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Vintage Tube Amp Ohms...PLEASE HELP

A

Amorica

Dec 19, 2011
43
0
#1
I have a 1979 Marshall Tube combo amp for bass. I'd like to install a period correct speaker. It's current speaker is a Music Man rated at 8 ohms. The replacement is a Celestion rated at 4 ohms. Can I make the switch without having a negative adverse impact on the amp?

Also, if this can be safely done, what benefits/drawbacks are there sonicly to making this change?
 
#2
Amorica said:
I have a 1979 Marshall Tube combo amp for bass. I'd like to install a period correct speaker. It's current speaker is a Music Man rated at 8 ohms. The replacement is a Celestion rated at 4 ohms. Can I make the switch without having a negative adverse impact on the amp?

Also, if this can be safely done, what benefits/drawbacks are there sonicly to making this change?
Click to expand...
Impedance switch?
impswitch.jpg
:bassist:
 
#3
Can you find a schematic for the amp? The output taps on the transformer are what are important. If you have a transformer with windings for 4/8 you could have a tech rewire the output to the 4 ohm tap and be good to go. Otherwise I'd say try to stick to the same impedance as the original otherwise you risk ruining the output transformer, especially considering that Marshall's are notorious for having low tolerance for impedance mismatch.
 
#5
coreyfyfe said:
Can you find a schematic for the amp? The output taps on the transformer are what are important. If you have a transformer with windings for 4/8 you could have a tech rewire the output to the 4 ohm tap and be good to go. Otherwise I'd say try to stick to the same impedance as the original otherwise you risk ruining the output transformer, especially considering that Marshall's are notorious for having low tolerance for impedance mismatch.
Click to expand...
Ain't that the truth.
:bassist:
 
#9
Your amp looks like it's similar to Invalid Link Removed one, but the Bass version, instead of the Lead version. Check the output transformer, it should have a stamp on it (like the picture I referenced). If it's got the 784-139 spec transformer like most marshalls seem to, then - at least as far as I can Invalid Link Removed - it should have 4/8/16 ohm taps. It may be worth having a tech take a look at it and seeing if they can figure out which leads are connected, based on standard marshall schematics (ie which wires from the OT are going to the speaker).
 
#10
coreyfyfe said:
Your amp looks like it's similar to Invalid Link Removed one, but the Bass version, instead of the Lead version. Check the output transformer, it should have a stamp on it (like the picture I referenced). If it's got the 784-139 spec transformer like most marshalls seem to, then - at least as far as I can Invalid Link Removed - it should have 4/8/16 ohm taps. It may be worth having a tech take a look at it and seeing if they can figure out which leads are connected, based on standard marshall schematics (ie which wires from the OT are going to the speaker).
Click to expand...

This was hands down the most useful post anyone's EVER given me. Thanks man...I owe you a beer if you find yourself in Atlanta.
 
#11
coreyfyfe said:
If it's got the 784-139 spec transformer like most marshalls seem to, then - at least as far as I can http://www.talkbass.com/forum/attachment.php?attachmentid=249904&d=1328686927 - it should have 4/8/16 ohm taps. It may be worth having a tech take a look at it and seeing if they can figure out which leads are connected, based on standard marshall schematics (ie which wires from the OT are going to the speaker).
Click to expand...

BINGO...784-139 installed! Can anyone walk me through verifying which leads are presently being used and if needed switching to 4 ohms? Could be in "just take it to a professional" stage if that's the case please speak up. http://www.talkbass.com/forum/attachment.php?attachmentid=249904&d=1328686927
 
#12
I would say this is take it to a professional stage. I was searching through Invalid Link Removed page trying to find a similar enough marshall that had that output transfomer to see which connections should be used and have seen Invalid Link Removed, Invalid Link Removed, or a switch on most of them. It could be that it should be using an 8 ohm speaker and someone just put in a 4 ohm speaker, or it could be that they swapped the taps. Either way, I'd go the safe route and bring it to a tech and have them sort it out. Shouldn't cost you more than the hour of bench time which will be around $50-60.
 
#13
Yeah, old Marshall amp impedance is something you don't want to get wrong (unlike old fender amps, which tolerate all sorts of abuse).
 

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