Bassman line out?

[Bassmasta]

Hello. New here on this forum. I did search but didn't come up with anything. I've got very little experience building amps. I built a small 8 watt per channel Stereo tube amp and I'm in the process of building a Fender Bassman 5f6-a.


I'd like to add a line out or even better a balanced line out. I've read a little bit about wiring some resistors at the output of the power stage
But wouldn't this run the amp at an odd impedence and effect tone? I read this thread but had some trouble following it due to my lack of knowledge.

http://music-electronics-forum.com/s...light=line+out

I haven't decided on speakers yet, but the load will be either 2 or 4 ohm. The amp will have a switchable impedance, 2,4,8ohm and i would like the line out to work with all three. Also, wouldn't a balanced line out require another OT? Thanks in advance!

[Chuck H]

A line out is just a matter of voltage division. It should be set up to put out about 1 volt (close enough to .775 volts).

If you downloaded the chart from the post you attached then you know that your Bassman build @ 4 ohms will put out about 13 or 14 volts if it will be a two 6L6 power amp. So just figure a 13:1 ratio with 500 ohms as the denominator. You get 6500/500. Since 6.8k is a more common resistor value you could just use that. So a 6.8k dropping resistor and a 500 ohm load will give you just under a volt of output. To make it adjustable use a 4.7k resistor and a 500 ohm pot. That way when your playing super clean and below the amps max output you can turn up the line out to improve the noise floor. And when your cranking it for overdrive you can turn it down to avoid clipping sensitive inputs.

This extra 5.2k to 7.3k parallel load presented to the OT secondary is mice nuts compared to the 4 ohm speaker load. I promise that you will not even notice it's there and niether will your amp.

As far as a balanced output. I've never done one so I don't have much info, but I wouldn't bother for this application. The noise floor of your Bassman and guitar pickups will be higher than any induced hum on the line out and the impedance of the circuit is low enough to handle long cable runs. If you like I suppose you could wire it to an XLR so it goes neatly into the mixing board.

Oh, if you go 2 ohms your amp will put out about 9 volts. So a 9:1 ratio gives you 4.5k over 500. Just use 4.7k (common value) for a dropping resistor and the same 500 ohm load for an output just under 1 volt. Use a 3.3k over a 500 ohm pot to make it adjustable.

HTH

Chuck

[Bassmasta]

alright cool. so if I ran it at 8ohms that would be 20:1. so I would need a 10k R and a 500 ohm pot right? Sense the dropping resistor's values are all different how would I make this work? Could I use a switch and have it switch between the resistors? I'd like to avoid this if possible seeing how i already have a switch selecting impedance.

Thanks for the response. I apologize for my novice questions.

[Chuck H]

Just pick one impedance to work with and wire the line out to that impedance tap, not the speaker jack. Same as you would for a NFB loop. That way the OT always "sees" the same thing.

Chuck

[olddawg]

I've done this old trick before to get a line level signal out of an amp, but it is not a true line out. It is a speaker level signal attenuated to a line level out. Usually what you end up with is a hellatious noise floor with lots of hiss and you risk any ripple noise being exaggerated hundreds of times. Now this isn't always bad. A healthy noise floor can cover a multitude of evils and add character when doing garage recording, but there has to be a cleaner way to do it.

[Bassmasta]

yeah thats probably what i will do. it'll be easier and cheaper.

I've done this old trick before to get a line level signal out of an amp, but it is not a true line out. It is a speaker level signal attenuated to a line level out. Usually what you end up with is a hellatious noise floor with lots of hiss and ou risk any ripple noise being exaggerated hundreds of times. Now this isn't always bad. A healthy noise floor can cover a multitude of evils and add character when doing garage recording, but there has to be a cleaner way to do it.

It'll work for what I'm going to use it for. Recordings and running a line to the mixer for live application. What defines a true line out if this isn't one?

I was looking at the schem. for a bassman 135 and Fender used this same technique for a line out.

[Chuck H]

The noise floor on this type of system is unavoidable. But the alternatives do not sound like your amp.

When you include the power amp in the line out signal you get all the hum, fuzz, hiss and even odd noises from the room that get amplified to the extent of the amps wattage. But if you take the signal from earlier in the amp you miss out on what the power tubes and OT to speaker kick back voltages are doing to the tone. Line out circuits that come from a preamp tend to sound dry and sterile. Not to mention that they lack in overdrive if your amp derives most of it's distortion from the PI and power amp.

All in all I'd say this is the best system going. The alternative would be to send a preamp signal to a different, isolated, mini power amp circuit that is designed especially for noise rejection, and then derive a line out from that. But who's going to trouble with that.

Chuck

[olddawg]

The noise floor on this type of system is unavoidable. But the alternatives do not sound like your amp.

When you include the power amp in the line out signal you get all the hum, fuzz, hiss and even odd noises from the room that get amplified to the extent of the amps wattage. But if you take the signal from earlier in the amp you miss out on what the power tubes and OT to speaker kick back voltages are doing to the tone. Line out circuits that come from a preamp tend to sound dry and sterile. Not to mention that they lack in overdrive if your amp derives most of it's distortion from the PI and power amp.

All in all I'd say this is the best system going. The alternative would be to send a preamp signal to a different, isolated, mini power amp circuit that is designed especially for noise rejection, and then derive a line out from that. But who's going to trouble with that.

Chuck

All true, but it still doesn't sound like your amp because it does not include the coloration from the speaker and cabinet. Then again, amps with speaker emmulated line outs sound like crap to me also. In the end an SM57 in the sweet spot is the best way to go IMHO.

[Chuck H]

Ahhh yes. But have you ever experimented with combining the dry signal from a "line out" with a mic'd signal. It can be golden. Plus, for club use having a line out on your amp means you don't need to have an extra mic, cord and stand. You just plug into the PA. Very convinient if you play a lower wattage amp like 5, 15, 20 or even 30 watts. You can get most of your tone from the amp itself and just a little boost from the PA.

I have always used a mic for recording electric guitar and always will.

Chuck

[olddawg]

Ahhh yes. But have you ever experimented with combining the dry signal from a "line out" with a mic'd signal. It can be golden. Plus, for club use having a line out on your amp means you don't need to have an extra mic, cord and stand. You just plug into the PA. Very convinient if you play a lower wattage amp like 5, 15, 20 or even 30 watts. You can get most of your tone from the amp itself and just a little boost from the PA.

I have always used a mic for recording electric guitar and always will.

Chuck

Yeah sure. You really have to watch out for phase cancellation though. I would usually buffer it through a tube direct box. BTW, since your'e up did you see my recent post on adding a reverb to an 18 watt clone. Any ideas Chuck?

[Bassmasta]

I have another question. I'm wiring the output transformer. My instructions say to wire the output transformer plate to pin 3 on one of the output tubes and the other plate wire to pin 3 on the other output tube. Which wires are the plate wire? i have black, red, and brown. it also mentions that one is a blue wire but i don't have a blue wire. The transformer is a Mercury Tone clone. I looked over mercury's website and couldn't find any information.

[Chuck H]

The MM Tone Clone OT's use wire codes like the originals they are "cloning". With the exception of the units that have multiple output impedance taps. Your OT should have:

Primary side: Red, blue and brown, blue and brown go to the power tube plates. The red goes to the B+ supply.

Secondary side: Black, and one to three other colored wires. Most likely green, yellow and grey. But it could be other colors. Black is usually "common" and goes to the output jack "-". The colored lead/s go to the output jack "+" or the impedance switch.

Usually the primary leads come from one "side" of the transformer and the secondary leads come from the other.

If your OT does not have at least 5 leads, three one side, which are red, blue and brown, and two to four on the other, at least one of which is black, something is wrong.

Chuck

[Bassmasta]

I have a total of seven leads. On the Primary side i have black red and brown. on the secondary side, black, orange( 2ohm), yellow(4ohm) and gray (8 ohm).

The transformer came with a wiring diagram and shows the red on the primary side being the center tap. Would this make the black and brown wires the plate wires? This is my first guitar amp. The wiring diagram doesn't say anything other then black red and brown next to their corresponding wires. on the secondary side its labeled which wire is which impedance.

[Chuck H]

I just had a post where I was bitching about how tranformer MFG's often seem to use whatever wire they have lying around. I guess it's possible that MM was out of blue the day they made your OT

Do a continuity test to see if the black brown and red wires are even related to each other. If they are I would assume there is a temporary fashion change at MM and black is the new blue

If you think the "black" lead is the "blue" lead then hook it to the tube plate that has it's grid tied to the PI on the 82K plate load side. This may save you the trouble of having to swap them later because they are backwards.

Chuck

[Bassmasta]

how would i know if they are backwards?