Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dpm309 on February 27, 2012, 05:36:59 pm

Title: Preamp Out on Gibson GA 5T
Post by: dpm309 on February 27, 2012, 05:36:59 pm

I am looking at adding a preamp out to a GA 5T for someone who wants to use the preamp stage and plug it into the Power Amp In of his SS amp.  Would this be same as adding 1/2 of an effects loop?  I have seen a few examples of effects loops added to amps and was wondering if I could just install the send portion of the loop.  Would I insert this before or after the PP driver transformer.  I have attached a schematic for reference.

Thanks,

Dan

Title: Re: Preamp Out on Gibson GA 5T
Post by: sluckey on February 27, 2012, 06:21:44 pm

If you want the tremolo effect you'll have to connect somewhere on the right side of T3. I suggest this simple line out that is derived from the speaker jack...

(http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/Images/line-out.gif)

Replace R1 with a pot if you need to adjust the level.

Title: Re: Preamp Out on Gibson GA 5T
Post by: dpm309 on February 27, 2012, 08:19:11 pm

Sluckey, thanks for the response and drawing.  I had thought of going that route but do not want any sound coming from the Gibson's speaker while it is plugged into the other amp.  If I just cut off the speaker, it will certainly fry the OT.  I would like to keep the tremolo effect so I am thinking I need to insert this right before the power amp section.  Any suggestions?

Thanks,

Dan

Title: Re: Preamp Out on Gibson GA 5T
Post by: blackcorvo on February 27, 2012, 11:26:30 pm

Quote from: dpm309 on February 27, 2012, 08:19:11 pm

Sluckey, thanks for the response and drawing.  I had thought of going that route but do not want any sound coming from the Gibson's speaker while it is plugged into the other amp.  If I just cut off the speaker, it will certainly fry the OT.  I would like to keep the tremolo effect so I am thinking I need to insert this right before the power amp section.  Any suggestions?

Thanks,

Dan

Use a 10R power resistor as a dummy load for the OT. Just be sure it can handle the power output of your amp!
Just one question... is the OT directly soldered to the speaker, or does the amp have a speaker out? Because you could just use a switching jack connected to the OT, and wire it so that the dummy load comes in play when there is no speaker connected, and it's taken off the circuit when you plug in the speaker.

Title: Re: Preamp Out on Gibson GA 5T
Post by: sluckey on February 28, 2012, 06:29:35 am

Here's a line out that was just posted in another recent thread...

(http://www.el34world.com/Forum/index.php?action=dlattach;topic=13303.0;attach=28577;image)

Title: Re: Preamp Out on Gibson GA 5T
Post by: dpm309 on February 28, 2012, 02:56:46 pm

Since the OT is wired directly to the speaker, I could use a switching jack for the line out but will I still need some kind of dummy load for the OT?  Does Sluckey's suggestion work if there is no speaker load?

Dan

Title: Re: Preamp Out on Gibson GA 5T
Post by: sluckey on February 28, 2012, 03:00:28 pm

Quote

Does Sluckey's suggestion work if there is no speaker load?

You really need a speaker load. An 8 ohm 25 watt resistor is only a few $$$.

Title: Re: Preamp Out on Gibson GA 5T
Post by: six el six on February 28, 2012, 10:10:58 pm

An 8 ohm, non inductive, 225 watt resistor can be had from Newark for $20.

They come in handy.

Title: Re: Preamp Out on Gibson GA 5T
Post by: sluckey on February 28, 2012, 10:25:20 pm

That's a bit overkill for a 5 watt amp! :wink:

Title: Re: Preamp Out on Gibson GA 5T
Post by: dpm309 on February 29, 2012, 10:45:53 am

I should be able to find an 8 or 10 ohm 25W resistor locally.  Since the amp is directly wired to the speaker, where would I place the power resistor and how would I wire up the switching jack?

Thanks,

Dan

Title: Re: Preamp Out on Gibson GA 5T
Post by: sluckey on February 29, 2012, 12:06:58 pm

You would need a special switch jack for the line out that will also switch speaker and dummy load resistor. Here's how I would do it with readily available parts...

Title: Re: Preamp Out on Gibson GA 5T
Post by: dpm309 on February 29, 2012, 01:15:16 pm

Steve, thanks.  I will probably go with your suggestion using a SPDT switch unless I can find a special switch jack.

Dan