B+ is over 450 volts. This much current is not recommended since it is way over the max spec for the Hammond build Allied 6K7VG. I have been running it this way off and on for about 5 years and the transformer is still going strong, although it gets too hot to touch at this current level.

The OPT's in that amp are designed for 300B tubes and are 3000 ohms with a 4 and an 8 ohm tap. The EH KT88's do not like 3000 ohms into my speakers. I get noticible distortion on bass peaks, so I run my 8 ohm speakers on the 4 ohm tap for a 6000 ohm load. I can actually play it louder since it doesn't distort. The speakers in question are above 8 ohms for most of the audio range and are well above 10 ohms below 100 Hz. If your speakers need good damping I would go with the 5K OPT's.

I have no experience with the GXE series Edcors but I can report that the cheaper XSE-15 series works just fine at 80 mA with 300B tubes. Those transformers will not work below 100Hz at 15 watts. I doubt that the GXE's will get down to 40Hz at 15 watts. The CXSE-25's will go to 25Hz at 15 watts.
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Old 8th October 2011, 06:14 PM   #13
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Originally Posted by tubelab.com View Post
I have an SSE that uses EH KT88's... and is powered by a 6K7VG. I usually run it with the tube rectifier and the B+ is about 440 volts if I am remembering right. I made some measurements and found that I get the lowest distortion and best sounding bass when I run the tubes at 100 mA each. With the solid state rectifiers enabled the B+ is over 450 volts.
Thanks for setting my memory straight. I thought the 6K7VG would sag quite a bit under that much load. Apparently I was mistaken.
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Old 9th October 2011, 02:26 AM   #14
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I forgot to mention my 122 volt line voltage. You might get a little less on a more normal wall socket. I am 1200 miles from my lab right now so my numbers could be off a bit, but I know it had plenty of power even heavilly overloaded.

Also note that Hammond and Allied transformers have changed a bit over the years. I get more voltage out of a new one than a 5 year old one in the same amp on the same test bench. The sag performance may differ too. The transformer I put in that SSE was a few years old when I built the amp, and the amp is about 5 years old.

Quite a while ago (7, maybe 8 years) Allied gave me a 10% off coupon and offered free shipping on orders over $100. That was before the price increases too. I got about 100 pounds of transformers on one order. I still have a few hanging around.
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Old 21st October 2011, 02:04 PM   #15
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I have bought a 193H choke with 65ohm DCR. Is it fine to use by itself or would I need more series resistance as suggested below. Also, if additional resistance is required, what would be the reason for it? Is it to drop B+ further as 65ohm wouldn't drop it to within a safe range?

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This one has a rather low DCR, but it will work. You should add some series resistance to it, like a 50-100 ohm 5W resistor. The 193J is a better choice (more inductance and 82 ohm DCR). The Triad C-14X choke from Allied is half the price and has a perfect 150 ohm DCR but is ugly.
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Old 21st October 2011, 03:58 PM   #16
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It will work, but some additional series resistance will improve filtering. In a CRC filter, which is the original design, increasing R will improve the filtering of the second C. Of course, the downside is a lower B+. A CLC filter gives you even better filtering, because the inductor stores energy along with the capacitors. 5H is not really enough inductance on it's own, but in combination of the higher DCR of the Allied choke it works out.

It doesn't hurt to try it. If you get more hum than you'd like, add more series resistance or increase the size of the second cap, The resistance also helps isolate the second capacitor from the rectifier. Since there is a choke there I don't think it is cause for concern.
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Old 21st October 2011, 04:25 PM   #17
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Is it fine to use by itself or would I need more series resistance as suggested below......It doesn't hurt to try it.
It should work fine. The amp I mentioned above has a tiny 1Hy choke with 50 ohms resistance because that is all that would fit under the chassis. There is no noticable hum on 96db speakers. If you have 100+ db speakers you might hear some hum.
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Old 21st October 2011, 05:30 PM   #18
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Thanks for your help. Much appreciated. I will try without first and see what results I get.
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Old 23rd October 2011, 03:44 PM   #19
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Could I please confirm the latest suggestions on how to connect the amp compared to the schematic. I understand that the standby switch should not be used as it can generate a voltage spike, so CT should be permanently connected to amp ground? Also in regards to the inrush limiter, I couldn't find a diagram in previous posts so could someone confirm I have the connections right for 374BX and 220V operation. If I am correct, the limiter should go in series with the transformer primary and also in between the CT and ground? Lastly, the thermistor I can easily buy is a 22ohm 2.8A and not the 120ohm recommended, that should be fine? Should the thermistor connected to the CT be of lower resistance to the one connected to the primary?
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Old 23rd October 2011, 05:34 PM   #20
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IRCL devices are specified by the cold resistance, hot resistance, and maximum current they can handle. For our type of applications, we generally want as high a cold resistance and as low a hot resistance as possible. The maximum current must be a relatively close match for the expected current draw.

The CL-90 is particularly well suited for the primary side of many vacuum tube power amplifiers. The cold resistance of 120 ohms is one of the highest values you will find among the CL type devices. At 50% of full rated load, the CL-90 has a hot resistance of three ohms, so it's getting close to being out of the circuit at that point. Finally, the 2 amp maximum rating fits well with the expected primary side current draw of many vacuum tube amplifiers.

For example, the Tubelab Simple Single Ended fitted with KT88 may require 170 mA of B+ at 450VDC, plus 3.5 amps of heater current at 6.3 volts and 2 amps at 5 volts. That's a total of 110 VA. Let's assume our power transformer is roughly 80% efficient (a number pulled completely out my back side), so expect a primary side current draw of 1.1 amps. That's well within the capabilities of the CL-90, but still close enough to its max load rating that the device will get hot.

I do not understand why people want to put IRCL devices on the B+ loop. Your current draw here will usually be less than 200 milliamps. A device like the CL-90 is, for all intents and purposes, still "cold" at this current draw. You may as well just stick a 120 ohm resistor between center tap and ground. Even something like a CL-140 (rated 1.1 amps max, 50 ohms cold, 5 ohms at 25%) is a stretch. IRCL devices do not function without heat, and to get them hot you need to have substantial current draw through them.

Just my two cents.

Oh, and yes you do have the CL-90 on the primary side in the correct location, although with your higher line voltage (220V) you can expect current draws of roughly half what we get in the States. You will probably be operating a closer to 25% of the CL-90 rated max load, where it has a resistance of nearly 8 ohms. Study the data sheets, and you may find an IRCL that is better suited for your application.
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