Using a guitar head as a bass amp. Suggested Mods etc.

I was having a discussion with 94Tremoverb via PMs about dual purpose rigs and it was getting interesting so I thought we could move the discussion here.


Short version:
Guitar amps CAN be used as bass amps. It is guitar SPEAKERS that are vulnerable to bass frequencies and will get blown easily.

1) Contrary to internet wisdom, an actual certified tech says that the components on a guitar amp ARE tough enough to work with a bass guitar. IN fact, the output transformer is stressed HARDER running high gain guitar distortion than it is running bass. You WON'T BLOW the components on the amp with a bass.

It's nonsense that you need more robust parts for bass - if anything, cranking a guitar amp flat-out into power stage distortion is far more stressful for it than clean bass. Back in the old days, Marshall built Super Lead and Super Bass amps (and PA amps) with exactly the same parts apart from a few preamp values, and the bass ones hold up just fine - the only difference is in the frequency response. It's the speakers that tend to blow if you use a guitar amp for bass, not the amp.

It's nonsense that you need more robust parts for bass - if anything, cranking a guitar amp flat-out into power stage distortion is far more stressful for it than clean bass. Back in the old days, Marshall built Super Lead and Super Bass amps (and PA amps) with exactly the same parts apart from a few preamp values, and the bass ones hold up just fine - the only difference is in the frequency response. It's the speakers that tend to blow if you use a guitar amp for bass, not the amp.

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2) I phoned Mesa Boogie and they said plugging a bass into my Dual Rectifier would not harm it.

They recommended I get a bass cab for my head so I went and shopped for one. The $220 I paid for the 210 (used) was as much money as it would have cost me to replace the g12m heritage in my 2 x 12. I'm glad I did, I have the most beefy practice rig EVER! (we aren't even going to discuss bass amp options in that price range)

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3) Run 6550 power tubes and a 12AT7 for a phase inverter so maximize headroom and minimize distortion

4) Use high efficiency speakers. A 100watt amp running 103db / watt / metre speakers is at LEAST as loud as a 300watt amp running 97db/ watt / metre speakers. More wattage simply increases clean headroom.

5) USE ENOUGH SPEAKERS! A 210 doesn't move enough air for bass. Run at least a 210 with a 115 or preferably a 410 with a 115 or 215 etc.

I know the 2x10s can be loud (esp. with neodymium speakers) but they just don't seem to fill a space like the big cabs do, in my experience... I've never heard one that can really cut it for a gig unless the amp is going through the PA too. 4x10"s are much louder - doesn't really matter if it's one cab or two, in fact an Ampeg 8x10" is actually four sealed 2x10"s in the same outer box!

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SOOO after replacing a dead 9v battery, I really discovered what the biggest problem was with my current setup. Fender P-Bass with EMGs into a Dual Rectifier (6L6s Bold Diode) into an Ashdown 210 with 102db / watt / metre speakers. There is a big difference dialing in sounds for bass on a guitar amp than there is for dialing in guitar tone on a guitar amp.

1) EQ settings. I find no matter what, DIME THE MIDS. That is a good starting point

2) Treble now means High Mids. It doesn't add a whole lot of cut to the tone. I leave it at 12:00

3) As you turn up, turn the bass down. When reaching near maximum output on the amp, I had the bass at 0 and it simply made the tone get even beefier with no cut. When the levels are super low, you have to add some bass to phatten it out but for rehearsal situations, zero-ing the bass will help you be much more audible.

4) Presence is the new treble. The presence is now the most important tone shaping tool on the amp, period. At 12:00 it makes for a fairly balanced tone but you can crank it up almost to full to add much more bite to the tone. I found between 12:00 and 5:00 to be the most useful region for increasing definition in the tone. You can drop the treble below 12:00 to get into more muted jazz tone territory.

I haven't tried hooking more bass speakers up since I have no more. I think that having more will make this bass 'rig' a much more viable solution where adding KT88s or 6550s would be just 'icing on the cake'. I definitely want to swap a 12AT7 for the phase inverter just to give a bit more clean headroom since they are only $10 or so.

Depends on how you're running it and how hard. I did a little internet research (how scientific :lol: ) because I was filling in to play bass for a week at a church camp and we played a couple times a day. I had to borrow a bass and the sound guy said that they tried just running the bass direct last time and that the PA was having trouble keeping up. I almost didn't get to borrow an amp but a buddy of mine came through last minute with his little 100w Ampeg 1x15. Back to what I was saying... of all the stuff I read through that had seemingly legitimate reasons to back up their opinions I found that the biggest caution that people were throwing out was the speaker handling capabilities. Bass speakers are not only rated for higher powered abilities but have an EQ curve that is closer to a full range (PA, keyboard or acoustic amp) system. I have an acoustic cab rated at 300w so I though I'd be fine. I also wasn't playing all that loud so the power section wasn't going to be pushed to extremes.

You seem to be on the right path with the bass cab already, I just don't know the full extent of your application of it. As long as you're not at a volume to where you're pushing it to produce more than it can take then you should be okay. If you're getting overdrive from the power section I'd back off. Best case scenario would be burning up your tubes real quick, worst case would be blowing a tranny and when that happens other stuff on the board can usually get messed up real bad along with it. Remember that bass amps are usually rated at least three times higher than their guitar counterparts. (ie Dual or Triple Rec vs Bass 400+)

If I remember right Paul D'Amour and/or Justin Chancellor from Tool used a Dual Rec in their bass rig earlier on and it helped give the bass a real grinding middy sound. "Opiate" has a crazy super present sounding bass.

See, this is the sort of miss-information that started this PM discussion in the first place. Basically, everything about guitar amps not being robust enough run bass is complete hogwash. The more watts you have, the more clean headroom you get, with the perceived volume increase getting less and less for each 100watts added. That is all. Modern bass amps are more powerful because people want clean tone with a furiously thumpy low end. A lot of people just talk on the internet and they really are not experts in what they are discussing.

Hey man, Do you mind posting a link from your talkbass forum? curious to see whats being said over there and dont want to dig through all the different topics lol. thanks.

http://www.talkbass.com/forum/showthread.php?t=661954

It is an interesting thread and an interesting forum. Just be warned that there is a lot of misinformation floating around the net about gear, specifically guitar amps being tasked as bass amps.

I saw a band last week where the bass player was playing through a Boogie Mark IIC+ guitar amp at 100 watts...you cant hurt the guitar amp but you need to be sure you have speakers that can handle the frequencies that you are pushing...this guy was using an Eden 410XLT cabinet...it sounded awesome

I could not have found this thread at a more perfect time. I'm planning on just doing recording and really don't feel like buying a whole bass rig. Would it be possible for me to just buy a bass guitar and a speaker? Not the whole cab, just the speaker. Could I put the speaker in my thiele? I know it's a pretty tough design meant to handle a EVM12L. Since it's only for recording, I really don't see the need to spend a whole bunch of money on a whole new cab. Besides, maybe I could pour all the money I'm saving into a really nice bass. Any suggestions on speakers?

As far as I understand, bass speaker enclosures are more 'complicated' to design than guitar ones. i.e. they function differently because of the lower frequencies. (Talking out my *** here, just regurgitating what I heard from a guitar cab builder)

Just find a good used 210 with a horn and you'll be set. Run that with a guitar head and a bass and it should sound great.

Tips: 1) turn up the mids. WAY up 2) Turn down the bass. 3) adjust Treble and Presence to taste. Treat Treble like High Mids and Presence like Treble. Keep in mind the frequency response of a bass guitar is very wide.

If you are using a mark series amp, I bet engaging the EQ and running it like a frown instead of a smile will have favourable results. For my Dual, I dime the mids, zero the bass, and then run 12:00 treble and like 3:00 presence. I also turn the gain way down to keep the preamp as clean as possible.

I was running 6L6 tubes with diode and the variac set to bold. Basically, the more headroom the better.

Hmmm. Not to worry. I talk out my *** all the time with crap I overhear. I'll need to ask around about that because now you got me worrying though. Although it is a Thiele cab so I'm just thinking that it MUST be fine for bass. I'll have to check though. I don't have a presence knob or a graphic EQ, although I do have a contour knob, so I'll mess around with that. Maybe I'll put a Tung Sol in the first gain stage just to liven things up.

SD Boogie said:

I saw a band last week where the bass player was playing through a Boogie Mark IIC+ guitar amp at 100 watts...you cant hurt the guitar amp but you need to be sure you have speakers that can handle the frequencies that you are pushing...this guy was using an Eden 410XLT cabinet...it sounded awesome

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I have that setup at home! I'll have to give it a try with my bass!

EtherealWidow said:

Hmmm. Not to worry. I talk out my *** all the time with crap I overhear. I'll need to ask around about that because now you got me worrying though. Although it is a Thiele cab so I'm just thinking that it MUST be fine for bass. I'll have to check though. I don't have a presence knob or a graphic EQ, although I do have a contour knob, so I'll mess around with that. Maybe I'll put a Tung Sol in the first gain stage just to liven things up.

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The thiele is designed for the optimum response of the specific guitar speaker you have in there. If you got that from boogafunk, then he would have modeled it for the exact guitar speaker model. Bass speakers have a surprisingly different voicing / response curve and you most likely won't get the results you want.