Will this work?

[Arthur Dent]

Is there anything preventing the possibility of making an amp with ten 6l6's or something similar? I would be winding the transformers myself, so I can make them large enough. How much power should I expect from that? Are there any tubes that would be better for a similar or higher power?

I'm making a bass amp for a project in school. I have a schematic drawn up which draws heavily on Ampeg's designs. I'm pretty sure it should work, but I wanted to get a second and third opinion on it before I build the thing. Thanks.

[Ray Ivers]

Arthur,

Is there anything preventing the possibility of making an amp with ten 6l6's or something similar?

Absolutely not - here's a schematic of a 400W, ten-KT88 power amp:

http://www.freeinfosociety.com/elect...ew.php?id=1152

How much power should I expect from that?

Ten 6L6GC's in push-pull? Anywhere from <150W to >600W - it depends on the design.

Are there any tubes that would be better for a similar or higher power?

IMO, yes. Although using lots of 6L6GC's will work fine, good-quality KT88's and 6550's would be better suited to getting a lot of power from a smaller number of tubes - there are also the transmitting-tube types (4CX250B/350B, etc.) but I assume you don't want to get involved with over-1kV B+ supplies and forced-air cooling.

I have a schematic drawn up which draws heavily on Ampeg's designs.

Can you post this, or links to the Ampeg circuit(s) it borrows from?

I assume you want to do this the easiest and/or cheapest way possible?

Ray

[Arthur Dent]

Hey, thank you very much. I think I'd prefer to use 6l6's rather than kt88's, mostly because they require less voltage to work well, and there are a lot of parts that I can get for free at school that can't stand up to that extra voltage.
The design is mostly derived from the Ampeg B-15n, except with a more Marshall-esque tone control, 12ax7's in the preamp (because I got a bunch of them for free), a master volume, and ten 6l6's.
The schematic for the Ampeg can be found at http://www.drtube.com/schematics/ampeg/b15n68-jp.gif

[Arthur B.]

Wouldn't it be be more efficient to use lesser number of tubes and drive into AB2?

[Tom Phillips]

Arthur,
You will find that the optimum number of 6L6s is forty two. Regardless of which design you choose, the bypass is still going through so it doesn't really matter in the big scheme of things. Does it?

[Steve Conner]

Mesa Boogie made a bass amp with 12 6L6s, the 400+

http://www.mesaboogie.com/Product_In...bass_400_.html

If you remember your towel and don't panic (I know it's hard with all these dumb hitch-hikers guide references flying around) you can make practically any size of amp with any number of tubes you want. The main limit is the cost of the output transformer, which seems to be about $1.50/watt. Tube bass amps often seem to undersize them to save weight and money. If you're winding it yourself though, you can easily make a full bandwidth monster that goes down to 20Hz clean and needs a forklift to load in at gigs. Having said that, the bigger you make it, the harder it'll be to get good treble response.

It's debatable if that low-end bandwidth is any actual use in a live bass situation though: I think you sometimes want your lower notes distorted by saturation in an undersized OT, so you can get better pitch and timing feedback from the harmonics. When I play through a clean solid-state amp, it's like "Bass you can feel, but somehow can't really hear properly".

With normal voltages used in guitar amps, one pair of 6L6GC will give about 50w of clean power before they start to clip. So the Mesa 400+ should do about 300w clean. 6550s or KT88s should give about 100w per pair, and might work out better value for money, now that Sovtek make them. Even if you don't use much more plate voltage, the big tubes seem to pass almost as much current as two 6L6s in parallel.

[Ray Ivers]

Arthur,

I think I'd prefer to use 6l6's rather than kt88's, mostly because they require less voltage to work well...

At what voltages have you heard KT88's not working well? FWIW, I've heard them sound excellent at B+ voltages from 420V to 700V, but if you're limited to B+ voltages below 400V, yes, you should do just as well with 6L6's.

Steve's post mentioning the 400+ reminded me of the (original?) Boogie Bass 400, so here's a print for that (six 6L6's, 540V B+, Class AB1; IMO they would be lucky to get 250W under these conditions, but the print reads "Bass 400"...):

http://www.freeinfosociety.com/elect...iew.php?id=505

Ray

[Arthur Dent]

Arthur,
You will find that the optimum number of 6L6s is forty two. Regardless of which design you choose, the bypass is still going through so it doesn't really matter in the big scheme of things. Does it?

Nice.

Anyway, I thought I heard somewhere that KT88's running at 450v wouldn't be as powerful as 6l6gc's running the same voltage. 450 volts is pretty much the upper limit, as the free stuff that I have access to is limited to that voltage. I definitely want to take advantage of that, largely due to the fact that I can get a few really cool big silver capacitors that cost alot. Will KT88's actually work better at 450 volts? If they will, I'm using them.

[Arthur Dent]

You know, it would actually be pretty awesome if I would actually use forty-two power tubes...

[Enzo]

Let's see, 42 6L6 tubes at .9 amp per for the heaters... Yep, you'd need a 40 amp 6.3VAC transformer JUST for the heaters. That is a 250 watt hunk of iron just for them. That would be in addition to the main power and output transformers. That amp ought to tip the scales at a good 500 pounds.

[Steve Conner]

You'd probably be looking at rewinding a pole transfomer for your output transformer too

I always wanted to make my own high powered tube bass amp. I got as far as scrounging a big transformer core (about 18lbs) to make the OT from. But when I started gigging, I found out that I could get by fine with a 50 watt amp I had already. I guess my band is fairly quiet, and doesn't need massive pant flapping bass.

Most other bands on the local circuit seem to have small bass combos too, I guess they just can't be bothered lugging heavy amps and cabinets. I recently did sound for a metal band whose bass player had a crappy 100 watt SS "Fender Rumble" amp and the guitarist had a Dual Rectifier with 4x12". I put as much bass through the monitors as I could, and never mic'd the guitar amp at all, but he still couldn't really hear himself.

[Arthur Dent]

Um, what exactly is a pole transformer?

I could probably get by okay with a smaller amp, as my band isn't terribly loud, but since I'm making an amp for a school project anyway and I can get a bunch of free stuff, I decided to make a very powerful one, mostly just because I can. It'll be fun to test it in school I guess I'll probably end up running a guitar through it too, as my band's guitarists have really lame amps.

I'm actually going to make the core of the transformers in the school's metal shop, so procuring one large enough shouldn't be a problem.

[R.G.]

Um, what exactly is a pole transformer?

It's the transformers that hang on the electrical power transmission poles. They're usually cylinders of about 12" diameter by 36" long, probably 150-200lb, maybe more.

Actually, Steve is being pessimistic. Pole transformers are about 30-50kW at 50/60 Hz, and I doubt you'll get to over 1kW. You may need 3-4kW for the power transformer, though.

I could probably get by okay with a smaller amp, as my band isn't terribly loud, but since I'm making an amp for a school project anyway and I can get a bunch of free stuff, I decided to make a very powerful one, mostly just because I can. It'll be fun to test it in school I guess I'll probably end up running a guitar through it too, as my band's guitarists have really lame amps.

You may want to consider the issues. It's likely that you will need a wheeled cart to move such an amp, as it will likely be too heavy to pick up. And it won't be all that much louder than another amp. The human ear has this funny thing where sounds that appear to be twice as loud are really ten times more powerful. A 1KW amp is in reality only twice as loud as a 100W amp, not ten times as loud. And it is more than ten times as likely to burn out a speaker if you make a mistake somewhere.

I'm actually going to make the core of the transformers in the school's metal shop, so procuring one large enough shouldn't be a problem.

You may have some problems there. The transformer cores really have to be laminated transformer iron. Transformer iron is low carbon steel with about 4% silicon, kind of a special mix. You really have to use laminations, otherwise the eddy current losses in the iron will choke off any power transfer. If you do have transformer iron in suitable sheets, it really needs to be punched to shape to get them all the same size so they can be stacked.

Have you ever wound a transformer?

[Arthur Dent]

The weight issue doesn't really bother me that much, as long as the thing isn't more than 100lbs, which I can't see happening. Two people will be able to carry it, I'm sure.
I do know what doubling the power of an amp does. I have a 4 ohm 200 watt powered Sunn mixer that was driving a single 8 ohm speaker for a while, which I have been told probably cut the power nearly in half. I then acquired a second speaker and wired it in parallel with the first. Before that, my band was running a mic and a guitar through it, and it had problems with clipping trying to be heard over the drums. Now, we run two mics and a guitar through it, and we have turned it down so that the drums could be heard over it.
The metal that I'm using for the core is indeed low carbon steel suitable for use as transformer iron. It comes in sheets, which will be glued together, and the final result will then be ground and sanded all smooth-like. No, I haven't ever wound a transformer before, but I know how to.

[Alex/Tubewonder]

The weight issue doesn't really bother me that much, as long as the thing isn't more than 100lbs, which I can't see happening. Two people will be able to carry it, I'm sure.
I do know what doubling the power of an amp does. I have a 4 ohm 200 watt powered Sunn mixer that was driving a single 8 ohm speaker for a while, which I have been told probably cut the power nearly in half. I then acquired a second speaker and wired it in parallel with the first. Before that, my band was running a mic and a guitar through it, and it had problems with clipping trying to be heard over the drums. Now, we run two mics and a guitar through it, and we have turned it down so that the drums could be heard over it.
The metal that I'm using for the core is indeed low carbon steel suitable for use as transformer iron. It comes in sheets, which will be glued together, and the final result will then be ground and sanded all smooth-like. No, I haven't ever wound a transformer before, but I know how to.

This is not a recommended way to make a transformer core. Core laminations must be electrically isolated from each other to minimize eddy currents in them. Just glueing sheets of metal together and then grounding and sanding the core stack is asking for trouble. Get proper transformer laminations for both power and oitput transformer together with correct bobbins.

Have you thought about total power requirements? Just for the sake of getting a rough estimate: 10 6L6 tubes wil draw near 10 Amps in heatre current only at 6.3V so it's 63VA there. Your B+ supply at 450V will have to supply ca 1.5A to the plates i.e. over 750W dc power. All in all you are looking at a power transformer rated for at least 1000VA power draw from the mains.

Not to mention interwinding and interlayer isolations requirements, calculating magnet wire diameters, turns etc. Buidling a transformer like that is no small task.

I have no idea how much you know about these things or if you have ever wound a transformer, but your suggested method of making the core makes me a little wary. This is not meant to discourage you, I 'm only pointing out where the traps are hidden.