73 Concert Bass questions

[Babs]

Hi,

Been a while since my last post. Had a couple of questions about my now-working-great 1973 Concert Bass head.

First, mine has two speaker outs but the second is covered and not at all connected internally. I have seen pictures on eBay and such of later Concert heads that have what appears to be 2 functional speaker outs. Was this the standard configuration of speaker outs in 1973? Does anyone know if it is possible to add a plug and wire the second out?

Also, was just curious if the preamp out/poweramp in on this head could be used as a poor man's effects loop; preamp out to effect to poweramp in. Anyone done it or know if it is possible?

[AnimosityTheGreat]

I have a similar model, and mine does have a functional 2nd speaker output.  If I am not mistaken, all they do is wire a 2nd plug inline to split the power and resistance.  Basically wire the ground of plug#1 to the hot of plug#2, then ground on plug#2 to original ground that was hooked up to plug#1.

and as for the preamp out/power amp in section, I think that using that loop bypasses the output section of the amp in favor of a different power source.  That may leave you with no power after the effects are applied.
I am surely no expert and may be wrong in this case.


I am having trouble with my concert bass, would it be possible to get some hi-res shots of the internal connection to the power amp board?

[Anonymous]

Animosity,

Thanks for the reply. Now that I think about it... pretty stupid question about the effects loop.

I checked out the schematic on this site and it appears to show only one speaker out. I suppose I could try your wiring, but I am concerned about frying the thing.

I don't have a hi-res camera, but I can send you some digital shots of the inside. Post your e-mail address and I'll send them.

[JoeArthur]

Animosity's wiring sounds a little fishy to me.  I'd never wire a "hot" to a ground lead.

My concert has parallel output jacks - the hot of one jack has a jumper to the hot of the other jack.

If you do wire one up, make sure you use an open circuit jack.  Check out this concert power amp schematic:

http://www.dangpow.com/~sunn/schematics/SunnConcertLeadNewPoweramp.GIF

[JoeArthur]

Oh... and yes, you can plug the preamp out into an effects unit (line level, not guitar level) and route the output of the effects unit back into the poweramp in jack.  I do it all the time.

[Babs]

Joe,

Thanks for the advise on the 2nd speaker out. I'm not is a hurry at this point. But if I every really need it I'll give it a try.

On the effects loop/preamp out-poweramp in thing, I have a followup. I assume you said to use a line level effect as opposed to a guitar level because either the preamp out would be too much for guitar level or the effect guitar level wouldn't be able to drive the poweramp in (which one?). How do I know what is the appropriate level by looking at the specs? For instance, check out this compressor:

http://digitech.com/products/BassSqueeze.htm

It says that it has two outputs, one for amp and one for a board. Is one of these appropriate for this application?

Thanks, Joe, for you response.

[AnimosityTheGreat]

My mistake, I was working from memory and usually and working with AC power instead of DC.

After checking the pair of outputs, there is a jumper hot-to-hot as Joe described it.

My email is borntoremain@yahoo.com
Thanks for all the help!

[JoeArthur]

Babs...

The unit you gave the link to is a guitar level (also called low-level) unit.  Both of the outputs require connection to a preamp type device to raise the low-level signal to line level.  

As a rule of thumb... power amps are driven with line level signals, preamps are driven with low level signals.  A low-level signal is typically around 50mv (0.050 volts) or less.  A line level signal is usually a 1.0 volt signal.  Since signals can have peaks exceeding these, consider this a rough average - just to see the relative difference in signal strength.

What would happen if you used a line level output (preamp out of your concert) to drive a low level unit and used the output of the low level unit to drive a line level input (concert poweramp input)?

Probably both of the situations you mention.  Turning up the volume on the concert preamp could drive the effect into distortion... and the effect might have too weak an output to drive the power amp to full output.  

Will it hurt to try it?  Not if you keep the concert preamp at a very low level and control the overall level with the effect volume control.  You will get sound through it... and you will have to be the judge as to the level being sufficient for your particular use.

Would I do it?  Nope.  This type of setup would give you much more noise out of the system then you would probably be confortable with.  

Especially with a compression effect - as it will raise the non-playing noise level (noise generated by the concert preamp) that would normally not be present with the effect directly driven by your bass and then into the concert as it was designed to be used.

As to how you can tell from the specs... a line level unit will say that it is a line level unit.  If it doesn't say so... and especially if it is a stompbox type of effect, it will be low level.

I use a Digitech Legend 21.  It has a "secondary effects loop" that allows me to plug into the second part of the unit which is line level and gives me effects like delay, echo, reverb, chorus... etc.  I bypass the first part of the unit which is a low level preamp since I don't want the unit's distortion, compression and tone controls.

If your speaker is a 4 ohm unit, you won't need the extra speaker jack, as it is not a wise idea to go lower than a 4 ohm load with the concert.  On the other hand, if you have two 8 ohm speaker enclosures, I'd do it in a heartbeat!!