Metropoulos Forum

Hi. I am not a native English speaker, so I hope I can explain my problem, and hopefully that you can offer me a solution.
I have a power transformer with 2 wires on one side, and 8 wires on the other.
These 8 wires are arranged in couples, and here are the voltage I have messured (first value is when messured with an analog voltmeter, and second with digital)

First pair: 1. and 2. wire = 390V; 415V
Second pair: 3. and 4. wire = 57V; 67V
Third pair: 5. and 6. wire = 5.6V; 7.3V
Fourth pair: 7. and 8. wire = 5.6V; 7.3V

So, I assume next is correct:
First pair: HT supply
Second pair: Bias supply
Third pair: Heaters supply
Fourth pair: Heater supply

And finally my problem:
Here is a link to schematics of an 1959 Marshall which I am trying to build.
http://www.drtube.com/schematics/marshall/1959u.gif" onclick="window.open(this.href);return false;
I am interested:
1. Can I use this Power transformer?
2. How would I connect HT supply, since my transformer doesn't have central tap wire and this schematics clearly uses one?

If the answer to the first question is NO, I would still like to find out the answer to the second question.

I hope you understand me... Thanx in advance...

First get more reliable readings. You need to trust what you are measuring, so find or fabricate something that is a known AC voltage. That being said the readings called 'analog' look about right to power a 1959 circuit assuming the input was 120VAC.

Nominal voltages are around those values (and your list makes sense to me), although the heaters being at 6.3VAC would be ideal. Please state for the record where you are measuring voltages in reference to (where is local ground?).

Input is 220VAC, since I am in Europe (Croatia).
I took all measures with transformer pluged into wall socket, and then connecting my voltmeter to the end of the wires.
No ground was connected.

I think your conclusions are correct.

Without a centre tap you will need to use the same method of rectification used in the 50watters but your dc on the plates will be very low with only 400v of ac (Vdc=2 Vmax/pi = 0.637 Vmax) so only just over 250v dc - very brown sound

I am not sure I understand your answer, but I read somewhere that my problem can be solved like this:

- Instead of central tap which connects between the capacitors, I should use ground connection
- on each ht wire add some resistor before the capacitors.

I am not sure were I read this and I cant find it anymore. Solution is something like that, but not sure where exactly to put the resistors and not sure what type and specs to use.

Does this sound familiar to you? Can you help further? Would this work?

thanx

Compare this 50watt:

http://forum.metroamp.com/download/file.php?id=11543

with this:

http://forum.metroamp.com/download/file.php?id=11538

You will see you have full wave rectification with the 100watt (centre tap) and half wave with the 50watter, without centre tap, but your voltages will be low with that PT. I do not know another way of wiring it to get what you want, maybe someone else will?

Don't need to use or even have CT in a Marshall 100 watt style PT. Marshall actually just stopped using the HV CT in the later JCM era 1959s (without series mains caps) as can be seen here:
http://www.drtube.com/schematics/marshall/1959pwrm.gif" onclick="window.open(this.href);return false;

Example of a Laney using the same style of HV PT winding with no CT:
http://www.drtube.com/schematics/laney/klipp100.gif" onclick="window.open(this.href);return false;

CT balances voltages on the (series) mains caps in a 100 watt Marshall 1959, the Laney above should have resistors in place to account for the loss of that balancing action, but point is that the CT does not make or break this rectifier circuit's operation.

Below is a general example of how a 1959 can be set up with a PT with no CTs. That said (and assuming the voltages you measured are correct), even though voltages may be correct for use with this style of rectification, are you sure the windings can supply the required current? If you indeed have two 6.3V non CT heater windings, you may need to use both. Will need to tweak the bias supply values to work with the lower starting AC there as well.

So, the PT may work out , but don't know enough about it yet here. Too many unknowns are still involved, but just the lack of a CT on the HV tap does not rule it out as potentially working well in this circuit.

Hope that made any sense.

I think this is what I was looking for. It looks like something I have read somewhere before.... Not sure only about that resistor on Heaters Supply.
Hope this will work.
Thanx a lot.....