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Old 18th April 2009, 05:12 AM   #11
RDL_AK is offline RDL_AK  United States
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Skipper, I orginally built the Tubelab's Simple SE with Allied 6K7VG. It works great with the Simple SE. I also bought the Edcor CXSE25-8-5Ks. These OPT's are outstanding!!! The Simple SE is very versitile. I experimented with a Hammond 282X 1KV CT @ 200ma 5VCT @ 3A 6.3VCT @ 6A with EL34s, 5U4 and 12AT7. (Bought PT on EBAY cheap less than half price including shipping to Alaska!) The Simple SE handles this with no problem. But sure generates a lot of heat. Presently I have the Allied 6K56VG giving 300 B+ so I can use more tube types. Also am using the Edcor cheap OPTs ($20 each) XSE-15-8-5Ks with Sovtek 5881WXTs. Sounds great! I find that the tubes take a few hours to "settle in" to different configurations when I make changes. The orginal Allied 6K7VG and Edcor CXSE25-8-5Ks are now in a SE KT88 design by Mikael Abdellah on diyAudioProjects.com . This is my favorite AMP with the 12AT7 instead of a 6N1P and Sovtek 5881/6L6WGCs instead of KT88s. I do want to try Edcor's power transformer in these amps so thanks for this thread. By the way I do have KT88s and they do sound good in both of these amps. Lots of bass. I guess the music I like suits the 5881's sound better. Also I have older 4 way speakers that make a big difference in the quality of the sound too. I am new to Tube Amps Still learning alot and experimenting to see what works and what doesn't. So far I have found that tweaking an amp and the tubes is how you make the amp to sound good to you.

Richard
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Old 18th April 2009, 07:09 PM   #12
skipper is offline skipper  United States
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Hi All,

Thanks for the great responses, this is my first tube amp and I am still trying to figure things out. I am still a little confused at how to choose the correct OPT for each series of tubes and how you get the final B+ number. From another thread I read there is a 1.41 factor you mulitly your OPT number with to get the higher resulting voltage?
The PWR transformers seem a little easier, 6.3V for the OPT/driver tubes and 5V for the rectifier tube.
From what I read the KT88 give the most bass, some other SE tube amp I heard, the base was really lacking. The 5881/6L6WGCs seem interesting too.
I was also thinking of building one of Bob Latinos ST-70 PP clones but after reading about the SE amps, I thought I would give an SE a listen. I still may build a PP amp later to compare.

Thanks again,
skipper
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Old 18th April 2009, 07:22 PM   #13
rknize is offline rknize  United States
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I have both amps...a Bob Latino "VTA-70" and a Simple SE...well the SSE was tested for a few nights and I am currently building a chassis. How sensitive are your speakers?

As to your question about OPT (output transformers), I think most folks go with 5K-ohm primary impedance. EL34s and 6L6s are a good match to that and biggers tubes like the KT88 enjoy the easier load to drive. If you know that you will only be running KT88s or similar and are going to turn up the bias current, then you may consider a 3K primary. You will get another watt or two of power and maybe lose some bass (really depends on your speakers more than anything).

I'm a bit confused on your question about the B+ voltage. The B+ comes from the power transformer. The output of the B+ supply goes to the OPT, where there is a slight drop in voltage, and then back to the tube plate. I think that most folks run about 450V B+. I got 460V with the Edcor XPWR035 with 6L6GCs biased at around 60mA (~34V across the 560 ohm cathode resistors):

Click the image to open in full size.

George ran a bunch of simulations that will give an idea about how your B+, OPT primary impedance, and cathode resistors affect power output:

http://www.tubelab.com/AssemblyManua...ormers_SSE.htm

Russ
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Old 18th April 2009, 08:24 PM   #14
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Quote:
Originally posted by skipper
Hi All,

Thanks for the great responses, this is my first tube amp and I am still trying to figure things out. I am still a little confused at how to choose the correct OPT for each series of tubes and how you get the final B+ number. From another thread I read there is a 1.41 factor you mulitly your OPT number with to get the higher resulting voltage?
The PWR transformers seem a little easier, 6.3V for the OPT/driver tubes and 5V for the rectifier tube.
From what I read the KT88 give the most bass, some other SE tube amp I heard, the base was really lacking. The 5881/6L6WGCs seem interesting too.
I was also thinking of building one of Bob Latinos ST-70 PP clones but after reading about the SE amps, I thought I would give an SE a listen. I still may build a PP amp later to compare.

Thanks again,
skipper
The 1.41 multiplier is used for the power transformer calculations, not the output transformers.

The power transformer provides both the B+ voltage for the tubes as well as filament (heater) voltage and current for the tubes. Yes, the max DC output of the power transformer can be 1.41 X the secondary AC voltage. The simple SE design uses a CRC or CLC (with choke option-recommended) PS design, which allows you to adjust the B+ voltage by changing the value of the first cap within reason. A small value of the first cap will give a B+ DC voltage close to the AC output of the transformer, and a larger value for the first cap will get closer to 1.41 X transformer secondary voltage. If you are using a tube rectifier, that will drop 30V or so, reducing the B+ volts available. You can also vary the R in the CRC to adjust the voltage somewhat. If you use SS rectification, the sand diodes only drop a couple of volts yielding a higher B+ voltage compared to tubes.

Also keep in mind that when the amp is first turned on, and the tubes are cold (not conducting yet), the B+ voltage can rise to 1.41 X the transformer secondary voltage, since no current is flowing yet. Once current starts to flow, the voltage drops to design levels. That's why the PS caps are rated for eithier 450V or 500V depending on the secondary voltage of your PS transformer, since they will see the full B+ voltage at startup. This is typically only an issue when SS rectification is used, since B+ comes up instantly and the output tubes are not conducting yet. Tube rectifiers warm up slowly allowing B+ voltage to rise over 10-20 secs or so and by then the tubes are starting to conduct as they are warming up, so B+ may not get all the way to 1.41 x the transformer secondary.

That's why most folks recommend a power tranny with 350-0-350 to 375-0-375 output at 175ma-200ma or so. The current requirements come from the current draw of the output tubes (50-80 ma or so per output tube plus a few ma for the driver tube)

Some output transformers have multiple taps for the primary, typically 3.5Kohm and 5Kohm, allowing lots of opportunities for tweaking and tube rolling, although these are typically more expensive than a single 5K or 3.5K transformer. Some output transformers also have multiple output taps (4 ohm/8ohm) for speaker connections, the budget transformers will typically have just one secondary impedance. Check out tubelab's transformers and output tubes page for a comprehensive discussion of the choices. Generally, a lower OT impedance will yield more power, and more distortion than a higher impedance. Tubelab George has sim tables at the bottom of the page that compare tube current (Ik) and B+ voltage for different output transformer loads (loadZ).

http://www.tubelab.com/AssemblyManua...ormers_SSE.htm
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