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Old 26th October 2013, 03:16 PM   #1
sherpa is offline sherpa  United States
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Default Tubelab SSE Power transformer option

About to pull the trigger on building one and had a question on power transformer option. I was looking at the Edcor XPWR033, 760V (380-0-380) at 200mA center tapped, 6.3V at 5A and 5V at 3A. I plan on running KT88's in it primarily with a motor run 40uf cap for the first cap along with the additional cap and choke. Anyone see any issues or limitations with this transformer?
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Old 27th October 2013, 03:41 PM   #2
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I'm running both kt88s and el34s with a 700v CT Edcor (same heater specs), a 5ar4, a 2h choke, and the same size motor run. Sounds great and no hum on 97db sensitive speakers.

What type of rectification are you planning? You may want to watch the b+ for solid state or 5ar4. More for tube longevity than anything else probably. This would of course depend on your tube and bias choice.

I'm guessing you've seen the tables on the tubelab website? You'll have to strike a balance that's going to depend on your rectification and the resulting b+, and the opt to choose if you intend to have a single fixed bias value. Have you decided on a opt?

Last edited by Sodacose; 27th October 2013 at 03:49 PM.
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Old 27th October 2013, 04:07 PM   #3
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Sorry, I'm killing time on the train on my phone and didn't notice you intend to use only kt88. Again, depending on your rectification, you may end up at the high end of the b+ range and kt88s seem to not mind lower load impedance (closer to 3k). Making both of these choices might give you an ideal kt88 amp, but also limit your ability to try other tubes if you have a single fixed bias value.

Important reference: http://tubelab.com/designs/tubelab-sse/applications/

I'd suggest deciding on your rectification and opts before your power transformer, if you haven't already. Once you know the voltage gained during rectification and the load on your tubes, you can choose a power transformer and bias value that will put you where you want on the table with regards to distortion, dissipation, etc.

I don't know your comfort level and you may have this all figured out, but just thought I'd share my approach for anyone else reading and thinking about building one of these great little amps.

Last edited by Sodacose; 27th October 2013 at 04:15 PM.
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Old 27th October 2013, 06:48 PM   #4
sherpa is offline sherpa  United States
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I was looking at using both tube and solid state rectification. As far as bias resistor i was planning on setting up a couple of options so i would be able to use multiple tubes. The main reason for looking at the XPWR033 was the 200ma B+ supply. I want to be able to experiment running the kt88's at different currents up to close to 100ma each without stressing the power transformer too much. the XPWR035's 175 ma rating seems like i would be running the transformer at it's limits. I also figured if the B+ was too high i could add another RC filter stage to drop the power down a little further.
I do plan on running at 500V B+ power levels. I'm not running unobtanium tubes so reducing tube life a little doesn't bother me.
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Old 27th October 2013, 07:28 PM   #5
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Sounds like you know what you're doing with regards to bias and b+ so I done see any reason that transformer wouldn't work. Fwiw, I went with the xpwr155. Also 200ma, but might not get you to 500v b+. With a 560 ohm bias, I've run both kt88 and el34 with no problems and 6l6s should work, but I haven't tried them.
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Old 3rd November 2013, 10:05 AM   #6
cjkpkg is offline cjkpkg  United States
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380 is high...I'm using 700v ct edcor and it runs high. You will have trouble finding electrolytic caps to manage the psu.
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Old 4th November 2013, 07:58 PM   #7
dmb5245 is offline dmb5245  United States
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It sounds like you know what you're doing, but I'll post this for the benefit of others who might not and are looking to save $20 by choosing the -033.

The Edcor -033 probably isn't a good choice. As rated, your B+ will be over 500V according to the simulators. Edcor purposefully wires their PTs a little hot, so there's a good chance it will be even higher. I purchased the -033 for my SSE and wound up with 520V of B+. I ended up selling it and replaced it with the -035. I think I got unlucky in how far off rating my PT was, but even a -033 that matches its rating is pushing it.

Anything over 500V is above rating for the most common power supply caps and will greatly stress your rectifier diodes and/or rectifier tube as well as your output tubes. My rectifier tube arced over like a bug zapper at 520V. In hindsight, I'm glad it did because that was a better result than the power supply caps blowing.

So my opinion, for what it's worth, is to stick with a 740V PT like the -035 or even go smaller to keep a stable power supply.
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Old 4th November 2013, 08:35 PM   #8
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Quote:
I purchased the -033 for my SSE and wound up with 520V of B+.
520 volts will make life a little rough on the 5AR4, but with the diodes in the new board, a good tube should hold up. Finding a good new production tube might take a few tries. Definitely add the CL90 on the primary side of the PT and the CL140 in the holes on the PC board.

I haven't tried the Edcor PT, but I did try an SSE board with an Antek 400-0-400 volt toroid. B+ was about 550 volts and I still had the Panasonic 500 volt caps in the board, so I didn't leave it on long. This was definitely too much B+ voltage since I had to cut the output tube current down to less than 65 mA to keep the KT88's from trying to run away. The tube current just kept increasing slowly. Tubes were EH KT88's from about 6 years ago. They have been totally reliable and stable below 450 volts. I have no experience between these extremes. Maybe the screen grid of a KT88 doesn't like 550 volts.

If you run this much B+ I would lean toward a 5K load impedance or higher.
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Old 4th November 2013, 08:59 PM   #9
dmb5245 is offline dmb5245  United States
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Quote:
Originally Posted by tubelab.com View Post
520 volts will make life a little rough on the 5AR4, but with the diodes in the new board, a good tube should hold up. Finding a good new production tube might take a few tries. Definitely add the CL90 on the primary side of the PT and the CL140 in the holes on the PC board.
At the time, I didn't have the new diodes in and was using a JJ 5AR4. Maybe the diodes or a better tube would have fixed the problem. Either way, the remote prospect of an exploding electrolytic was enough to scare me back to a PT with less voltage. I'm pretty sure my fiance would kick me and my toys out of the living room over that!

George, thank you for a great product and taking the time to answer questions on this forum. Your SSE was my first DIY amp and I had a ball putting it together.
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