SWR Workingman's 15 blow fuse

[52 Bill]

So that is an MM made transformer?
Who bought it, you or the former owner?

Juan, pre Fender SWR amps had Mercury transformers as original equipment. I won't argue that they were under sized, but they were made by Mercury for SWR.

[agat38]

A friend come to see me yesterday
him : oh you have a new amp
me : yes
him : fine a SWR 120W/160W
me : how you know that
him : it's just write on the front
me :

So the first version of Workingman's 15 are 120W/160W.


I

[Jazz P Bass]

The 120 watt rating is for the internal 8 ohm speaker connection.

The 160 watt rating is with an external 8 ohm cabinet also attached.

[agat38]

Ok, my power amp power have the same référence ( 170021) then then earlier version ( 160w/200w) so the différence can be the transformer ?

[J M Fahey]

These pictures, were very necessary to confirm:

1) that is the original, OEM made, SWR fit MM transformer , clearly made when MM was just a regular industry transformer supplier, probably won contracts based on cost , supply times, sales conditions, etc. like any regular Industry supplier, manufacturers don't buy "mojo".
They often sell it, to the end user, but that's a different story.

2) there is no space there for a larger transformer .
To be more precise, there is not space for a larger lamination size , clearly shown by the strip of spongy rubber glued to the top, so it does not buzz against cabinet "roof" , so you are stuck with lamination #112 .

And there is no way such a small core can feed a 160W RMS amplifier.

I've never had a Workingman 15 on my bench, but have sold my 100W amps to Workingman 12 users, (which apparently are the same but with a 12"speaker, at least the power amp schematic I have says Workingman 112/115) which could not hold it against a regular drummer , while my 100W ones did, so although not measuring them, I have practical evidence thay they are much more in the "100W" class than in the "160W"one.

I did also replace SWR Bassic Black.
Why do I mention it?
Because it is also a 120/160W model , quite probably same electronics although a better speaker, packaged in a different shape combo

so same general conditions should apply.
And this amp was in a rehearsal room, could not cope with a loud drummer with an extension cabinet added and to boot heated so much that another Tech had added an external 220V fan blowing into it full time.

These repeated examples seem to confirm, at least to me, that these "small" SWR amps were meant for Jazz players and maybe Pros , think wedding band or session players backing a singer, or recording TV commercials, etc.
In that case, an under rated transformer would be acceptable, sort of, because the player would never run it hard.

Definitely not meant for sweaty hairy rockers.

3) If I had it on my bench, I would not replace the transformer with the same one (I already know it doesn't cut it) but would custom wind a larger one, period.

Same #112 lamination because that's what the height allows, but there are available "longer" winding forms/cores/bobbins

which allow much thicker stacking of same size lamination.

You would need to mount it with conventional "legs" and move that terminal strip further away (the one supporting a 10W resistor and a yellow cap) but no big deal.

I checked with my suppliers, #112 is available from 29x29mm hole (what you have) to 29x60mm , twice as much core and magnetically 4x the power (unfortunately same window space as the smaller one so you can't fit thicker wire) :

11229/0 Carretel N° 112 29 x 29
11233/0 Carretel N° 112 29 x 33
11240/0 Carretel N° 112 29 x 40
11246/0 Carretel N° 112 29 x 46
11260/0 Carretel N° 112 29 X 60

In a nutshell: I strongly suggest you check how much extra space you can get by moving away that terminal strip and ask a couple French suppliers what transformer can they custom wind for you, with the only restriction that EI laminations must be American #112 or if only European size available, the one with 30mm center leg width which is only very slightly larger and *should* fit.

Trying to get "more of the same" will only end in frustration.

[agat38]

Thanks lot Juan Manuel, now i need to find real secondary windings transformer voltage for this amp to look for other one.

Phil

[J M Fahey]

Well, it's not set in stone

There is some flexibility in its value and in a way it depends on the power you will use which in turn depends on the space available for a larger or smaller transformer.

You are already making the amp work with +/- 41V rails or so, the classic rail voltage in a ton of 100W class amps by Marshall, Peavey, Crate, Laney, my own: some 70/80W into 8 ohms, 100W into 4, in that case a transformer same size as the original will do, with windings: 30+30VAC - 2 to 2.5A - 120 to 140VA (tops)

Such a transformer will work forever , because that's the honest maximum power you can get from such a core (square 29x29mm)

36+36VAC means +/- 50V rails, a classic for 100/8 and 150/4 and perfectly meets the biasing and setup sheet specs:

You would need a core of same EI size (no space for anything larger but stacked to at least 40 mm, if you can fit 45mm even better).
Not hard to wind and what should have been the core used for the original version.
This is the one I suggest.
Current some 2.5A or around 250VA (and that's stretching it to the limit).

See that even if the amp is capable of more power, the limiter will not let it surpass 100/8 and I think that was the logic behind the SWR design: the amp is rated up to 160W RMS, but is limited to 100W RMS, the net result is having a very loud and clean 100W amp, perfect for Jazz and similar stuff.
Slappers, which love SWR, use high peak power but much less average.

Now enter a Rock Bass player fighting a loud drummer and guitar player (or both) and the amp either does not deliver or overheats.

If you want to meet or surpass the "official" specs, although I think they were more a creation of the Marketing dept than anything else, you might try to use +/- 55V rails, a classic Peavey value for the TNT300 ,Nashville steel guitar amps, etc, some 200/4 and 300/2 (IF the transistors, heat sink and power transformer can take it, not your case), and probably what the modern, Fender built version uses, some 150/8 or 200/4 are possible, but notice they use a much larger chassis and probably better heatsinking, no such thing as a free lunch.

The transformer would have a long and narrow, much harder to wind 29x60 mm core.
Your transformer maker will certainly bark at it, charge a lot and, really, may prove to be too much for your amp.
I mentioned a similar spec SWR Basic 160 which had a kludgy noisy fan bolted outside to survive rehearsal room work.

Almost forgot: all transformers suggested should be rated at 2.5A , even the higher VA ones, because the small window does not let you use thicker wire.

I would go for the middle one (36+36 VAC)

Please post what your winder comments on this, I'd love to hear his comments when you tell him that the tiny original transformes was officially rated for 260VA or better

Je trouverai ses insultes três amusants, en Français s'il vous plaît

[costipato]

Hello from Italy, I would like to thank Juan Manual for following his suggestions, I had my Workingman 12 (1994, old model) main transformer blown and replaced it with this one

TST150W/2X35V Transformer toroidal 160VA 230VAC 35V 35V 2.28A 2.28A TST160/020 | eBay

( TST150W/2X35V Toroidal Transformer 160VA 230VAC 35V 35V 2.28A 2.28A TST160/020 )

Now it works like a charm after replacement, I've followed the bias adjusting guide monitoring the output signal as described but it didn't need any fixing,
though I had to move some parts to leave enough space for this little toroidal (diameter is 96mm) and change tweeter load resistor with an Alcor wirewound resistor.



Thanks! ;D

[J M Fahey]

Thanks
That was certainly a *tight* fit