DI bass and guitar

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A.David.MacKinnon
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Re: DI bass and guitar

Post by A.David.MacKinnon » Wed May 27, 2020 11:42 am

I've never owned one but have recorded a bunch of them. Almost every one I've tracked has been a bit rattle-y and the speakers have had various degrees of farty-ness. The heads always sound great but the cab and speaker are often really beat. Once in a blue moon I record one that's in great shape and it's a real treat.
I would guess that having the head on vs off would be similar to having a closed or open back cab. You'll probably get a bit more low end with the top on.

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Re: DI bass and guitar

Post by joninc » Thu May 28, 2020 10:18 pm

i'm trying out this EXT amp output and its not feeding anything out of it...I have a dummy load on the speaker cable but I am getting nada - doesn't matter whether it's on standby or not either.

What am I doing wrong?
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Re: DI bass and guitar

Post by A.David.MacKinnon » Fri May 29, 2020 5:47 am

Hmmmm. I’m not sure. You’re sure your plugged into the ext. amp jack not ext. speaker?
The external amp out just taps into the pre-amp-power amp connection. Apparently you can also feed signal to the power amp by plugging in there as well.
What’s the rest of the chain? You’ll need to run to a DI and pre to get the signal up for your interface.

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Re: DI bass and guitar

Post by joninc » Fri May 29, 2020 10:35 am

I double checked that it was the correct output - i was patched into the DI in of a preamp in and saw no input level of any kind.. My B15 is the later era B15s - i wonder if it has a change in the circuitry or design?
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Re: DI bass and guitar

Post by joninc » Fri May 29, 2020 11:19 am

further investigation finds that i get no output to speaker if I have a cable plugged in to EXT AMP....

I'm using a patch chord - does it need to be a balanced cable?
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Re: DI bass and guitar

Post by A.David.MacKinnon » Fri May 29, 2020 11:28 am

Hmmmm (again). It should be an unbalanced connection. I wonder if the later models implimented an insert style jack there. What happens if you pull the cable out half way like you can with a trs jack?

Also, what era is yours? The only manual I can find online is for the heritage B15. It has the ext amp jack and says it can be used to get a feed from the preamp or feed into the power amp. None of the schematics I can find look much different as far as that jack is concerned. They all show a mono 1/4" jack.

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Re: DI bass and guitar

Post by joninc » Fri May 29, 2020 1:31 pm

i think mine is early 70's. It's a higher wattage version than the B15n. no dip switches. 2 channels.

looks like this: https://www.google.com/imgres?imgurl=ht ... egUIARDsAQ
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Re: DI bass and guitar

Post by markjazzbassist » Fri May 29, 2020 2:01 pm

that's the B15S it's actually a different model, it's more like the B18 head in that it puts out 50+ watts. the b15n is just 25 watts. i'm guessing it lacks that option. i used to have one of those. powerful and large.

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Re: DI bass and guitar

Post by joninc » Fri May 29, 2020 2:31 pm

yep, that's what i said a few posts back B15s.
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Re: DI bass and guitar

Post by The Scum » Fri May 29, 2020 2:50 pm

Looking over the schematics, they're all pretty similar (N, S and X models), particularly in regard to the Ext jack. It's indeed a spot where both preamp channels come together to feed the power amp. If you're not getting signal there, the thing overall shouldn't work. It's off standby, right?

https://drtube.com/en/library/schematic ... eg-schemas

As to your question about running with the trapdoor fully removed from the cabinet - there's some good and bad.

Getting the hot electronics off the thing that vibrates mercilessly will help them last longer. Bass amps as a category are playing a game of cooking & vibrating themselves apart.

Having the cabinet open will diminish the low end a bit - the wave from the back of the speaker will meet the wave from the front, and they'll cancel. If it's a tone-ring era cabinet, it'll completely undo the tone ring porting.

So my recommendation might be to take the head off the board, and seal the cab back up.
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Re: DI bass and guitar

Post by joninc » Fri May 29, 2020 3:23 pm

My tech is saying that I only need a stereo cable to connect to the EXT AMP out and then one of the L or R will provide the connection at the other end.
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Re: DI bass and guitar

Post by joninc » Fri May 29, 2020 3:26 pm

I'll cut a board to seal the cab - thanks for that. I like having the heads on this side of the glass :)
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Re: DI bass and guitar

Post by A.David.MacKinnon » Fri May 29, 2020 5:24 pm

joninc wrote:
Fri May 29, 2020 3:23 pm
My tech is saying that I only need a stereo cable to connect to the EXT AMP out and then one of the L or R will provide the connection at the other end.
Interesting. It's running like an insert connection then.

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Re: DI bass and guitar

Post by The Scum » Fri May 29, 2020 5:24 pm

That Ext Amp jack is a bit odd - certainly when seen from the perspective of today, with our TRS "insert points" that are an input and an output hidden on one plug.

That Ext Amp isn't one of those. It's just a point in the heart of that circuit between the preamp and power amp. If you plug into it, you can drive a signal into the power amp, or extract the signal that's the mix of the input channels. If you connect two of these amps together at this jack, all 4 inputs are mixed together, and it drives both power sections.

So a standard TS cable should do the trick...Or the signal should be at the tip of a TRS.
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Re: DI bass and guitar

Post by A.David.MacKinnon » Fri May 29, 2020 5:30 pm

The Scum wrote:
Fri May 29, 2020 5:24 pm
That Ext Amp jack is a bit odd - certainly when seen from the perspective of today, with our TRS "insert points" that are an input and an output hidden on one plug.

That Ext Amp isn't one of those. It's just a point in the heart of that circuit between the preamp and power amp. If you plug into it, you can drive a signal into the power amp, or extract the signal that's the mix of the input channels. If you connect two of these amps together at this jack, all 4 inputs are mixed together, and it drives both power sections.

So a standard TS cable should do the trick...Or the signal should be at the tip of a TRS.
Similar to the RCA jack on the back of Hammond organs.

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