Adding line-in to B-100R?

velo-hobo · #1 10-06-2011, 01:52 AM

Or Effects Return or Power Amp In, whatever you wanna call it.

I've had one of these for about 13 years and just picked up a second one because I like it so much. I want to have the option of stacking them as a few others here have done.

I've tried using the line out of one into the input of the other, and it works pretty well, but setting up the controls can be tricky. Also, I'd rather not buy a buffered A/B or DI box if it can be accomplished with <$5 worth of parts.

I have almost never used the headphone jack in all the time I've had this amp, so I am thinking one could convert that jack location to a line-in, which would seem much more useful (effects loop, slave amp, etc).

I'm not an expert on electronics, but I tinker with amps and such quite a bit and it seems like it would be pretty simple to implement. Looking at the attached schematic, the preamp is in the top half of page one. Could one add a switching jack (and any necessary accompanying resistors and/or caps) after C21 and just before the Master Vol (extreme right side of the preamp diagram)?

This seems to be the logical location as it is in the same place as the tap for the line-out, and would allow you to control the output level of the slave amp with its MV while making all the gain and EQ adjustments on the primary amp.

Before I go monkeying with my amp, anyone have any informed opinions to contribute?

BTW, I also have a V4B and Hartke 3500 that I run into a 410-HLF, but I love the tone of the B100R so much that I want the option of playing louder gigs with it. I often play shows without PA support so it's a relevant factor to consider.

RickenBoogie · #2 10-06-2011, 11:19 AM

This is a case of, if you have to ask, it's over your head. best to find a qualified amp technician near you, and have it done safely and correctly. Good idea though, best of luck.

velo-hobo · #3 10-06-2011, 12:34 PM

That's a reasonably cautionary reply, but with all due respect I don't personally find it helpful. I've done recap jobs and other minor repairs on tube amps and have overhauled solid state devices and power supplies plenty enough times to feel confident modifying my B100R. The amp is long out of warranty so that's not an issue either.

I'm hoping that this thread will serve as a useful reference for other DIY types who are interested in the amp's possibilities. It's often mentioned that this amp lacks a line-in so I would like to foster a discussion on the smartest and simplest way to implement one.

Sure, I could pay my tech $100-$200 to figure it out but the amp itself isn't worth too much more than that. If I felt the need to go that route I might as well just use that money to invest in a good A/B box, but that still wouldn't accomplish what a line-in would do for the functional versatility of the amp.

anderbass · #4 10-06-2011, 06:01 PM

Hey cool idea velo-hobo, I've gigged my pair of B100r combos and always ran into drawbacks just like you.

I cant offer any tech help but I agree this would be the best method of chaining a pair of these combos.

velo-hobo · #5 10-06-2011, 06:27 PM

I took another glance at the schematic just now and realized some minor work would need to be done around the headphone jack connections as well, since plugging in to it mutes the speaker.

Looks like it accomplishes this by opening up the speaker's connection to ground, so a small jumper would do the trick.

Another thing I am seeing is that one could add a level control to the line out by replacing R31 with a pot wired as a variable resistor, possibly with a resistor wired in series to provide a ceiling for the signal level. Don't know where that knob would end up - I suppose it could go where the headphone jack is instead of a line-in jack.

I'm pretty busy with other repair projects but when I get the chance to delve into this some more, I will report what I find. In the meantime I'm hoping some knowledgeable folks will chime in with their ideas.

Shadi · #6 10-06-2011, 08:30 PM

Subscribed!

billfitzmaurice · #7 10-06-2011, 08:42 PM

Quote:

Originally Posted by velo-hobo

Could one add a switching jack (and any necessary accompanying resistors and/or caps) after C21 and just before the Master Vol (extreme right side of the preamp diagram)?

Yes. I'd put an output jack post C21 on one amp, the master, and a normally closed jack pre C21 on the second, the slave, so that with nothing inserted it operates normally, with a jack inserted it bypasses to pre and takes the signal from the master amp. You'd retain separate master volumes.

velo-hobo · #8 10-06-2011, 08:51 PM

Quote:

Originally Posted by billfitzmaurice

Yes. I'd put an output jack post C21 on one amp, the master, and a normally closed jack pre C21 on the second, the slave, so that with nothing inserted it operates normally, with a jack inserted it bypasses to pre and takes the signal from the master amp. You'd retain separate master volumes.

Do you think the existing line out as wired would be insufficient as a feed for the slave amp? Just asking as R31 is in series with the hot side of the existing jack. I'm assuming Ampeg put this in as a means of setting the output level of the line out jack. [edit: ok, I see why putting this before R31 makes sense: you want the slave amp to see the same signal it would be expecting from its own preamp]

Also, what is the reasoning behind putting the line in jack before C21 on the slave (instead of after C21)? I'm not clear on the function of C21 so maybe you could elaborate.

lowend1 · #9 10-06-2011, 09:31 PM

Radial Big Shot ABY - I use it with my B100R/B50R stacked(and a Yamaha NE-1 parametric), and it makes it all work. Have you tried using the attenuated input? When I use my tube preamp in front of the B100R, that's where I go in.

velo-hobo · #10 10-10-2011, 10:23 PM

So, unrelated to the idea of this thread, my older B100R decided to start blowing fuses. I'll need to figure out what is shorting or pulling too much current in there and do some repair work before proceeding with the mods we've been discussing here.

billfitzmaurice · #11 10-11-2011, 06:36 AM

Quote:

Originally Posted by velo-hobo

Also, what is the reasoning behind putting the line in jack before C21 on the slave (instead of after C21)? I'm not clear on the function of C21 so maybe you could elaborate.

C21 is an interstage coupling cap, to prevent DC flow to the next stage. If you put the insert post C21 you'd want to do so with a cap, the same size as C21, to serve the same function.

beans-on-toast · #12 10-11-2011, 06:57 AM

Quote:

Originally Posted by velo-hobo

I've tried using the line out of one into the input of the other, and it works pretty well, but setting up the controls can be tricky. Also, I'd rather not buy a buffered A/B or DI box if it can be accomplished with <$5 worth of parts.

Rather than run the line out to the input of the other amp, you can use an instrument cable and run from the -15db input on the amp you are plugging your instrument into to the 0db input of the second (slave) amp. Set the tone controls the same on both amps. Adjust the volumes to balance. Worth a try since no modifications are involved to see how it sounds.