bass amp project

[John]

So, sometime in the coming year I'm going to build a tube bass amp for a buddy. I know SS makes way more sense, but he'd like me to build him a tubester, and I've often wanted to anyway.


I figure I'll pretty much copy the Ampeg B15 circuit, one that uses 6SL7 in the preamp and PI, and the 2 6L6 for output. Gonna keep it as simple as possible, so probably cathode bias.  This is supposed to be pretty portable, so a 10" Weber lightweight bass speaker, most likely. I think I have a PT and OT that will work well enough.


Ennyway, before I get into the layout and noodling around with the schematic, is there anything obvious I'm missing? Asking because of my penchant for overlooking the obvious.

[kagliostro]

Ciao John

Have you considered one of this "ready to build" projects ?

(schematic - layout - chassiss, all is ready to be realized)

http://ctgelectronics.weebly.com/custom-amplifier---fcn-51-b.html



this is an evolution of the previous

http://ctgelectronics.weebly.com/custom-amplifier---fcbn-51b.html



you must only build it and .... voilà le jeux sont fait

Franco

[John]

Hey Franco, that's very similar to the circuit I'll be using for sure. Except just the one channel/input.

[Paul1453]

Those designs K has posted look good.

A Sweetwater catalog showed up in the mail last week.

Looking at the bass amp section, most are SS.

But Mesa has the Eight:88 and Four:88 with big wattage output.

A slight modification to K's designs, to use KT88's or 5881's might be something to consider.

I like 6L6's but the others can take higher voltage and put out bigger wattage.
2 KT88's can give you 100W no problem, 6L6's maybe half that.

It seems that the higher wattage is required to move big speakers and give that big bass thump.   

[ernie_jr]

If you are looking for bigger power, look at a SUNN 200s schematic. With solid state rectifiers it will give you about 80 watts from 2 x 6550 or kt-88's.
Ernie

[John]

Thanks for the replies. Not looking for more power, he's looking for something to take along to jam sessions, something lower powered and (fairly) light weight. He and I both know that SS is much more practical, but he'd like me to build him a tube amp. And I need a nice slow project. ;)

[Cobbler]

I need a nice slow project. ;)

Don't go too slow...I have about the same project in the hopper...

[Paul1453]

Thanks for the replies. Not looking for more power, he's looking for something to take along to jam sessions, something lower powered and (fairly) light weight. He and I both know that SS is much more practical, but he'd like me to build him a tube amp. And I need a nice slow project. ;)

It seems that even 100W is considered lower powered for a bass amp in that catalog.   

I think that the weight difference between a higher voltage, higher wattage KT88/5881 amp vs. the 6L6 design would be rather negligible. 

If 50W or less of output power adequately thump the bass speakers he will use, then regular 6L6s is what I would use.
Since I only have 2 5881s and no KT88s. 

My silly kids have just recently discovered how good my tube amps can make their rap music sound.
They want a bass response that nearly pumps the air out of your lungs.   

[EKDENTON]

As long as you can make one that has the tone that he likes .....it can be small for jamming in small rooms. If he does a concert or plays somewhere that he needs more volume he can mic the cab into the PA or soundboard and be as loud as he wants and retain whatever tone the amp has. Almost everyone mics their cab nowadays anyway. I hardly ever see bands where everyone is using solely their own amp for volume. They would be called the "Rogue Band" LOL. Sound guys/gals now like to have complete control of everyone on the sound board.

[John]

I believe this is what I'm going with. Bias is a mix of the B-15 Ampeg, and Sluckey's Sunn amp with the individual bias pots (Thanks Sluckey!) So far I've only tested the PT for voltages (540'ish unloaded DC) and the bias - adjusts from -35 to -65 each pot.


PT and OT are from an HH Scott hi-fi carcass I had bought years ago when I was just getting into this. It was running 6V6's not the L6s. However, both trannies are much beefier than what would normally be in a guitar amp with those tubes, so I'm crossing my fingers that they'll supply the current needed for the 6L6 bottles. That amp also used a 5Y3 for rectifier, and I'm going with SS diodes. If not, then I'll buy what I need.


I plan on breadboarding this before I start the actual build. If nothing else, wire up the power tube sockets and a dummy load and see what kind of voltages I get. If it sags down to 350 I'll go to Plan B. If heater draw is the only problem I'll stick a filament transformer in there.


Not all component values are listed in the schematic, but I plan on going with Ampeg's values on their schematic. My buddy Dave says that his first bass amp was a B-25. This is pretty much the B-15n but with just one input.


I'm currently in the middle of putting new galvanized roofing on the loafing shed, so this is not going to progress very quickly. But, the milk house does have AC in it now, so.... 

[Paul1453]

You've got individual bias pots, but they don't seem to be fail safe.

If the pot wiper fails your tubes will burn up quickly.

Some other bias designs eliminate that issue.   

[sluckey]

That bias circuit has served Fender very well for over 50 years.

[John]

You've got individual bias pots, but they don't seem to be fail safe.

I was thinking of putting a high value R between wiper and ground, but figured I'd ask about that later. The bias pots I'm using are what Doug sells, those little white ones.

[sluckey]

Put it between high side and wiper, not wiper and ground.

What happened to the simple cathode bias?

[John]

Put it between high side and wiper, not wiper and ground.

What happened to the simple cathode bias?

I changed my mind!     It appears that 35'ish watts are better obtained with fixed instead of cathode.... just from what I fathomed on the innerwebs.


(and I wanted to try something "new")

[John]

Well, with a pair of 6L6's hooked in, I'm getting 420 at the plates,and 385 at the screens (going through a 10K dropping resistor between node A and B) I know adding a couple 6SL7's will drop a tiny bit more, but I *think* if I can maintain 400 plate voltage I should be good? Keeping in mind this will never be gigged, maybe a sit-in at the local music store jam session. Picture of my .... test hookup attached for your Monday laugh.

[SILVERGUN]

Well, with a pair of 6L6's hooked in, I'm getting 420 at the plates,and 385 at the screens (going through a 10K dropping resistor between node A and B) I know adding a couple 6SL7's will drop a tiny bit more, but I *think* if I can maintain 400 plate voltage I should be good? Keeping in mind this will never be gigged, maybe a sit-in at the local music store jam session. Picture of my .... test hookup attached for your Monday laugh.

That lowered screen voltage will significantly effect full output volume.
Why the 10K dropper?...trying to limit output power to protect that OT?
 
The lowered downstream voltages will also result in less headroom potential for a big bass signal to pass through

[PRR]

> 10K dropping resistor between node A and B

Might be interesting for a guitar amp with 6L6 or EL34. Gives a funny overload limit.

May be "too much" for a Bass Amp which should be solid.

Don't fret much about the no-signal G2 voltage. Modern 6L6 will take it.

[John]

Good questions about the 10K. I put that in there because that's on a perfboard, and I can hook up my sub box to it (parallel) to lower R and increase voltage. And also because the 10K is what I had in 5 watt. 

[John]

So, I do have a question. On the schematic, it shows the 1K CR coming off the PI tube, pin 6, being connected to the common wire on the OT, which AFAICT is "ground". Why is it not just grounded to "ground", like the 25uf cathode cap?


 Does the NFB coming off the 16ohm tap have something to do with, then, grounding the cathodes to the OT as well? Hum reduction? Something I'm just too dim to see? Or, does it not really matter? Link to schem below, and thanks in advance!


http://el34world.com/charts/Schematics/files/ampeg/Ampeg_b15nf_portaflex.pdf


Oh, and what is up with all the extra resistors on the PI? I'm building to this schematic, but are all those types of PI's wired like that? And if so, why?

[sluckey]

So, I do have a question. On the schematic, it shows the 1K CR coming off the PI tube, pin 6, being connected to the common wire on the OT, which AFAICT is "ground". Why is it not just grounded to "ground", like the 25uf cathode cap?

That's a safety feature. Notice that the speaker plug must be inserted for that 1K resistor to be grounded (thru the jumper between pins 2 and 3)? If the speaker is unplugged the cathode of the PI will no longer have a path to ground, thus killing any drive signal to the output tubes.

[John]

Ohhh, that makes perfect sense now . I do believe I'll build it just like that. thanks!

[John]

Okay, my brain is freezing up. With a normal shorting jack, there is no "normally open" with no plug inserted.


Or wait, I don't use a shorting jack. I simply connect the 1K to the tip ring.... with plug in, it's connected to positive which is almost ground. With jack out, there's no connection.


Is this right? I'm feeling dumb right now!

[sluckey]

That plug is not a phone jack. It's an Amphenol plug. Looks similar to a tube socket. I would not bother with this little circuit. I would just use the simple 12A jack and connect the switch to the sleeve like Fender does.

[John]

Well, the amp part is done. The transformers are plenty beefy, at least so far. We'll wait 100 hours and see again. It even played perfectly on the first start up . Amazing. Schematic is attached. I used the dual bias pots per Sluckey's diagrams, so that should let him use tubes that aren't matched. The pair I have in I can tell are pretty far apart, but they're both set at 48 mA with 435 on the plates, which is around 21 pdiss per tube at idle, if my calculations are correct. I have to build a cabinet for it and the 10" Eminence Legend 10" speaker I got for it.


Testing it through my milk house speaker and el. guitar, I didn't want to turn it all the way up. It's loud! So anxious to hear it with a bass guitar and the bass speaker! I have a feeling it's going to take as long to build the cab as the amp. ;)

[SILVERGUN]

 

It even played perfectly on the first start up .

....give the guy in Quality Control a nice Christmas bonus
 

I have a feeling it's going to take as long to build the cab as the amp. ;)

Hey, 5 months aint bad...
I've had a piece of poplar sitting here for a year now, awaiting it's destiny.
 
-Is it gonna be a combo?
Let me know if I can be of any assistance with the cab....I'll gladly, easily cut you a baffle board (I've got some extra 3/4" birch ply. sitting around here in smaller cuts). I enjoy cutting the circles with my router and it's all setup to go. I've got some others to do, so it would kick start my ass in gear.
If that helps drop a month off of your time, it'd be well worth the effort..
 
Congrats on gittin 'er dun!
 

[John]

If I get a layout that I know I haven't messed up, I just might have you cut the pieces & do the dovetails, I'll glue everything together. It'd be money well spent, for sure!


Oh yeah, build pictures are here. STILL not Sluckey quality ;)
https://www.facebook.com/pg/Loafin.Shed/photos/?tab=album&album_id=577450432448577

[John]

Well, for anyone following along, the pair of 6L6's will drive my 12" 50 watt speaker juuussst fine, with excellent bass response. They don't do so hot with the 10" Eminence Legend(built to handle 200W) Now there's something I didn't plan on. So, more research for me!


*edit* I'm not so sure it's not the speaker itself actually. I get plenty of movement from the cone, but not nearly the volume,and it breaks up really early. Maybe it just needs broken in?


*another edit* Okay, so I'm a dummy. It needs an enclosure to move air. Just laying on top of the bench pointed at the ceiling doesn't really work well. 

[PRR]

The Legend Tens are NOT bass speakers. Resonance higher than bottom note of guitar.

And yes, unbaffled the air just slides around the frame, does not make actual bass-waves in the air. To do a good job of 42Hz with a naked diaphragm you need to be 10 _feet_ diameter.

[John]

http://www.parts-express.com/eminence-legend-bp102-10-bass-guitar-driver-200w-8-ohm--290-471


Sorry, it's the Legend BP102 bass speaker. I should have specified that. But yeah, it was funny watching the cone moving what seems like 1/2" but not getting the thump!

[SILVERGUN]

91.8db sensitivity on that Emi. is probably much lower than the 50W 12"er you were using. (which model?)
That could be a factor.....db's are important
 
 

If I get a layout that I know I haven't messed up, I just might have you cut the pieces & do the dovetails, I'll glue everything together. It'd be money well spent, for sure!

Just give me dimensions and a 2-3 week lead time and I'll give you the friends and family discount $

[SILVERGUN]

A while back I put an Eminence EM12 in an EV designed TL806 ported enclosure and it rocks...plenty of deep bass. Commonly referred to as an EV "Thiele" cab.
 
I'd consider just scaling this cab down for the 10"....turned on it's side there would be enough width to house your chassis
 
You 'could' do head/cab, or I can try to incorporate a combo approach with the speaker being housed in the sealed bottom portion (see pic)....would probably turn out to be fairly heavy if we stuck with 3/4"birch.
Not sure how it would go with some nice light pine 
Suggestions welcome...

[John]

Still trying to decide on a combo or head&speaker separate. If a combo, pine would be good to keep the weight down.


That 12" speaker is a Celestion bargain basement thing out of a cheap SS amp, so it can't be that good. lol The 10" is highly reviewed all over, it's why I picked that one. With the way the cone was moving, I really think I just need to stick it in a box instead of laying it on the workbench. I'm gonna knock together a quick one out some scrap wood, that way I'll know for sure though!

[SILVERGUN]

Still trying to decide on a combo or head&speaker separate. If a combo, pine would be good to keep the weight down.

I like it. 1 piece combo will be much cheaper (...and lower PITA factor) than 2 piece head/cab...just a thought.

That 12" speaker is a Celestion bargain basement thing out of a cheap SS amp, so it can't be that good.

Probably still at least 97-100db sensitivity....big audible difference
 
That 10" in a decent ported box will do well.
 
-Chassis dimensions?
 
My dovetail jig only lets me go to approx. 11" wide which equates to total max cab depth.
Try to limit the depth of your experimental cab to get comparable results.
 
I'd love to get away with using 1x10"s and not having to rip it down...so that would equate to a 9.25" cab depth.
Any deeper and I'll have to rip down some 1x12"s (no problem just higher labor cost  )
 
I hope this isn't too much mumbo-jumbo
 
Here's the TL806 plan in case you want to get "real" experimental (don't forget, this is for 12" speaker)

[PRR]

> Legend BP102 bass speaker

OK, missed that, sorry.

This is a "bass speaker", sure. Low Fs and long excursion. Mostly a trade-off against high midrange efficiency. (Add mass {voice coil length}, midrange drops more than bass, it is "flat" to a lower frequency.)

However to handle say 50Hz well you *must* have over 10 feet separation between one side of the cone and the other. An 8'x8' plywood bill-board comes close. A sealed box has infinite separation front to back. The vented boxes arrange to have a "just right" separation over a narrow frequency range (venting does not improve midrange output unless you redesign the driver).

And IMHO this is not a "bass guitar" speaker. Even at 1/4" displacement (each way) it lacks the cone area to move heaps of air. It is probably fine for rich-bass voice, much wedding and party PA. As a guitar-only speaker it seems a bit power-hungry. It also has nothing above 2KHz, suggesting it is for 2-way PA not full range instrument amp.

I didn't run the numbers in detail. A cubic foot box will be a good (maybe not screaming good) guitar speaker. You don't want a perfect cube (sounds like a box), try like 9"x12"x16" or 10"x12"x14" inside. If you are not bass-hungry, it will "work" in 10"x10"x6", hardly even its own shipping-size. Fb=106Hz Q=1.35.... it will boom another 3db in the second half-octave of guitar's bottom notes. Not great for kick-drum but otherwise a killer dorm-room system for its size.

Plot of what this speaker will deliver with no baffle other than its own ten inches:

[John]

Dang, that is good info. Well, I don't have much invested in it, so if I have to go another route(speaker) it's no biggie. I know pretty much nothing about speakers, it's why I don't spend much money on 'em. TBH, if I DO go with a different speaker, just might be a 12" anyway. I was shocked at how good the one I have sounds, and it ain't much.


SG, my chassis 19" long by 7" wide, so using 1x10 should be fine. I'll have to get through Christmas before tackling anything much, the next 2 weeks are my hay making time! Thanks for your thoughts, much appreciated.

[birt]

if you go for a single 12" these are nice for both bass and guitar (as is your amp): https://www.jensentone.com/vintage_ceramic/c12k

[John]

So I'm going to show my utter ignorance about speakers    Why is that a good choice for bass, as the freq. response shows only down to 90 Hz? Just good bottom end regardless of the specs?


Like I said, that cheapo 12" has excellent bottom end too, so what do I know?

[birt]

i know it's nice out of experience, not numbers. (i am clueless when it comes to numbers)

[John]

i know it's nice out of experience, not numbers. (i am clueless when it comes to numbers)

Oh, so you and me take the same bus then! 

[PRR]

> Why is that a good choice for bass

Good for its size. Especially for full-range (whole band) program.

Bass players have a 42Hz string, but the vast majority of listeners have ~~80Hz speakers. The excursion and power handling will give a LOT of output 80-90Hz up in a small or very-small box. They happy.

As an instrument amp, some bass players will note the lack of bottom-octave fundamentals. Some bassists don't work hard down there and I have seen some "light" speakers behind bass players. Others expect deep strong boom and would be disappointed in this or any Ten.

[John]

Thanks for the explanation, PRR.


I banged together a box for the 10" today out of some  1/2" birch plywood I had laying around. It's 1.15 cu. ft. inside. While it's not exactly to specs I think I can safelysay I don't like this speaker.


However, I do like the amp! I'm very pleased with how it sounds (in the 12") given I didn't know what to expect out of the transformer set I used. And with the volume half up it's almost dead silent.


SG,I'll shoot you a pm about the cabinet.

[John]

So, if I can pick your brain just a little more..... I'm looking at 2 Eminence speaker with the neo magnet. My buddy doesn't have a great back, so I'm trying to keep weight down. Both of these are very well reviewed, but so was the 10". Turns out he doesn't care whether a head & speaker separate cab or combo. Anyway...


http://www.eminence.com/speakers/speaker-detail/?model=DeltaliteII_2512     the DeltaLite


http://www.parts-express.com/pedocs/specs/290-518-eminence-basslite-s2012-specifications-45373.pdf    The BassLite


Biggest differences between the 2 that *I* can see is 2" vs. 2.5" voice coil, and 97 db sens. to not-quite 100 db.


Before I buy one I will trim down the enclosure for the 10" justtobesure, but it's really underwhelming me. As always, any thoughts are appreciated!

[PRR]

> differences between

They are the SAME over most of the range; astonishingly so considering the larger voice-coil on the higher-Watt rated job.

> doesn't have a great back

Switch to singing. Much less to carry.

Bass or drum == bad back. I am sitting here in pain due in part to youthful folly. Don't ruin your later years.

Neodymium is a mild help. I laugh at a "4 ounce magnet", we used to think 40oz was skimpy. But in fact these magic magnets do the job. The few pounds saved is diluted by still needing a large expanse of plywood to baffle the cone.

[John]

Thanks for the reply! I'm actually surprised these speakers don't cost very much more than the regular ones. I wouldn't pay twice the price, but for an extra 20/30 bucks, it's worth it.

[shooter]

fwiw, here's a handy "cheatsheet" for cab/port/vent sizing, hunt a little bit and you can find some "get the muck boots out" sound engineering calculators

https://www.jblpro.com/pub/manuals/enclgde.pdf

[John]

Cool beans! And I think the Emi has data sheets too. "Building cabs for dummies"  lol

[silverfox]

I think the consensus so far is, trying to keep the discussion simple. In the interest of a little expansion of the topic: The design of an acoustic system is a very complicated subject with many dependencies such as room size, speaker size for the cabinet, frequency response in relation to open or closed back design and economy, to name a few. Even after all the years of research, new designs are created around a system that is for the most part a coil, magnet and cardboard cone mounted in a conical shaped frame.

The Thiele cabinet plan is a little complicated but from what I've read, works very well. JBL, ElectroVoice and others use it.  I believe a scaled down version of the Sunn Bass cabinets would be simpler and work well also. What is key to the design is fitting the low frequency average wave length, or even multiple fraction thereof, into the cabinet for proper loading. Similar to getting the best sound out of the soda bottle as it is consumed. diy audio has a huge section on this topic and there is a section here that would I'm sure meet most of your design needs. Here's one design for a 12" bass speaker, but a simple search will reveal an endless assortment of plans for free.

silverfox.

[ampedup]

I built a sunn 2000s for my brother but he never used it but my buddy that jams with us has been using it. Now he can't stand solid state amps anymore, he's ruined. This amp is overkill because we just jam in my living room. My brother gigs all the time so I think he likes the portability of the solid state amp. This thing is huge and heavy.
There's nothing like tube amp bass tone.

[John]

I can't imagine lugging around a 100W + tube amp, even just the head. Those transformers are monsters. This one's not too bad though, only 35'ish watts so less iron to heft.