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  #1  
Old 23rd October 2018, 11:14 AM
karellen karellen is offline
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Default kel84 halved on two

Hi!
I have an old kel84 and thinking about rebuild it split on two tiny monobloks, 15w each (just for fun and a different aesthetics)
So i suppose i need two main transformers, with the same voltage rantings but half the current,
and a pair of chokes , 1H but half the current.
Is this ok? How critical is the rating of the choke?
what do you think?
Any comments will be very welcome

Peter
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  #2  
Old 23rd October 2018, 12:45 PM
colin.hepburn colin.hepburn is offline
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Default Re: kel84 halved on two

Hi Peter
Why half the current . the same current or more will give a stiffer PSU. with less heat and stress on the psu and better sound . thats the way i build my amps dont mean to say its right thou the other will chine in on this soon i would think
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  #3  
Old 23rd October 2018, 02:26 PM
bikerhifinut bikerhifinut is offline
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Default Re: kel84 halved on two

My first comment would be.............if it ain't broke dont fix it as you are unlikely to get any real sonic advantages out of this.
Now thats over with and you want to rebuild the circuit as 2 mono blocs lets think this through.

Option1. Keep the original mains transformer and choke for one side and get an identical replacement for the other amp. This would give you identical PSU on each side. Downside, if its an early one then getting the same transformers and choke will be nearly impossible.

Option2. Sell the original transformer and choke and put the money towards a pair of suitable mains transformers and Chokes. there are many options here. for what its worth something like a 10H choke rated at 150mA or better will be plenty assuming the EL84s are taking around 100 to 110 mA maximum on each side. Like wise the mains transformer needs to be able to supply heater current for 2x EL84 plus whatever preamp valves you choose. This is not a trivial purchase, you will more or less pay double the cost of a single transformer for a pair and again the chokes will cost you nearly double that for a single one.
You could, at a pinch use something like a 100 ohm resistor in place of a choke in the PSU. this was common practice in many commercial designs but why would you want an inferior HT supply?

Option 3. keep the original transformer and choke, build 2 mono blocks and use the transformer and choke to make a separate PSU connected by an umbilical.................. and introduce a whole new set of potential problems.

I am sorry if this all sounds negative but I honestly think you are going to turn a silk purse into a sows ear to paraphrase an old English proverb. And destroy any inherent value in the WAD/WD KEL84 design which sells well on ebay etc.

So if you are determined to make a pair of mono push pull EL84 amps my advice would be to sell the KEL84 and use the proceeds to buy the parts for your own bespoke amps. You may be able to buy a pair of WD output transformers from matthew here and then hunt around for a coiuple of mains transformers and some chokes. Bluebell audio here in UK do a range of hammond ones at reasonable rates and there is also outfits like VVT who have a large range of transformers suitable.

A lot to take in? Perhaps but I think you should think it through very carefully.

Andy.
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  #4  
Old 23rd October 2018, 03:18 PM
colin.hepburn colin.hepburn is offline
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Default Re: kel84 halved on two

yes i would do a new build rather than convert as well

10 Henry 200 mA DC 66*66*64 mm³ 160 Ohm DC.£17.89each plus shipping you cannot go wrong with there s pro made and test as speced

ihave uesed them for years and would not bother with any others i have four in my stock for my next Amp build maybe a KT88 SE
Choke build QC is topend check him out nice guy
here https://www.die-wuestens.de/eindex.htm
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  #5  
Old 23rd October 2018, 06:18 PM
bob orbell bob orbell is offline
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Default Re: kel84 halved on two

Well my suggestion would be to sell the KEL, as Andy (bikerhifinut ) has said, they do sell well, and buy or build a new WA 88 amp, most on here will tell you the latter is very very good. Now how about a bit of commission if this comes off Matthew . Bob
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  #6  
Old 24th October 2018, 08:44 AM
snowman_al snowman_al is offline
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Default Re: kel84 halved on two

No seriously?
Where will you 'split' the PWBA? You will have to hard wire instead.
You will need two new chassis etc. as stated.

So there is little choice other than to build from new and leave what you have now alone...

Last edited by snowman_al; 24th October 2018 at 09:04 AM. Reason: refresh
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  #7  
Old 24th October 2018, 01:55 PM
karellen karellen is offline
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Default Re: kel84 halved on two

Many thanks to you all; very good and useful advice.

The idea was to recicle the output transformers and the tubes, wind myself the new power transformers and chokes, and go hardwire…
A way more expensive and labour intensive, but then just was an idea.

I’ll keep dreaming (with tubes, of course!)

Peter
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  #8  
Old 24th October 2018, 11:34 PM
bikerhifinut bikerhifinut is offline
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Default Re: kel84 halved on two

So if you have the skills to wind your own mains transformers etc why not do just that?
Sell the KEL84 to help finance the project.
Buy a pair of output transformers off matthew or you can buy a set of EL84PP jobs from several suppliers.
And build it as a hardwired pair of monos.
You don't have to stick with the WD circuit either as there are a lot of choices of circuits to drive a pair of EL84 in push pull.
I dont know if there are any issues about using the intellectual property of world designs either when building something that is based heavily on that amplifier.
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  #9  
Old 25th October 2018, 08:59 AM
bob orbell bob orbell is offline
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Default Re: kel84 halved on two

As Peter said in his last post Andy, ( I'll keep dreaming ) . Bob
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