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Old 29th March 2009, 11:50 PM   #1
Forsman is offline Forsman  Sweden
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Default Building RH84 SE tube amp.

Hi, as the newbie I am I would appreciate some advice on building my first amplifier. Iíve been reading, both on this forum and in other texts recommended on this site, and found that the RH84 amplifier has a very good reputation and seams fairly easy to build for a first timer.

There are pretty many things things that I have problem with comprehending and one is how to choose the output transformer. What is important when it comes to choosing OPT? As far as I understand (that goes for all this text) there are a couple of criteria that need to be met and some that doesnít. For me it need to be affordable, I donít have the budget to purchase the best one on the market so my question is witch gives me most value for money without ruin me?

The impedance of the OPT should be 5 KΩ, according to the drawing. That is easy to find but how is it with the current it is designed for. Claus Byrith states in his article on building up on Lundahl transformers that one should choose an OPT designed for the right current for it to operate at its best. Iíve been looking at Hammondís OPTīs but I canít find any current marking. Do I need to pay attention to this or is it subsequent to sound quality? On the other hand I donít knew what the current over the OPT is in the RH84 and Iím not skilled to calculate it either (yet ).

As Iím planning to use this Amp with a pair of back loaded full range horns (Ron Clarkeīs Austin 166) I guess that the frequency response is of importance. The speakers rolls off at approximately 40 hz (have I read somewhere) and I want the Amp to mach that. With the Hammond 125SE-serie OPT the frequency response is stated to 100-15 000 hz. Is this OPT suitable for the task at all? Can I tweak it to go full range or should I choose from a different series of OPTīs? The Hammond 125ESE caught my eye on the Boozhoundlabs site but in a different amplifier with different tubes (6V6GT).

/Fredrik
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Old 30th March 2009, 12:41 AM   #2
mach1 is offline mach1  Australia
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I have built two RH84 to date. One with sony 5k RtR transformers, the other with some Japanese RCA Victor. Both are modestly sized and work very well.

I reckon the budget Edcor 5k:8 trannie (under $20) would be a great match.
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Old 30th March 2009, 03:00 AM   #3
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any single ended output transformer rated for at least 50ma. should do just fine.

i have built six versions of this circuit with salvaged parts using everything from an akai reel-to-reel, zenith s.e. console amps, magnavox, rca consoles. ive used the edcor gxse-15-8k (rated 75ma.d.c.) and it sounds great! the akai was a real gem with two pairs of great power and output transformers of high quality.

i use these amps with some fostex ff225k and theres plenty of power.

very nice circuit, cheers to mr. kitic for sharing with us!
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Old 30th March 2009, 03:02 AM   #4
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For a single ended EL84 amp, assuming budget is the most important criteria, I'd make sure the output transformer matches specs on the primary impedance and the intended output load. In your case, if the schematic says 5k ohms and your speakers are an "8 ohm" load, then buy a set of transformers specified for 5k:8 usage. Also make sure the transformers are intended for single ended usage, and that they have an air gap to minimize the effects of core saturation. Beyond that, go for the heaviest transformers you are comfortable affording. They'll have more iron and copper in them, and will have better low frequency extension than smaller outputs.
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Old 30th March 2009, 04:03 AM   #5
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For new i'd pick Edcor over Hammond. I also wouldn't be afraid to look on eBay for some vintage iron... we've had good luck with German consoles, Nordmende (they have big ones & little ones -- for a FR amp you need the big ones) Grundig, Lowe-Opta. Some of these will reflect more than 5k, that is OK.

dave
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Old 30th March 2009, 04:42 AM   #6
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EL84 SE? Here ya go:
http://www.edcorusa.com/Products/Sho...ct.aspx?ID=428
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Old 30th March 2009, 02:59 PM   #7
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I built one with 125ESE's. Works fine. I was so impressed with it that I rebuilt the amp on a proper chassis with better components and Lundahl opt's, I don't remember the model but they are 5K primary with a single 5 ohm secondary (I think). That was a few years ago. I still have not finished it

Someday...
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Old 30th March 2009, 03:53 PM   #8
tim614 is offline tim614  United States
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Default transcendar

i also have built the rh84 with great result using edcor ($20 opt)
but when i upgrade to the transcendar it was night and day.
so if i was to built another one transcendar is a great deal, you can get them on ebay for about 115 a pair. edcor for $20 is a great deal sounded decent but i would highly recomend buying
better OPT for great result, after all OPT is the heart of the amp.
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Old 31st March 2009, 10:08 AM   #9
Fuling is offline Fuling  Sweden
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I donīt know how Hammond tests their transformers but when I measured an 807 SE amp with 125CSE OPTs I got something like 35Hz to 30kHz @ 1,5W.
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Old 31st March 2009, 10:20 AM   #10
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Hey Fredrik,
Build a pair of 6B4G SEs instead. Better to go for the real thing! Not so much more work either. The russian version 6S4S are cheap at eBay. I have an extra pair of 125ESE lying around, mail me if you are interested.
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