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Old 16th October 2010, 05:58 AM   #1
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I am a newbie to all of this,Glass Audio, but have always have been pokeing around at amps. I have been looking for weeks and want to make a oddwatt 225. I bought the two 6SL7GT from an old tv guy for $10.00Can NIB nad I just bought 4 jj kt77 on ebay for $35.00+10.00 for shiping. I need help to find an power transformer and out put ones as well new or used I do not want to buy the wrong thing. And I want to add 2 EM80 eys they look so cool. I will postr pics in the section when I start . I have walnut for the corners and birds eye maple for the front, back,and sides. With these tubes what kind of watage am I looking atas well? you can email at driftwood@live.ca my name is Jason and Iam 35years old. thanks for the time and the help
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Old 16th October 2010, 04:09 PM   #2
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Assuming you mean this one:

OddWatt 6SL7 SRPP / KT77 Push-Pull Class-A Ultra-Linear DIY Tube Amplifier

Reading the description, built per the schema you're looking at 25 wpc.

As far as the output transformer goes, Bruce is using a 25W 8K primary Edcor with two sets of output taps, 16R and 8R. Important note on the schematic: He is connecting 8 ohm speakers to the 16 ohm taps, or 4 ohm speakers to the 8 ohm taps. This is effectively halving the load that the output tube see from 8K (the transformer spec) to 4K. You could also use a 5K or possibly even a 3K output transformer it you want. The 5K will reduce output power & distortion a little bit, and the 3K will increase them a bit.

You may want to consider output transformers a little larger-say 35W; more iron gives a little more bass headroom.

The amp is configured in Ultralinear (UL) so your transformers will need screen grid taps (this shouldn't be a problem). You can also configure the outputs to run in triode if you want....

What is your budget? Two obvious choices for output transformers are Edcor and Hammond (esp since you are in Canada). Edcors are great bang for the buck, however, their lead time is commonly 4-6 weeks. They have 3 series to choose from from $ to $$$. The XPP, GXPP, and CXPP. Bruce is spec'ing the CXPP, and after a quick glance at Edcor's product offering, there is possibly one XPP transformer suitable (25W/7.6K) using the speaker tap trick mentioned above. There should be lots of choices in the CXPP series.

Seems like a straightforward project to me, although IIRC the SRPP 6SL7 front end is a bit out of fashion around here presently. The 6SL7 has lots of voltage gain at the expense of current drive.

As far as the power transformer goes, Bruce is spec'ing the Edcor XPR101, so there is an obvious choice. You can use other power transformers if you want; you'll need to tally up the current draw of the tubes, and use Duncan Amps free PSUDII software to help you model the PS to get the correct B+ voltages. Getting a B+ of 350V out of a 240V secondary seems like a little stretch to me, but the secondary of the XPWR101 is rated at 2A!!

I would do the PS a little differently, but that's just me.......You certainly don't need 2A (2000 ma) secondary current, and you also won't be using the 60V bias tap, so there are other good power transformer choices for less than $103 for the XPWR101. I would also use a choke in place of the 25R/5W resistor in the PS.

Last edited by boywonder; 16th October 2010 at 04:27 PM.
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Old 18th October 2010, 04:00 AM   #3
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will this work E-Bay #320586931410for the power transformer Item number:320586931410Item number:320586931410Item number:320586931410
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Old 18th October 2010, 05:17 AM   #4
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Howdy Jason,

Not sure of your situation, but I would just use the Edcor power transformer (barring any problems with shipping from the US) if I were building this particular design. The eBay transformers, while inexpensive, are 1) a relatively unknown quantity and 2) only marginally up to the challenge. Although the seller seems to have done a pretty good job of testing the limits of his wares, such substitutions are best left to those with a bit more experience.

Boywonder has given you some excellent advice, and I'll second the use of that choke in the power supply. I'd just like to add that at around $103 USD, the Edcor iron is just cheap insurance; it'll be just loafing along in that particular design. Yes, it is a bit overkill, but it'll more than likely produce the voltages needed for your design, and do so coolly, quietly and reliably - just like the designer intended.

Just call (575) 887-6790... that's (575) 887-6790... Again, that's (575) 887-6790! Plus if you act now, we'll include this set of steak knives - a $99 value - ab-so-lute-ley free...
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Old 18th October 2010, 04:00 PM   #5
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Dont build the oddwatt 225. Its out dated. Use is newest odd watt that uses el34 and kt88's. He no longer uses the 8 ohm on the 4ohm tap trick and a few other errors. You can still use the 350 volts b+, but he has since switched to higher transformer impedance with lower current and higher voltage then the old oddwatt 225
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Old 18th October 2010, 04:16 PM   #6
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Driftwood: There is a thread here on that Ebay transformer:

10 pounds of power for $15

They were $15......the selller upped the $$ to $20 recently.
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Old 19th October 2010, 03:29 AM   #7
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I allready have the 6sl7GT and the 4 kt77 tubes what other amp could I build money is a little tight If I do not like the amp I have a buyer for it . I have $130.00 Can for the three transformers
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Old 19th October 2010, 04:28 AM   #8
rmyauck is offline rmyauck  Canada
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Hi,

Look at Georges tubelab.com. He has a Form under Vendors on this site. In his Form a reader has built a Simple SE PP amp with 2 boards using them together with a differential splitter type of circuit. It is titled "Dual Simple SE's in PP". Your KT-77's would work and 12AT7's are only about $10 ea. You would also need 2 5AR4 rectifier tubes, but may work with good quality SS rect. diodes to save $. Startup is harder on other tubes without slow startup from 5AR4'S however. Add inrush limiters to help compensate. Bass may be improved with SS rect. however. Lots of help from other tubelab builders on this site. Should sound nice!
Keep 6SL7 for a preamp.

Last edited by rmyauck; 19th October 2010 at 04:58 AM.
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