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Old 1st May 2006, 10:51 PM   #11
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Quote:
Originally posted by lndm
Steel is not as good a conductor as copper or aluminium, and by a larger margin. Magnetic fields would also permeate the steel and spread and induce eddy currents (I think). Not sure how successful this would be.

Good point. I think I'll go with aluminum. Sorry for all the goofy questions but this is my first amp and I don't want to muck it up too bad if I can help it. Do you have any thoughts or opinions on the schematic as far as bass response? My wife tells me that chamber music doesn't have much going on in the way of bass. I'm hoping that the extra bucks for the OPT's may pay off in the long run.
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Old 1st May 2006, 11:13 PM   #12
lndm is offline lndm  Australia
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Quote:
Originally posted by jaymanaa
bass response?.... extra bucks for the OPT's
Can't argue with this.
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Old 2nd May 2006, 02:20 AM   #13
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Well this is going to be an amp someday I hope. One question I have is with all my 6.3v heaters connected I have 7.38 volts. This transformer ran more heaters in it's previous life and probably had some sag that is now non-existent. What if I were to rectify the 6.3 with a simple diode bridge? Wouldn't that give me a little drop, and isn't DC more desirable for the heaters? Any thoughts and ideas would be appreciated. I think I am learning from this project after sort of hitting a wall with the books. Thanks, Jay
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Old 2nd May 2006, 02:37 AM   #14
lndm is offline lndm  Australia
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Awesome.

Just my opinion but the extra voltage will allow you to use a series resistance. The resistance of a cold heater is low and when hot it is high. Therefore the resistor will have most effect when the heaters are cold. It will reduce the inrush current and prolong the life of the heaters.

AFAIK, AC on the heaters is an excellent choice due to the simplicity of the circuitry, except for the issue of hum. I use AC and have no significant hum (except in my phono stage). I actually get more hum from my supply rails (fixed with a few neat tricks).
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Old 2nd May 2006, 02:44 AM   #15
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Quote:
Originally posted by lndm
Awesome.

Just my opinion but the extra voltage will allow you to use a series resistance. The resistance of a cold heater is low and when hot it is high. Therefore the resistor will have most effect when the heaters are cold. It will reduce the inrush current and prolong the life of the heaters.

AFAIK, AC on the heaters is an excellent choice due to the simplicity of the circuitry, except for the issue of hum. I use AC and have no significant hum (except in my phono stage). I actually get more hum from my supply rails (fixed with a few neat tricks).

Could you recommend a value and wattage to start with, or maybe there is a formula (I have to remember math is my friend). With 2 el34, and 2 6sl7 heaters I am pulling 3.80 amps at 7.38 volts.
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Old 2nd May 2006, 03:07 AM   #16
lndm is offline lndm  Australia
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Add the currents for all the heaters. Then, 7.38V - 6.3V leaves 1.08V in excess. 1.08 divided by the total current gives the resistance (0.28 ohms). 1.08 squared, divided by the resistance gives the power dissipated by the resistor (use 5W for example).

In practice, this will only be a starting point and you'll need to tweak.
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Old 2nd May 2006, 10:39 AM   #17
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Thanks indm, This is how I learn. By trying to build something and getting guidence along the way. I could read that 100 times in a book and it won't stick. Actually using it to solve a problem seems to make it soak in. I'm going to try and get by the electronics store today as I have very few resistors in the 5 watt range. I will keep you posted. Thanks Again, Jay
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Old 2nd May 2006, 09:23 PM   #18
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That seemed to do the trick and it never hurts to be kind to the tubes with prices what they are for the good ones. I picked up an assortment of 5 watt resistors and tried a .22 first which gave me 6.75 volts. Then I tried a .33 which hit dead on at 6.32v. I picked up a nice piece of aluminum today 1/8 thick and 11" wide by 19" long. I plan on trying to do some layout work tonight. I may mark it all out and post a pic for scrutiny. I plan on having mains and signal at opposite ends and rotating opt's 90 degrees from pt. I will have to look at the sockets to figure out how to keep evrything short and tidy (this may take some time and thought). Is there any problem with putting the choke directly in front of the pt with a 5U4 on each side of it. I have some coke bottle ones and thought it would look good that way. Any ideas are allways welcome, and thanks for the help so far. Jay
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Old 2nd May 2006, 10:47 PM   #19
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Hi Jaymanaa,

A few words from me. Firstly, it is fine to experiment but laudable to ask for advice. A waste of time to re-invent the wheel, and experience is fine, but second-hand is cheaper!

I would also have chosen the alu chassis. Although most commercial amps are built on steel chasses, I often had problems with hum (magnetic induction), even when using the recommended single star-type earth. With alu I could earth all over the place without problems. Well, to qualify, not earth so that the transformer-rectifier-filter capacitors are far apart; those you earth at one point, but I had no problem with lesser-current earths at the most convenient point.

Just as an example, I recently had to re-organise someone's Mullard 510 on a steel chassis (hum) and when all else failed tried to experiment with different power transformers. You would not imagine the difference in "hum induction" I found. Best orientation was often at an angle somewhere in space. In the end I had to rebuild comepletely on an alu chassis to solve the problem.

Then to repeat what was mentioned on other threads, that you need to view the "dc on heaters" with care. Just simply 6+V through a bridge rectifier followed by a large C can cause more harm than good. You can easily get more interference from the "turn-on" current spikes than the original ac would have given. Unless you can smooth the dc to a fair degree, stay with ac. Often the problem is not with the 6V ac as such but with ill conceived wiring. I have used ac heaters even down to pre-amps with phono input without too much trouble (although with regulators etc. available nowadays I would now use a serie heater string with dc for that).

A little more than you wanted for now, but hopefully of useful guidance in future projects.

Regards.
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Old 5th May 2006, 02:55 PM   #20
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This is a rough Idea of the layout I have in mind. Anyone see any pitfalls or concerns? Anyone ever use "Krinkle paint" for trans. end bells? I don't know how this amp will sound, but it may be in the running for the heaviest 8 watt amp in the world. Thanks for any and all advice, Jay
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