converting amp output to line in

[Guest]

i bought this old challenger pa amp today that has an rca (phono) input and 1 old school mic input. it also has bass and treble. i am thrilled with it on guitars into a 2 12'. like you told me before prr just plug the thing in the mic input from the POD modeler, and it works great. i have shown a few people that are guitar and amp collectors, they can't believe some junk i bought at a pawn shop for 75 bucks sounds this cool. all i did was knock out the old amphenol terminal and stuck in a 1/4" female. the phono input works best with the bass pod, as it is cleaner, but identical sound, they didn't put eq on the phono in's. the mic in has about 7x gain over the phono, and appears to distort quicker even at low input levels. so i am runing the guitar through the mic, it works out nicely.

when i was testing it at the store, i plugged a mic in it, and it had a nice character on the voice too, so that got me thinking i might try to make use if it as a not-so-pretty mic pre. i want to run the outs
(there are 4, 8, or 16 ohm options) into the ptools and see what kind of effect it will have for a dirty rock vocal. would anybody kindly indulge me on a way to properly get the outs to the xlr inputs? i don't want to screw it up by running some output it doesn't like. if it doesn't hurt it, i can already just make up a cable from the spades to xlr male and go in unbalanced(obviously).

another question, it has 2 12ax7's and 2 6v6GT's, and 1 6AX5GT. i went ahead today and replaced the 12ax7's with groove tubes. does anyone know of a better substitute for the other guys? i couldn't find them at guitar center or sam ash.

also, i don't know if it is because it sat for a year or mnore in that shop, but this thing smell funky. hope it fades!


thanks for any help

 

[PRR]

> i bought at a pawn shop for 75 bucks

You got robbed.

> replaced the 12ax7's with groove tubes

The Bogen tubes were BETTER than anything you can buy today, and are probably still good.

> this thing smell funky.

It will either burn-off the dust, or burn-up something irreplacable. So enjoy the odor while it lasts.

> run the outs (there are 4, 8, or 16 ohm options) into the ptools

Even a puny 2x6V6 will smoke a line input.

Also it may not be happy, and sure won't sound the same, without a load.

What are the other connections on the back (or top)? List them and jog my memory. There is one specific connection found on many bigger Bogens that will do what you want, safely. If you are too lazy to tell me, I'll make you go inside and follow the circuit to find it.

 

[Guest]

yeah it was too much money but i didn't feel like driving all over and looking for something else.

i pulled the cover off, took the tubes out to clean the dust and broke one of the original 12ax7's, thats why i went ahead and replaced them both. one is in a metal shied, the other is exposed. i can put back one of the originals and see what it sounds like.

there is(was) the old school style mic in on the front, on the rear right is a rca in, then the spade lug panel with com, 4, 8, 16, 70v, with the wire that goes inside to the plastic speaker connetcor. i am just comnnecting at the 4 ohm terminal. after that there is only as aux AC jack.

on the front is mic in vol, mic in jack, phono vol, bass, treble, on off.

i dont see any other option as far as external connections.

thanks

 

[PRR]

Use one of NYDave's handy tables to figure a 600Ω 20dB attenuator, and put that between 4Ω/0 and the line-in.

You want a load. Either a speaker or a 10Ω 10Watt resistor on the speaker taps.

 

[Guest]

thanks prr i will look up the tables.

i can just keep the speaker as the load.

 

[NewYorkDave]

Rat Shack used to sell an 8-ohm, 20W noninductive resistor that was OK to use as a dummy load on a very low-power amp. (I wouldn't recommend running it anywhere near 20W). I don't know if they still stock them. If they do, four of them in series-parallel would make a decent dummy load for medium-power tube amps if the resistors have enough airflow around them.

 

[PRR]

> an 8-ohm, 20W noninductive resistor

Good part, and it really will swallow 20W for an hour or so.

But he's got a 2-6V6 amp. 10W, maybe 14W if Bogen pushed it.

And a singer can't keep an amp saturated for very long. There won't be 10W of heat in the resistor. (Fuzzed-gitar just might hold totally-clipped output for long periods of time and exceed 10 or 20 Watts enough to smoke in the middle of a long solo.)

And non-inductive is really more about getting accurate test loads for precision measuements: ALL loudspeakers are inductive as heck, much more so than any "NOT-non-inductive" resistor.

And if we are not going to model speaker impedance (a "8-ohm" speaker really swings from 6Ω to 50Ω and down and back) then the exact value is not at all critical. It won't clip/sag the same in a speaker as a resistor, but it will be near-enuff the same for 6, 8, or 10Ω on the "8 ohm" screw.

I'd fearlessly run 10Ω 10W wirewound. That used to be a stock RS part, though who knows anymore?

 

[Guest]

i tried a test late last night with materials "on hand", i made a voltage divider using a 10k and a 1k, which is around 20db atten as you had suggested, left the speaker in for the load. it sounded pretty bad. it worked as far as dropping the signal to the right level. i found a circuit somewhere that suggested that the resistor wattage could be low with the speaker in. maybe bogus info, i googled

here is the circuit i found:

http://www.epanorama.net/circuits/speaker_to_line.html

maybe a better way could improve the sound, i guess i had hoped i could use it as a crude mic pre(for effect mostly).

as another option, can't i just tap off inside somewhere prior to the power output tubes? and use that as a line level out? it shouldnt be to hard to follow the the signal path, there is hardly anything inside it.

plus, half the purpose of this is to learn a bit about tube circuits.

thanks for the previous ideas. nothing like learning some new stuff, even on old not-so-esoteric gear.

 

[Guest]

i tried putting the 10ohm as a load instead of using the speaker, and i then took a wire off the 16 ohm output to an avalon compressor, then to protools and did some vocal tests compared to the avalon mic pre. i am having some hum that is not terribly bad, it could easily be gated. it is really not audible with other information present. there is also some slight noise floor of hiss due to the fact that the output runs wide open, you can only adjust the mic input, not amp output. so i have been poking around inside this thing trying to find a point that i can take a signal out before it hits the output tubes. i haven't had any luck so far, got shocked pretty good once though. i have no schematic yet, but they are available i think, so soon i'll get one.

my question is, should it be theoretically possible to access a point before he output tubes that i can get a signal? point being to avoid some noise.

in my tests tonight i was really shocked that i chose the bogen over the avalon easily for the character of sound. if i can solve the hum and noise floor i will have made a cool mic pre out of it. of course, the sound may not be as pleasing when i tap off earlier in the amp, but i want to try it anyway. thanksto anyone if you know whether this is possible. preferably, i'd like to just tap in somewhere, put in a jack to run outside the box, leave everything else as is for use on guitars.