Primary windings and some information

[Bernardduur]

Hello all!

I have some issues with an old MIG 100 head I got from a friend. He stated that there was some work done of it before but that it keeps burning fuses, tubes and resistors and that he needs to find out what is wrong with it.

After some thorough examination I can state that the unit is functioning fine, only that the primary windings are the origin of the problem. The amp has a 230 / 110V switch that swicthes two primary windings of the power trafo OR in series (230V) OR in parallel (110V).

Something there in the primary must have gone wrong as the voltages in the amp are always double of what is required. The heaters all run on 12V AC and the plates and screens run on 900V (even loaded with tubes, I accidentally noticed). I removed the whole 110 / 230V switch and hardwired the 230V setting (windings in series) but still the amp behaves like it is on the 110V. I measured the windings with a multimeter and both windings give a reading and both give the exact same reading (very low ohmage as expected). When I put the windings in series the resistance is exactly the sum of the windings alone. I attached an AC voltage to the secondary and measured the exact same thing as mentioned above. So I can state that on the 230V setting it will behave like running on 110V and thus I have doubled the secondary voltages.

My only option is to replace the transformer I first thought..... but then I got an idea;

- The full wave rectifier in the unit (w/ center tap at the juntion of two large caps from V+ to ground) can be modded to a half wave rectifier with the tap going to ground giving me 1/2 of the voltage
- The preamp tubes can be run on 12V by putting them in series
- The 12V AC can be easily made into 6V DC by using a halve wave resistive load design by adding one diode to one tap and grounding the other tap

My main issue here is the safety and other design flaws...... is this idea any good? I know I'm trying to keep a wreck from sinking but still, it'll make the amp usuable.

[Chuck H]

Clever solution but it doesn't seem like a smart idea to use a compromised power transformer by redesigning the circuit around it's failure. The PT is just a ticking clock at that point. Any number of available PT's could be used with some circuit modification.

JM2C

[R.G.]

The short answer is to replace the PT. Any defect in primary power wiring - especially a defect of an unknown nature - leaves the amp prone to failure, and possibly a dangerous failure. Think electrocution and fire.

The longer answer adds onto this that the fixes you propose are not necessarily workable.

- The full wave rectifier in the unit (w/ center tap at the juntion of two large caps from V+ to ground) can be modded to a half wave rectifier with the tap going to ground giving me 1/2 of the voltage

The power available from the change in rectification is not the same. If I understand your description correctly, the original uses a full wave bridge and the CT to keep the DC across two caps balanced. Changing this to a full wave center tap style with two diodes instead of a bridge does give half the voltage, but makes much poorer use of the transformer windings, as only half of the winding is active on each half-cycle, not the whole secondary on each half-cycle. This could well give temperature rise issues with the PT windings - or not, if the original PT had lots of extra capability. Hard to say, but there is an exposure there.

But kudos on observing that it can be rewired this way. Clever.

[Jazz P Bass]

If there is something screwy going on with the transformer, as it appears, I would not mess with it.

Replace the tranny.

Safety first.

[Enzo]

Well, what they said^^^^


But the problem seems very strange to me. You set it for 230 but it acts like it is on 120? OK, disconnect the secondary wires so we don't further damage anything, now turn the primary switch to 120v. Are the voltages now FOUR times what they should be, or perhaps the switch was wired backwards and they fall into place?

Maybe I am old and cranky, but I have a hard time finding a transformer that neatly doubles all the voltages. Somehow that doesn't sound like the transformer's fault.

[olddawg]

Do you have a variac? I would pull the transformer completely out of the unit, put it on the bench. I would input a lower voltage (30vac or something) on the primary and measure the output on the secondary, then bring it up in increments. Btw, most PT primaries I have seen with multiple taps use either all or half of the primary in practice, not paralleling anything. You have something weird going on. If you have a 240vac mains and are doubling it, I would expect much more to be blowing in the amp than a fuse, like exploding caps.

[g1]

I'm with Enzo, seems too coincidental that the voltages are exactly double.
The schematic shows 5 wires for the primary, please draw out how you are wiring it up, or post some pictures or something.
Here's the schematic, maybe it will give somebody an idea!

[eschertron]

I'm with Enzo, seems too coincidental that the voltages are exactly double.
The schematic shows 5 wires for the primary, please draw out how you are wiring it up, or post some pictures or something.
Here's the schematic, maybe it will give somebody an idea!

That's clearly not the tranny that the OP has in his amp, his has two sets of windings, the schem shows a single winding with taps on the primary. Was the PT replaced at some point with the wrong component?

[g1]

The Mig 50 shows the separate winding type, maybe that will help.

[Bernardduur]

Well, what they said^^^^


But the problem seems very strange to me. You set it for 230 but it acts like it is on 120? OK, disconnect the secondary wires so we don't further damage anything, now turn the primary switch to 120v. Are the voltages now FOUR times what they should be, or perhaps the switch was wired backwards and they fall into place?

Maybe I am old and cranky, but I have a hard time finding a transformer that neatly doubles all the voltages. Somehow that doesn't sound like the transformer's fault.

Yeah, I had the same feeling and checked, double checked and completely removed the switching part. Even without it it still behaves the same......... I think I'll have to open the bell ends and check the connections inside.

[Bernardduur]

I'm with Enzo, seems too coincidental that the voltages are exactly double.
The schematic shows 5 wires for the primary, please draw out how you are wiring it up, or post some pictures or something.
Here's the schematic, maybe it will give somebody an idea!

Well, I got two different schematics for MIG100 amps, both differ and both are not the amp I have on the bench (values, wiring, etc). It is much more like the posted MIG50 schematic

[Enzo]

OK, so you understand how the primary switching SHOULD work, and you have identified the two primary windings? Can you power just one primary winding and see what voltages result. Use a variac.

[Bernardduur]

OK, I've removed the transformer from the amp and did some measurements. The readings I get are the same as mentioned before. But I now not think the unit doubles the voltages...... the preamp voltages distracted me because they are DC, not AC.

The transformer behaves just quiet, no hissing, noises or other weird things.

When I power her up she behaves normally around a voltage of 210V, around 20V less compared to the net here. My best guess is that the weird voltages are mainly due to the raise of the net voltage that gives me weird readouts. I'll try to lower the net voltage to a lower value by using a second transformer and substract approx 12 to 18V from the 230 as described here

[Bernardduur]

OK, to close this thread.......


When doing these kind of things, always check your equipment



Turns out my multimeter, my trusty meter that has been with me for so long, was the main problem. I was checking another amp last night and got the SAME values....... With my backup meter it all measures OK! No issues here..........