Let's Talk about step down transformers

[Randall]

Nothing that big at Mouser that I could find. Would this work?
Hammond Manufacturing - 185E24 - Dual Sec:Ser 24VCT,Par 12V Pri:115/230VAC Chassis 80VA Transformer, Power - Allied Electronics

And can I just use one of the 12v taps and still maintain the 80VA rating?

[The Dude]

I found this at Mouser. Would this not work?

FD8-12 Triad Magnetics | Mouser

[Randall]

Perfect!! Thanks Dude! I don't know why I couldn't see that from my end. I appreciate the help everyone. Now..... what value fuse should I use? The amp has a 6.3A 250v.

[The Dude]

I see no reason to change the original fuse value.

[potatofarmer]

Um... KT66s are rated for 400V on the screens. Does this model have screen stoppers?

Seems to me they replicated an amp designed to eat tubes and it's functioning properly.

Me, I'd increase the screen stoppers and/or add some resistance in series with the choke, (in addition to the bucking transformer) but it might make a noticeable difference to the tone.

[Chuck H]

Um... KT66s are rated for 400V on the screens. Does this model have screen stoppers?

Seems to me they replicated an amp designed to eat tubes and it's functioning properly.

Me, I'd increase the screen stoppers and/or add some resistance in series with the choke, (in addition to the bucking transformer) but it might make a noticeable difference to the tone.

Agree. I've never been opposed to altering a flawed design. Even when it's from a major manufacturer. I'm often surprised at how a clearly faulty design gets into the publics hands this way. I know it's a reissue. I know authenticity is one of the goals. It's still bad form. If Marshall is using kt66's that will survive in this model, how do they know that tube will even be available in the near future?!? That "s" changes monthly these days! So this is the opinion of two potato's now.

[Leo_Gnardo]

Um... KT66s are rated for 400V on the screens. Does this model have screen stoppers?

Seems to me they replicated an amp designed to eat tubes and it's functioning properly.

Me, I'd increase the screen stoppers and/or add some resistance in series with the choke, (in addition to the bucking transformer) but it might make a noticeable difference to the tone.

Still that KT66 amp? 400V Vg2 max is one reason I keep mentioning KT88 instead. KT66 are wonderful tubes whether NOS, China Russia or Slovak made, but a bit of a hot house flower, won't take to high voltages very well. As potatofarmer says [this is] "an amp designed to eat tubes." By chomping up their screens like a hungry Everglades gator. I've found KT88 to be a good deal more robust by comparison.

Just for curiosity what amp is it 'zackly? So we have some advance warning should one come this-a-way.

[Randall]

1. I am not proposing changing the fuse in the amp, rather what value fuse should I install in the transformer box?

2. It has 1k 5W screen stoppers.

3. This is the 40th Anniversary JTM 45/100 Dual Output Super Lead Head. 250 made I think. Customer paid $2,500 used. Keeping it stock is a priority. I ordered a set of Ruby's burned in for 72 hrs, and matched at 500v, they have the 6 mo warranty. So we shall see.

[Leo_Gnardo]

This is the 40th Anniversary JTM 45/100 Dual Output Super Lead Head. 250 made I think. Customer paid $2,500 used. Keeping it stock is a priority. I ordered a set of Ruby's burned in for 72 hrs, and matched at 500v, they have the 6 mo warranty. So we shall see.

Then it's wise to go the bucker transformer route. If I'm not mistook it's the secondary of that transformer that does the heavy lifting in the line adjustment dance. IMHO 6.3A fuse in the amp is 50% oversize, after all 4A Slo blo is the norm in 100W Marshalls.

But the crustomer MUST have his amp absolutely stock, unaltered in any way! So 6.3A it is in the amp. Should keep him satisfied and likely will never blow that fuse no matter what happens. So... put the real safety fuse in your bucker transformer box. 4 amp slo blo just like every other 100W Marshall, gets my vote.

If it hasn't been mentioned before, it's sorta funny modern day Marshall made the most of a production glitch they encountered when they were a wee little company just starting out, mid 60's either they couldn't get 100W OT's or didn't want to pay the price for them back when it was early days. Maybe they had a demanding customer who just had to have a 100W head, but they didn't see much of a future in it imagine that! So Ken slung together a 100W amp with two 50W OT's. Also loaded the amp with GEC/Genelex KT66's, the original manufacturer, there were no others then. Last I saw the original items were $150 each and that was 15 years ago. If you could find 'em.

[Randall]

I got an 8 amp 12v transformer, and will mount it in a re-purposed piece of dead test equipment. Should be cool. Anyone care to suggest what fuse I should use? The amp has a 6.3amp, but I am going to suggest my customer use it on all of his vintage amps.

[g1]

Any amp will have it's own fuse, so you just want to protect the 12V xfrmr itself. I'd go with 7A just to err on the safe side. That should make it usable for all except real high power stuff.

[Randall]

Thanks guys, I was thinking 7 amp as well. I always appreciate how much help is offered here, such a great place. I'll post pics of my "minus 12" box when it's completed. It's going to be cool! I can't drop names, but my customer has been on over 20 million records, and has a gold and a platinum record, so it's important that I get this sorted out.

[Randall]

OK then! I built a 12v bucking transformer per RG's design, and housed it in a re-purposed vintage piece of automotive test gear. I used an 8 amp transformer, fused it at 7 amps, put in a 120v amber lamp, and cut up an extension cord for the power in and out connections. It puts out about 108- 110vac depending on what the wall voltage is. Maybe that's a slight bit on the low side, but it brings the amp voltages down to where I am a lot more comfortable. My plates are at about 455v now when biased at 70%. And it looks cool! Here it is

[R.G.]

OK then! I built a 12v bucking transformer per RG's design, and housed it in a re-purposed vintage piece of automotive test gear. I used an 8 amp transformer, fused it at 7 amps, put in a 120v amber lamp, and cut up an extension cord for the power in and out connections. It puts out about 108- 110vac depending on what the wall voltage is. Maybe that's a slight bit on the low side, but it brings the amp voltages down to where I am a lot more comfortable. My plates are at about 455v now when biased at 70%. And it looks cool! Here it is

Nice work!!