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Old 25th July 2007, 11:55 PM   #1
chrish is offline chrish  Australia
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Default Now I have caught the bug - next project

Hello all,

A little while ago I built Tubelab's SimpleSE amp. I have been very happy with the results. Thanks George (Tubelab). The ease of construction, proven design, good web page and great end result has whetted my appetite for building another tube amp. I have decided to built another amp, but this time I would like to get my hands a little dirtier, and build from scratch rather than a cct board.

My girlfriend listens to music in her office off her laptop computer with its crappy internal speakers. I would like to build an amp for her, and speakers, that will be appropriate to work from a laptop sound card (ie does not need to be hi-end) in a quiet office environment.

My work has me travelling a fair bit and I just had the opportunity to pick up a cheap power transformer. Not knowing which design I was going to build, I had to guess as to an appropriate voltage. I picked up a transformer with outputs for 240-0-240VAC and 275-0-275VAC (and 2*6.3VAC and 5VAC) 150mV, so I am guessing that B+ voltages will be around 285VDC or 330VDC?

I am thinking of making bookshelf speakers from Fostex FE103E or FE126 or similar to go with the amp.

If anyone has some suggestions of what would be a suitable project for the transformer and application I would be most grateful. Also, I live in Australia, so sourcing parts can be problematic (and expensive if I have to mail order) so designs without esoteric parts would be more desirable.

Thanks in advance for your suggestions!

Chris
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Old 26th July 2007, 12:18 AM   #2
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Chris,

275-0-275 rectified by a 5AR4 and CLC filtered should yield a satisfactory B+ rail for an "El Cheapo" variant. ANY 6V6 family tube, instead of the 'AQ5 shown, will work without parts value changes.

If the current capability of a single 6.3 V. filament winding is sufficient to power all 6 signal tubes, the 2nd 6.3 VAC winding could be voltage multiplied to yield the B- rail. The 6 signal tubes draw 2.4 A. of heater current.
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Old 26th July 2007, 02:51 AM   #3
chrish is offline chrish  Australia
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Thanks for the response Eli, will investigate.

What about something like this? A clone of the Decware Zen http://www.decware.com/se84cschematic.pdf

Looks reasonably simple, think I read a post that said that worked better with a lower impedance output transformer than the stock 9K, so thought the Edcor XSE15-8-5K like I used in my SimpleSE might work.

Chris
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Old 26th July 2007, 11:02 PM   #4
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Quote:
Originally posted by chrish
Thanks for the response Eli, will investigate.

What about something like this? A clone of the Decware Zen http://www.decware.com/se84cschematic.pdf

Looks reasonably simple, think I read a post that said that worked better with a lower impedance output transformer than the stock 9K, so thought the Edcor XSE15-8-5K like I used in my SimpleSE might work.

Chris

Chris,

I own a pair of DECWARE SE84Bs. They are nice. The speakers used INFLUENCE which O/P trafos work best. DECWARE's "standard" O/P trafo works well into low impedance loads and speakers with dips in the impedance curve.

Keep the need for EFFICIENT speakers in mind. Approx. 2 WPC is what you should expect.

If you follow the SE84 topology, I suggest you use a 5965 or 7062 for voltage amplification, instead of a mu 33 tube. That way, you definitely will not need a line stage.

The SE84 topology works with the 6V6 family too. FWIW, the 6V6 family makes a more linear triode than the EL84 makes.
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Old 27th July 2007, 09:26 PM   #5
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Since your've got a working amp - why not take the opportunity to design your own amp from scratch. Its not that difficult and you can learn a hell of a lot in a relatively short amount of brainstorming. Select a really obscure valve that no-one else is using (some of the nice TV tubes would make cheap candidates) and you can have a cheap and excellent amp. The sense of achievement will be huge.
I would suggest a PP amp, but thats just my preference. SE amps are nominally easier.

Have fun, theres no rush.

Failing that, the RH84 or RH807 are excellent SE designs (the RH807 will probably work out cheaper and gives about seven watts).

Shoog
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Old 28th July 2007, 09:25 PM   #6
chrish is offline chrish  Australia
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Thanks Shoog, I think that is what I will do. My Dad is visiting for a week, he is an electronics engineer and we will sit down and design an amp. He has convinced me I should build a push-pull amp. To keep the tube count down we are thinking of running 6BM8s for power and phase split and driven by ECC83 and (probably) a solid state rectifier.

Will post schematic when we get that far.

Thanks for the help guys!

Chris
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Old 28th July 2007, 10:18 PM   #7
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Quote:
Originally posted by chrish
Thanks Shoog, I think that is what I will do. My Dad is visiting for a week, he is an electronics engineer and we will sit down and design an amp. He has convinced me I should build a push-pull amp. To keep the tube count down we are thinking of running 6BM8s for power and phase split and driven by ECC83 and (probably) a solid state rectifier.

Will post schematic when we get that far.

Thanks for the help guys!

Chris

FYI, the triode in a 6BM8/ECL82 is similar to those found in the 12AT7. A look at the data sheet curves shows you should have no problem achieving full power using a CDP as the signal source into the "El Cheapo" topology.

Given the limited power O/P available, I suggest you use pentode mode and regulated screen grid B+. Pick your O/P trafos with care.
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Old 29th July 2007, 06:55 AM   #8
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The triode in the 6BM8 is rather soft sounding and has to high gain and to low bias (-1V). The pentode is rather harsh, even when triode strapped. I have messed about with one for a few projects and never been totally happy with the results. Going PP will probably help things a bit.

Good Luck

Shoog
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