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Old 26th April 2010, 08:24 PM   #1
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Default Help me choose output transformers

I am gathering parts for my first tube amp and am ready to order some OPTs, but I'm wavering on which ones are best for the job. The amp will be ~4.5W output, SET, with a good quality build. The load should be approximately 7k ohm. The speakers are ~93dB, single driver, with a useful freq response of 40-20k Hz.

I like the Edcor line because they are affordable, good looking, and USA made. So lets stick with them unless there is a compelling reason not to.

The relevant choices from their catalog are:
Model, Power, Primary, Secondary, FR
GXSE10-8-8K, 10W, 8K Ohms, 8 Ohms, 40-18k
GXSE15-8-8K, 15W, 8K Ohms, 8 Ohms, 40-18k
CXSE25-8-6.5K, 25W, 6.5K Ohms, 8 Ohms, 20-20k

I've eliminated the XSE series because the freq response is narrower than I would like.

The questions coming to mind that I am unable to answer are:

The two GXSE models are identical in specs other than power handling and weight. I've read that generally, the heavier the OPT the better, but it sounds in this case like the increased weight is just due to the increased power handling. For a 4.5W amp, should there be any difference in sound quality?

The CXSE has better FR and a massive 25W power handling. Would these provide any benefit for a 4.5W amp feeding speakers that are only good to 40Hz?

Which would you choose for a project like this? Feel free to suggest other brands if you feel they would better meet my needs.

Thanks for the help!
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Old 26th April 2010, 08:54 PM   #2
Zoran is offline Zoran  Serbia
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Hi I am not sure did You mention the output tube type You want to use in SE?
...because of the tube internal resistance that is of merit when we talk about the OT primary inductance and that is from importance for the proper bw
not the transfer ratio...
.
probablu some pentode in pentode mode operation?
that is could be read from 7K and 4.5W power?
.
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Old 26th April 2010, 08:58 PM   #3
tomchr is offline tomchr  United States
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When designing my first SE amp, I bought the XSE15-8-5k and CXSE25-8-5k transformers and tried both in the same configuration. The XSE15 is really quite good -- especially for the money ($20). The CXSE25 offers a bit tighter bass, and much more transparent highs. At least that was my experience.

If you are just looking to get your feet wet with the tube amp design and aren't overly confident in your design skills, don't want to spend too much money all at once, get the XSE or GXSE. But if you're fairly confident that you'll like the tube sound and can handle the design, want a solid amp that offers really good performance, I'd opt for the CXSE-series of transformers.

Hope this helps.

~Tom
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Old 26th April 2010, 09:00 PM   #4
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The output tube is the 6N6G. A somewhat uncommon dual dissimilar triode, but makes for a very simple, low parts count amp. I am building the recommended configuration from the data sheet here:

http://www.mif.pg.gda.pl/homepages/f...084/6/6N6G.pdf
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Old 26th April 2010, 09:08 PM   #5
jlsem is offline jlsem  United States
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The 6N6G has way too high a plate resistance for that load. What else can I say to convince you that using that tube in a single-ended output mode is a bad idea?

John
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Old 26th April 2010, 09:22 PM   #6
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John,

Could you explain that a little further? I do not understand the relationship between plate resistance and load, and what happens when they are not correctly matched.

Surely the 6N6G is able to operate in SE mode since the data sheet indicates that is what it's intended for? Is it then a matter of sourcing an OPT with the appropriate characteristics?

I have a box of these tubes, and would like to make something useful of them if possible, but if it's really a bad idea I suppose I could scrap the design.
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Old 26th April 2010, 10:10 PM   #7
2wo is offline 2wo  United States
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I am not familiar with that tube but According to the data sheet it is looking for a 7k load, As you already have them, I say go for it. If it doesn’t work out, there are plenty of tubes that will work well into a 8k transformer and you will have lost almost nothing. I have used the GSXE10-8-8k in a 6V6 amp and they sound very good for the money…John
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Old 27th April 2010, 01:17 AM   #8
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Quote:
Originally Posted by roundel325 View Post
The output tube is the 6N6G. A somewhat uncommon dual dissimilar triode, but makes for a very simple, low parts count amp. I am building the recommended configuration from the data sheet here:

http://www.mif.pg.gda.pl/homepages/f...084/6/6N6G.pdf

Dude,

In addition to John's warning about RP, think about 5% distortion and inadequate power O/P. Paul Joppa's 102 dB. rule tells us the minimum power O/P for 93 dB. sensitive speakers is 8 WPC. If you want to use 6N6Gs, go push/pull. A 12AT7 based LTP will give you the 15 V. of drive needed from slightly less than 1 V. of I/P. That's excellent and a line stage will not be needed, if a CDP is the signal source. Edcor's GXPP15-6-10K should be fine with 8 Ω single driver speakers. You might think about a small amount of NFB, for damping factor and distortion reasons. Roll extreme LF info. and infrasonic noise off with a cap. at the amp's I/Ps. A 30 Hz. F3 appears reasonable.
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Old 27th April 2010, 04:06 PM   #9
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Thanks for the suggestion Eli. I'm a bit scared of the P-P scheme due to several circuit elements I don't understand well enough to design myself, like how to implement NFB and what an LTP is.

I'm not too worried about the power output. I have a small room and have been perfectly happy with the 5wpc t-amp I've been using on these speakers.

I'd love it if someone could drop some knowledge on me and explain why a high Rp is a bad thing.
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Old 28th April 2010, 01:29 AM   #10
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Quote:
Originally Posted by roundel325 View Post
Thanks for the suggestion Eli. I'm a bit scared of the P-P scheme due to several circuit elements I don't understand well enough to design myself, like how to implement NFB and what an LTP is.

I'm not too worried about the power output. I have a small room and have been perfectly happy with the 5wpc t-amp I've been using on these speakers.

I'd love it if someone could drop some knowledge on me and explain why a high Rp is a bad thing.

It's routine to use a load of 3X RP in combination with a "conventional" triode. The much loved 2A3 has an 800 Ω RP and it's frequently combined with 2.5 KOhm O/P trafos. Sylvania's 6N6G data sheet shows a RP of 24 KOhms, which suggests (sic) a 75 KOhm primary. However, that's anything but a "conventional" triode. Frankly, I'm still trying to make sense of the wiring diagram. FWIW, I see a cathode follower directly coupled to a common cathode power triode. Perhaps the data sheet has incorrect numbers and the RP is 2.4 KOhms and the μ is 5.8, which does "jive" with a 7 KOhm O/P trafo primary. Also, μ = 58 is very large for a power O/P type.


LTP = long tailed pair, AKA cathode coupled, AKA Schmitt, is a phase splitter based on a differential gain block.

The small signal circuitry in the attached "El Cheapo" schematic should serve you well, with few adjustments. With a combined draw slightly in excess of 200 mA. from 4X 6N6s, the PSU needs to be rethought.
Attached Images
File Type: gif elCheapo-23jun06-map.gif (38.8 KB, 297 views)
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