Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

Building Marshall Lead 100 MOSFET
Building Marshall Lead 100 MOSFET
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 8th November 2018, 09:50 PM   #31
jrow is offline jrow
diyAudio Member
 
Join Date: Jan 2018
Quote:
Originally Posted by Mooly View Post
I have seen two very obscure problems cause distortion like that.

1/ Having a ferrite cored inductive component in series with the load. This can be something as small as a ferrite bead slipped over one of the output leads somewhere. The material saturates and gives this weird distortion... more likely an issue at high frequencies though.

2/ Another inductive issue can be having a shared magnetic component (such as a dual wound ferrite core) in series with each rail. Mutual coupling between the supply currents again causes this odd effect.

Beyond that and its a case of scoping rails and seeing if anything is apparent. The volt drop is normal, however the rail should still remain clean though and with only a little ripple showing.
1st thing you say is impossible, I mean, there is no ferrite coil in series- parallel- or anything.

When I scoped output, and I refer to '8 ohm speakers connected', the load is 2x12" Celestion V-Type (16ohm) in parallel. It's not a problem of scoping, because distortion appears at same level (showed in screen), whether scoping is connected or not.

The only inductive problem that could be happening is that toroid transformer is inside of metal box. Idk if I should put it outside, but if I'm not wrong, toroid transformers make a very low noise...

This is how the diagram looks like. Please take a look to discover any issue. Thanks!!

circuit diagram.jpg

Last edited by jrow; 8th November 2018 at 09:54 PM.
  Reply With Quote
Old 8th November 2018, 11:03 PM   #32
JMFahey is online now JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
Just as an advance, too busy now to develop my answer: your amplifier is at present *heavily*current limited.
No problem in reaching rails and "voltage" clipping like any normal amplifier without load, but it can NOT drive an 8 ohm load.

Why?
Dunno yet, let me analyze the schematic (which I hope you posted earlier; if not, do now)


If I donīt remember wrong, that amp had transistor clamping current limiters (a.k.a. "short protection") ; if malfunctioning you will get that waveform.

But there are other possibilities too, so let me look at schematic first.

The waveform shows a square wave ... a *current* squarewave that is; the "weird shape" comes from driving an inductive load such as a speaker ... specially a guitar type one.
__________________
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
  Reply With Quote
Old 9th November 2018, 07:34 AM   #33
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Building Marshall Lead 100 MOSFET
Try removing the current limiting (the 1N4007's) and see if it is OK.
  Reply With Quote
Old 9th November 2018, 04:15 PM   #34
jrow is offline jrow
diyAudio Member
 
Join Date: Jan 2018
Quote:
Originally Posted by Mooly View Post
Try removing the current limiting (the 1N4007's) and see if it is OK.
Okay, but do you mean just removing them and leave it?? or removing them and connecting instead directly the transistor to GATE??

Isn't it dangerous?? Thanks!!
  Reply With Quote
Old 9th November 2018, 06:50 PM   #35
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Building Marshall Lead 100 MOSFET
I mean remove them completely as a test and see if it 'fixes' your issue. The distortion may be the limiting cutting in to early. If that is the cause then you need to either run it without and accept the risk if the amp is abused, or redesign the limiter.

But first you need to prove that is or is not the cause.
  Reply With Quote
Old 11th November 2018, 05:16 PM   #36
JMFahey is online now JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
Tr10 and TR11 are current clamping/limiting transistors,for short circuit protection.

A damaged circuit might work too early, strongly limiting output current even when unneeded, so for testing purposes, letīs remove it .

Just remove diodes D10 and D11 , and retest amplifier.

A second possible problem is that you replaced source ballast resistors with wrong value ones, so recheck R67/R68.
Remember meters add own internal and wire resistance to displayed measurement so first short test leads , read display, and substract that value from what you measure across resistors.

Very poor precision, but if your end result is anything about 0.4 or 0.5 ohm, we can assume they are fine; if you measure 1 ohm or more they are not.

A third possible problem, after cheching and discarding 1 and 2 above, is that Mosfets are fake, because the ones you bought, if real, must work very well there.

4) just for kicks, measure DC resistance through the speaker cable, one end plugged in the cabinet, you measure at the free plug, to check what does the amplifier actually "see" ... we expect around 8 ohm there (again remember to substract meter parasitic resistance).

Do all tests and please post results, whatever you find.
__________________
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
  Reply With Quote

Reply


Building Marshall Lead 100 MOSFETHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Marshall Mosfet LEAD 3210 blowing power fuses - help locating capacitor(s) jmilch Instruments and Amps 13 20th August 2009 06:31 AM
Marshall 2196 lead cab alexidoia Instruments and Amps 1 6th May 2008 09:16 PM
Marshall Super Lead MkII microslav Instruments and Amps 5 15th March 2005 08:59 AM


New To Site? Need Help?

All times are GMT. The time now is 04:49 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki