Pat_rocks92
Tele-Meister
I mean it would be too big and heavy and i'm already using a 18w chassis for the build to save some space :
- Width: 430 mm
- Depth: 170 mm
- Height: 65 mm
Going to build a JMP 50w head need help and recommendations for filtering
- Thread starter Pat_rocks92
- Start date
Lowerleftcoast
Poster Extraordinaire
It could look something like this. You will need to choose the cap values, and choose a ground scheme.
Len058
Tele-Meister
If your goal is to make something that sounds like the 68 Superlead but is smaller than the original, I'd find out what the differences are. Changing the rectifier and filtering will not make the 50 watt circuit sound like the 100watt version.
For example the Bright caps and some coupling caps are different. The NFB is probably different because the 100 watt nfb has the same or less resistance but the output of the 4 tubes is higher. They have the same choke, as far as I can find, but the voltage demands are quite different, so that might change the response of the amp.
Just some thoughts...
For example the Bright caps and some coupling caps are different. The NFB is probably different because the 100 watt nfb has the same or less resistance but the output of the 4 tubes is higher. They have the same choke, as far as I can find, but the voltage demands are quite different, so that might change the response of the amp.
Just some thoughts...
Pat_rocks92
Tele-Meister
my objec
thanks i think i will try this way ! But i'll go with 2x 50uf and 2x 16uf my only concern is where do i wire the 27k resistor to ?(i don't have a bias Tap)
Lowerleftcoast
Poster Extraordinaire
You will probably change the 27k for 100k and take it directly from one side of the PT HT secondary. (I would ditch the standby switch). OMMV.
dan40
Friend of Leo's
The way that you drew your layout will work fine for tapping the bias voltage. It's basically the same as the Metroamp layout. You will notice that in the Metro layout, they used a 150k dropping resistor for the bias circuit. If your PT delivers the same voltage as the Metro PT, 150k will work fine. If the PT is slightly different, you will need to adjust the value of the resistor slightly. I have also had to lower the value of the 47k in the bias circuit to 33k in every one of these that I have built. Just wait until you start the amp to see if your power tubes will bias properly before changing any of the values. The 100 watt amps have a lower voltage bias tap on the PT so it uses a lower value for the dropping resistor.
The 50 and 100 watters use the exact same preamp except for a different value nfb resistor so your board will work fine. Just be sure to follow the rectifier and power supply/filter cap wiring for the 50 watt model and you should be fine.
The 50 and 100 watters use the exact same preamp except for a different value nfb resistor so your board will work fine. Just be sure to follow the rectifier and power supply/filter cap wiring for the 50 watt model and you should be fine.
dan40
Friend of Leo's
I forgot to mention that the rest of the wiring in your rectifier drawing is incorrect. You attached a blue wire between the negative end of the filter cap and the end of your diodes that will not work. This will send all of the current from that pair of diodes straight to ground. You also connected the positive end of the filter cap to the junction of D1/D2. This wire will not work in this spot either. Wire the four diodes like lowerleftcoast's diagram and then take your power from the junction where all four diodes connect together.
Pat_rocks92
Tele-Meister
will this work better ?
That's the pt i got : https://shop.amppartsdirect.com/pro...-laydown-power-transformer-upgrade-apd-8089-h
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dan40
Friend of Leo's
Yep...that will work fine. The PT is also correct as long as you purchased a chassis for a laydown PT. All of the early 50 watters had the laydown style but they changed to an upright model somewhere around 1971-72. Good luck with your build and be sure to start a build log once you get going.
Pat_rocks92
Tele-Meister
Thank you a lot Dan ! yes i'll try as it will be my first high watt marshall build. I contacted merren for a transformer but got no answers so i went with a cheap one https://www.banzaimusic.com/Transformer-T-OP-M50A.html . We will see how it will sound. i wanted to add an extra preamp tube and stage and i don't know if it is possible with a 50w amp ? i know that it works with 100w amps :
that's the exact mod i'm looking to do with my build :
i'll try to follow larry grounding scheme too.
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Lowerleftcoast
Poster Extraordinaire
I just ran across a thread from 2011. Larry said he does not use the *Larry ground scheme* in his own amps.
In this Metropoulos forum he is using the username *novosibir*
Bad idea to use Chassis for secondary current! Build one solid return current bus or use one wire for each amp stage.
For us europeans Piemme Electra in Italy is place for high quality transformers which often are priced reasonably too.
Pat_rocks92
Tele-Meister
I went on their website and i didn't find any replica of 50w transformers :I. I tried primary windings and demeter however they couldn't help me neither.
so what would you recommend me for grounding scheme ?
Lowerleftcoast
Poster Extraordinaire
Straight from Larry in 2011:
"the best grounding scheme is:
- each stage does have its own filter cap
- all grounds of every stage are connected to their corresponding filter cap negative (sub star)
- all sub stars are routed together in the order, as the circuit is running (ground bus) - still isolated from the chassis (!)
- the ground bus is connected to the chassis at only one point
That's the way, how I'm doing the grounding in my own amps."
"the best grounding scheme is:
- each stage does have its own filter cap
- all grounds of every stage are connected to their corresponding filter cap negative (sub star)
- all sub stars are routed together in the order, as the circuit is running (ground bus) - still isolated from the chassis (!)
- the ground bus is connected to the chassis at only one point
That's the way, how I'm doing the grounding in my own amps."
On page 7 there are black and chrome Marshall Drake 784-139 replica Output Transformers but they have 3.4k primarys. Price is about the same what your Banzai was. Piemme seems occacionally discount 15% everything.
I think everywhere in Europe it is forbidden to use chassis for operative currents because it increase electromagnetic disturbance to Mains. It also increase the hum amps tend to produce when chassis surround amp circuits.
OT/Anode feed B+1, Screen feed B+2 and Bias feed B-1 combine to cathode current which then change to return current. Just think it as zero volt which use filter capacitor negative terminal as "ground" if you must use historical term
B01 is correct label for it. There often are two electrolytes but B01 and B02 are as often wired together. From this electrolyt negative terminal both Bridge rectifier diodes anodes alternate taking current back to power transformer.There are two equally good methods to construct return current wiring. One, often stiff bus, which is installed thru the board where it is mounted to turretts which in board operate as 0V. It start from input jack and finish to B01. All which in schematic have upside down T are wired to this bus.
If input jack is steel without insulation it connects 0V to chassis and when mains Safety Earth wire is connect to chassis it connects 0V to clobe Earth. In europe the SE is separate wire in building wirings so there is less disturbance.
If input jack is insulated a ground lug is screwed to chassis here where 0V is soldered...
...another method has one wire from each other stages filter capacitor negatives B03, B04, B05...etc to B01/B02.This is called Star-principle. Also here input stage 0V is connect to chassis. Now when this amp input jack is only place where 0V and chassis meet there can't leak secondary current to chassis! This means less distortion to mains and less distortion from mains. Also less distortion to guitar thru cable shield and less distortion back to amplifier input thru cable hot. When chassis does not operate as active circuit it operate better Shield!
So return current circuit is even simpler than feed current circuit which has voltage dropper resistors!
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dan40
Friend of Leo's
I used the "Larry Grounding" method in a couple of my 50 watt builds with good results. These days I have switched over to a single ground bus scheme, especially on higher gain builds. I drill two holes in the chassis between the board and the pots. One hole at one end of the board and another hole down near the input jacks. I will then mount a 3 terminal tag strip in each hole and run an 18awg bus wire between each. On the end of the bus wire closest to the PT, I solder the bus wire to one of the insulated outer terminals on the strip. The other terminal strip near the input jacks will get a star washer on the mounting screw for solid chassis contact. Solder the other end of the bus wire to the center grounded terminal of this strip. I will then solder the two power transformer centertap wires to the end of the bus wire closest to the PT. The ground wires from the filter caps and speaker will also attach on this end. The rest of the ground wires from the board and pots will be soldered to the bus wire as you move closer to the input jacks. The ground wires from the input jacks will be the last wires soldered to the bus. This method has worked well for me in all of my last few builds.
PCollen
Friend of Leo's
Just follow the JMP 50 watt schematic of your choice (I suggest the Model 1987) located here:
www.thetubestore.com
Marshall Amp Schematics - www.thetubestore.com
Marshall Amp Schematics to assist you with your amplifier repairs and modifications.
www.thetubestore.com
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