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Old 24th June 2007, 01:36 PM   #1
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Default Pre-amp power iron advice??

Hi all i need some advice,

I am rebuilding my year-old hand wired, tube regulated (wish it wasn't, it causes big thump when they strike) botched up Aikido amplifier.
It has served me trouble free a long time but i still get nervous switching it on... so time to build it nice and relax a bit to the music.

I'm a student on a tight budget and have been considering power iron for the amp.
Is hammond a good choice?
the 2xx series power transformers are about half price of the 3xx series, i can work around the primary voltage issues no probs but i read the 2xx series are worse built and leak more flux?

I am also looking into dual power supplies per channel, this will eat all my cash up.. i can just about afford it but is it worth it?
I already have an old 350-0-350 @ circa 150mA transformer that works if i run both channels from it.

Many thanks!

P.S. im planning on ordering Aikido pcb's + sockets + attenuator kit from glass-ware, is the attenuator a worthwhile purchase?
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Old 24th June 2007, 05:38 PM   #2
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Dual power supplies are not really worth it for a preamp. As long as the channels are properly decoupled there should be no problem.

If the amp thumps when powered on you could always wire up a time delay relay to close after a few seconds.
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Old 24th June 2007, 11:12 PM   #3
jnb is offline jnb  Australia
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As an enthusiast I can sympathise with you wanting to use separate power supplies for everything, I like to too.

You don't have to use separate power transformers though for two reasons:
1) You are powering a pre-amp which is probably not pushing the transformer hard and is probably drawing a fairly steady current and 2) You have regulators.

Simply use a separate regulator for each section if you want a completely independent supply.

or

Group up stages within your amp that you feel will probably be OK on the same supply and use separate RC legs coming off a single regulator.


BTW, what's the attenuator for?
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Old 25th June 2007, 10:34 AM   #4
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Thanks for replies guys!

Ill go with both your advice and go single supply, i know 2 supplies is overkill but when your building somthing overkill is an attractive option .

I will consider regulating each channel independantly, probably with some 0A2's/0B2's controlling a series pass tube like a 6V6 maybe? but ill see how it goes with a C-L supply first as i have 3 x 20H +1 x 8H chokes and loads of 1000uf@385v caps to experiment with.

The attenuator is going to be for the input to the Aikido amplifier to allow adjustment of signal into the circuit. I could use a dual POT but i used this before and it crackles and doesnt track each channel evenly (2-3dB's difference!) very annoying.

If i do go regulated then relays for anti thump seem attractive, probably best as a safety precaution anyway....

Craig
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Old 25th June 2007, 11:59 AM   #5
SY is offline SY  United States
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Two small bits of advice: if possible, use a separate transformer for heaters and B+. If possible, use a split-bobbin construction for the power transformers.
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Old 25th June 2007, 12:08 PM   #6
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Cheers SY, noted. I can obtain filament trannies cheap from a local shop.

What is split bobbin? I think my tranny uses E-I core laminations.

I remember you posted a lot on the Aikido a while back when it was new, you gave me a lot of help too which i am grateful for.


Regards
Craig
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Old 25th June 2007, 12:15 PM   #7
SY is offline SY  United States
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Split bobbins are those where the primary and secondary are either would on completely separate bobbins or one that is segmented (split). The primary and secondary do not overlay. This greatly reduces capacitive coupling between primary and secondary, a plus for reducing line noise coupled to the secondary and rectifier hash coupled back to the primary (and then on to other circuits).

Broskie is not a stupid guy. He sent me Aikido boards and parts knowing that, even though I don't need a preamp with gain, I'd build it anyway out of curiosity, measure it, play with the circuit a bit, and then save him a pile of work by troubleshooting the early builds here.
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Old 25th June 2007, 03:39 PM   #8
kevinkr is offline kevinkr  United States
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I wouldn't recommend the use of a Hammond 2XX series transformer on 50Hz as they already run fairly warm under load on 60Hz, 50Hz operation will make this much worse. The 3XX is really a much better transformer, runs quieter, cooler and since it has multiple taped primaries unlike the 2XX series you can avoid the need to use an autotransformer.

Growing up overseas using 120V appliances with autotransformers became a bit of a nuisance, I'd avoid that at any cost, but that's just me.
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Old 27th June 2007, 09:15 AM   #9
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I see.. nothing irritates me more than a buzzing transformer, if i need to buy a new transformer ill certianly get a 3xx series.
See how my old one tests out on both channels, its used to running 12ax7's in aikido but now its going to run 6sn7's which run with a lot more current so well see how it copes.

Cheers
Craig
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Old 27th June 2007, 01:34 PM   #10
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Default My 2 bobs worth

Hi Craig
I used filament transformers from local electronics shop. 12Volt ones to supply home made pc boards sporting LM338k regulators no problems at all with hum and they can handle 5 amps each with a decent heatsink. i use one trans and one regulator per channel on my preamp which runs 2 6sn7's and one sl7 per side. I love overkill.. its a geeks paradise.
I built dual regulated B+ using one transformer but with chokes in parallel one for left and one for right channel
If you want a piccie of heater psu's let me know
Good luck
Nick
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