Aw man - my brand new output transformer is dead!

H

HamishR

Well-known member
I finally received all the bits I needed to build my JTM45 amp recently. I got a lovely perforated turret board and all the chassis bits from Valvestorm and a set of transformers from Mercury Magnetics. It costs a small fortune to ship transformers to Australia but I have never been disappointed by Mercury - until now.

I finished building the amp this afternoon and went to test things, bias it up and see how it sounds. All the tubes were glowing, the voltages seemed ok - except for plate voltage. One tube was getting everything and the other zero. I disconnected the dead side's primary and get no continuity between it and either the centre tap or the other end. They have continuity. There must be a break or something inside there. It's such a PITA ass to replace an output transformer. One of the mounting bolts is under the board!

Sorry - had to vent. This is unusual and extremely frustrating!
 
HamishR said:
I finally received all the bits I needed to build my JTM45 amp recently. I got a lovely perforated turret board and all the chassis bits from Valvestorm and a set of transformers from Mercury Magnetics. It costs a small fortune to ship transformers to Australia but I have never been disappointed by Mercury - until now.

I finished building the amp this afternoon and went to test things, bias it up and see how it sounds. All the tubes were glowing, the voltages seemed ok - except for plate voltage. One tube was getting everything and the other zero. I disconnected the dead side's primary and get no continuity between it and either the centre tap or the other end. They have continuity. There must be a break or something inside there. It's such a PITA ass to replace an output transformer. One of the mounting bolts is under the board!

Sorry - had to vent. This is unusual and extremely frustrating!
Click to expand...
That Sucks!
Could you replace it with a nut insert so you don't have to remove that one bolt in future builds?
 
That sucks big time. Mercury should own up to this and help you out with a replacement OT I would say. They are a pricey brand with a high reputation, so they should hoour that me thinks.
Years ago I had a blown PT on a Weber amp after some hours of running just fine and they sent me a replacement no worries. Hope that sort of customer service still exists.
 
I haven't heard back from Mercury yet. Yes I would hope they will send a replacement. They're not cheap! I'm really surprised - it's probably just a bad connection inside, but I'm not about to look in case I get blamed for the fault.

I don't think I've ever had a faulty transformer before. It seems that they are having trouble holding onto their workforce because on their homepage they are still advertising for workers.
 
HamishR said:
I haven't heard back from Mercury yet. Yes I would hope they will send a replacement. They're not cheap! I'm really surprised - it's probably just a bad connection inside, but I'm not about to look in case I get blamed for the fault.

I don't think I've ever had a faulty transformer before. It seems that they are having trouble holding onto their workforce because on their homepage they are still advertising for workers.
Click to expand...
That is very odd, they do test each and every transformer they make, that's what the ends are stripped and usually frayed as a result of testing.

edited: it's sunday night here, I'm only about 5 miles from them.
 
Hammond for the win.

here's a funny story.
went to setup my rig for band practice yesterday.
and to my horror - it immediately became apparent that at the last show we played - i had forgotten to change the impedance switch to match my speaker cab (8ohm) from the speaker cab at the practice room (16 ohm)

turns out I had played a whole set (30mins) with the amp output set to 16 ohm, into an 8 ohm 4x12. holy shit. whoops. (not the good kind of mismatch)

somehow the OT didn't appear to care. was running beautifully all afternoon yesterday.
 
Hey Mr Retired - I remember you saying you lived close by. It IS really weird - I mean transformer manufacturers don't have much of a better reputation than MM. I've bought a LOT of transformers from them and would never have expected this to happen. They were well packed and arrived in near perfect condition. There's always a corner which gets bent but these were in excellent condition as I have come to expect.

But it doesn't matter what I do - there is zero continuity from one of the ends to either CT or the other end. The other end has continuity with the CT. So it's not my wiring at fault. Baffling. I'm hoping to hear from them tomorrow (Tuesday) because where I live is 15 hours ahead of LA.

Hey Mr Owl - a 100% mismatch shouldn't do too much harm. Fender amps have had extension speaker sockets on them for years, and if you use it with the correct impedance speaker that results in a 100% impedance mismatch.
 
HamishR said:
Hey Mr Retired - I remember you saying you lived close by. It IS really weird - I mean transformer manufacturers don't have much of a better reputation than MM. I've bought a LOT of transformers from them and would never have expected this to happen. They were well packed and arrived in near perfect condition. There's always a corner which gets bent but these were in excellent condition as I have come to expect.

But it doesn't matter what I do - there is zero continuity from one of the ends to either CT or the other end. The other end has continuity with the CT. So it's not my wiring at fault. Baffling. I'm hoping to hear from them tomorrow (Tuesday) because where I live is 15 hours ahead of LA.

Hey Mr Owl - a 100% mismatch shouldn't do too much harm. Fender amps have had extension speaker sockets on them for years, and if you use it with the correct impedance speaker that results in a 100% impedance mismatch.
Click to expand...
Right, sounds broken! Ends-to-CT resistance should be around half-ish of end to end.
 
HamishR said:
But it doesn't matter what I do - there is zero continuity from one of the ends to either CT or the other end. The other end has continuity with the CT. So it's not my wiring at fault. Baffling. I'm hoping to hear from them tomorrow (Tuesday) because where I live is 15 hours ahead of LA.

Hey Mr Owl - a 100% mismatch shouldn't do too much harm. Fender amps have had extension speaker sockets on them for years, and if you use it with the correct impedance speaker that results in a 100% impedance mismatch.
Click to expand...
i always assumed the rule was you could mismatch from lower impedance output (amp) to higher impedance cab/speaker, but not the other way around.
[ 8 -> 16 not 16 -> 8 ]

that issue with your mercury OT sounds so odd though, what a bummer. i’d be so mad
 
owlexifry said:
i always assumed the rule was you could mismatch from lower impedance output (amp) to higher impedance cab/speaker, but not the other way around.
[ 8 -> 16 not 16 -> 8 ]
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Nope, that's true for solid state but not for tube amps with an output transformer. An open load is the worst use case for tube amps and is to be avoided pretty much at all cost. 16-->8 will typically cost you around 1/3rd of your max output power, increase distortion, and change play feel pretty noticeably, but it's safer and usually better sounding than going the other way, IME.
 
I heard from Mercury Magnetics this morning - they're sending a replacement - yay! This is a huge relief. I'm not even going to think about what is involved in replacing it until it arrives, because that will be a bugger of a job. I've replaced a few power transformers and they're a breeze.
 
HamishR said:
I heard from Mercury Magnetics this morning - they're sending a replacement - yay! This is a huge relief. I'm not even going to think about what is involved in replacing it until it arrives, because that will be a bugger of a job. I've replaced a few power transformers and they're a breeze.
Click to expand...
I use the bottom mount tube sockets so I can access under the board without having to mess with all that soldering, they are the hardest part.....
 
HamishR said:
I heard from Mercury Magnetics this morning - they're sending a replacement - yay! This is a huge relief. I'm not even going to think about what is involved in replacing it until it arrives, because that will be a bugger of a job. I've replaced a few power transformers and they're a breeze.
Click to expand...
Have they shipped it express or Slow Boat from USA?
I ssee that Lucas Miles stocks them for reasonable money but probably not the ones you want?
 
I just received the replacement OT a few days ago. With some cunning work with a flat spanner I managed to replace the OT without too much swearing and no mess. Yay! The amp immediately fired up and my god it sounds good, if I do say so myself! I only deviated from stock in a few places. I made the first bypass cap 25µF rather than the crazy 330µF, left off the bright cap on the bright channel volume pot - there's a big 500pF cap on the mixer which is all it needs. And I added some KT66 appropriate grid stoppers. I like to bias to around 60% because it sounds much like 70% but maybe I'll get some more life out of the rather expensive KT66s.

At playing levels (plugged into the bright channel, vol around 9.00ish) it's a full, warm, sweet sound with some lovely crispness from that cap on the mixers. The normal channel is identical apart from that cap and can sound a little dull in comparison, but I could get close by raising the treble and presence. Bass is huge, so it stays around 9.00. With a Gibson it's clear, crisp and beautifully defined in a way that some smaller amps can struggle with in comparison. Using an OD pedal I seem to need a touch more gain to get where I want to be, but the amp works really well with everything.

It's almost like Vox made a 5F6-A Bassman. It has that British chime. I guess the extra NFB and bigger transformers are working with the KT66s to fatten everything up. With a Gretsch it's near perfect. The mids work with Filter'trons in a glorious way.

Cranked with the Les Paul it's predictably bloody loud and a great old school howl. Too loud for my house unfortunately! I'll go take some pics...
 
HamishR said:
I just received the replacement OT a few days ago. With some cunning work with a flat spanner I managed to replace the OT without too much swearing and no mess. Yay! The amp immediately fired up and my god it sounds good, if I do say so myself! I only deviated from stock in a few places. I made the first bypass cap 25µF rather than the crazy 330µF, left off the bright cap on the bright channel volume pot - there's a big 500pF cap on the mixer which is all it needs. And I added some KT66 appropriate grid stoppers. I like to bias to around 60% because it sounds much like 70% but maybe I'll get some more life out of the rather expensive KT66s.

At playing levels (plugged into the bright channel, vol around 9.00ish) it's a full, warm, sweet sound with some lovely crispness from that cap on the mixers. The normal channel is identical apart from that cap and can sound a little dull in comparison, but I could get close by raising the treble and presence. Bass is huge, so it stays around 9.00. With a Gibson it's clear, crisp and beautifully defined in a way that some smaller amps can struggle with in comparison. Using an OD pedal I seem to need a touch more gain to get where I want to be, but the amp works really well with everything.

It's almost like Vox made a 5F6-A Bassman. It has that British chime. I guess the extra NFB and bigger transformers are working with the KT66s to fatten everything up. With a Gretsch it's near perfect. The mids work with Filter'trons in a glorious way.

Cranked with the Les Paul it's predictably bloody loud and a great old school howl. Too loud for my house unfortunately! I'll go take some pics...
Click to expand...

Yay! Congratulations on the new baby!

Speaking of 5F6-A Bassman, I'm almost done with the board. :cool:

Circuit board done 99pct.jpeg
 
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