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Old 4th October 2011, 06:15 PM   #1
kkcinc is offline kkcinc  United States
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Default Simple SE build help/ parts selection

Hello,
I am starting this project. I already have the Allied 6K7VG Transformer, but haven't ordered the opt's yet. My intention is to build it as simply as possible for now. I'll temporarily leave out the choke and cap. I'll also stick with ss rectification for now. (those items are likely to change in the future) My goal is the most power I can get because of my large inefficient 4ohm speakers. I'll use UL and CFB. I can and probably will stick with kt88's. My real question is, should I go with a lower impedance opt and eliminate the ability to use el34's opting for a couple more watts on the kt88's? Or, is the power gain insignificant and should I just stick with the 5k so I have all the tube options? Here are my tranny choices. All Edcor because of the value;
GXSE15-4-3.5K for strict kt88 use and budget consciousness.(I'll need help on resistor selection)
GXSE15-4-5K for use of all tubes 34/88/6l6 and budget consciousness.
CXSE25-4-2.5K all out balls to wall kt88 only(with resistor help from you guys)
CXSE25-4-5K This will be a no compromise/build exactly as laid out on tubelab sight.
Please help because I want to get started.
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Old 5th October 2011, 01:50 AM   #2
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If it were me, I'd go for the 5K output transformer. Match the secondary to whatever fits your speakers best; in your case 4 ohms. The GXSE15 is a fantastic value. If you don't mind paying more, the CXSE should be just a little bit better.
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Old 5th October 2011, 02:47 AM   #3
kkcinc is offline kkcinc  United States
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If it were me, I'd go for the 5K output transformer. Match the secondary to whatever fits your speakers best; in your case 4 ohms. The GXSE15 is a fantastic value. If you don't mind paying more, the CXSE should be just a little bit better.
Thanks. The thing is, with the 15w transformer, will it be ok with the kt88's in ul with cfb and ss rectification? Do you know the wattage that combo will produce using the 560r or 510r? If that transformer is safe I wouldn't mind saving the money. My speakers don't do much below 40hz anyway.
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Old 5th October 2011, 03:41 PM   #4
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You could also consider a 5K-8 ohm transformer (or a 6.5K-8 ohm CXSE); your 4 ohm speakers on the 8 ohm tap would be equivalent to a 2500 ohm load for KT88 use. You could run 8 ohm speakers on the 8 ohm tap for a 5K load for EL34 use. Just a thought.......

Last edited by boywonder; 5th October 2011 at 03:46 PM.
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Old 5th October 2011, 05:21 PM   #5
rknize is offline rknize  United States
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If you need to drive full range, I would spring for the CXSE's. As to the primary impedance, that depends on your speaker. You say it's 4 ohms...how flat is the impedance curve? You might be better off with the better DF of the 5k primary over the couple of watts of power. boywonder also has a good idea.
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Old 5th October 2011, 05:30 PM   #6
kkcinc is offline kkcinc  United States
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If you need to drive full range, I would spring for the CXSE's. As to the primary impedance, that depends on your speaker. You say it's 4 ohms...how flat is the impedance curve? You might be better off with the better DF of the 5k primary over the couple of watts of power. boywonder also has a good idea.
Except for the crossover point, the response is a flat 4.5 ohm all the way.
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Old 5th October 2011, 05:33 PM   #7
kkcinc is offline kkcinc  United States
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Originally Posted by boywonder View Post
You could also consider a 5K-8 ohm transformer (or a 6.5K-8 ohm CXSE); your 4 ohm speakers on the 8 ohm tap would be equivalent to a 2500 ohm load for KT88 use. You could run 8 ohm speakers on the 8 ohm tap for a 5K load for EL34 use. Just a thought.......
How would that effect the bass response? Usually using 8ohm tap with 4ohm speakers causes you to lose bass.
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Old 6th October 2011, 06:21 PM   #8
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How would that effect the bass response? Usually using 8ohm tap with 4ohm speakers causes you to lose bass.
'Lose bass' can have different meanings - the usual expectation would be to have a bit more power and distortion and perhaps more, but 'looser' bass. Any time I experimented with this, I found the difference was less dramatic than I expected.
This may have to do with the impedance vs freq characteristic of the speakers being used, of course.
It would be handy (though more expensive, I guess) if Edcor offered multi-output taps as standard.
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Old 6th October 2011, 07:21 PM   #9
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VictoriaGuy has a good point; reducing the effective output transformer primary impedance (ie connecting a 4 ohm speaker to an 8 ohm tap) usually reduces the damping factor and makes the bass "looser" or "flabby". This is sometimes perceived as more bass. I notice this effect quite a bit when I run my full range speakers (half changs) on a SS amp the bass is very thin to non-existent compared to using my TubeLab SE. The SS amp has a much higher damping factor than the Tubelab SE. Conversely, some commercially designed speakers will sound bass heavy when driven by a no feedback, SE triode amp.
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Old 7th October 2011, 02:44 AM   #10
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...with the 15w transformer, will it be ok with the kt88's in ul with cfb and ss rectification? Do you know the wattage that combo will produce using the 560r or 510r?
I tend to try to focus less on the claimed wattage rating of the output transformer, and instead seek to learn the actual maximum current rating of the primary winding. Especially with single ended transformers, you need to make sure you aren't exceeding the max current spec, otherwise you'll drive the core into saturation. You can most likely assume your KT88 tube will idle somewhere around 70 mA or less. Even if the 6K7VG can keep the B+ up to 450VDC under that kind of draw (optimistic) you're talking about 30+ watts of idle plate dissipation per tube. That's plenty enough for KT88, especially modern production. So the only question that remains is this - is the GXSE15 sufficiently gapped for 70mA standing current? Phyllis might be able to answer that.

I'd wager the answer is yes.
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