Yamaha B100 Amp Question

[Jimbowhit7]

Hi all! I have an old first-gen Yamaha B100 that I want to use some parts from for a project, but I need a little help with determining what all I need to use. I'm wanting to pull the preamp and house it in a pedal enclosure - I love the sound of the preamp, but the amp is bulky and it only puts out 100 watts.

Now, for the question:

Is the rectangular PCB on the bottom that the pots are attached to the only one I need to pull, or do I need to pull the other PCB as well? It looks like that larger board has more to do with the power amp, but I don't want to disconnect wires and find out after the fact I needed both parts. Any help at all would be great. Thanks!

 

[agedhorse]

Do you know how to read schematics?

You are going to need a regulated ~36V power supply at a minimum. Not a standard pedal power supply voltage.

You are WAY better off to buy a pedal preamp kit if you really want to build something and sell what you have. That's 100 watts at 8 ohms which is how the combo speaker is configured. It has more value as a combo than several pedals IMO.

 

[JimmyM]

What Andy said. Amp preamps are designed to run on AC, not 9v DC. Add that to the very real possibility that if you don't even know what wires you're supposed to pull, it's very possible to build a pedal that electrocutes you in short order.

 

[JW56789]

It's none of my business, but if you open up an amp and have to start asking very basic questions with high voltage staring at you, I think I'd re-install the back panel and consider another way to accomplish what you're after. But it's up to you.

 

[Omega Monkey]

Instead of trying to DIY this (might work, might kill you), maybe reach out to some of the "boutique" pedal builders (JHS, etc...) and see if they might do a custom pedal version of the pre, by taking the schematic, and then doing whatever it would need to work in a pedal format (ie making it work with a more typical pedal power supply voltage like 12, 18, or 24). I don't know how doable that even is, but presumably it would be possible. Would probably be expensive, but would also come out a lot cooler.

Another idea would be to have someone look at the schematic and figure out "what makes it tick", ie what gives it the sound you see to like.

Also, have you run the preamp out (assuming there is one) to a different power amp/cab to see if the "magic" is indeed in the preamp and not in the whole combination of everything?

I can't say I've used one of these Yamahas, and if I ever did, it was probably 20+ years ago. But I will say some of these old simple bass amps just sounded "like bass". Not usually super hifi, or tons of low end, etc... but just kind of musical sounding and with usable tone controls. Kind of the same concept as with my Roland Jazz Chorus for guitar (or bass too actually). The controls are just in a good range to get what you need out of them, and not trying to do too much or be too gimmicky.

 

[Jimbowhit7]

Instead of trying to DIY this (might work, might kill you), maybe reach out to some of the "boutique" pedal builders (JHS, etc...) and see if they might do a custom pedal version of the pre, by taking the schematic, and then doing whatever it would need to work in a pedal format (ie making it work with a more typical pedal power supply voltage like 12, 18, or 24). I don't know how doable that even is, but presumably it would be possible. Would probably be expensive, but would also come out a lot cooler.

Another idea would be to have someone look at the schematic and figure out "what makes it tick", ie what gives it the sound you see to like.

Also, have you run the preamp out (assuming there is one) to a different power amp/cab to see if the "magic" is indeed in the preamp and not in the whole combination of everything?

I can't say I've used one of these Yamahas, and if I ever did, it was probably 20+ years ago. But I will say some of these old simple bass amps just sounded "like bass". Not usually super hifi, or tons of low end, etc... but just kind of musical sounding and with usable tone controls. Kind of the same concept as with my Roland Jazz Chorus for guitar (or bass too actually). The controls are just in a good range to get what you need out of them, and not trying to do too much or be too gimmicky.

I have a friend who builds pedals, and I'm gonna see if he can take a look at it. It has a preamp out I've used, that's how I discovered the tone I like. The volume knob is a pre gain knob, so when it overdrives it has a distortion sound I like; that's why I was considering doing this.

 

[Jimbowhit7]

It's none of my business, but if you open up an amp and have to start asking very basic questions with high voltage staring at you, I think I'd re-install the back panel and consider another way to accomplish what you're after. But it's up to you.

I'm under a shelter in place order where I live, and have very little to do, so I figured why not try and learn? If it's not feasible I ain't gotta do it, I just thought it would be a learning experience to try - I've been wanting to try my hand at diy pedal builds before now, I just have the time to try now.

 

[Bassdirty]

Im not much help, (like some ACTUAL designers/engineers here that work on this stuff...that ya should listen to)

BUT, if you must..

Just keep one hand in your back pocket at all times.. this way if/when you get electrocuted, the current doesn't run directly across your heart and down the other arm. It could still kill ya, runnin down your legs, but these a better chance of survival.

I'm here to help.

 

[agedhorse]

I'm under a shelter in place order where I live, and have very little to do, so I figured why not try and learn? If it's not feasible I ain't gotta do it, I just thought it would be a learning experience to try - I've been wanting to try my hand at diy pedal builds before now, I just have the time to try now.

Then why not learn from building a kit or two? Study the circuit first, then build and test. It will be a better experience imo and you can sell the amp to find several pedals.

 

[IamGroot]

I owned one of these along with an Acoustic 2x15 cab 20 years ago and it was loud!
And heavy.

 

[BassmanPaul]

I owned one of these amps in the Nineties. For what it was it's a good amp. It would be a shame, in my book, to destroy it for some out there idea. Leave it be - there are plenty of pre-amps out there already. You just have to chose one.

 

[BassmanPaul]

What Andy said. Amp preamps are designed to run on AC, not 9v DC. Add that to the very real possibility that if you don't even know what wires you're supposed to pull, it's very possible to build a pedal that electrocutes you in short order.

To be fair the pre-amp section does run on DC, typically plus and minus 15 to 18 volts. This is supplied by the power transformer after rectification and smoothing. You would need a large pedal enclosure to fit the PCB and a suitable power supply. IOW it can be done but it won't be easy or small.

 

[agedhorse]

To be fair the pre-amp section does run on DC, typically plus and minus 15 to 18 volts. This is supplied by the power transformer after rectification and smoothing. You would need a large pedal enclosure to fit the PCB and a suitable power supply. IOW it can be done but it won't be easy or small.

The original amp is powered by an AC source, this is what Jimmy is saying.

This amp is also a little bit different in that it uses a single +36V rail for preamp power. It's just how Yamaha did it back in the day.

 

[A Train NH]

Two things. I would just like to agree with all and sundry replies regarding this is an ill conceived first electronics project: it is unlikely to work as you envision in the best case, and in the worst case it burns down you house or kills you- or someone you know. Secondly, as a B100 aficionado (bought my first gen B100 head and matching 1x15 cab in 1989), it would be a shame to rip it up when it could be played as is.

 

[funkmasterron]

This is an old thread, but I'm restoring one of these Yamaha B100 (original version) bass amp heads, and am looking for an accurate schematic of it. Through online searches, I can only find later versions which visually appear to be based on a different circuit design. Anyone have a schematic for the Yamaha B100 bass amp head pictured by the OP? TIA!

 

[agedhorse]

This is an old thread, but I'm restoring one of these Yamaha B100 (original version) bass amp heads, and am looking for an accurate schematic of it. Through online searches, I can only find later versions which visually appear to be based on a different circuit design. Anyone have a schematic for the Yamaha B100 bass amp head pictured by the OP? TIA!

There are at least 3 different versions, and they are all quite different.

 

[funkmasterron]

There are at least 3 different versions, and they are all quite different.

@agedhorse My Yamaha B100 bass amp looks exactly the same as the one in this post by the OP. I have been working with this schematic (https://www.synthxl.com/wp-content/uploads/2021/10/Yamaha-B100-115-Bass-Amp-Service-Manual.pdf) which appears to be correct for the preamp section at least (page 5 schematic), and the parts placement on page 7 also match my hardware.

I wanted to get your take (and anyone else that would like to chime in) on what I'm seeing.

The overall state of the amp is that it is powering up and producing an output, but the output is very low. All knobs / tone shaping all work. All jacks and pots have been cleaned. The issue appears to be in the preamp section as the "preamp out" (labeled as 01, aka "Distortion") on the schematic, has a low output.

I have run a diode test on all FETs in both the preamp section and power amp section, and all check out. Power to the preamp board (aka B+ is 41VDC as expected).

When tracing a single (a 2.70Vpp 1kHz sine wave) injected on the high input jack, it is immediately being squashed to a 0.2Vpp 1kHz sine wave on the input jack PCB before even going to the preamp board. Then the signal I'm tracing is lost in the noise as it makes it's way to TR1. I am seeing similar behavior when injecting a signal on the other (aka low) input jack.

My initial conclusion is to just replace the input jacks. But odd they are both squashing the input signal by 10x.

 

[agedhorse]

It's probably not the jacks themselves but measure each jack with an unterminated plug installed in each jack to see if perhaps something might have become broken or a piece of conductive debris is in there.

I would want to know what's going on with the input coupling cap, and the input FET. If the input FET is shorted gate to source, it will behave exactly like this. It will also behave similarly if the FET is in cutoff or saturation, but not at the input jack (after the FET the signal will be almost zero or heavily distorted).

 

[funkmasterron]

It's probably not the jacks themselves but measure each jack with an unterminated plug installed in each jack to see if perhaps something might have become broken or a piece of conductive debris is in there.

I would want to know what's going on with the input coupling cap, and the input FET. If the input FET is shorted gate to source, it will behave exactly like this. It will also behave similarly if the FET is in cutoff or saturation, but not at the input jack (after the FET the signal will be almost zero or heavily distorted).

Thanks, I removed TR1 (N channel JFET) and the input signal is now making it from the input jack to the gate of TR1 fine now. TR1 passed the diode check on my multimeter, but that's not the full picture. I'm ordering a replacement. For anyone else with this issue that is looking for a replacement part:
The schematic says 2SK30A but those are obsolete. The actual part on my board is an J231 but they are expensive (a few dollars each). I'm going with an Onsemi J113 which are currently <$0.40/each on Mouser/Digikey, but they have a different pinout (so just need to connect the G S D pins properly per the J113 datasheet, which isn't a big deal).

 

[agedhorse]

Correct, but the 2SK30 is a different, more linear part. It may require a bias adjustment. I will have to look at what replacement works better, though I may have some original parts left.