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Old 26th December 2008, 10:55 PM   #1
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Default Finally, back to my Simple SE project

Well, after a few months of being shoved into the corner, I've finally dusted off my stuff and goten back to my Tubelab Simple SE project. I was quite busy with work, having to travel for a couple months both around the US and to Europe.

If anyone's interested, here's my previous thread on the project:
http://www.diyaudio.com/forums/showt...26#post1544426

Before I had to put everything away, I had the amplifier working (with a power transformer to the recommended spec's) on the kitchen table, temporarily assembled. So, I can say that I had the amplifier working in a "known" configuration as we had discussed in the previous thread. TubeLab was nice enough to send me this power transformer to get everything working - Thanks again!

While I do the woodworking for my chasis over the next couple of days, I'm hoping to generate some discussion in this thread in order to finalize everything before final assembly. My hope is to use my Edcor power transformer (rated at 400 V, 200 mA) mainly because it matches my output transformers nicely. However, I realize that I might have to "upgrade" a few parts/components to do this. Here's what I'm planning so far:

- Replace C1 and C2 with off-the-board motor run capacitors that I bought. Their ratings are ----> C1 = 35mF 440 VAC, and C2 = (qty 2) 70mF 440VAC paralled
- Install an in-rush current limiter

I'm also considering removing the diodes permanantly, as I don't think I'll ever use a SS rectifier. I have a feeling the failure(s) I had before were related to the diodes, and I might prefer to not have to deal with the beast all together

I realize that this higher voltage will, at a minimum, shorten the life of my tubes, and maybe even some other components. I am willing to put up with the purchasing new tubes every once in a while, as long as it's not going to cause any safety issues, such as electrical safety or fires.

Suggestions that anyone else may have are more than welcome.

Thanks!
-Eddie
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Old 26th December 2008, 11:38 PM   #2
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Hi, welcome back. Thanks for the payment for the transformer. it was not expected or necessary, but welcome considering everything that is going on. I am currently at my mother in law's house which is not too far from Pittsburgh. We have made several trips to UPMC for her cancer treatments lately, but they postponed everything until the middle of January, so I will start the long drive home early tomorrow morning.

I believe that the 400-0-400 volt transformer can be made to work since I had a similar transformer running in one of my Simple SE's. I know that the cathode bias resistors (and maybe bypass caps) will need to be changed, but I don't know the values yet. I need to know what output tubes that you plan to use (type # and manufacturer) and I can connect the big transformer back to a Simple SE board to see what else may need to be changed.

I should be back home Monday or Tuesday depending on the number of stops, and it will be a day or two before I can get to it.
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Old 29th December 2008, 02:44 PM   #3
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Tubelab - I hope that your stay in Pittsburgh was pleasant, despite the circumstances; and that you and your wife had a safe drive home. You may have even passed near my house during your travels... I live near the turnpike (I-76) exit 67, and work very close to the I-76/I-70 interchange.

Well, as for the tubes I'm hoping/planning to use, they are Electro Harmonix EL34, JJ KT88, and some Chinese EL34s that I got for cheap. I intended the cheap EL34s to be installed when I powered everything up for the first time, basically just to make sure everything was working OK.

If it's possible to get both tube-ypes to work, I can certainly wire in a switch (whichever type is necessary) for extra resistors/capacitors to get the proper voltages/currents. If it's just doing some math to determine the correct values, just let me know which components are controling, and I can certainly give it a shot myself... although it's been about 10 years since my circuits class.

Thanks!
-Eddie
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Old 29th December 2008, 11:14 PM   #4
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Quote:
You may have even passed near my house during your travels... I live near the turnpike (I-76) exit 67, and work very close to the I-76/I-70 interchange.

My stays in Pittsburgh were confined to the UPMC Montefiore hospital (don't remember the route but I remember seeing Heinz field off in the distance as we went over a bridge), not particularly fun. I spent most of my time doing some PC board layouts on my laptop while my mother in law was being tortured in the name of medical science. My mother in law lives in northern West Virginia not far from I-70. I am now back in Florida, but Sherri is still in WV for the time being. I can now cook up said PC boards on the kitchen stove without her intervention.

The increase in voltage supplied to the Simple SE causes two things to happen. First the power supply electrolytics will be overloaded. Off board capacitors of a higher voltage rating will solve this issue. Second, the increased voltage will cause the output tubes to draw more current. The additional current and the additional voltage across the tube will cause the tube to dissipate more power, a lot more power. This will lead to red glow and a short unhappy life for the output tubes. The usual solution is to increase the value of the cathode bias resistors to cut back the tube current to a safe value. Raising the value of the cathode resistor will increase the voltage across the resistor. You may need 50 to 60 volts with the KT88, so the 50 volt cathode bypass cap will fry. A 63 volt cap, is likely marginal as well. The higher voltage and current raises the power dissipated in the cathode resistor too. I have found that the white "5 watt" power resistors really don't like to run at or over 5 watts. I have blown one at 5.5 watts which then caused the bypass capacitor to spew its guts all over the PC board. I blow stuff up so that you don't have to!

So, the cathode resistors and capacitors need to be changed. To what values, and how are they determined? I usually start with component values that are derived from the tube curves, but I am lazy and don't like to think so I use a circuit simulator. The Simple SE output stage was initially designed using SE amp cad from Tubecad. I created the often misunderstood "tubes and transformers" table on my web site when I was designing the amp. Unfortunately the table only goes to 500 volts, and I am guessing you are going to get about 525 volts, and SE amp cad won't run on this Vista POS, so we can guess from the table.

First the EL34. The hottest I like to run an EL34 is 25 to 27 watts. With 500 volts the table shows that an 820 ohm cathode resistor will get you to 25 watts with a cathode voltage of 46 volts. A 910 ohm resistor is probably a safer bet due to the higher B+ voltage.

Second the KT88. These tubes can operate at 40 to 50 watts of dissipation depending on the manufacturer. I have run the EH tubes to 50 watts, but I haven't tried JJ, so lets shoot for 40 watts. The table shows a 510 ohm resistor and 45 volts gets you 39 watts with a 500 volt B+. 560 ohms is probably OK, but 46 volts is too close for a 50 volt cap. A 63 volt cap may work, but I would go higher.

So much for all the theory and thinking, this is what I would do:

Install the inrush current limiter (2 may be a good idea, 1 in the transformer primary, and one in the HV secondary center tap), and new capacitors, don't forget to remove the existing 500 volt caps. Install the 1K ohm cathode resistors and 100 volt bypass caps outlined below. Test the amp, then add the "turbo" switch outlined below.

It is possible that your 5AR4 may object to the additional voltage. This has been an issue with some new production tubes. Some batches of JJ and Sovtek tubes have been unreliable, while others have been trouble free for years. Some users have reported two or three instant failures before finding one that will work. Some users have switched to a 5U4GB to reduce the B+ slightly, a discussion of inrush limiters and different rectifier tubes can be found in these threads:

Rectifier tube 5AR4 vs. 5U4GB?

Another Simple SE builder

525 volts of B+ is a guess since I don't have the same power transformer that you do. Since you may actually get more voltage, I would put some conservative component values in the PC board. I would use a 1 K ohm 5 watt resistor for the cathode resistor (R17 and R27). I would use the biggest (microfarad value) 100 volt electrolytic capacitor that will fit in the PC board for the bypass cap (C12 and C22). Then I would wire up a 2 or 3 position (DPDT or DP3T) switch to add additional resistors in parallel to increase the current with the KT88 tubes.

The actual resistor values will need to be determined by trial and error. I would guess that a 3.3 k or so resistor could be used with the EL 34 and a 1.5 k or lower resistor could be used with the KT88. These resistors would share some of the current making life easier on the 5 watt resistor in the PC board. The "switched" resistors can be 3 watts since they will "share" less than half of the power. I would get 2 each of 1.2K, 1.5K, 1.8K, 2.2K, 2.7K, 3.3K, 3.9K, 4.7K and 5.6K. Start with just the 1K that is in the PC board. Assuming that the amp works, let it run for a few minutes, then look at the output tubes in a dark room. If there is no glow on the plates (outer metal surface of the tube) then a higher current can be tried. Start with the highest resistor value (4.7K), tack solder it across the 1K that is in the PC board and test the amp again. You can try progressively lower resistor values until a dull red glow in noticed on the plates, then back up two or three resistor values.

This has been discussed here:

Cathode Bias for SimpleSE

I got in late last night, and haven't even unloaded the car yet. I will try to hook up my Simple SE later this week to see if I can provide a better guess as to the actual resistor values.
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Old 30th December 2008, 11:42 AM   #5
chrish is offline chrish  Australia
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Great support from George as always.

I noticed a thread about using a bucking transformer to reduce B+ voltage. Would using a small bucking transformer under the hood perhaps be a simpler solution?

Good luck with the project, it is worth it!

Also, best wishes to George and his family...

Chris
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Old 30th December 2008, 06:07 PM   #6
n_maher is offline n_maher  United States
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Am I the only one reading this who thinks that the better course of action would be to buy the proper transformer and not jump through the 20 hoops necessary to make the 400V unit work? Save the trafo for another project or sell it (even at a loss) to recoup funds. Better that than have to rework an entire project to make one part fit. George's advice is great and will no doubt work but my goodness, what an effort it would be.

That's my 2 anyway.
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Old 30th December 2008, 07:05 PM   #7
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He's already got the "right" transformer. Now he wants to make the "wrong" one work... just because.
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Old 30th December 2008, 08:27 PM   #8
chrish is offline chrish  Australia
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Or you could buy a can of paint and paint the endbells of the working transformer the same colour as the Edcor
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