Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: GarlandKelley on December 06, 2023, 09:51:48 am
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Hey everyone,
Noob amp builder here. Apologies if this has been addressed elsewhere. I am getting ready to build a Hoffman 18 Watt Stout and was planning to use the stock power and output transformers from my Deluxe Reverb Reissue that I had replaced. Will these transformers work for this project? Also, do you know how I would map the transformers? Thank you all for your help.
https://el34world.com/Hoffman/files/Hoffman_Stout.pdf
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The DRRI output transformer will work but the power transformer puts out way too much voltage for EL84 tubes.
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Thanks so much! That’s exactly the information I was hoping to get.
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Another question, does anyone have recommendations for resources where I could learn to test and map power transformers? I have a few from old radios I have salvaged and don’t have their specs. It would be great to be able to test whether they are functional, what the specs are, and sort the cabling.
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Let's use that DRRI as an example. Put the PT on the bench. There are 10 wires. We're gonna measure resistance between wires. Pick any wire and connect one meter lead to that wire. Now check resistance between that wire and ALL the remaining wires. Make notes about any wires that measure any resistance. Twist all wires together that show resistance readings. This will be a winding.
Now pick another free wire and connect one meter lead to that wire. Now check resistance between that wire and ALL the remaining free wires. Make notes about any wires that measure any resistance. Twist all wires together that show resistance readings. This will be another winding.
Repeat this procedure until you have all wires grouped. There should be no single free wires. For the DRRI PT you should end up with 2 black wires in one group, 2 yellow wires in another group, two green wires in a third group, and 2 reds, one red/yellow, and one red/blue in a fourth group.
Black (or black and white) is typically primary winding. Yellow is typically 5VAC filament winding. Green is typically 6.3VAC filament winding. And reds are typically high voltage winding with red yellow being a center tap and red/blue being a bias tap.
You can confirm that you got it mapped out correctly by looking at the schematic available in Hoffman's schematic library. (Click link below this message.)
Apply what you learned in this exercise to your other unknown transformers. I could probably write a 10 page essay on this topic, much of which comes from training and real world experience. But you have enough info to get started. And you can always ask questions about a specific transformer, posting pics of course.
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Wow, I really appreciate you taking the time to provide this info. I’m fascinated by amp building and value the guidance from more experienced builders like yourself. 🤘
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I have a 5Y4GT that I pulled from an old radio and have read that you can modify the socket pin soldering to have it function as a 5Y3. Do you happen to know if this is possible when building the Hoffman Stout? I love using salvaged parts when possible.
https://el34world.com/Hoffman/files/Hoffman_Stout.pdf
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Jumper pins 2 and 7.
Jumper pins 3 and 4.
Jumper pins 5 and 6.
Plug in either tube.
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I have a 5Y4GT ......function as a 5Y3.
Look up the tube data. Franks. https://frank.pocnet.net/ Bookmark it. Pass it on.
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PRR (or anyone else), do you know why the pin diagram for the 5Y4 is so different from the 5Y3?
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Nope.
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There is a data-page showing both 5Y3 and 5Y4, same data except height and base.
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Wild speculations (some of it may be right-ish).
https://rec.antiques.radio-phono.narkive.com/FSswYGoT/the-official-sylvania-explanation-for-the-5y4g
https://antiqueradios.com/forums/viewtopic.php?f=19&t=238557
The idea of Philco wanting a sole-source position, or RCA sucks, resonates through tube lore.
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Thank you all for the help and advice! Would I need to modify anything in order to use the DRRI output transformer in the Hoffman build? I’m going to attempt to use the 5Y4G in the build using the tube socket pin jumper wiring suggested above.
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Also, does anyone have a relatively straightforward explanation or suggested resources to learn how to calculate the wattage of a power transformer?
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Here is a wiring diagram for wiring your rectifier sockets to accept different rectifier tubes.