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edsmith 19th March 2004 05:50 AM

Building a Marshall Plexi
 
Just wondering if you guys could tell me what output and power transformer i should purchase for the marshall plexi i am going to build.

As you can probably tell I have not built a tube amp before, I have gained my electronics knowledge from stompbox building.

The schematic is found at www.drtube.com in the marshall section. It is the 1987 JMP 50 watt head.

Oh and also what does a choke do and what one should I get?

Your help greatly appreciated,
edsmith:)

Tim Wyatt 19th March 2004 05:20 PM

Hey
 
You may find this place of interest. http://launch.groups.yahoo.com/group/JTM45/

pingfloid 19th March 2004 05:22 PM

Hi,

For a 50W push-pull with EL-34 I think a Hammond 1650K could be a choice for the output transformer, but I've heard that OT at guitar amps are deliberately under-rated. żany tip here?

About the choke, you need it to smooth the power supply current.

Cheers,

pingfloid 19th March 2004 05:25 PM

Quote:

You may find this place of interest. http://launch.groups.yahoo.com/group/JTM45/
WOW! That's a great place for the Marshall fans :cool:

stormy 19th March 2004 08:21 PM

Hi
Just had a look at the plexi schematic and it looks as if it might
work (A lot of schematics contain errors, even the origonal engineers version,so copying wont always work)

As to a clue about the rating for the choke, theres a 500ma fuse
in line before it so that should tell you a lot.

personaly i would go for 10-20hy


stormy.

SHiFTY 19th March 2004 10:25 PM

As far as guitar output trannies go, you only need one which can do about 100-15000 hertz at 30-40W. You have quite a few choices, try and get one that can handle a decent amount of current in the primary. A cheap one may be ok, but it is probably best to get a decent one, even original marshalls might be available on eBay.

As far as power tranformers go, what you need is something like 400-0-400 @ 180mA and 6.3V at 6A. This will be a nice cool running, long lasting transformer as it is over 150% of minimum. Those are pretty much the only requirements, you can just about use any power transformer that is close to that. Check the VA rating- i.e 400V*0.18 + 6.3V*6A = 109.8 so around 120VA. What is the B+ rail of the amp?

For a choke, the only current that will be passing through it is screen current of the EL34s and current for the driver stage. Probably a 40mA choke would be adequate here, around 10-15 H would be good.

Should be a grunty amp! :smash: I am building a copy of an Ampeg with 7591s that I have.

sad mafioso 19th March 2004 11:59 PM

Check out OEI. I think one of the guys who makes them is (relatively) local to you, too.

stormy 20th March 2004 01:31 AM

Quote:

Originally posted by ShiFtY
As far as guitar output trannies go, you only need one which can do about 100-15000 hertz at 30-40W. You have quite a few choices, try and get one that can handle a decent amount of current in the primary. A cheap one may be ok, but it is probably best to get a decent one, even original marshalls might be available on eBay.

As far as power tranformers go, what you need is something like 400-0-400 @ 180mA and 6.3V at 6A. This will be a nice cool running, long lasting transformer as it is over 150% of minimum. Those are pretty much the only requirements, you can just about use any power transformer that is close to that. Check the VA rating- i.e 400V*0.18 + 6.3V*6A = 109.8 so around 120VA. What is the B+ rail of the amp?

For a choke, the only current that will be passing through it is screen current of the EL34s and current for the driver stage. Probably a 40mA choke would be adequate here, around 10-15 H would be good.

Should be a grunty amp! :smash: I am building a copy of an Ampeg with 7591s that I have.

Hi are you shure about a 40mA choke theres a 500mA fuse
Just before it in series with the choke seems a bit expensive to
protect a fuse with a choke

I would go for at least a 200mA choke


stormy

adamamp 20th March 2004 04:43 PM

The coil only carries the EL34 screen grid and preamp current. The center tap for the output transformer is connected right after the fuse and takes most of the current. The fuse is slightly over- sized prolly to compensate for inrush current. The choke could be 8H/100ma like the Vox AC50 but a 20H would be better(Hammond 193C). This 100ma rating allows for a nice safety margin. Also Hammond transformers would be fine. I have used many for guitar and HiFi. A good pair for that amp maybe a 1650H(rated at 40wt UL but will do 60 wt. pentode) for audio and a 272JX for power. Hope this helps. :cool:

gajdack 13th May 2004 06:25 PM

Tube Amplifier Transformers
 
Hi to all!

I am a manufacturer's representative for a domestic custom transformer manufacturer. They made the transformers for many of the classic guitar amplifiers that folks are trying to duplicate or rebuild today. We are still supplying these transformers to distribuitors around the country.

If you will e-mail me with the part you are looking for, perhaps I can send you in the right direction or have the transformer made for you.

Best regards,

Curt Gajda
Cummings and Associates North
gjada@comcast.net


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