Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: alange5 on October 08, 2017, 10:16:07 am
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I was given a homemade tube amp to play around with. It has octals in the preamp, a pair of 5881's, and a 5Y3. I'm thinking about building a low-power twin - either a 5C8 or a 5D8. The former uses a 5U4. The latter uses two 5Y3's in parallel.
The PT is a Western Electric model 360A. I can't find any documentation or specs. It's massive and heavy - one of the bigger PT's I've seen.. My gut tells me it'll handle a 5U4 just fine, but the 5Y3 currently installed has me second-guessing. Because it's a home-made amp, I have no schematic, and the original owner is unfortunately deceased.
Is there a better test for max current on the 5V winding other than a pass/fail "wait and see how hot the PT gets"?
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Why not use a 5AR4? It has the same filament current requirement as a 5Y3 and will certainly carry a pair of 6L6s, 6550s, etc.
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I would just shove a 5U4 in it. It can only be 1/4W more heat in the PT. But Sluckey's idea is good too.
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Ah, I hadn't thought of a 5AR4. Thanks for the suggestion.
The closest documentation I can find is the model 359A PT, which has matching terminal designation but only shows 2A max current for the 5V winding.
I may audition a 5U4 and see what happens (since I have one on-hand).
I've never experimented with pushing the limits of a PT. Generally speaking, are they built a bit over-spec? If I end up pushing the current limit, what should I look out for? How hot is too hot, and should I expect overheating to happen immediately, or with prolonged use?
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I've never experimented with pushing the limits of a PT. Generally speaking, are they built a bit over-spec? If I end up pushing the current limit, what should I look out for? How hot is too hot, and should I expect overheating to happen immediately, or with prolonged use?
You have an unknown PT. There's no way we can give you a definitive answer to these questions. 5AR4 only needs 1.9A of filament current. Be brave. You're the only one in a position to answer your question.
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How hot is too hot
I used a PT from a '39 zenith that had 12 9pin'rs n 1 6V6. I made a 2 tube pre and did PSE 6V6, lasted almost a year, but after 1 hr of play she was too hot to hold and smelled like a '39tranny in distress.
hope that helps
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5V4GA is another option and has load lines similar to 5U4 (slightly more B+). not a as readily available, but a LOT less NOS than a NOS 5AR4/GZ34, not that you'd want to use NOS 5AR4/GZ34.
consider it if you want/need lower B+ than with the 5AR4.
--pete