Bass amp to Power amp conversion? (70's Yamaha B100II)

[jaywalker512]

I was given an old SS Yamaha bass amp (B100II) from the early 70's, problem is I am not a bass player, nor do I own any bass speakers. I am curious to find if it would be possible/practical to mod it to bypass the EQ and everything that makes it a "bass" amp and just use it as a power amp for a nice tube preamp that I can run my guitar through and end up in a guitar cab. It is doable now with the EQ set right, but it still has loads of low end and not much treble/mids, and if someone bumps up the 60hz knob on the amp, I can say goodbye to my guitar speakers -_- Thoughts?

[Enzo]

Does it have an effects loop or a power amp in jack?

[jaywalker512]

No it doesnt, it has a high and low impedence input and 2 speaker outputs. If it had an effects loop I wouldnt have posted this question

[Enzo]

Well, jay, you are new here. Welcome to the place, by the way. SO I don;t know what you know or don;t know.

I don't have my ancient yamaha files handy. Do you have a schematic?

[jaywalker512]

Right, I understand. Its all good No I dont have a schematic and every place I've found with one wants to charge $20 for it, which im not really willing to pay for a little booklet (including the yamaha site... thought about emailing asking for a schem but then i saw the "order a manual" link) :/ I found a simplified diagram of the B100III circuit on their site, but Im not sure how much help that would be in this case.

[Enzo]

Oh, OK, I'll try to dig mine out.

In defense of Yamaha, they do charge $20 or whatever for a service manual, but they are excellent manuals. This amp is pretty basic, but some products are pretty complex. Their manuals cover disassembly and assembly - pictures of which screws etc. Schematics, plus board layout - often showing trace art as well. Parts lists. Specs and adjustments - bias in the example perhaps. Diagnostic procedures too. That is cool on any digital based product because the self tests are all explained. For keyboards especially it is cool, because they show how to extract and install the keys. That can be pretty frustrating to figure out if you had never done it.

SO, I'd rather not pay either, but if you have to bite the bullet, at least you know they tried to give you as much useful information about the product as they could.

A block diagram is not helpful really. And the III is not the II.


OK< here is the schematic. it is a scan of a scan, so the graphic section is a bit hazy.

The power amp plugs into the power supply board. Three wires connect to that board in the corner - IN, E, and +. E for earth (ground), + for power supply, and IN means input. The white wire to IN is the input to the power amp. I'd try tapping in there, you could mount a cutout jack, wire it there, and use that as a power amp in jack. That would allow the thing to remain the bass amp it always was when not using the power amp jack.

That white wire comes from O on the preamp board. It is the output from the graphic. There is also a white wire at TO. it goes to the transformer for the XLR preamp out. I think you could run that wire to the powr amp input instead of the graphic out wire. That would bypsss the graphic but leave the preamp.

The preamp is simple. The basic BMT tone stack has bass values, but you could reduce the cap values and make it more guitar friendly. And it ought not be difficult to brighten up that preamp with a few changes. For that matter, I don;t see why a few cap and resistor changes in the grapahic wouldn;t make that more useful, and you can keep it. You could just convert the 60Hz band to a 2500Hz band. Change the 6.8uf and .027uf caps to .047 and .001, I think.

[jaywalker512]

Thank you very much for the schematic and instructions! I plan to attempt these mods in the morning since I havent slept in a day or two and that wouldnt be the best idea to go modding amps in this state

Is there a specific formula I can use to determine what frequencies will be affected by whatever capacitor/resistor combinations are in the tone stack? That would help me adjust it all to my own tastes and I wont need to keep coming back and asking for help.

What is the best way to drain these massive caps so I dont kill myself mucking around in this? Would shorting the positive lead to the chassis work?

And lastly (sorry for asking so many questions), can you think of any other mods that might improve the performance/sound quality of this amp?

I greatly appreciate your help

[Enzo]

Why don;t you just check with a meter for voltage stored. I think they discharge pretty fast.

It is never a good idea to just short a filter cap to ground. Always use a resistor. otherwise you run the risk of burning out the internal connection.

I have no idea what to do with it. The circuits are very basic, so the sorts of things you might do to any amps should work here as well.

Go over to Duncan's Amp Pages and download his tone stack calculator. Fun to play with and informative, it lets you fiddle with tone circuits.

[jaywalker512]

Alrighty, did everything you described (power amp in jack, 60hz to 2500hz band, custom tone controls after fiddling with the tone stack calc) and everything works great Sounds much more guitar-friendly and functions great as a power amp when I need it. Thanks again for your help!

[Enzo]

Well, cool. I love it when a plan comes together.

Isn't that tone stack calculator great?

[jaywalker512]

Yep, no trouble is always good. I did actually have the TSC before you told me about it, I was just hoping I could learn a little more of the math behind the magic so I dont have to rely on what the TSC provides me with, incase I wanted to make a completely unique tone control or something, or when working on other electronics where the TSC cant help me.

Edit: Hmm thought this up a moment ago, how easy do you think it would be to mod in a decent sounding overdrive? If doable, do you have any recommended circuits to use? Im fairly new to amp building/modding and the only opamp I've had the opportunity to play around with was an LM386, which sounded... ok as an overdrive. Perhaps I could build a little board with an lm386 on it that would go after the preamp and before the poweramp, and have a switch controlling whether the signal goes through the 386 or not (thus turning OD on or off).

[Enzo]

You can make all kinds of interesting things with an LM386, but it is not an op amp. The LM386 is a power amp. Only puts out about a quarter of a watt, but that is what it is. I think it was originally designed for things like headphones drivers.

You could add a send jack right next to the power amp in jack you just installed, making an efects loop. But generally ODs are better in front of the amp rather than in the loop.

OD is so subjective, there are a million little project OD circuits, you could try a bunch. I don;t know what you'd like.

But for my money, ther are so many excellent OD pedals on the market and some are dirt cheap, like the DanElectro stuff, why not just use a stomp box?

[socalscott]

You ever look into the Tech21 character series pedals? They work best with a power amp. Any famous flavor you like.

[olddawg]

Consider a modeling pedal. If you have built a tube preamp into this amp you will only get overdriven preamp distortion with an overdrive pedal (or built in boost) or SS clipping from the poweramp at a very high volume. IMHO both of those are not a desirable type of distortion. You would have a much more versitile setup with a modeling pedal like a Zoom, Vox, Line 6, Digitech, etc. But, like Enzo says, and he is rarely wrong if ever in this field, it's all very subjective.

[Black Thumb]

Hey can somebody please repost the schematic for the Yamaha B100 ii? I can't open the attachment in JayWalker512's post. Thank you.