JTM45 / 50 Build

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Spencerleehorton

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Hi All,

Now moving onto the JTM45 build from the success of the fender champ.

First of all I wanted to try and use this line 6 spider MK2 power txf.
I have the following voltages on it:

Winding 1 is 370vac with centre tap
Winding 2 is 60vac
Winding 3 is 20vac with centre tap
Winding 4 is 6.7vac but in 4 leads?
1+3 = 6.71v
1+4 = 6.73v
2+4 = 6.72v
2+3 = 6.71v

My issue seems to be on the 6.3v windings. (And yes I know I could just buy a 5v txf)
By process of elimination, would I be right in guessing that on winding 4, taps 1 and 2 could be the CT on the 6.3v windings?
I need one 5v winding and one 6.3v winding for this txf to work and wanted ideas to get the 5v?
Could I use a high wattage resistor to lower it?

My other option could be if I can split the centre tap of the 20v winding and then parallel the two 10v winds to give me 5v?
 

Spencerleehorton

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I could use a voltage divider of 10k and 6.8k to drop 1.47v, which would get me close but I'm also concern I dont know what wattage these 6.3v windings are?
Is there any tests I can do to establish was they are?
As I need I think at least 2A possibly 3A windings?
Be good to know a few more techniques on this if anyone could share please?
 

abbey road d enfer

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Spencerleehorton said:
I could use a voltage divider of 10k and 6.8k to drop 1.47v, which would get me close but I'm also concern I dont know what wattage these 6.3v windings are?
You seem to ignore the basics. With 10k in series, tehre will be almost nothing when it is loaded with the rectifier filament. The GZ34 filament behaves like a 2.6 ohm resistor when hot. But when it starts cold, it's like a 0.2 ohm resistor.
That means it would receive about 6mA, which would never start it heating.
If you want to lower the voltage with a resistor, it should be about 0.68 ohm.
However I wouldn't recommend it, because the heater voltage may change when teh tube heats, maybe significantly enough to put it in unsafe operation area.
You must definitely learn about Ohm, Norton, Kirchoff & Thevenin.
 

Spencerleehorton

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Yes my calculation attempt was way off, I am starting to learn it though!!
My second attempt I worked out that I needed 3.15ohms which would give me 2A, which would involve a 0.8ohm 5watt resistor!!
 

Spencerleehorton

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I appreciate it is very frustrating reading my mindless attempts but I am trying to learn this, having been helped in the past I need to like you say get the basics right.
Ohms laws seems to makes sense now but I do need a lot more experience with it all as havent dealt with these particular calculations before.

Would you be able to explain the any possibilities of using this txf for this purpose please? How you would go about it and any calculations would be fantastic.
 

Spencerleehorton

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So from what you have said, if I put a 2.6ohm resistor on one of the 6.3v windings then measure the voltage and see what I've got?
I think I have some 2.2ohm 2W , i definitely have some 7ohm 8watt.
 

abbey road d enfer

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Spencerleehorton said:
So from what you have said, if I put a 2.6ohm resistor on one of the 6.3v windings then measure the voltage and see what I've got?
I think I have some 2.2ohm 2W , i definitely have some 7ohm 8watt.
Please read again, carefully now.
0.68 ohm.
 

Spencerleehorton

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I'm not talking about within the circuit I'm talking about just testing the transformer.
So are you saying to test the txf just use a 0.68ohm resistor and see what voltage drop I get?
 

abbey road d enfer

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Spencerleehorton said:
I'm not talking about within the circuit I'm talking about just testing the transformer.
So are you saying to test the txf just use a 0.68ohm resistor and see what voltage drop I get?
With just one resistor and no load there is no voltage drop. The GZ34 must be in circuit.
 

Spencerleehorton

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My aim was to test this txf with a dummy load first, then see what voltage I had.
Just to be clear.

I took what you said about heater filament as if I'm testing without tube that the filament is 2.6ohm, then the load would be 0.68ohms to reduce down to 5v, so 3.2ohms total?
I dont mean to offend but you have not explained how you got to this, could you explain please?
 

abbey road d enfer

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Spencerleehorton said:
My aim was to test this txf with a dummy load first, then see what voltage I had.
Just to be clear.

I took what you said about heater filament as if I'm testing without tube that the filament is 2.6ohm, then the load would be 0.68ohms to reduce down to 5v, so 3.2ohms total?
I dont mean to offend but you have not explained how you got to this, could you explain please?
5V 1.9A gives 2.6 ohms.
Drop 1.3V at 1.9A gives 0.68ohm.
 

Spencerleehorton

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Ok, well it seems I maybe able to move forward with this txf by the use of some diodes on the 6.3v winding to bring it to 5v, I just need to make sure that I have two completely separate 6.3v windings.
So one winding could heat the filaments at 6.3v and the other with two sets of two anti-parallel diodes rated to 5A connected in series with the rectifier filament. The diodes should be placed on a heatsink.
Thanx Milan for this.
 

abbey road d enfer

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Spencerleehorton said:
Ok, well it seems I maybe able to move forward with this txf by the use of some diodes on the 6.3v winding to bring it to 5v, I just need to make sure that I have two completely separate 6.3v windings.
So one winding could heat the filaments at 6.3v and the other with two sets of two anti-parallel diodes rated to 5A connected in series with the rectifier filament. The diodes should be placed on a heatsink.
Thanx Milan for this.
Simulation shows that with two sets of two diodes, the RMS voltage would be 4.9V; close enough.
You don't necessarily need two separate windings.
The diodes introduce some distortion of the voltage; how significant it is depends very much on the DCR of the xfmr and the sensitivity of the audio circuit.
Note that, even using a separate winding, the waveform would also be distorted, although slightly less.
 

Spencerleehorton

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This is the schematic I'm going to follow but I'm going to use diode bridge for B+ and will put diode bridge on the rectifier socket like what I did for the champ.
The output valves I have will be EL34s so there also maybe a few changes to components?
Will need to verify these changes.
 

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Spencerleehorton

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Its seems I may go for the JTM50 build and put switches on the cathode bias of V1a and V2a.
Will check that this power txf can get 320v ac so after the diode bridge I can get 440v dc.
 

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