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Old 6th November 2005, 08:17 AM   #11
tonecat is offline tonecat  United Kingdom
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U mean the soldering iron in the pix. (Antex 'C' 15W) or transformer iron - a company over here used to sell kits for power transfomers in 20VA, 50VA and 100VA sizes - the output transformers were created from 2x100VA kits by creating a larger winding former using the cutting and glueing method!

BTW, the first amp weighs nearly 7 kilos. I generously sized the output transfomers (and was rewarded with deep tight bass)
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Old 6th November 2005, 08:31 AM   #12
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[QUOTE]Originally posted by tonecat
or transformer iron /QUOTE]

That... te OPTs... the 12BH7 amp the same? What primary you using?

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Old 6th November 2005, 08:33 AM   #13
tonecat is offline tonecat  United Kingdom
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Cal, I just noticed an unused 20VA kit on the shelf similar to those I used for my amps. The E I laminations can be interleaved or stacked, gapped and glued with a magnetically inert spacer (err.. cardboard). I had to use studding to replace the bolts for the first amp's output transformers.

The book is the Coil Design and Construction Manual by B.B. Babani - an excellent all-round guide to DIY coils and transformers for radio, audio and power. As you can see, it has fallen apart due to frequent use!
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Old 6th November 2005, 08:35 AM   #14
tonecat is offline tonecat  United Kingdom
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Oops, sorry, that was addressed to Dave.
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Old 6th November 2005, 08:44 AM   #15
tonecat is offline tonecat  United Kingdom
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Dave,
have a look at http://headwize.com/projects/showfil...frazer_prj.htm - scroll down to Amplifier Circuit Note 8 which describes how I made the transformers for this amp

I don't have the 12BH7 specs to hand but I did arrive at the impedance ratio of 2k7:8R by empirical means rather than finger-in-the-air!
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Old 6th November 2005, 05:49 PM   #16
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Quote:
Originally posted by tonecat
impedance ratio of 2k7:8R by empirical means rather than finger-in-the-air!
Indicative that a PP OPT for 6BM8 might work OK with this tube

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Old 6th November 2005, 06:38 PM   #17
tonecat is offline tonecat  United Kingdom
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Dave,
I haven't used the 6BM8 before (triode/pentode like ECL82) - I found examples of transformers on www.tubehifi.com suggesting 5K primary for both SE and PP operation.
Fow a low power amp like this, using such a transformer may be possible. SE transformers need to be a bit larger due to the magnetising DC current flowing in the primary but in PP transformer each arm of the primary cancels this out. Hardly a problem in this low power amp.

It should also be possible to build a neat PP amp using a couple of ECL82s with the triode sections as preamp and phase splitter.

I think the ECL82 was a popular valve for record player amplifiers or guitar practice amps. There is an interesting example at

http://www.tubebuilder.com/images/sc.../ecl82_se1.gif

Tony.
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Old 6th November 2005, 06:50 PM   #18
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6BM8 = ECL82

Japanese are in particular a big fan of this tube. Here, Gingertube has an interesting PP amp with its big brother the ECL86 and Morse has a nice 6BM8 SE (called The Gnat)

I have quite a few donors for both, some from budget hifis, so a lower power tube would be like an OPT upgrade.

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Old 7th November 2005, 07:07 AM   #19
tonecat is offline tonecat  United Kingdom
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Dave,

The Gnat looks like a great project - I like the elegance of simple designs.

The basic circuit could easily be adapted for other tubes too - 2xECL86 or maybe an ECC83 and 2xEL84. The ECL86 pentode is similar to EL84 and was also a common SE audio amplifier in radios and record players.

Tony.
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