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Old 6th Mar 2021, 8:20 pm   #41
Ed_Dinning
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Default Re: Transformer calculations app

Hi Doug, were the lams originally TIG welded, they appear to have grooves in them.
One measurement you may like to make is the no load input current at nominal voltage. Not sure if your spreadsheet will predict this and it will vary on type, grade and thickness of lam used.
The repurposed lams from microwave oven transformers tend to run warmer than normal if in continuous use as they are a thicker lam with higher mag losses.

Ed
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Old 7th Mar 2021, 10:09 am   #42
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Default Re: Transformer calculations app

Quote:
Originally Posted by Ed_Dinning View Post
Hi Doug, were the lams originally TIG welded, they appear to have grooves in them.
One measurement you may like to make is the no load input current at nominal voltage. Not sure if your spreadsheet will predict this and it will vary on type, grade and thickness of lam used.
The repurposed lams from microwave oven transformers tend to run warmer than normal if in continuous use as they are a thicker lam with higher mag losses.

Ed
You're spot on Ed, they were welded so I had to carefully angle grind the weld out. At least I knew the power consumption of the amp it powered, 200W, there was no the VA rating on the transformer.

Also, as a fit for purpose test I should really run the power transformer with its estimated max load on all the windings and measure the voltage drops and temperature rise over time.

Maybe measure the inductance of the primary with secondaries open and shorted.

I have a couple of microwave oven transformers stripped down for lams (angle grinder required here too). I'll guess that these used the lowest cost materials they could. The wire is aluminium, no holes in the lams so no need for bolts. At least they are quite large.

Doug
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Old 20th Mar 2021, 4:35 pm   #43
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Default Re: Transformer calculations app

I've created a similar app this time to help with calculations for rolling your own LowFi output transformer. It's good for simple 3 layer primary-secondary-primary push pull guitar amp type builds. For SE use I've not got my head around calculations for the air gap yet...

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Old 20th Mar 2021, 5:13 pm   #44
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Default Re: Transformer calculations app

I've tested a few builds with the OT calc app (based of info from RDH4 Ch 5).
I used a 10W 100v line transformer from CPC as a donor for the bobbin and laminations for a small 1 watt push pull amp based on £5 worth of EF80 valves, pre amp and PI use them triode strapped.

this one, about £6

https://cpc.farnell.com/eagle/p634t/...10w/dp/LS01701

Here's a sound sample of it in the amp:

https://soundcloud.com/dougie-tamson...-push-pull-amp

Surprising amount of low end comes through for such a small OT, running it into a 2 x 12" 16Ω load, called it my 5 & dime, see photo

I've used this 30W one as a donor for a 2 x EL84 amp

https://cpc.farnell.com/eagle/p635f/...30w/dp/LS01703


Doug.
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Old 20th Mar 2021, 5:54 pm   #45
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Default Re: Transformer calculations app

Here's a photo of the completed OT for a push pull based 2 x 6V6 or 2 x EL84 amp.

Again, the donor was a 100v line transformer, 30W from CPC.
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Old 21st Mar 2021, 10:26 am   #46
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Default Re: Transformer calculations app

Nice work Doug, how does it perform HF wise? Did you run a sweep? Also did you interleave the windings?

Andy.
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Old 21st Mar 2021, 12:22 pm   #47
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Default Re: Transformer calculations app

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Nice work Doug, how does it perform HF wise? Did you run a sweep? Also did you interleave the windings?

Andy.
Hi Andy, the OT is for a guitar amp so just scatter wound the primaries with the secondary sandwiched between. I used stiff card between the pri and sec winds. Quite a tight fit in the end, the plastic bobbins help. Also I re-used the terminals from the donor. I didn't use the wire from the donor.

I think the donor transformer is a bit on the small side to do any useful interleaving unless your target is low power.

I also used some of the wire you kindly sent to wind the primaries

Doug
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Old 21st Mar 2021, 1:40 pm   #48
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Default Re: Transformer calculations app

Ahh, guitar amp, got you. How does it sound with a guitar?

Quote:
I also used some of the wire you kindly sent to wind the primaries
Glad it came in handy.

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Old 21st Mar 2021, 2:06 pm   #49
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Default Re: Transformer calculations app

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Ahh, guitar amp, got you. How does it sound with a guitar?

Quote:
I also used some of the wire you kindly sent to wind the primaries
Glad it came in handy.

Andy.
I put a link to a sound sample in post #44 above, this was recorded using the small EF80 push pull amp I built (all 5 valves are EF80)

I was hoping for output valve distortion, the pre-amp valves are 2 triode strapped EF80s, kept quite clean.
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Old 22nd Mar 2021, 7:40 am   #50
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Default Re: Transformer calculations app

Had a listen, sounds kushti, nice finger work Doug. Try biasing the IP valve differently and increasing the gain by using a bigger anode resistor. You need to over drive the OP stage, so,would think you'd need over 10v RMS or so, though not familiar with the EF80 as an OP stage.

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Old 22nd Mar 2021, 10:59 am   #51
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Default Re: Transformer calculations app

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Originally Posted by Diabolical Artificer View Post
Had a listen, sounds kushti, nice finger work Doug. Try biasing the IP valve differently and increasing the gain by using a bigger anode resistor. You need to over drive the OP stage, so,would think you'd need over 10v RMS or so, though not familiar with the EF80 as an OP stage.

Andy.
The current off-the-shelf offerings from the likes of Marshall et al use an ECC82 as the output stage, 2 triodes in push pull and will put a dent in your wallet of about £220 for the amp head not a combo.

My idea was to use pentodes and keep the costs more like £30 including the donors for the PT and OT (excluding the chassis). Solid wood case/box to be built from scrap palettes
The EF80 was chosen as I had a few old Mullard pulls, still got some life in them their new service duty! I can buy 5 EF80 (or the successor EF184) valves for the same price as a single Chinese ECC83.
Also, using the EF80/EF184 is not a problem for folks who want to keep old radios going, I'm sure there are many thousands sitting doing nothing and worse, millions of them would have ended up in landfill.
I was aiming for an amp under 1 watt so not to break my ears and keep the neighbours (and wife) happy.

I'm planning a design change, the 2nd gain stage, currently both EF80s are triode strapped. There is a 1M pot between the 2 stages as a gain control. If I change it to pentode mode then the gain for stage 2 will be a lot higher and the 1st EF80 will be able to push it into clipping.

Plenty to keep me busy
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Old 31st Mar 2021, 11:12 am   #52
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Default Re: Transformer calculations app

I've uploaded alpha test versions of my simple audio output transformer design app based on chapter 5 of the Radiotron Designers Handbook v4.

This can be used to help roll your own OTs for non HiFi use. I've included some templates, I've successfully built the Fender one from the '50s using the laminations from a '50s power transformer.
For the low watt EF80 based amp, I rewound a common £5 100v line transformer.

Windows/Mac/Linux versions can be downloaded from here:

https://github.com/dougietamson/output-transformer

Doug.
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Old 5th Apr 2021, 6:08 pm   #53
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Default Re: Transformer calculations app

https://www.agw.co.uk/agw-calc-app

Another App that miy be of interest.
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Old 5th Apr 2021, 8:38 pm   #54
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Default Re: Transformer calculations app

Quote:
Originally Posted by Syrinx1 View Post
https://www.agw.co.uk/agw-calc-app

Another App that miy be of interest.
Good spot, I've got the android version working nicely, no luck with the iPhone one.
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