Will these transformers work?

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Stetson
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Will these transformers work?

Post by Stetson »

I have on hand both power and output transformers made by Classictone. I'm currently planning an 18 watt build; I originally bought them for a Vox AC15 build that I have since abandoned, and I'd like to use the iron for the 18 watt build. The power transformer is #40-18079 with a secondary of 600V CT @120mA. The heater winding is 6.3V @4A. Can I wire the rectifier with the heater wiring for rest of the tubes? The 15 watt output transformer is #40-18081 and has a 6K ohm primary. It would be great if I could use these, but I'm sure I could sell them if I can't. Thanks, Ed
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TriodeLuvr
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Re: Will these transformers work?

Post by TriodeLuvr »

I thought the AC15 used an output transformer with an 8K primary. That's the magic number for a pair of EL84 or 6V6. You could use the 6K in a pinch, but I don't know how it might affect the tone.

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Jack
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Stetson
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Re: Will these transformers work?

Post by Stetson »

Thanks. I figured that might be the case with the OT. How about the power transformer? 4 amps should be adequate for the heaters and the EZ81, right?
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Re: Will these transformers work?

Post by Bieworm »

Stetson wrote:
Mon 05/31/21 5:47 am
Thanks. I figured that might be the case with the OT. How about the power transformer? 4 amps should be adequate for the heaters and the EZ81, right?
given the 4 amps on the 6.3V filament winding you should be okay wiring them heaters of preamp, powertubes and EZ81 all together. that's what I read on EL34 about the topic.
https://el34world.com/Forum/index.php?topic=10824.0

that said, I still think it's better to put in a solid state rectifier. Less room for error and tube failure... had a brand new EZ81 cr@p out on me and it took me ages to find the problem. I wasn't expecting that new tube to be bad.. should have known tho, it was a JJ :)
anyway, I do my builds with SS rectification and a 150R /25W wirewound in series to get that sag... sounds the same, cheaper in parts, less wires crossing at that rats nest @ the PTX
rectifier.png
about the OT... I think the original M@tchle$$ Lightning has a 5k primary, and word is out that gives them higher clean headroom ( READ: less nice OD)
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Stetson
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Re: Will these transformers work?

Post by Stetson »

I read that Vox did the same using the 6K- to get a "cleaner" tone. Do you build your own SS rectifier?
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Re: Will these transformers work?

Post by Bieworm »

Stetson wrote:
Mon 05/31/21 6:21 am
I read that Vox did the same using the 6K- to get a "cleaner" tone. Do you build your own SS rectifier?
yes, it's easier than wiring an EZ81.
rectifier JMP layout.png
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Stetson
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Re: Will these transformers work?

Post by Stetson »

Cool!
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Bieworm
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Re: Will these transformers work?

Post by Bieworm »

watch out with the orientation of the UF4007 diodes. And don't overheat them while soldering. You can clip an alligator clip on the leg you're soldering to take the heat of the soldering iron.
If you can't find UF4007 diodes you can use the 1N4007 ones. I can't get UF4007 in my area, so I use UF5408.. they're larger and harder to bend around turrets though..
rectifier JMP layout.png
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Stetson
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Re: Will these transformers work?

Post by Stetson »

I might give this a go.
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geoff 1965
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Re: Will these transformers work?

Post by geoff 1965 »

4 amp is plenty for the filaments,i’ve got the ez81 wired in series with the power & preamp tubes with no problems.you should find your HT in the 18W B+ sweetspot area as well without having to use zeners to drop the voltage.
like Jack mentioned ideally you want 8K primary on the OT
good luck
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Stetson
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Re: Will these transformers work?

Post by Stetson »

Thank you
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TriodeLuvr
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Re: Will these transformers work?

Post by TriodeLuvr »

If you replace the tube rectifier with diodes, don't forget to check all the capacitor ratings to be sure they'll handle the unloaded peak voltage. Peak voltage across the filter caps in your case will be almost +425V (300 X 1.414). It will come down significantly when the tubes heat up and conduct, but the caps everywhere need to be able to withstand the initial turn-on surge. That includes preamp decoupling caps located in strings of dropping resistors. In most cases, a tube rectified amp will have caps ratings based on the voltage drops across those resistors created by the conduction of all the tubes.

Jack
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Stetson
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Re: Will these transformers work?

Post by Stetson »

Good info. Does using the standby switch help with the surge?
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Re: Will these transformers work?

Post by TriodeLuvr »

Stetson wrote:
Wed 06/02/21 12:44 pm
Good info. Does using the standby switch help with the surge?
Only if you remember to use it every single time the amp is powered on. Personally, I wouldn't have an amp that relies on routine operator intervention to prevent damage. This issue is the reason people have devised so many ways to delay the application of B+ until after the tubes heat. Delay relays, timers, separately switched plate transformers, etc. Or, like I said, just be sure to up-rate all the caps to handle the surge. :)

Jack
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