A couple of questions

a 3W single ended EL84 amp for home, work & school

Re: A couple of questions

Postby TomMcNally » Sun Oct 04, 2015 3:26 pm

Graham -

Since your filament voltage is low, it seems to me that the best way to bring everything up is to change the power transformer primary connections. I'd use blue/yellow instead of blue,and connect brown to black/red, leave white as is. Check things then. If it's too high, connect black and brown back together and tie black/red off. Kicking up the promary should fix your voltage problems. I'd measure the filament and look for 6.3 to 6.5 volts.

... tom
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Re: A couple of questions

Postby mike567 » Sun Oct 04, 2015 6:21 pm

Sometimes 50 cps line will yield a lower voltage on a transformer than our 60cps.
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Re: A couple of questions

Postby GrahamJ » Mon Oct 05, 2015 7:23 am

Gents,
Thanks for the assistance.

Tom,
I decided to try the 230v Primary option first (leaving the black and brown alone and just changing out the blue for the blue and yellow).
So blue/yellow and the existing white for the supply.
Some voltages,
ECC83 anodes-172v. Cathodes-1v
6P14P anodes-310v. Cathodes-10v
All heaters now around 6.8v.
Dammit!
6P14P plate current now about 39.4 mA and dissipation over 98% or 11W
The B+ at J6 for the output transformers is 319v.
If only there was another primary option in between, we would be about spot on.
This one is not giving in without a fight.
If you can direct me towards any other possible options I will give them a go.

Thanks,
Graham
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Re: A couple of questions

Postby mike567 » Mon Oct 05, 2015 6:00 pm

use a filament transformer or a spare winding as a buck or boost transformer.
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Re: A couple of questions

Postby GrahamJ » Tue Oct 06, 2015 10:55 am

Mike,
Thanks for your suggestion, don't know a thing about 'bucking transformers' so I have some internet research to carry out.
In the meantime I am hoping that Shannon will come to my rescue with a solution that is both easier and cheaper to put in place. I thought that as the Hammond 269JX was identified as a 'drop in replacement' I was on safe ground using the 369JX.
Oh well, I'm off to try to check out bucking transformers.
Graham
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Re: A couple of questions

Postby TomMcNally » Wed Oct 07, 2015 7:01 am

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Re: A couple of questions

Postby GrahamJ » Wed Oct 07, 2015 12:51 pm

Hi Tom,
Thanks for the link. I have been looking at another article which has the same drawing.
I'm sure this is doable and probably better than messing with Shannons PCB.
Here's my plan.
Reinstate my original power transformer wiring, so the primaries are wired to 240v. I will then try to pick-up a 240v to either 6.3 or 5v filament transformer and use this for a bucking transformer to get the primary down to around 235v.
I think a rating of 2.5A for the bucking transformer should work???
Finally, do you think that in the meantime it will be OK to use the Budgie sparingly with the 6P14P's running at 7W and the heater voltages down at 5.6v?
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Re: A couple of questions

Postby TomMcNally » Wed Oct 07, 2015 4:38 pm

I'd run it hotter rather than cooler. It's only 8% high ... the tubes won't care. To be honest, I've build about 100 amps and only get the voltmeter near them in the rare case of a problem. They aren't critical.
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Re: A couple of questions

Postby mike567 » Wed Oct 07, 2015 6:26 pm

The primary current on the transformer will be less than one amp. No need for something very large.
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Re: A couple of questions

Postby GrahamJ » Thu Oct 08, 2015 7:04 am

Hi Tom,
Yes that makes good sense and the heaters at that higher voltage are a little closer to specification as well.
I'll get the altered wiring tidied and get the amp back together.
Many thanks,
Graham
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Re: A couple of questions

Postby GrahamJ » Thu Oct 08, 2015 7:24 am

Hi Mike,
Apologies, could you just briefly explain your comment. I'm on a steep learning curve here.
The way I looked at things was to add the current demands which got close to 2A. I then just used the voltage drop/addition required and went with that.
I hope to pick up a reasonably priced older transformer but Hammond have two possible candidates 266M2 2.5v @ 3A or266M5 5v @ 3A. Do you think that I could make do with either less current or voltage?
Just spotted another cheaper Hammond, 266JB6 which provides 3.15v @ 2.4A when paralleled.
Thanks,
Graham
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Re: A couple of questions

Postby mike567 » Thu Oct 08, 2015 8:12 pm

If you are using the recommended power transformer this amp draws 50 t0 60 watts. You only HAVE to match the primary current of your transformer.
In the land of 230 ish volts this is less than .25 amps. You don't need a very large bucking transformer. Feel free to use what you're comfortable with.
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Re: A couple of questions

Postby GrahamJ » Fri Oct 09, 2015 4:55 am

Hi Mike,
Thanks for the clarification, I was only focusing on the secondary load.
I've purchased a new 6.3v/2A filament transformer via E-bay that should do the job. I'll use the layout described in Fig. 1 of Toms link above.
It seems that the tricky issue will be the phasing, so I hope to put it together then test the output voltage and switch the secondary leads, if necessary.
Thanks for your help.
Graham
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Re: A couple of questions

Postby GrahamJ » Thu Oct 15, 2015 11:47 am

Hello again,
I've assembled the parts for the bucking transformer and roughly cobbled them together for testing.
Throughout the tests I get strange output readings, 242vac at the outlet socket regardless of whether or not I switch the brown live wires on the secondary? I have uploaded a picture so that hopefully someone may identify a wiring error or some other problem. I'm using brown for the lives, blue for neutral and green/yellow for earth.
The following link is for the 6.3v 2A filament Transformer that I am using.
http://vvttransformers.co.uk/vtf12887-1 ... asheet.htm
The basic circuit used is as detailed in the link that was provided by Tom on the 7th October.
Thanks,
Graham
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Re: A couple of questions

Postby GrahamJ » Fri Oct 16, 2015 3:16 am

Do you ever wish you hadn't posted a message? I do.
I'm embarassed to report that I had linked both secondary terminals.
Moving quickly on, the link is removed and the outlet voltage is now 136vac which is about correct as the mains is currently 142vac.
I hope my previous exploits have not damaged the filament transformer but I will leave it on test for a while to check.
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