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FDP Forum / Fender Amps: Vintage (before 1985) / '78 SFSR Line-Out Question

silverfacefox

USA

May 9th, 2016 01:37 PM   Edit   Profile   Print Topic   Search Topic

I just recently inherited a '78 silverface Super Reverb. It's in great condition, only had the tubes replaced. I've come across a weird issue - if I hook into the line-out, run that to my mixer, and then disconnect the speakers, the signal for the line-out becomes almost inaudible. I can crank both master and regular volume and my mixer won't even register a blip on the LED VU meter.

Plug the speakers back in, nice strong signal, master and regular volume at ~2.

Am I going to be stuck taking a 100w resistor and making a hotplug for any time I want to run this to my mixer and use headphones to play guitar, or is there some other way to utilize the line-out without having to have the speakers connected as well?

Hammond101
Contributing Member
*********

So. Cal. USA

May 9th, 2016 03:22 PM   Edit   Profile   Print Topic   Search Topic

Do not run your amp without the speakers connected. Tube amps of this vintage are not designed to be run without a speaker load. Damage will result. Major damage.

This line out was designed to send a signal to a mixer or the input of another guitar amp. When you disconnect the speakers you are engaging the shorting type output jack of the amp and the power tubes are running directly into that short. not good at all.

You could run the amp into a dummy load but is not reactive like speakers and you will eventually kill the amp.

Roly
Contributing Member
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Whitehorse Canada

I don't get out much
May 9th, 2016 04:40 PM   Edit   Profile   Print Topic   Search Topic

Pull the power tubes?

Hammond101
Contributing Member
*********

So. Cal. USA

May 9th, 2016 05:13 PM   Edit   Profile   Print Topic   Search Topic

It's been a while since I had an ultra linear amp on the bench. I believe the line out is power amp based (green wire on OT), not the preamp. Pulling power tubes may not work if this is the case.

silverfacefox

USA

May 9th, 2016 07:39 PM   Edit   Profile   Print Topic   Search Topic

I won't be running without some load, I know how to not blow my amp. I would have just thought this would be a more true line-out.

I can make a reactive load to plug into the speaker plug. That's as easy as three resistors, two inductors, and a cap.

So I guess I am indeed stuck making a hotplug device when I want to not wake my neighbors.

silverfacefox

USA

May 9th, 2016 08:37 PM   Edit   Profile   Print Topic   Search Topic

If this will help in identifying, here's a picture of the internals.

Link to pic of internal wiring

Steve Dallman
Contributing Member
*********

Merrill, Wisconsin

Age is just a number...mine is big
May 10th, 2016 02:16 PM   Edit   Profile   Print Topic   Search Topic

The line out takes the speaker signal, and lowers it via two resistors to line level. The line level is not speaker compensated (made to sound like a mic'd speaker) and is better used for driving other amplifiers than recording or running to a mixer.

The amp HAS to have a speaker load. Pulling the power tubes would defeat the line out. Running the amp without a speaker load will soon cause destruction of the output transformer.



Roly
Contributing Member
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*********

Whitehorse Canada

I don't get out much
May 14th, 2016 05:44 AM   Edit   Profile   Print Topic   Search Topic

Line out is post output tubes.....yes?
If that's the case...my suggestion won't work.

cheers

Peegoo
Contributing Member
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Eat. Sleep. Guitar.

Repeat
May 14th, 2016 06:28 AM   Edit   Profile   Print Topic   Search Topic

Correct. Without the power tubes in there, there's no output.

The resistors off the 'speaker out' step down the output voltage from speaker level to line level.

On modern amps tho, the vast majority of line outs are after the pre-amp and before the power stage.

FDP Forum / Fender Amps: Vintage (before 1985) / '78 SFSR Line-Out Question




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