Bass amp suggestions?

[firemedic]

I have a Twin chassis on the way, the price was right.
But I already have a BF amp.
I wanna build a bass amp head out of it.

I'm thinking Ampeg since I love my Reverberocket. Any suggestions? Hiwatt? Fender? 

[G._Hoffman]

Portaflex.

Easier than an SVT, and sounds better most of the time.


Gabriel

[firemedic]

Let me check this out.......

Looks good. How difficult would it be to make it a 4-tube output? EL34 compatible??

I also have some 34s laying around but no amp at all to use them in. All my sockets are wired right but I'll need to install an extra Xformer for the heaters.

But back to the bass amp: AFAIK the 7027 operates at roughly the same specs as a 6L6GC but with different pinouts. Might I need more driver current for double the output tubes?
   

[tubeswell]

B15N

[firemedic]

Freakin Ampeg. They couldn't seem to make an amp with tubes that I have.
I should clarify: I have a blackface Twin chassis coming. i.e. 4 output tubes. Should I try another design or will the B15N get me where I need to be with minor mods?

[spacelabstudio]

Freakin Ampeg. They couldn't seem to make an amp with tubes that I have.
I should clarify: I have a blackface Twin chassis coming. i.e. 4 output tubes. Should I try another design or will the B15N get me where I need to be with minor mods?

I would maybe start with Blackface Twin power amp as that just gets lots of nice clean current to the speaker, for the most part.  Then mate it with whatever preamp you like.  I have a Bassman Ten whose "studio" channel I'm quite partial to, personally.

Chris

[Danskman]

AB165 preamp with 4x6L6. My next "bassman 100" re-build... I think this one will growl nicely.
I don't ever heard a b15N live, though, but I surely will try to build one next year.
BR,
Danskman

[kagliostro]

An Hiwatt with 4 el34 instead of 6 ?

Kagliostro

[firemedic]

Okay, I'm nebulizing ideas....How about a Twin Reverb output but with a Marshall 100w OT? Clean is the desired effect. Also to be used as a generic huge (for me) power amp.
I imagine I could drive it w/ something like a B15S preamp- 12AX7s- I'm really liking the Ampeg sound. Tho I dig the paralleled stages of the Hiwatts! And I do have the socket spaces.
Suggestions? Misgivings?

[ernie_jr]

You could try a Sunn 2000s with 4 6l6's instead of the 6550's.
This would give you about 100 watts and it is a simple build.
Ernie

[firemedic]

That looks good too. This is gonna be some kind of hybrid of several different amps, no doubt. Simple is good.
Since I'll have the chassis space I'll make some sort of guitar channel too, with a tube-buffered effects loop.
 

[sluckey]

What kind of Twin? I've been assuming blackface, but there are lot's of Twins--- tweed, Twin Reverb (black and silver), evil, etc. And is the chassis complete and working?

[SoundmasterG]

You could try a Sunn 2000s with 4 6l6's instead of the 6550's.
This would give you about 100 watts and it is a simple build.
Ernie

The only problem with doing a Sunn 2000S is the cost and size of the OT....which in a stock early 2000S is as big as a baby's head and amplifies cleanly down to 10 Hz! I doubt if the stock OT would fit on the Twin Reverb chassis.

I vote for the stock Twin Reverb output section, with a bigger OT since you want to use the amp for bass, and a B15N preamp in one channel and a Fender AA864 bass channel preamp in the other channel. You could stick with the Fender LTP PI or go with the Ampeg paraphase PI depending on taste. This would give you a bass amp with some versatility in the tones. I would stay away from a guitar channel in there as a bass amp design is quite a bit different than a guitar amp design and to make either work well you tend to compromise both.

Greg

[firemedic]

Sluckey, it's just a chassis. "AC80" I think is the designation. Weber is selling them for $15 each, I had to bite. It looks like a BF style but I don't have exact specs; I only know I've got 4 octal spaces to work with & I suppose 6 novals. I know I'll have to drill it at least for the OT.
This will be a scratch build. When I get the chassis I'll know more.

Ernie Jr I appreciate the reality check.
My only problem w/ B15N is I want to use tubes I have on hand, 12A_7s, 12DW7 &/or 5879s. Is there an easy conversion from 6SN7s?

BTW, any thoughts on the 100w Marshall style OT Doug sells? Somewhere it seems I heard they are more finicky with matching impedances, or is that just with EL34s?
I just want to get that little nagging question out of the way.

[birt]

a long time ago i started a similar topic where i came up with a bass amp that has 3 preamps. fender, sunn and ampeg. it's going slow (too much touring, no time in the workshop) but i now have the cabinet ready, the chassis (super twin reverb) has the PI and PA and i'm drawing the preamp layout.

[firemedic]

I'm not a bass player (I don't even own a bass) so 2 preamps will suffice.
BTW any suggestions for a decent low-end bass guitar?

[ernie_jr]

Look at Fender Squire line of basses

[6G6]

Hey FM,

 How serious are you about building a bass amp?
I am a bassist, so I can tell you headroom is your friend
on the bottom end!

Since you are starting from a bare chassis, you can go in a lot of directions.
I'd start with finding how much P/S you can sqeeze in, then select output tubes
to take full advantage of that.
4 big octals;
KT88s, 6550s, maybe you could be amoung the first to see what you can squeeze
from KT120s?
Shoot for 120-200W.
More wouldn't hurt.

Pre could be B15N, Sunn 200/2000S, or Fender.
Fender?
Don't laugh, the famed Alembic preamps are nothing more than AB763 pres!

Also, you can't scimp on speakers.
I'd recommend you check out fEARfuls.
It's a roll your own design, full range, high power, no compromise.

Plan on heavy, expensive...and fun when yer done. 

[6G6]

fEARful link:
http://greenboy.us/forum/index.php?sid=771c057aa974392ea1906be93f2c0ab7

[firemedic]

Serious as a heart attack.
Since the chassis is for a TR I was just planning on a TR PT w/ a 100w Marshall OT (twice as heavy).
I have a 3A 6.3v filament tranny I guess I could squeeze in there for bigger tubes though I hadn't really planned on that.
I was hoping to use tubes I have laying around; I have a pair of 6ca7 & a pair of EL34 I still may use but I'm still bouncing ideas around.
I'm meeting a guy on Tuesday about 2-custom 15" speaker cabs w/ no speakers. That's probably the most I will be using w/ this amp so even 4-6L6GCs would probably get the job done. 

[6G6]

For good example of what you can do with 4 octals, see the Fender 300PS.
It uses an interstage trans, instead of a more common PI, but it does
squeeze 300 watts out of a quad of 6550s.
Quite convincingly, too.

There is a guy on evil bay who was an engineer at Fender, working on
that and the 400PS.
He still has a stock of left over transformers.

[blown240]

For good example of what you can do with 4 octals, see the Fender 300PS.
It uses an interstage trans, instead of a more common PI, but it does
squeeze 300 watts out of a quad of 6550s.
Quite convincingly, too.

I absolutely agree.  I have a 300PS as my main bass amp.  I used to play in a LOUD punk bank. We would mic the drums for shows and the guitarists has 3 100watt Marshall stacks.  The guys would always ask me to turn down the 300ps because it thundered!

[firemedic]

Well I'm not likely going to build a 300 or 400PS, sorry guys. That is a level of complication I don't want to take on. 100W is plenty for my needs.
But I will have a look at the schems.

[6G6]

If you only want 100 watts, a 100 watt Bassman
would be simple to build in that chassis.

If you wanted to go big, a single channel version of the 300PS
isn't that much of a stretch.

[firemedic]

The 300PS looks like it has a 6V6 as a driver? is that right?

[blown240]

Yup, it sure does.

[firemedic]

Yeah, that's a little over the top for me.

My current plan is to use the output section of a bassman 100, except bigger gridstoppers & screen resistors. I'll probably use a 5814 or 12AU7 instead of a 12AT7. And EL34s instead of 6L6GCs. I'll use an auxiliary filament tranny. Will I need to alter the PI circuit or power tube grid leaks?

One preamp will be AA864 Bassman.
The other is open- I'd like to do a portaflex but the B15s is the only one w/ noval preamp tubes. Everyone is pushing the B15N. Is the B15S lame or something?

I'll make the power rail as beefy as space permits.

I'm meeting some guys from craigslist next week- one has 2-custom 15" cabs that are empty, and one has a new in box 15" Peavey Black Widow for $100. So this is kind of coming together. I'll be using one of the cabs occasionally as an extension speaker w/ my Vibroverb build.   

[SoundmasterG]

Everyone is pushing the B15N. Is the B15S lame or something?
  

They are just different than the B15N, and the B15N is one of the best sounding bass amps of all time, so people tend to look at that one only when they consider any of the Portaflex models. You could grab the normal preamp channel from an SVT...it is similar to a B15N but uses the smaller tubes instead of octals. You would have to go with something more normal for the tone stack though instead of the inductor unless you really wanted a challenge.

Greg

[birt]

you can sub the 6SL7 tubes with 12AX7. they are not identical but a close match.

[ernie_jr]

Look at the ampeg b26. use the 2nd channel pre amp. it has a 12ax7 and it sound great for bass. used it in one of my builds and i am using it in a current build
ernie

[firemedic]

OK, I will. Back to the B15N for a second. What kind of PI is that? It's almost like a paraphase inverter but not quite. I'd just as soon use the B15 PI if that is part of the sound, but would I need to modify it for 4 EL34s?

I know I'm being needy here with perhaps obvious questions, thanx for helping me with this gentlemen!

[sluckey]

It is a paraphase inverter and is probably part of the sound. I wouldn't think you would need to change anything about the PI just to use it with EL34s.

[firemedic]

I meant in terms of needing to increase drive current or voltage.

[sluckey]

Me too.

[firemedic]

Four EL34s? Double the power tubes?

[firemedic]

The heck with it. I've got enough Fender bulids.
Sunn 2000s power amp & preamp. Looks like a tetrode feeding a concertina PI, I should be able to use a 5879 to feed a triode. (BTW is it safe to leave the unused triode totally disconnected w/ the heater on?)
B15N preamp for 2nd channel.
Maybe a tremolo circuit. Why not.

 

[sluckey]

The Sunn is a nice amp. I'd definitely keep the original 6AN8 circuit. It's a proven design. Yes, you can leave an unused triode section totally disconnected.

What parts do you have that you must use on this project? Just a chassis? The 2000S needs 6 octal sockets, a looong chassis, and has a unique layout that could be hard to achieve on your chassis even if you go SS on the power supply. Iron and tubes for the 2000S will probably set you back $400-500 and may be difficult to find. I'm not trying to say you can't or shouldn't build a 2000S in a TR chassis, just trying to get you to think about it from a logistics viewpoint. And also ask yourself, "Do I really want to put this expensive experiment in a $15 bargain bin chassis?"

Sunn 200S is the same amp but only a pair of 6550s and iron to match. It will be loud enough for most any venue except maybe Shea Stadium. And it could easily be adapted to your chassis.

If you already have a Marshall 100W OT and must use that, you may consider building a Marshall PI, power amp, and power supply. And build the preamp of your choice in front of it.

I'm just throwing out ideas since you don't seem to be locked in on anything specific yet.

[firemedic]

SS bridge recto, EL34s instead of 6550s. I'd be using a 100w Marshall OT and TR PT. A 6.3v trans will power the EL34 heaters. I was really looking for an EL34 quad output I could just copy. And I like the split load driver, don't ask me why.

I'm going to try a 5879/12a_7 just to see if it sounds good.

I was going to use a regular screen supply instead of the UL taps. And I use lots of shielded wire.
 
The chassis is actually pretty nice. It was in Weber's bargain bin. Not perfect but almost.

So i guess it won't really be a Sunn 2000s but if it works & is clean all the way up w/ good tone, well, that's what I wanted and that's why we're here, right?

[sluckey]

that's what I wanted and that's why we're here, right?

Yes. Plus, the coffee ain't bad either!   

[SoundmasterG]

The stock Sunn 200S and 2000S sound fantastic until they distort, and then they still sound fantastic, but different. They tend to distort around 5...you can control this by limiting the gain in the preamp....lower voltages on the supply can be done to accomplish this. Listen to any early Rush stuff to hear distorted Sunns with roundwound strings on a Rickenbacker or Fender Jazz. The 2nd Jimi Hendrix album "Axis: Bold as Love" is a Sunn bass amp...roundwound strings...Fender jazz bass, but it is generally cleaner recordings. A lot of America's stuff was a Sunn amp with flats on the bass. Some of the Doors later recordings were also Sunn amps. A large part of the Sunn sound though is the huge and very good quality transformers used. You won't get that sound with a Marshall transformer....just not big enough or good enough quality. I could share some pics of the stock Sunn transformers and you should shit bricks...they are HUGE, and very expensive.

Another thing you may consider....the Marshall Super Bass....basically the bass version of the plexi. That would be four EL34's and a bass amp that was often used back in the day. Another amp that was used with that tube configuation is the Vox AC100...these were used for guitar and bass without changes and even uses a floating paraphase inverter, so it is similar to what the B15N is using. I built myself a clone of an AC100 that isn't quite done yet, but haven't tried it a lot for bass. It sounds ok, but my Sunns and Ampeg SVT sound better for bass than the Vox design. The Marshall was used a lot though by a lot of bands back then and sounds good....so you may check it out.

Greg

[firemedic]

Ok, thanks SG! I already ordered the transformers so let me peruse the Marshall SB.

[nateflanigan]

My most recent build was a sunn spectrum (same as 200s but with a guitar tone stack), I used BF bassman transformers and changed a few things at the tail end of the output section to jive better with that iron as opposed to the UL baby head on the original.  It's a really cool amp, clean but by no means sterile, really big and articulate sounding and definitely different from a fender.  I think you'll be very happy with your build.

[firemedic]

My latest plan is this: Marshall SB output, PI values for a 12AU/T7 (closer to a Bassman 100 PI).
Sunn pa for one channel & a modified B15s pa for the other channel. I chose the "s" because I do not have octal pa sockets and it's a known circuit.

The transformers arrive next week and I've procured 2 15" MDF cabs. Anybody have opinions regarding the use of folded horns (w/ crossovers) in bass cabs? I could either keep the horns or cover the openings & install ports. I'm leaning towards porting.

[6G6]

Get the best bass speakers you can and use WINisd to find the best porting for the cabs you have.
It will help if you use speakers that you know the T-S parameters of.

[firemedic]

Please peruse my design, I'm sure there are some mistakes but you get the idea. It's a hybrid B25b, AA864 and big Marshall output section. If there are any glaring issues please be brutally honest.

[sluckey]

Your AA864 tonestack is different from the original.

[firemedic]

It's the AA864 bass channel tonestack with no changes. I increased the cathode bypass values in that and the B25 channel is a hybrid of the 2 channels found in that amp.

I'm not real worried about the preamps, but the output section is a real frankenstein of a Bassman 100 PI w/ Marshall NFB and modified Marshall power tubes, even the bias section will have to be tweaked.

I plugged my 40w TOS into the Peavey Black Widow cab, with a bass guitar, and it was immediately apparent I'm going to need a lot more power to drive this speaker.
Granted, the TOS rolls off a lot of bass. But still.
That's why I'm trying to design for EL34 and 6550. 

[sluckey]

It's the AA864 bass channel tonestack with no changes.

Yes but your drawing is incorrect. There are three 0.1µF caps in the tonestack. One side of each cap ties together. Then one cap goes straight to ground. Another cap goes to the Deep switch which is also connected to ground. The third cap connects to the junction of the treble and bass pots. Take a look...

 

[firemedic]

See, I told you there'd be mistakes. But any thoughts on the power section & PI?

[sluckey]

But any thoughts on the power section & PI?

Looks like it will work. But...

Why 12AU7 and 330K grid resistors? And resistors across the power supply diodes? And such a big input cap?