Help With Transformer colours.

[OdgeUK]

Ok,

This is a Miles Platting 50W head. A friend of mine is replacing the OT with a Dagnall D2507 TXOP 000005.

As you can see (click on picture). The OT on the Miles Platting has FIVE wires.



Yellow and Purple as one pair going to pin 6 and 7 of a power tube?
Blue and Pink as another pair going to pin 3 and 4 of a power tube?
Red going to a circuit strip.


The Dagnall has SEVEN wires, which I think are as follows:

Orange: ground
Yellow: 4ohms
Green: 8ohms
Grey: 16ohms

Red: Primary
White: Primary
Brown: CT

Please could someone tell me, which wires on the Miles Platting are Primaries, CT etc? And where the Dagnall colours should connect to on the amp?

[OdgeUK]

This is the Dagnall 50W replacement

[OdgeUK]

Anyone?

[SDM]

Ok,
Yellow and Purple as one pair going to pin 6 and 7 of a power tube?
Blue and Pink as another pair going to pin 3 and 4 of a power tube?
Red going to a circuit strip.

Actually looks like the power tube sockets are just mirrored there, so the yellow/purple pair go to pins 3 and 4 as well.

Not familiar with this amp at all, but the wires above appear to be your primaries, with red being the CT. You've got two extra primary wires there that your Marshall OT doesn't have. These are the wires to your pin 4's, and thus I would have to assume the original OT has ultralinear taps, which is what these extra wires are. You'll note there are no screen grid resistors on the tube sockets, as the ultralinear taps take care of the screen supplies.

So to use the Marshall OT which does not have ultralinear taps, looks you'll need to add screen grid resistors and tap the B+ line at the appropriate point to supply them.

I have no idea on the secondary color codes or wiring for the original OT, I can't tell from the pic if the secondary wires go to an impedance switch of some sort, or are just jumpered together on a circuit strip there to create only one available output impedance. If the latter, and the amp only provides one output impedance setting, you can forgo the circuit strip the old wires connect to, and just select the output tap you need from the known replacement OT. The orange (ground) wire would go to the speaker jacks sleeve lugs (nearest chassis) as it looks like a brown(?) wire now connects to. The desired impedance tap wire would just go to the tip lugs on the speaker jacks as the blue wire connects now.

Hope that helps a bit at least, and someone familiar with the amp will chime in.

[OdgeUK]

Cheers Steve!

The schematic for the amp is here:

http://web.telia.com/~u27805779/images/ ... f6-V50.jpg

Not sure if this helps identify the Secondaries? Does look like they go to an Impedance control on the back.

Better picture of an identical amp here:

http://web.telia.com/~u27805779/images/ ... %20009.jpg


I was kinda hoping it would just be a case of working out which wire was the equivalent on the Marshall OT. Looks a bit trickier!!

[SDM]

Yeah the red short wire is your CT, the wires to the pin 3's and 4's are the primaries. The original OT does indeed have ultralinear taps, so you will need to do some tweaking. For starters I edited the schem to show a reasonable and simplest way to do it here. Green circled bits are changed/added things. This schem eliminates some sag due to moving the PT center tap to before the 200 ohm resistor (you can always add another resistor back in), and this will make the amp react more like a Marshall.

So the red and white primary wires of your new PT will go to pin 3's of the power tubes now. Which to which I don't have enough info to tell you. If you get them reversed the amp will literally howl at you, so 50% chance of getting that right. If you get the howl, you just swap the two pin 3 wires around and all will be well. The brown CT wire of the new OT will not connect at the same point on the terminal strip as before (the old red wire), but rather to the 200 ohm resistor/filter cap terminal. You'll connect that CT wire on the end of the 200ohm resistor that has the wire running straight to the fuse to be more specific there.

Then you'll need to add the screen grid resistors, can do that Marshall style. When you pull the old OT out pin 4 and pin 6 will be empty on each power tube socket. Attach a 1K 5 watt resistor between these pins on each socket. Then run a wire between both pin 6's. From the most convenient pin 6, also run a wire to where the old center tap used to be on the terminal strip (was short red wire). That'll take care of the screen supply.

The bias circuit may need some tweaking with the 1K 5watt screen resistors used now, but I haven't looked at that yet.

So that should cover you on the primaries, the secondaries you'll need to figure out. Easiest way may be just to pull the old OT and measure the switch to see what lug connects to the lug that has a blue wire on it running to the tip lugs of the speaker jacks, note that and then hook up your known color code taps accordingly. The orange wire (ground) will indeed go to the sleeves of the output jacks as the brown wire does now. The blue wire on the impedance selector that runs back to the main terminal strip is your NFB wire, which according to the schem is the 16 ohm tap. No need to move that blue wire and the wire (looks white?) from the old OT to that same lug would then be the 16 ohm tap. So a clue there and thus where you would attach the grey secondary wire (16 ohm) of the new OT to. So that just means you gotta figure out where the yellow and green wires go of the new OT, which again can be determined from measuring the switch once the old OT is removed.

Also, just to note the schem may be different than your amp, so the NFB may originate from a different tap than the 16 ohm., but again measuring the switch should tell you what needs to connect where.

Hope that makes any sense!

[OdgeUK]

Steve - above and beyond in your help. Thank you very much for taking the time on this. Appreciate it.

I'll give this a good bash. I'm certain to have more questions as I go though!

Thanks again

[SDM]

No problem OdgeUK, actually nice to see something different once and a while. Just to make the above description a tad easier, I very quickly edited your pic for the new OT and wiring. It's rough (excuse the poorly drawn 1K 5watt screen grid resistors especially ) but hopefully will help you along.

Do keep in mind the red and white wires to the power tube pin 3's may need to be swapped around, so leave them a tad long just in case. Also keep in mind the secondary connections as drawn (except the orange wire, and probably the grey) are not certain so make sure to check what tap should go where by measuring the switch contacts before connecting the new taps and after the old OT is removed.
Good luck!