A question about a line out on a tube amp

[70sSoundQuality]

Long time lurker here. The members here were a lot of help when I had a question about tubes a while back...so I figured this would probably be the best place to ask this question.

I have a '60s Bogen tube amp (Bogen CHB-100) and It has been "modded" by adding 1/4" ins and outs (instead of amphenol WWII connctions and strange speaker outs). I use it for my bass. The amp works just fine and there are no issues with the machine. However, I do not fully understand the whole business of the "line out".

Right where the 1/4" line out lies is a wire tap, a flying lead to change between 4, 8, and 16 ohms. The amp is getting plugged into a cabinet at a 4 ohm setting and that is fine.

But I asked my friend a question, who knows more about this stuff than I do but still claims to "not be an expert". I asked him what would happen if I plugged directly out of that 1/4" output and ran straight into my recording console, computer's stock soundcard, or even Plugged a set of headphones into that 1/4" line output. He said, "I really don't know".

So I decided I'd come here and ask you guys. What would happen if I ran straight out of that 1/4" line output and went into any of these 3 things I've mentioned? While I don't plan on doing any of this without some knowledge, I am still very curious as to what would actually happen in any of the instances. As anyone here can probably tell, I don't have the slightest clue about impedance matching and that sort of thing.

The thing is, the amp sounds good plugged into the cabinet, but I'd really like to go STRAIGHT out of the amp and into my console (its an oldie) and not have to use any loud cabinets (a number of reasons: a) it would be a quieter recording instead of using my cheap mics to record the cab, and b) not having a loud cab in the room makes it possible to record drums without a cabinet leaking into the drum mics).
Can anyone here point into the right direction ?

I appreciate you reading this.

best regards

PS - here's the manual for the amp in PDF format in case anyone is curious. It's a good cheap point to point amp.

http://www.bogen.com/support/discontinu ... HB100m.PDF

Thank you

[Bob-I]

No... DO NOT take the speaker output and plug it into your console. You'll damage both the amp and the console.

In order to do what you describe you'll need to add a voltage divider to the speaker output, plus you'll need to use a speaker along with the direct line out.

[70sSoundQuality]

Thank you bob-1. As I said, I had no plans of randomly plugging cables into things out of fear of the very things you've mentioned. I do have one question though.
What if I built a dummy resister load? This way I can go straight out of the speaker output, into a little box with a resistor tree inside of it, and out of that dummy load resister box into a console. Wouldn't that work?

Wouldn't that help me achieve what I want...ie not using actual speaker? Isn't a dummy load like this simulating the speaker cabinet in a way?

I appreciate any help!

70s Sound quality

[slajeune]

Hi 70s,

this is exactly what I have done:

dummy load -> voltage divider -> very simple tone shaping (to grossly simulate a speaker) -> out.

The way it's setup currently, you can use it as a hertzog unit (i.e. take the output of the first amp, plug it in the load box, take the output of the box into another amp). But, you can also use the line out and put this in the microphone in of a console.

Here is the project:

http://www.sopht.ca/mambo/index.php?opt ... &Itemid=54

The sound clip on the page was recorded straight into a mic in of a zoom ps-04. BTW, the amp is a single ended trainwreck type amp.

Cheers,
Stephane.

[70sSoundQuality]

Wow ! Great post. That sound clip is really well done. You get a damn good sound.


Here's is the bogen amp I've been talking about.

[img:720:540]http://photographs.homestead.com/Bog-3.JPG[/img]

[img:720:540]http://photographs.homestead.com/Bog-1.JPG[/img]

[img:648:486]http://photographs.homestead.com/Bog-2.JPG[/img]

[img:720:540]http://photographs.homestead.com/Bog-4.JPG[/img]

The speaker out is that fuzzy looking 1/4" out between the tap and the auxilary electrical out. Are you telling me, and I hate to seem redundant, that I can simply run a cable out of this 1/4" out, into a box with resistors to take the load, and out of that box into a console without worrying about anything?


Here are some schematics I found for 4,8 and 16 ohm loads @ 100 watts. The bogen in this picture is 100 watts. If I built one of the first two resister loads on this PDF, would this acomplish what I'm needing?
See, I wouldn't build it, but my friend who is in the elctronics department in the Coast Gaurd. He's interested in helping. You guys are also helping and I really appreciate that. Thank you.

Here's the dummy loads I found:
http://www.aikenamps.com/DummyLoad.pdf


Let me know what you think. The thing is, I don't see an "out" on those schematics.


Absolute best regards

70sSoundquality

[slajeune]

Hi 70s,

the dummy load pdf from Aiken is just that, a dummy load. It is ment to simulate the load of a speaker so that you can test an amp without hearing sound (i.e. injecting a signal and tracing it). Therefore, there is no output.

A line out has basically 2 sections:

- a dummy load to simulate the load of the speaker, and therefore use the amp 'quietly' without a speaker connected to it
- a voltage divider (with a control) to bring down the signal to a usable signal for a console. My box can be used as a hertzog, you can take the output of my box and plug it into another amp. I plugged my box into a mic in of the zoom ps-04 recorder to record the clip.
- (optional part) something to smoothen the highs and cut off a bit of the lows to simulate speaker response

So, looking at my box, R1 is the dummy load. You can use something more elaborate (as in aikens document). R2 and R3 is the voltage divider required to drop the signal. That is all that you need to bring down the signal to usable volume. The rest (C2, R5, R4, C1) are for tone shaping (i.e. simulating of a speaker). This is optional.

Cheers,
Stephane.

[drz400]

It isnt going to sounds the way you think
You will have a lot of high end, it'll be buzzy
You need some extreme tone shaping to get close to sounding like a mic'd cab. The load resistor is also a fixed load, not reactive like a speaker so the amp isnt going to respond the same at all.
what you need is a reactive load and tone shaping like the Red Box or probably better the Palmer stuff.

[70sSoundQuality]

slajeune - awesome post. Since that circuit is designed for a 50 watt, I assume my friend simply has to multiply the values by 2 in order to accomodate a 100 watt amp, such as mine?
Either way, we'll build your circuit (tone shaping and all). I have no idea why I didn't latch on to your site sooner. Thanks a million.

And drz400 - I don't mind if it doesn't sound like a mic'd cab. In fact, for bass, I was hoping to get the straightest cleanest signal path possible. I like that direct clicky tube sound (a sound I used to get running directly out of my super reverb pre-amp). Surely it can't sound any worse than just plugging directly into a console, right?

Thanks again.
slajeune, I'll update when we've built your circuit.

[slajeune]

You can leave the values as is.

Simply make sure that the 8ohm load can handle 100watt. To be safe, aim for 250Watts or more. 4 x 2ohm rated 100watt each would be a good start.

Cheers,
Stephane.