Converting a guitar amp -> bass amp!
shteyven
Thread Starter
#1
13th May 2012
Old 13th May 2012
  #1
Gear nut
 

Thread Starter
Converting a guitar amp -> bass amp!

Hi everyone,
Hope this is the right forum.
It's a long post, tl;dr - what do I need to do to the amp head to make it bass suitable?

I have a Marshall MG HDFX100 and matching 4x12 cab, and I want to convert it into a bass head/cab.
I think I'm sorted on the cab end of it, such as changing the build of it, swapping the 4x12" for 4x10", though I may do 2x10x2x12 haha.
I guess I'm knowledgable about ohmage, speaker type/build etc, I just need help with the head!
I'm hoping I get to open it up and really delve into it, just have one channel and put a more in depth eq in (using the spare knobs from the 2nd channel etc)
What do we reckon?
Hope you can help guys!
You'll get to decide how it looks ;D

Sent from my GT-I9100 using Gearslutz App
#2
14th May 2012
Old 14th May 2012
  #2
Lives for gear
 

Is 100 watts going to be enough? (I mean, this is a Marshall, it is going to be a little blurry and distorted for bass, but I can only assume this is what you want, yeah?)

If so, leave it alone and put some tone shaping in front, a parametric or something.

I made a horrible mistake in the 1970s and modded a Marshall bass head into a guitar head. Stupid move.
#3
14th May 2012
Old 14th May 2012
  #3
Lives for gear
 

Plug in a bass cab and see how close that gets you without modifying the head. I would suggest a cab without a tweeter.

Again like the above poster said it probably won't be very clean especially at high volumes...
shteyven
Thread Starter
#4
14th May 2012
Old 14th May 2012
  #4
Gear nut
 

Thread Starter
Quote:
Originally Posted by kslight View Post
Plug in a bass cab and see how close that gets you without modifying the head. I would suggest a cab without a tweeter.

Again like the above poster said it probably won't be very clean especially at high volumes...

Yeah, I was going to avoid a tweeter anyway, forget that hassle!
Haha. So you suggest upping the power if I'm wanting it to stay clean?

Quote:
Originally Posted by Bill@WelcomeHome View Post
Is 100 watts going to be enough? (I mean, this is a Marshall, it is going to be a little blurry and distorted for bass, but I can only assume this is what you want, yeah?)

If so, leave it alone and put some tone shaping in front, a parametric or something.

I made a horrible mistake in the 1970s and modded a Marshall bass head into a guitar head. Stupid move.
I imagine so!
If not, I'd only need to up the power of the power amp right?
That'll probably be the easiest bit haha.
As I'm buying new speakers I can make sure they handle the power.
A quick side note, any particular reputable manufacturers for them?
It's more of a see how it sounds, if it ain't right, fiddle with it! Haha.

Sent from my GT-I9100 using Gearslutz App
#5
16th May 2012
Old 16th May 2012
  #5
Marshall guitar amps are voiced for guitar with a 90 hz roll-off in the first tube stage, the .68 cap across the 2.7k resistor. Replace that cap with a 22 uf electrolytic.

The .022 uf mylar coupling caps are also a bit small, replace them (not in the tone stack) with .1 uf 600 volt mylar or polyprop film.

Then you will have bass, Noel Redding style.
#6
16th May 2012
Old 16th May 2012
  #6
Lives for gear
 

Pushing bass frequencies at the same SPL as guitar requires more power. The output section will have to be beefed up for stage volume. Without a beefier output transformer and sufficient power tubes you will burn them up. Ask me how I know.

Also more power means more heat you have to dissipate. Is the original chassis and head cabinet large enough to dissipate the heat? Blackface Fender Dual Showman heads were originally too small and the chassis got so hot that you would burn your fingers reaching for the power switch on the back. If it is solid state are the heat sinks for the output transistors sufficient?

Converting guitar amps to bass amps is a lot harder than the other way around.
#7
16th May 2012
Old 16th May 2012
  #7
Lives for gear
 
JohnRoberts's Avatar
 

If you haven't been talked out of this yet let me try..

Bass and lead guitar amps are apples and oranges.

The lead guitar is more an extension of the instrument, not linearly reproducing the input but. making pleasing guitar sounds. Same for the cabinet and speakers.

The bass guitar amp OTOH is more linear reproducing tones. The bass amp covers lower frequency range and requires more power.

They are just two entirely different animals.

JR
#8
17th May 2012
Old 17th May 2012
  #8
Lives for gear
 

^ Talking about modern amps, sure. OTOH, one of the most venerable bass amps in the studio (the B15) shares the same circuit as Ampeg's guitar amps. I don't know enough about that particular amp to say how much merit your project has. Perhaps check out the schematic for the jcm800 bass amp for some interesting tone options.
shteyven
Thread Starter
#9
17th May 2012
Old 17th May 2012
  #9
Gear nut
 

Thread Starter
Cheers for joining in guys!
It literally is my first, tacky guitar amp. It's about 8 years old and and solid state!
I've got a TSL now, so I have nothing to lose regarding this amp
Let's go crazy.

Quote:
Originally Posted by Jim Williams View Post
Marshall guitar amps are voiced for guitar with a 90 hz roll-off in the first tube stage, the .68 cap across the 2.7k resistor. Replace that cap with a 22 uf electrolytic.

The .022 uf mylar coupling caps are also a bit small, replace them (not in the tone stack) with .1 uf 600 volt mylar or polyprop film.

Then you will have bass, Noel Redding style.
There are no tubes!
Do you know anything regarding Marshall's solid state stuff?



Quote:
Originally Posted by The Real MC View Post
Pushing bass frequencies at the same SPL as guitar requires more power. The output section will have to be beefed up for stage volume. Without a beefier output transformer and sufficient power tubes you will burn them up. Ask me how I know.

Also more power means more heat you have to dissipate. Is the original chassis and head cabinet large enough to dissipate the heat? Blackface Fender Dual Showman heads were originally too small and the chassis got so hot that you would burn your fingers reaching for the power switch on the back. If it is solid state are the heat sinks for the output transistors sufficient?

Converting guitar amps to bass amps is a lot harder than the other way around.
It has a fan I'm replacing anyway, which will help, I haven't looked regarding heat sinks yet, but I'm sure I can sort that out

Quote:
Originally Posted by JohnRoberts View Post
If you haven't been talked out of this yet let me try..

Bass and lead guitar amps are apples and oranges.

The lead guitar is more an extension of the instrument, not linearly reproducing the input but. making pleasing guitar sounds. Same for the cabinet and speakers.

The bass guitar amp OTOH is more linear reproducing tones. The bass amp covers lower frequency range and requires more power.

They are just two entirely different animals.

JR
The cab, by the time I've finished it, will be for all intents and purposes, a dedicated bass cab, both build and speakerwise.
Regarding the amp, that's why I enquired about any circuitry changes, if it needed any! Haha.
I know it might go belly up but it's just a bit of fun and maybe a cheapish bass amp ;D
Do you have any ideas regarding circuitry?

Quote:
Originally Posted by TAVD View Post
^ Talking about modern amps, sure. OTOH, one of the most venerable bass amps in the studio (the B15) shares the same circuit as Ampeg's guitar amps. I don't know enough about that particular amp to say how much merit your project has. Perhaps check out the schematic for the jcm800 bass amp for some interesting tone options.
Thanks, will do!
I shall be up to my knackers in schematics! Haha

Sent from my GT-I9100 using Gearslutz App
#10
17th May 2012
Old 17th May 2012
  #10
Lives for gear
 
JohnRoberts's Avatar
 

Quote:
Originally Posted by shteyven View Post
Cheers for joining in guys!

The cab, by the time I've finished it, will be for all intents and purposes, a dedicated bass cab, both build and speakerwise.
closed back?
rigid sides?
different drivers?
Quote:



Regarding the amp, that's why I enquired about any circuitry changes, if it needed any! Haha.
I know it might go belly up but it's just a bit of fun and maybe a cheapish bass amp ;D
Do you have any ideas regarding circuitry?



Thanks, will do!
I shall be up to my knackers in schematics! Haha

Sent from my GT-I9100 using Gearslutz App
Extend frequency response lower than lead guitar pre...

perhaps less gain, does fuzz (clipping) distortion sound good on bass?

JR
shteyven
Thread Starter
#11
17th May 2012
Old 17th May 2012
  #11
Gear nut
 

Thread Starter
Quote:
Originally Posted by JohnRoberts View Post
closed back?
rigid sides?
different drivers?


Extend frequency response lower than lead guitar pre...

perhaps less gain, does fuzz (clipping) distortion sound good on bass?

JR
Yep, yep and yep
Though I'm thinking of how to put a port/vent in the cab, might make it a 2x10, so I have room for one, that'll surely make for a punchy sounding cab.

I'm going to attempt (ha) to modify the amp so it's just using one master channel, make it as clean as possible. No need for any kind of fuzz/distortion, I doubt any kind of clipping will sound nice with solid state haha.
I think i've got a solid state bass amp I can borrow to look at as well. See what the crack is.

Starting the cab this weekend all being well!


Sent from my GT-I9100 using Gearslutz App
#12
18th May 2012
Old 18th May 2012
  #12
Lives for gear
 
jimcroisdale's Avatar
 

Don't put a port in the cab.

Unless you know exactly what size port you need it would be complete guesswork and would probably not improve matters. Tuning a cab with a port requires a bunch of calculations done with software, and you would need the TS Parameters of your speaker drivers, as well as the internal volume of your cab. Get it wrong and your speakers will end up in the bin.

Closed cabs, on the other hand are much more forgiving, although they are less sensitive than ported cabs, and with less bass extension.

In all likelihood though, your guitar speakers are not up to the job of handling low frequencies at significant volumes - they will start to fart out quite early on.

Wattage ratings and speaker driver size mean very very little in the world of bass speakers, so those figures are nothing to go by.

A 100w solid state amp is also woefully underpowered for anything other than a practice amp, and if that's all you're needing, a 4x12 is unnecessary.

Not wanting to dampen your spirit, but I think this has been tried too many times without success - as one poster says, guitar and bass cabs/amps are apples and oranges.

Jim
Topic:
Thread Tools
Search this Thread
Search this Thread:

Advanced Search
Similar Threads
Thread
Thread Starter / Forum
Replies
antiguru / Geekslutz forum
26
FFTT / So many guitars, so little time!
70
pan60 / So much gear, so little time!
12
Gezrick / Geekslutz forum
2
drBill / So much gear, so little time!
12

Forum Jump
 
Register FAQ Search Today's Posts Mark Forums Read

SEO by vBSEO ©2011, Crawlability, Inc.