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Old 14th January 2008, 11:51 PM   #1
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Default power transformer rating and tube current - SimpleSE

I am going to build a SimpleSE (www.tubelab.com). I think that I can build a great amp for relatively little money and get between 4-8W.

I have sourced the parts, but am curious about one thing - the current rating of the power transformer. On his website, George presents a list of tubes and configurations with estimated power output and current. If I want to run the SimpleSE at 400V at B+ at 81mA current, then I should get something like 5.45W.

Now George has recommended the Allied 6K7VG transformer which is 750V center-tapped at 150mA. OK, so if I run a stereo amp using two KT88's, at 81mA, then I draw too much current for a single Allied 6K7VG don't I??? Never mind, the current drawn by the input tube. Is this correct? Does the current rating of the trafo have to be greater than the current drawn by the circuit? I always thought it does.

George recommends choosing components that match one of the actual configurations that he's already built, but for all my stupidity, I cannot find any presented with sufficient detail on his website - I am sure they're there somewhere!

If anyone else has built a two-channel SimplSE that uses a shared PSU, please could you let me know your specs.

Thanks,
Charlie
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Old 15th January 2008, 01:11 AM   #2
chrish is offline chrish  Australia
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I am running a 374BX Hammond on mine. 750 Volt Centre Tapped, 175 mA. It does run a bit hot, but have had no problems with it. I think that George (Tube Lab) mentioned that the Allied you mention is rated less than required, but like most things he tests, he runs it pretty hard on his setup and has had no problems. If he recommends it, go for it.

As an aside, I am very happy with my SimpleSE. If I could offer some advice, some of us have had problems with the standby switch setup causing problems and also with the solid state diode setup causing problems. I have lost two sets of diodes and one JJ brand rectifier tube. You might be OK, but I would suggest not bothering with the standby switch and not bothering with solid state rectification.

I did some online research, and it looks like the JJ ECC81 is a good quality new manufacture tube (I cannot hear the difference between this and a NOS Mullard), as are the JJ E34L (higher plate dissipation than EL34) power tubes. I have had a failure with my JJ GZ34 rectifier, but since changing to a Sovtec and also old Svetlana 5U4G have had no issues.

In my setup, I have a separate active crossover splitting the signal at 80Hz. The SimpleSE does the 80Hz up running a pair of Ariel speakers and the sub 80Hz is sent to a chipamp that is running a stereo pair of woofer boxes. Having tried JJ KT88, Winged C EL34, and JJ E34L, I think I prefer the E34L. They also happen to be the cheapest pair of tubes. They are more detailed in the mids. I found the KT88s a little boomy in the bass and a little less detailed in the mids. This may have a lot to do with my setup, as others have reported that they like the KT88s.

Good luck with the build, it is a great project.

Chris
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Old 15th January 2008, 01:16 AM   #3
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Chris,

OK, maybe I'll go with the Hammond and spend a little more. What bias current / B+ are you using with your EL34's?

I will probably not use the standby switch and rely on slow'ish warm-up time of the rectifier tube, and I doubt that I'll use SS rectification for the B+ at all.

Thanks,
Charlie
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Old 15th January 2008, 01:33 AM   #4
chrish is offline chrish  Australia
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Hmm, it was while taking measurements for someone that I blew the thing up last time

Think it is around 450-500 Volts, depending on rectifier. I am running the 560R cathode resistor and using the Edcor 5k cheap 15watt open frame output transformers. Note I am using a 300 series Hammond as I live in Australia and we have 240 Volt mains power. These 300 series are more expensive than the 200 series, and probably twice the price of the Allied. George has mentioned on several posts that the Allied works fine.

The 5U4G rectifier tubes are Svetlana from USSR on Ebay and will drop the B+ some if your voltage is a little high compared to the GZ34.

Cheers,

Chris
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Old 15th January 2008, 02:17 AM   #5
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Chris,

OK, I won't ask you to make any measurements! I guess that with 450V and 560R, then you're running the tube at around 70mA, so 140mA for two tubes, plus whatever yoou have the inout tubes at (I would guess 5-10mA ea) for around 160mA.

The Hammond 374BX comes out at something like $90 US, which is twice the price of the Allied 6K7VG. However, it allows for 25mA more current and the extra $45 may be worth a little more peace-of-mind. I could even run EL34's at 55mA and still get 8W output according to George's simulations.

Now can the cheap Edcor OPTs be run ultralinear? Or does it require an off-center center-tap?

I have just looked at the schematic for the SimpleSE. It seems that the CCR is set at 10mA (from the datasheet for the regulator). the 100K before the regulator drops 100V at 10mA. I cannot quite reconcile this math with the 12AT7 datasheet. ANYWAY, it seems that each half of the 12AT7 runs at 10mA.

So, for one channel with EL34s at 55mA and a 12AT7 at 10mA requires 65mA, which is 130mA total - the Allied trafo would work with this combination.

If anyone can see errors with my cranky math, please let me know.

Charlie
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Old 15th January 2008, 02:46 AM   #6
chrish is offline chrish  Australia
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Can't comment on the math, but I have done a search of my old posts and found that I was getting 480 volts B+ with the GZ34 rectifier. With the 5U4G I guess it will be closer to 450. Here is a link to that thread, there might be something useful for you there. Edcor OPT's w/ SimpleSE

Here is a link to the Edcor site. These are the transformers I used. The price has gone up to $20. They do have an ultralinear tap. http://www.edcorusa.com/products/tra...se15-8-5k.html

Cheers,

Chris
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Old 15th January 2008, 03:54 AM   #7
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I have the Allied trafo in my Simple SE with triode-strapped EL34 outputs and 560R cathode resistors. I removed the 120 uf electrolytic cap (C2) in the B+ supply, so the PS is 47 uf 'lytic - 6 H/150R Triad choke - 100 uf PPIO motor run.

B+ with a GZ34 rectifier is 440V IIRC. The transformer does not get overly hot. There is a bit of mechanical hum, but I don't notice it unless the house is silent and I'm close to the amp. Likewise there is a trace of hum at the speakers, but I have to be right close to the cone to hear it.

Like chrish noted, my SS rectifiers fried from using the standby switch. I have not replaced them.

Bill
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Old 15th January 2008, 05:10 AM   #8
Zap is offline Zap  United States
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Just another option to consider, Edcor makes a Power Transformer that I just ordered a week ago that is 760V @ 200mA. George said it should work just fine, and it probably provides a bit more headroom. The additional plus is that it costs only about $60, so its rated higher than the Hammond, and cheaper to boot. If you're ordering OPTs from Edcor, you'll probably save some cash on shipping too.

The part number is XPWR033. Full specs, 760VCT @ 200mA, 6.3V (No CT) @ 5A, 5V @ 3A.
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Old 15th January 2008, 05:54 AM   #9
w5jag is offline w5jag  United States
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Charlie,

The current iteration of my Simple SE uses a 10 pound-ish horizontal mount transformer that I picked up at a hamfest for a buck (my first transformer delivered better than 600 volts to the plates, but I managed to let the smoke out of that, and some other parts). The main HV tap is 330-0-330 and it has a screen supply tap at 200-0-200, that I haven't got around to using yet. I use a Bendix 5H choke that is 150 ohms DCR, iirc, and Transcendar 3K output transformers (don't recall the DCR on these) and loaded up I get about 410 volts at the plate pin with a 5AR4, and about 355 with a 5Y3 with my current setup.

I use 470 ohm cathode resistors, and typically run some variation of the 6L6 family with a 5Y3 or 5R4 rectifier. I tried some higher values of cathode resistors, but didn't like the way the amp sounded at the modest voltages I'm running, and below about 470 ohms, some of the new production tube types would show some color in the plates with 5AR4 or 5V4 rectifiers.

I think if you want max output at minimum current draw, EL34 is the way to go.

I use my Simple SE as the amp in our home theater setup, so it runs VERY long hours on weekends and over holidays when everyone is at home. Even after 15-16 hours, the transformer and choke are cool to the touch with the 6L6/EL34/6CA7 type tubes and a 5Y3/5R4 rectifier. When I use KT88/6550 and higher voltage rectifiers, the transformer tends to get warmer than I like after 7 or 8 hours, but I'm not sure how much of this is actual heat generated in the transformer, or pickup of radiant heat from the output tubes. When I fry this transformer, if I can't find a suitable hamfest replacement for a buck or two, I will use the One Electron power transformer from AES.

FWIW, the rough rule of thumb I use to predict B+ with a C input power supply is to take the AC RMS voltage and multiply by 1.11 for a 5Y3, 1.14 for a 5R4, 1.19 for 5U4/5T4, 1.22 for 5V4 and 1.26 for 5AR4. I use 47uF for my first capacitor value, which is way over the data sheet value for all of these but the 5AR4, but I don't care.

I'm sure you'll enjoy George's Simple SE.

Win W5JAG
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Old 1st April 2008, 11:57 PM   #10
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Default re TUBELABS SimpleSE amp- tube choice

am about to start on assembly and am curious as to the best recommendation for tubes etc. Will be using a Hammond 374BX ( maybe a clone) and Hammond 1628SEA also might be a clone ( a Chinese maker has offered to copy etc- we'll see) . - or might go with the Edcors that George suggested, 240V Aus power etc- A total newbie to all this and input appreciated

Also curious as to suppliers here in OZ for parts etc- the big winner so far is not Mouser or Digikey but Fedex and the US postal service- any suggestions?- have trodden the worn path to RS Jaycar Wagner etc and dont really want to accumulate too many bags of surplus with bulk buys .
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