Building a Marshall Plexi - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 19th March 2004, 05:50 AM   #1
edsmith is offline edsmith  Australia
diyAudio Member
 
Join Date: Mar 2004
Location: Australia
Question 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
  Reply With Quote
Old 19th March 2004, 05:20 PM   #2
diyAudio Member
 
Join Date: Mar 2004
Location: Wichita,KS
Default Hey

You may find this place of interest. http://launch.groups.yahoo.com/group/JTM45/
  Reply With Quote
Old 19th March 2004, 05:22 PM   #3
diyAudio Member
 
pingfloid's Avatar
 
Join Date: Jan 2004
Location: Bangkok
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,
  Reply With Quote
Old 19th March 2004, 05:25 PM   #4
diyAudio Member
 
pingfloid's Avatar
 
Join Date: Jan 2004
Location: Bangkok
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
  Reply With Quote
Old 19th March 2004, 08:21 PM   #5
stormy is offline stormy  United Kingdom
diyAudio Member
 
Join Date: Mar 2004
Location: Scotland
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.
  Reply With Quote
Old 19th March 2004, 10:25 PM   #6
SHiFTY is offline SHiFTY  New Zealand
diyAudio Member
 
SHiFTY's Avatar
 
Join Date: Jul 2002
Location: New Zealand
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! I am building a copy of an Ampeg with 7591s that I have.
  Reply With Quote
Old 19th March 2004, 11:59 PM   #7
diyAudio Member
 
Join Date: Jan 2004
Location: San Diego, CA
Check out OEI. I think one of the guys who makes them is (relatively) local to you, too.
  Reply With Quote
Old 20th March 2004, 01:31 AM   #8
stormy is offline stormy  United Kingdom
diyAudio Member
 
Join Date: Mar 2004
Location: Scotland
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! 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
  Reply With Quote
Old 20th March 2004, 04:43 PM   #9
adamamp is offline adamamp  Canada
diyAudio Member
 
Join Date: Mar 2004
Location: East of Eden
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.
  Reply With Quote
Old 13th May 2004, 06:25 PM   #10
gajdack is offline gajdack  United States
diyAudio Member
 
Join Date: May 2004
Location: California USA
Lightbulb 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
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Building Marshall Plexi zxx123 Tubes / Valves 1 19th June 2004 03:01 PM
50watt Plexi Again edsmith Tubes / Valves 2 23rd March 2004 07:14 PM
Extra gain stage Marshall plexi 50watt edsmith Tubes / Valves 1 22nd March 2004 04:58 PM
Plexi WCC_Burnout Multi-Way 7 24th February 2003 05:22 PM


New To Site? Need Help?

All times are GMT. The time now is 12:59 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2