Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 72Blazer on July 26, 2021, 10:50:09 am
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Is there anyone/business that repairs transformers?
I have a 13 year old Mercury Magnetics FBFDR-P PT (was in my deluxe reverb) that smoked.
I doubt it's worth it but thought I'd ask y'all.
VR
J.
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The good news is that power transformers really don't matter. Nobody needs a MM power transformer.
Any reputable brand DR replacement will do just fine and probably cost the equivalent of the hourly rate of anyone who might repair the old one.
It takes real effort to smoke a power transformer. Have you changed your ways? :icon_biggrin:
If not, you could buy 2 for the price of one MM, just to be safe.
290BX Hammond Mfg. Enclosures | Hawk Electronics (hawkusa.com) (https://www.hawkusa.com/manufacturers/hammond-mfg/enclosures/290bx?gclid=EAIaIQobChMI3ZuYkpSB8gIVhd7ICh231gStEAQYCCABEgL59fD_BwE)
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Since you asked, here's what happened. I left my amp at my dear friend/jam band leader's house. I was on vacation in Texas. He's 70 and misread the fuse value and put a 7 amp fuse in which blew again. That's when I got a phone call from him saying it blew a fuse. I told him too not put another fuse in but he had already done that. When I got home and picked up the amp I discovered the blown 7 amp fuse.
I actually started a thread about this amp blowing fuses and Sluckey and some other folks helped me troubleshoot down to the PT.
Sux...
I installed a replacement PT and have been using the amp for a month with no issues.
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Since you asked, here's what happened. I left my amp at my dear friend/jam band leader's house. I was on vacation in Texas. He's 70 and misread the fuse value and put a 7 amp fuse in which blew again. That's when I got a phone call from him saying it blew a fuse. I told him too not put another fuse in but he had already done that. When I got home and picked up the amp I discovered the blown 7 amp fuse.
I actually started a thread about this amp blowing fuses and Sluckey and some other folks helped me troubleshoot down to the PT.
That kind of information would have been nice to share in your original thread!
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Yes I admit I should have mentioned the 7 amp fuse from the git go. I sure wish I knew why the original fuse blew in the first place.
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stick your smoked transformer up on reverb for twenty bucks plus shipping and see if anyone will bite. make sure to mention it's smoked in the listing
other option is to scrap it. i keep buckets for scrap metal and when i have enough i just leave at the curb so that they magically disappear. doesn't even have to be garbage day!
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Hey, maybe a paper weight :icon_biggrin:
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Sung to the Melody of an old Meat Puppets Song: Lake of Fire (version by Nirvana Unplugged)
Where do PTs go when they die
They don't go to heaven where the Angels fly
Stick em round the neck of the ones who made em fry
And make them where it till the 4th of July....
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The good news is that power transformers really don't matter…
I disagree.
Many modern equivalent / replacement PTs have a much lower HT winding resistance than the original. This results in better regulation, ie high voltage output when loaded, and less heat generated by the copper loss.
All that may seem like a good thing, and I’m sure transformer manufacturers think they’re doing their customers a favour, but really they’re not.
Fundamentally, amps are just modulated power supplies, and so the characteristics of their power supplies to varying loading conditions are key to the response perceived by the user.
Amps with saggy HT supplies respond differently to amps with stiff HT supplies - it isn’t just a valve rectifier - silicon rectifier thing, rather the sag (poor regulation) of the HT winding plays a significant part in this.
Also the HT winding resistance (and total effective supply resistance, which also includes the reflected primary resistance) plays a key role in limiting charging current in the reservoir cap. With valve rectifiers that especially significant, as that’s also the peak anode current, a limiting value that can easily be exceeded.
When the valve rectifiers fail due to insufficient series supply resistance, it’s crappy current production valves that get the blame, rather than the incorrectly specified PT.
@HotBluePlates has shown that some modern replacement PTs have a much lower HT winding resistance than the PTs in the original amps that they’re supposed to be equivalent to.
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I'll tell you truthfully (and maybe I just have old ears now), but I've never heard a difference between MM transformers, mojo, and Weber, which I've used before. I've only bought MMs on eBay when someone was selling for a reasonable price-but they are still very expensive.
I'm a novice to building-but when playing-I know a crappy sounding amp when I hear one. At the end of the day, I think there are way too many variables inside the amp affecting performance and tone and FWIW and too me personally, the Weber PTs and OPTs have been great and are less expensive. I've got a twin reverb I built 10+ years ago with Weber's that is still kicking but. It's just to heavy to take out of the man cave these days.
When people talk about sag, I don't really know what they are talking about. I've heard them reference the difference between tube rectification and Diodes or something along those lines. Not a big deal to me @tt, but are there any good sound clips out there that demonstrate sag in general?
Vr
J.
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When people talk about sag, I don't really know what they are talking about. I've heard them reference the difference between tube rectification and Diodes or something along those lines. Not a big deal to me @tt, but are there any good sound clips out there that demonstrate sag in general?
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Sag means the drop in HT voltage, from its level at idle to the amp putting out its max unclipped power, and then further to the max clipped output.
The latter may be 80% of the former.
It very much affects the feel and response of the amp, it’s basically a power compressor.
Unfortunately that sort of thing doesn’t work well on YouTube etc, it’s perhaps to subtle to be readily apparent, given that the sound there goes through compression algorithms anyway, and the feel / response thing doesn’t scale down well to low SPLs.
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The good news is that power transformers really don't matter…
I disagree.
Many modern equivalent / replacement PTs have a much lower HT winding resistance than the original. This results in better regulation, ie high voltage output when loaded, and less heat generated by the copper loss.
All that may seem like a good thing, and I’m sure transformer manufacturers think they’re doing their customers a favour, but really they’re not.
I think there's something to this.
Tube amps with a tube rectifier turned up to the sweet spot will sag/bloom = compression = sustain = singing lead.
If that's what you want, and I do, then **** IF **** new PT's behave differently, then that could be a problem.
If the new PT's just shift the sag/bloom up a little on the volume control that's 1 thing, but if the new PT's change the sag/bloom envelop that's a different ball game.
And I think it's very possible they do change the overall sag/bloom envelop, because of, as pdf64 pointed out, the difference in winding resistance, self regulation, and heat generated by the copper loss.
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> if the new PT's change the sag/bloom envelop
I was just reading an analysis of the Soldano. Isn't the SLO known for sustain? The analysis hints the PT has very little sag. I wonder if we may have been barking up a wrong, or alternative, tree for decades.
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> if the new PT's change the sag/bloom envelop
I was just reading an analysis of the Soldano. Isn't the SLO known for sustain? The analysis hints the PT has very little sag. I wonder if we may have been barking up a wrong, or alternative, tree for decades.
There's different ways to get sustain and they come with a different tone.
Soldano, Mesa Boogie, etc, do it with preamp gain and master volume. Randy Bachman, Guess Who/BTO, did it with the Herzhog. Others stack 2 or 3 distortion pedals in series. But with all of these, then there's a ton of distortion to go with the sustain.
To me the trick is to get sustain without massive amounts of distortion, squaring off the sine wave.