Output transformer fall down go BOOM

[Frank]

Well it looks like my lovely Marshall power amp (9100, the dual monoblock EL34 50 Watt a side version) just decided to cr@p out an output transformer. All the techs I've spoken to have said "oh yes, they do that a lot" which is informative but not entirely helpful so I was wondering if there's a way of making these things a bit more reliable ... I mean, is it even worth replacing the OT with another Marshall unit which will most likely go the same way in a few months or is it worth shelling out the extra for a Mercury Magnetics version?

Any other suggestions for ways to calm it down a bit without utterly wrecking the sound would be appreciated.

[jpfamps]

Whilst Marshall OTs aren't the sturdiest, I wouldn't say they are particular bad in the 9100.

The main issue with replacing the OT is getting one that will fit without having to do some irriversible metalwork to your amp.

Hammond and Mojotone make excellent repro Marshall units that are availble more cheaply that Mercury Magnetics OTs in the UK.

[Frank]

After a lot of digging around, turns out Bluebell audio do the Hammond 1750P for a very reasonable £31. Although it did cross my mind that it may be possible to fit a 100 Watt transformer and connect the secondary as though it's a 100W amp with two tubes removed. If the Hammond's a reliable unit then I'll go with that, I'm certainly up to a bit of good old-fashioned chassis bashing to get it to fit in there.

[jpfamps]

Don't forget the VAT...

Hammond transformers are excellent quality; I've used loads of them with very good results.

When Marshall ran out of transformers last year I installed a 1750P in a JCM900.

From what I remember the phasing of the primary taps is the opposite to the stock transformer, so you may need to reverse the primary leads to the power valves if your amp howls when turned on.

[Frank]

Thanks for that, you win today's gold star for techie advice!

[HTH AMPS]

the 1750P is the JCM900 replacement and looks to be a laydown transformer.  I think the 1750N would be a better match for your 9100 (I've got one of those poweramps too).

alternatively, the Mojotone ones Frank was telling you about are made by Heyboer and the quality of their stuff is really nice.

also, do you actually use both sides of the poweramp? - I find that I just use one side and the other is there as a backup.

you might also consider converting the side with the blown OT to 6L6s/KT66s/6550 etc. etc. just for more tonal variation.

food for thought.

[Frank]

The 9100 uses laydown transformers, are you thinking of the HUGE 9005 power amp there? 9005 is the grey fronted 6 (?) rack unit job with the standy-uppy (1750N) transformers, 9100 is the 3U gold front with laydown ones.

As for the 6L6 thing, yes I am thinking about that, especially as the preamp has a more Fendery clean tone and a bit of variation could be useful ... but I do love the sound of the EL34's.

No wait, not the sound of them, the feeling that every chord is kicking me in the guts.

And yes, for most gigs the second channel is just a backup but really I'm playing with no backup now so it needs fixing.

[HTH AMPS]

The 9100 uses laydown transformers, are you thinking of the HUGE 9005 power amp there? 9005 is the grey fronted 6 (?) rack unit job with the standy-uppy (1750N) transformers, 9100 is the 3U gold front with laydown ones.

As for the 6L6 thing, yes I am thinking about that, especially as the preamp has a more Fendery clean tone and a bit of variation could be useful ... but I do love the sound of the EL34's.

No wait, not the sound of them, the feeling that every chord is kicking me in the guts.

And yes, for most gigs the second channel is just a backup but really I'm playing with no backup now so it needs fixing.

ahhh!!!!  PDT_038

yep, I'm thinking of the 9005 (which I have) - its a 4U monster, but sounds great.

as you use the other side of the power amp as a backup, I'd go for a different set of valves to give you more tonal options.  the way I was thinking of doing the same was to run both sides into a stereo cab, but since you have an EL34 and 6L6 side, you can balance the volumes accordingly.  Of course, it doesn't need to be 6L6s, you could use 6550s, KT66s, KT88s etc...  I'd probably try KT66s if they'll fit in your amp - they're a great tone that have all the best parts of the EL34 and 6L6 tone, imo.

[Frank]

Not sure the KT66 tubes will fit inside the 3U case. These things were originally designed for 5881s then changed with EL34s and I believe some brands of EL34 are too tall to fit inside. I'd really like to do all kinds of mods to it like adding a choke in the power supply but there's just not room in the case to play around much.

Marshall should really do a rackmount mono power amp, 50 Watt 2 Unit with the tubes mounted horizontally. That'd sell like hot cakes, I'd buy one just for the fun of modding it. I might even add that to my list of projects and have a bash at building one.

[HTH AMPS]

I think you'd be hard pushed to fit a 50w OT in a 2U rack case - toroidal transformers would be the way to go imo for saving space.