Marshall DSL401 power transformer

[DavidC]

Hello!

First of all, great forum.

I've got a problem and maybe someone here can help me.

My DSL401 blew up (fried) the power transformer, and I dont want to buy the same from Marshall since I think it's an accident waiting to happen.
The mercury magnetics is way out of my price range, and I'd have to pay extra shipping costs and custom duties since I'm in Europe, Croatia.
Thankfully I have a local transformer company which can make a transformer to my specifications but the problm is that I dont have the full transformer specs.
I've contacted Marshall and they say that they only have the voltage tap ratings (310, 23.6, 6.5) but not the VA rating.

I suppose it's something like the following but I'm not sure:
310 - 0.25A
23.6 (switching, reverb, bias) - 1A
6.5 - 6A


What are you suggesting me to do?

[Phil_S]

This amp has 4x EL84? No preamp tubes?

To be on the safe side, for the high voltage winding, I'd allow 90mA per pair or 180mA, plus something to run the rest of the amp. I'm not sure, but I think you'd want a minimum of 200mA, and more is better. They are telling you 250mA, so I don't think the rating of the secondary is the problem. I am guessing it is the quality of the build.

To get the primary VA rating, you already have the information to back into this. If you give those specs to the winder, he will know what to do. Basically, you add them up (rounded):
250mA *310V = 78
23.6 *1 = 24
6.5 * 6 = 39
78+24+39 = ~141VA. I think, if you are having a custom wind, 150VA as a primary rating should be more than adequate.

[Structo]

4 x 12ax7

4 x EL84

[DavidC]

It has preamp tubes. The problem is that I dont know the ampere rating, the states above are just guessings. :-/

http://www.drtube.com/schematics/marsha ... 2-iss9.pdf

[DavidC]

I'm soooo dumb. I can easily take the fuses ratings after the PT as an applicable information.

310V - 300mA fuse
23.6 - 250mA fuse
6.5V - 6.3A fuse

For the transformer increasing the ratings a bit would do the job, right?

310V - 350mA
23.6 - 300mA
6.5 - 7A
=
161VA


Another question, what are the advantages of copper foiling?

[Phil_S]

4 12AX7's probably use no more than 3-5mA each. Let's figure 30mA for the lot and that's plenty. I think 160VA is plenty and will allow for some unknown loss between the primary and secondary.

As you increase the VA rating, you are going to increase cost. It requires more turns of wire on both primary and secondary, and more laminations. The wire is not so expensive, but the lams might cost a bit. Also, the transformer gets bigger and heavier. You want this to fit where the old one was.

The copper flux band? It is a shield. Here's something I found...
http://newsgroups.derkeiler.com/Archive ... 00495.html

If you are going for a custom wound transformer, bring in the old one. The winder should be able to improve on what it was and with the secondary specs stated, he should have no trouble giving you what you need. He can probably re-use the lams, maybe saving you some money? I'm not sure because there will be time required to take apart the old transformer. You should ask.

[DavidC]

The winder is a local company and they use only their cores and price them with a standard price. :-S I mean, I asked already for rewinding the old one, they don't wanna do it. They want to make a bigger profit.

I have another question though. Will the 6.3V from the transformer be 6.3V in the tube under load? Is it safer to use something like 6.6-6.7?

[DavidC]

SOLVED

Mercury Magnetics gave me the specs for their DSL401 PT:

0-335V - 200mA

6.7V - 6A

27V - 1.5A


They are a bit strange though... 335 at a borderline 0.2A (marshall says 310V), 27V at 1.5A through a 0.25A fuse!

What do you think about it all, guys?

[Phil_S]

If that is what MM says, those are voltage under full load and they should work well. Just give it to the local winder and he should be able to make you what you want. BTW I guess MM got to the same 200mA on the HT winding the same way I did. It's just simple math.

I am not surprised they are not interested in rewinding. It is a great deal of work to take the old one apart. In the time it takes someone to take it apart -- work that must be done by hand -- they can probably wind more than one new one.

[DavidC]

BTW I guess MM got to the same 200mA on the HT winding the same way I did. It's just simple math.

How did you did that?


Also... I have a 3.8H 100ohm choke laying around I and was thinking to put it in the dsl401, but the replacing resistor is 470ohms. So the difference would be 370ohms, I figured thats around 37V, which is not little imho since the el84s are running already on more than 420V at around 26mA (40W+)

Since now I'm in a position of changing the transformer specs, do you suggest me to lower the HT voltage tap by 30 volts?

[Phil_S]

I don't have the technical knowledge to comment on making a change in the PT and the choke. I think it will change how the amp sounds, but can't say how.

For the VA rating, this is rather simple. You will need a tube reference to look up how many mA each tube requires. You can download the TDSL from Duncan: http://www.duncanamps.com/tdslpe/

You'll see that a pair of EL84 needs 72-92mA. A 12AX7 needs ~3-4mA.
(2*92mA) + (4*4mA) = 184+16 = 200mA. That's your 200mA for the high voltage secondary. Add a little something if you want a cushion, but it should be fine at 200mA.

Assuming about 300V @200mA = 60W. Adjust as appropriate, but 300V for EL84's is probably upper end of the relevant range.

Next, add what's needed to support the filament winding. EL84's need .76A each and 12AX7's need .3A each. (4*.76) + 4*.3) = 3.04 + 1.2 = 4.24A. 4.24A * 6.3V = ~27W. If you also have a 5V winding or a second 6.3V winding, include that as well.

27W +60W = 97W. Let's round to 100W for discussion. Allow for some power to be lost as heat. Amps are not all that efficient. I'm not an engineer, so I am unsure of the proper de-rating factor here. I was thinking a push-pull amp is about 75% efficient and you need ~150VA on the primary. A single ended amp is only about 25% efficient, so, if you are doing this for SE, de-rate appropriately.

If you take the MM data and add it up:
355V *200mA = 67
6.7V * 6A = 40
27 * 1.5A = 41
67+40+41 = 148VA. I believe the 6.7V winding is generously sized, which allows for less of a de-rating factor. 150VA seems perfectly reasonable.

I hope this makes sense. Your transformer winder should be able to do this with his eyes closed.