Let's Talk about step down transformers

[Randall]

The stand alone ones that plug between the wall and an amp, designed to lower the line voltage to either bring a vintage amp back to it's expected voltages, or to reign in an amp that has B+ that modern tubes can't handle. I have a Marshall re-issue 45/100 in my shop that I have posted about for other reasons in other threads. Today I measured 124VAC at my bench, this puts the B+ at 545V, and this amp is eating tubes. I can find nothing else wrong with it. It also has 6.85V on the filaments. Conventional wisdom says modern KT66s can't really live long at that high of a B+.

Today I put the amp on a variac and found that a line voltage of 108v put the B+ at 495v, a value I can better live with. I hear of, but have not ever seen an auto transformer one can buy to insert between wall and amp to solve this problem. I'd like to find one, but thus far have not had success.

Does anyone have any experience with this? I think it would not be a bad idea for this customer to also use it for his dead stock 68 plexi 100 watt head as well.

[Enzo]

545v with all tubes installed and reasonably biased? Wow.


If you can find an old RCA VIZ WP-26A isolation unit, it might be just what you need.

[nosaj]

The RCA VIZ WP-26A just allows you step up or down voltage in increments a variac with a voltmeter you could dial in a reasonable voltage.
I'd be using a variac on your bench all the time if your voltage swings high alot and checking it periodically.

Check with your local hams for a variac or RCA VIZ WP-26A(we use ours to drop the voltage to the christmas tree lights on the tree.)

The filament voltage is that with tubes in or out? What is the input voltage specced at for the re-issue?


nosaj

[Randall]

I don't want a variac for this amp solution. I want a set fixed step down value transformer. I don't want the customer to be able to adjust it, not knowing. The filament V is with all tubes in. This thing just runs high.

[Chuck H]

Using your filament voltage of 6.85V with the 124V mains voltage I think I've successfully surmised that Marshall, for some stupid reason, straight up used a 115V primary for this model. Because at 115V the filament would be 6.3V. If this is true Marshall expected high voltages with modern mains voltages. This seems to be a trend lately with reissue transformers too. Rather than adjust for modern mains voltage they are often made with the vintage primary spec. That said...

If the amp were run with 115V you could expect about 500V (actually 506V by the math, but once you adjust for bias...). But I'll reiterate that Marshall must have expected the higher voltage. It's not as if the engineers at Marshall don't know about US voltages!?!

Maybe check out this geofex article by our own R.G. It could be used to knock 12V off the wall voltage. So, 112V when the wall is at 124V and 108V when the wall is at 120V. That should get you in range and the parts are manageable and cheap.

http://www.geofex.com/article_folder...t/vintvolt.htm

[Tom Phillips]

I don't find the link in Chuck's post.
Look at New Page 1 and search for "Vintage voltage adapter"
You can also search for "Bucking transformer" on the web.
These approaches will give you the same result as a step down transformer.
It is an inexpensive and small size solution and it can be made in an outboard adapter or often permanently installed inside the amp chassis.
Cheers,
Tom

[Chuck H]

I don't find the link in Chuck's post.
Look at New Page 1 and search for "Vintage voltage adapter"
You can also search for "Bucking transformer" on the web.
These approaches will give you the same result as a step down transformer.
It is an inexpensive and small size solution and it can be made in an outboard adapter or often permanently installed inside the amp chassis.
Cheers,
Tom

Ah.!?. Thanks Tom. My poot has been acting strange tonight. I just edited the post and re added the link.

[Enzo]

Nosaj, if you look at the Viz iso I linked, you will see it is switchable in relatively small steps through the range of useful mains voltages, so you can correct for a few volts difference without a lot of effort. Easy enough too keep people from changing the setting, remove the knob.

[Leo_Gnardo]

I was gonna suggest . . . but Chuck's got the solution in post 5 link. Wired this way the transformer acts as a bucking autoformer. For your case I'd advise a 12.6V transformer with a bit more current capability than the amp calls for, that way it should withstand the typical switch-on surge without damage, and the amp's line fuse will still serve as a fuse, not the windings of your bucking autoformer. And 12V tran's are easy to find. The transformer should be small enough to fit somewhere on the amp chassis, hard wired so nobody can fiddle with it.

[J M Fahey]

It happens all the time in Brazil, for 2 reasons:

1) they used real 110V (not 113/115/117 as US later did) so a lot of not that old old tube amps (the Giannini ones which were literal photocopies of Blackface Fenders and actually sound very good) were made for that.\
besides, they copied the "good" Fenders, powered by the "good" 6L6 (Sylvania and such) so +V voltages were already high to begin with.

2) now they are switching to 127V lines (long story) and old amps are blowing allm over the place.

So much so that the required 127>110V autotransformer is a common, over the counter device.

Now they added a 100V tap, because Brazil has the largest Japanese colony in the World (around 1.7 Million) and it's customary for Nisei kids to go work in Japan for 1 year after High School.

They earn salaries in hard currency and typically buy " aaaaaaa Maaaaaarshaaaaallllll !!!!!!!!!! "

Only problem, it's the Japan version, and primary is *only* 100VAC .

Then they proceed to blow it back home, by plugging in now 127V mains.

[jbltwin1]

Here's the pic from the RG site.

I built one for my bassman. Works just fine. Dropped it something like 12-15 volts. don't remember as I haven't used it for a while.

[Randall]

Hey, that's pretty slick! I don't exactly understand how it works, but I just tried it with a 12-0-12v I had lying around, and by using the CT and one secondary I lowered my 124v down to 110.5v. I'll build one in a box so this customer can use it with his other vintage amps.

Another question, looking around mouser I see transformers are rated by VA, not amps. How to figure what rating I need? I went to a conversion site, but I got 756VA, and some how that doesn't seem right. This amp has a 6.3A 250v mains fuse. Can someone enlighten me? I'm looking at 120v/12v trannys.

[mhuss]

VA is basically 'virtual watts' - so just multiply the secondary rated current times rated voltage, e.g., 12v @ 2A would be 24VA. As alluded to above, I'd suggest a 6A x 12v unit if you're amp fuse is 3A just so it never gets stressed.

[J M Fahey]

Easy, VA means Volts multiplied by Amperes, in your case 12Volts * 6.3Amperes=some 75VA .
Pick the next higher, from 80VA up.

[Leo_Gnardo]

I just tried it with a 12-0-12v I had lying around, and by using the CT and one secondary I lowered my 124v down to 110.5v.

Ah it's nice when things work out. The voltage reduction may not be so much when you start pulling current through it, as transformer secondaries read more than the rated voltage when they're passing no current. But no worries, you're very much on the right track. Also wonderful to see you're getting some good work in your new location.