Marshall input problem

gerald · 28th March 2004, 03:44 PM

Hello,
I have a Marshall 5275, which has some problems:
It is a transistor amplifier (75 W Reverb) which was manufactured in 1985(or before).
The problem is that there is distortion in the highs if the input signal is too strong.
The guitar is a Gibson Sg-x (the microphone is a 500-T, which has a high level of output).
Would somebody have an idea from where that can come? (I am ready to repair it)

picture: http://images.google.fr/images?q=tbn...marshall75.jpg
schematic(

): http://www.drtube.com/schematics/marshall/5275.gif

PS: I've readed distortion in preamp kit : is it the same problem?
sorry for my bad english

chris wood · 29th March 2004, 06:19 AM

Has this distortion always been there, or is it a new problem? Have you recently changed guitar to the SG?

From the schematic, the amp has a high gain input stage (x10) so with a high output guitar pickup, this could be driving the amp too hard. Is there a gain control? If so try turning it to the minimum.

gerald · 29th March 2004, 07:28 AM

Hello,
No, this is a second hand amplifier, and it always did that with the SG.

indeed, when I lower gain(normal volume?), saturation drops a little, but it is always there...
Is this typical of Marshall??

Thank you

padamiecki · 30th March 2004, 07:20 AM

sorry,
i am araid that you have to turn off the volume in your guitar.
sg has extremely high output and from the schematic i could guess that the full output from 1 stage may be over +-15V (10Xgain 2 V input = 20Vpp)

gerald · 30th March 2004, 09:11 AM

Thanks for your replies,

so I think that I will have to create an attenuator between the guitar and the preamp...

A kind of de-premplifier

runebivrin · 30th March 2004, 09:22 AM

If you just change the 1M resistor (R3) on the input opamp to a lower value, you'll get lower amplification and lower noise. Start with maybe 220k. That'll give you ~1/5 of the gain you have now.

Rune

padamiecki · 30th March 2004, 09:55 AM

Quote:

Originally posted by runebivrin
If you just change the 1M resistor (R3) on the input opamp to a lower value, you'll get lower amplification and lower noise. Start with maybe 220k. That'll give you ~1/5 of the gain you have now.

Rune

CORRECT!!!
but the maximum overdrive will be weaker!

gerald · 30th March 2004, 10:26 AM

You said R3? I will test that.

In the worst case, I will not use any more the distortion, or it will be a little more "bluesy"

Thanks!!

gerald · 31st March 2004, 04:24 PM

cool! it works fine

The distortion is slightly less aggressive.

28th March 2004, 03:44 PM	#1
gerald diyAudio Member Join Date: Feb 2004 Location: france	Marshall input problem Hello, I have a Marshall 5275, which has some problems: It is a transistor amplifier (75 W Reverb) which was manufactured in 1985(or before). The problem is that there is distortion in the highs if the input signal is too strong. The guitar is a Gibson Sg-x (the microphone is a 500-T, which has a high level of output). Would somebody have an idea from where that can come? (I am ready to repair it) picture: http://images.google.fr/images?q=tbn...marshall75.jpg schematic(): http://www.drtube.com/schematics/marshall/5275.gif PS: I've readed distortion in preamp kit : is it the same problem? sorry for my bad english

29th March 2004, 06:19 AM	#2
chris wood diyAudio Member Join Date: Feb 2004 Location: Adelaide, Australia	Has this distortion always been there, or is it a new problem? Have you recently changed guitar to the SG? From the schematic, the amp has a high gain input stage (x10) so with a high output guitar pickup, this could be driving the amp too hard. Is there a gain control? If so try turning it to the minimum. __________________ Old becomes new with time.

30th March 2004, 07:20 AM	#4
padamiecki diyAudio Member Join Date: Mar 2004 Location: Krakow	Re: Marshall input problem sorry, i am araid that you have to turn off the volume in your guitar. sg has extremely high output and from the schematic i could guess that the full output from 1 stage may be over +-15V (10Xgain 2 V input = 20Vpp) __________________ regards, Pawel

30th March 2004, 09:22 AM	#6
runebivrin diyAudio Member Join Date: Jul 2003 Location: Huddinge, Sweden	If you just change the 1M resistor (R3) on the input opamp to a lower value, you'll get lower amplification and lower noise. Start with maybe 220k. That'll give you ~1/5 of the gain you have now. Rune __________________ Do wizards use spell checkers?

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29th March 2004, 07:28 AM	#3
gerald diyAudio Member Join Date: Feb 2004 Location: france	Hello, No, this is a second hand amplifier, and it always did that with the SG. indeed, when I lower gain(normal volume?), saturation drops a little, but it is always there... Is this typical of Marshall?? Thank you

30th March 2004, 09:11 AM	#5
gerald diyAudio Member Join Date: Feb 2004 Location: france	Thanks for your replies, so I think that I will have to create an attenuator between the guitar and the preamp... A kind of de-premplifier

30th March 2004, 10:26 AM	#8
gerald diyAudio Member Join Date: Feb 2004 Location: france	You said R3? I will test that. In the worst case, I will not use any more the distortion, or it will be a little more "bluesy" Thanks!!

31st March 2004, 04:24 PM	#9
gerald diyAudio Member Join Date: Feb 2004 Location: france	cool! it works fine The distortion is slightly less aggressive.

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