Going to build a JMP 50w head need help and recommendations for filtering

[Pat_rocks92]

Hello my main objective is to build a 50w 68 spec super lead.

however i want to know if it is possible to use the bridge rectifier and the filtering for a 100w version on a 50w version like in the pdf below ?

this is the classic 1987 50w layout :

could i use the same exact 100w filtering board but with lower values ?

my objective was to keep close to the 100w construction type.

 

[2L man]

You can use only two diodes of the bridge rectifier but its voltage rating must match the double voltage a CT connect power transformer produce! Same board construction work fine. That first double electrolyte can is parallel connect. I would install a resistor between capacitors and then there come RC filtering. For example 33 or 47 ohm improve PS.

I would buy 1000V diodes and mount them to circuit board. I understand mounting bridge to chassis can be nice.

I use Ultra Fast diodes because they are found to broduce less distortion.

When I install a stby/HV switch I always install a resistor over its NO contacts and then capacitors pre charge. It also lessen pop sound volume and make tubes conduct some current. For example 47k/5W.

 

[Pat_rocks92]

You can use only two diodes of the bridge rectifier but its voltage rating must match the double voltage a CT connect power transformer produce! Same board construction work fine. That first double electrolyte can is parallel connect. I would install a resistor between capacitors and then there come RC filtering. For example 33 or 47 ohm improve PS.

I would buy 1000V diodes and mount them to circuit board. I understand mounting bridge to chassis can be nice.

I use Ultra Fast diodes because they are found to broduce less distortion.

When I install a stby/HV switch I always install a resistor over its NO contacts and then capacitors pre charge. It also lessen pop sound volume and make tubes conduct some current. For example 47k/5W.

so it would not be possible to use the 4 diodes in x like in the 100w version ?

 

[dan40]

The 100 watt circuit uses a "voltage doubler" circuit because the PT voltage is much lower than the 50 watt PT. The 100 watter uses a "full wave bridge" rectifier to bring the HT voltage up into the correct range. The 50 watter uses a higher voltage PT with a standard full wave rectifier. In this 50 watt layout, it shows a bridge rectifier but it is not wired up as a "full wave bridge. You could use two diodes in series on each leg of the HT winding instead or just follow the layout and use the bridge wired as it is shown.

You can use any filter values that you want in the 50 watt. George went with 50uf everywhere except for the preamp (32/32uf) in his layouts because he was copying a 68 circuit. Later amps used 50/50uf in the preamp. Feel free to choose any values you like within reason. Many folks prefer to lower the filtering quite a bit to loosen the amp up but you may experience some ghost notes at higher volumes.

 

[dan40]

could i use the same exact 100w filtering board but with lower values ?

my objective was to keep close to the 100w construction type.

It's not that easy. As I mentioned above, the 100 watter has an entirely different power supply so you need to stick to the 50 watt layout for the power section. You can still achieve the same amount of filtering that the 100 watter uses.

 

[dan40]

so it would not be possible to use the 4 diodes in x like in the 100w version ?

You can use individual diodes on each side of the PT winding but you cannot wire them up the same as the 100. You will want to use two in series, facing the same direction on each one. You will then join the ends together, solder a wire to them and run the wire to the standby. The PT centertap will connect directly to ground.

 

[mountainhick]

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www.electrical4u.com

For 50W, Like Dan says:

Full wave rectifier:

VS For 100W, this produces twice the voltage.

full wave bridge rectifier

 

[Pat_rocks92]

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www.electrical4u.com

For 50W, This will run as shown, but as Dan says you can add one more diode in line in series in the same direction on each side. You'd then have D1a, D1b, D2a, D2b

Full wave rectifier:

VS For 100W, this produces twice the voltage.

full wave bridge rectifier

i just don't understand then how there seems to be 50w super lead with 4 diodes :

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images.reverb.com

is that 3 diodes only for the rectifier and one for the bias ?

 

[mountainhick]

i just don't understand then how there seems to be 50w super lead with 4 diodes :

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images.reverb.com

is that 3 diodes only for the rectifier and one for the bias ?

I just updated the first diagram. That's how... for the power supply.

Your link doesn't work. I'll dig up my 50W schematic...

 

[mountainhick]

I think this came from Robrob's website. You can see the same thing as I posted above but orientation is switched left to right. The diode in the bottom of the diagram is for the bias voltage supply

 

[dan40]

i just don't understand then how there seems to be 50w super lead with 4 diodes :

It's the way the wires from the PT are connected to the diodes and where the PT centertap is grounded that makes them different. Wiring the diodes like a 100 watter using a 50 watt PT, you would end up with extremely high B+ approaching 1000v. Mountainhick's image of the full wave rectifier only shows two diodes but most folks will use two in series on each side of the PT. This increases the peak ratings for the diodes. I have seen people use three on each side for even more protection. The 100 watter must use four because of the way the bridge is designed. The diode for the bias circuit is a separate thing.

 

[dan40]

Check out the PT in this picture from the Marstran website. You can clearly see two diodes in series on each of the PT's high voltage wires and then the ends are joined together into a single wire that connects to the HT fuse.

68 10135

www.marstran.com

 

[dan40]

This pic is a 71 model with the diodes on the turret board. It still uses two diodes in series attached to each HT wire and then the other end is joined together with a short piece of wire between the two turrets.

71 1620C

www.marstran.com

 

[Pat_rocks92]

This pic is a 71 model with the diodes on the turret board. It still uses two diodes in series attached to each HT wire and then the other end is joined together with a short piece of wire between the two turrets.

71 1620C

www.marstran.com

Thanks that's what i wanted to know so i can use this kind of layout for my diodes :
- two diodes in series attached to each HT wire and then the other end is joined together with a short piece of wire between the two turrets

I acquired 4x 10d10 vintage diodes, i suppose i can use them safely ?

 

[Pat_rocks92]

This pic is a 71 model with the diodes on the turret board. It still uses two diodes in series attached to each HT wire and then the other end is joined together with a short piece of wire between the two turrets.

71 1620C

www.marstran.com

this is an early small layout but would this work or are there some wires which are wrongly connected ?

Yellow would be ct.

 

[Len058]

I'm curious. What do you expect to achieve by doing this?

 

[Pat_rocks92]

I'm curious. What do you expect to achieve by doing this?

being close to the construction type of a 100w sl from 68. I have a full 68 board of nos components which test okay. I want this 68sl 100w sound badly but i don't have the space for a 100w version of it... So i finally ended up chosing to build the 50w version of it.

 

[Len058]

I'm no expert but why don't you copy the pre-amp circuit of the 100 but keep the rest of the 50?

If you want to replicate the power supply behaviour of the 100, you will not achieve it by just copying the schematic and just use 2 instead of 4 power tubes . You are missing the power consumption of two power tubes. So the amp will not behave the same, without the current draw of those extra power tubes, pushed hard.

 

[Pat_rocks92]

I'm no expert but why don't you copy the pre-amp circuit of the 100 but keep the rest of the 50?

If you want to replicate the power supply behaviour of the 100, you will not achieve it by just copying the schematic and just use 2 instead of 4 power tubes . You are missing the power consumption of two power tubes. So the amp will not behave the same, without the current draw of those extra power tubes, pushed hard.

I will add an extra tube with an extra stage that will perhaps help.

 

[Len058]

I will add an extra tube with an extra stage that will perhaps help.

But it's a push pull amp. You need to add 2 tubes for an extra output stage, but then you'd have a 100watt with the pre-amp section of the 50 watt version.

What do you mean with

i don't have the space for a 100w version of it...

Do you mean that the 100watt would be too loud, or you don't have a chassis big enough for the 100W amp?