Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Keyboy on September 16, 2023, 12:14:24 pm
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Hello,
I am planning a Fender Tweed Bassman 5F6-A build and I have an old power transformer that might work, but wanted your opinion. The power trans only has a few markings on it, "a large (twice the size as the numbers) "V" and a smaller "V" plus the numbers "4653" and "366728". The "orphan" power trans has a 353-0-353 secondary with 6.3v (green/green) and 5.0v (yellow/yellow). I have attached some specs which compare this unknown trans with drop-in TBassman replacement trans from Hammond. This could be a long shot, but your thoughts would be great.
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Do you know what the tube compliment was from the amp that the transformer was pulled from?
Dave
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It's quite a job to wade through all the specs and compare them, but it is a great education too. Been there, done that, many a time. But still don't know much :think1:
As Dave says, if you knew what your orphan came from, all you have to do is be sure you don't exceed what the old amp had in terms of tubes.
But also, I'm gonna go out on a limb here and try to sound technical. A higher DC resistance probably indicates a higher number of winding turns on the bobbin (or whatever else the coils happen to be wound on). I'm also guessing that if a secondary tap reads, for example, 355VCT, but it has a higher DC resistance than a different PT that also reads 355VCT, then that might indicate that the orphan has the capacity for a greater current draw. Meaning you could use a greater number of tubes of one sort or another.
If I'm wrong, I hope one of the many bigger brains here will point it out, but that's one possible conclusion I draw from a winding with a higher DC res but with the same output voltage.
In any case, I suspect your tranny's current draw is speced (specked?) somewhat higher than the comparison trannies you list. As is often the case, more is probably better. Unless I'm wrong :think1:
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Hard to say. The higher DCR could mean the wire is smaller and has a lower current capacity. Comparing dimensions may be more meaningful than comparing voltage.
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Hard to say. The higher DCR could mean the wire is smaller and has a lower current capacity. Comparing dimensions may be more meaningful than comparing voltage.
Ah. Thanks for that Steve.
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What, no weights?
Higher DCR at similar voltage is probably lower power.
The 117VAC winding tends to suggest overall power capacity at whatever secondary voltages.
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I really appreciate the feedback - thanks a ton.
I felt like using the old power transformer would be a stretch, but wanted your opinion. I understand there is a good bit involved in engineering the transformers, and I just don't know enough about it to share with you all.
I do have many parts for the build, but not the expensive ones yet (transformers, speakers) so, I might as well keep it simple and procure the good stuff.
Thanks again
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I don't think anyone is suggesting that it won't work. It's just kind of a who-knows sort of thing.
Do you have a method by which to compare the size and weight of the transformer to one that you know will work. IE a Super Reverb or a Bassman or other transformer that runs a 5AR4, a pair of 6L6's and a handful of preamp tubes?
Dave
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So you've plugged the primary into the wall, and those are your voltages without load?
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I really appreciate the feedback - thanks a ton.
I felt like using the old power transformer would be a stretch, but wanted your opinion. I understand there is a good bit involved in engineering the transformers, and I just don't know enough about it to share with you all.
I do have many parts for the build, but not the expensive ones yet (transformers, speakers) so, I might as well keep it simple and procure the good stuff.
Thanks again
From what you've shared, your PT sounds fine. Here's what ya hafta do when learning: build it with what you've got. Once built and running, if the PT gets so hot you could fry and egg on it, change it out for a bigger PT. If you can hold your hand on it firmly for 20 seconds or more when it's been fired up for 20 minutes, it'll be fine.
If you simply start spending money in a sort of guessing game, in my snooty opinion, you're missing an educational experience. Again, in my snooty opinion, don't expect your first builds to be perfect. Instead, see them as opportunities to learn and grow. If you just buy what everyone tells you to buy, what do you learn? It's when things don't go perfectly that we learn. My first dozen builds were crap. Yes, it frustrated the hell outta me, but my latest build give me a sense of achievement because I know why they're good.
Sorry of that seems opinionated. But how else are you going to know what that PT can deliver? Some of my best builds resulted from some crappy old PT that I thought was past its prime but turned out much better than expected. I even sold some of those amps. I'll shut up now....
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Thanks Shaun,
You have changed my mind. Honestly, I just put the old transformer in the trash an hour ago - then I read your post. I'll try it. If it works I'll save $175-$260, and if it doesn't - like you said - I'll learn something. I better go get it back :laugh:.
Thanks again, and I'll let you know how it goes, Glenn
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Hi AINewman,
Good question earlier, the voltages I mentioned for the orphan trans were read when plugged in and not under a load. I have literally no other info about the transformer - I made a quick attachment with the dimensions/appearance of the trans if that's helpful.
Thanks AINewman
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Weight is the most important parameter. Even approximate: like one apple? like 3 liters of milk? like a small dog?
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thx PRR,
The power amp in question is 5lbs and 14oz. (checked on two scales).
Any thoughts?
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Sorry, meant power transformer!
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5lbs and 14oz.
Bassman PTs seem to run over 7-1/2 pounds. So this seems a bit light.
https://www.mojotone.com/amp-shop/electronics/transformers/power-transformers?page=1