Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chocopower on April 30, 2014, 08:01:08 am
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Hi there. A friend of mine is using a "Crestline" Gibson Falcon GA 19 RVT. Sometimes he need more volume on stage and is using the Gibson "monitor" out connected to the effect return of a Crate combo (2xEL34, 2x12" speaker). So he is, basically, using a power amp to have more volume.
He is very happy with the sound, but in some situations, the Crate is not loud enough. A amp with 2 EL34 and two speakers should be a lot more loud.
I think the reason and the solution for the problem is in the voltage divider line out in the Falcon, who is the classical 47k / 470 ohm combination connected to the 8 ohm speaker out.
I"know" that rising the 470 ohm resistor will rise too the level signal, but:
How high can i go?
I know too that installing a pot give me adjustable level, but (again):
Which value could be ok?
Thanks in advance!
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Do you have a model # for the Crate? Or a schematic? You'd need to know something about the input on the Crate, to know what signal level will drive the secondary amp to full output power.
There is conflicting info on the power output of the Falcon, but one source says 14w. That implies √(14w*8Ω) = 10.58v RMS across the speaker at full output (of the Falcon). The monitor output reduces that to 470Ω/(47kΩ+470Ω) = ~0.01 times full voltage, or ~105mV RMS. It appears Gibson assumed you would plug this into the input jack of another amp.
*IF* the Crate expects a line level signal of about 1v RMS, you could swap 4.7kΩ in place of the existing 47kΩ. This now gives ~9.1% of full speaker voltage, or 0.96v RMS.
The above assumes the Falcon is cranked to full clean(ish) output power. You might exchange a 1kΩ pot in place of the 470Ω to get a bit more output voltage, and to allow a variable level.
The right way to know for sure is to look at the Crate schematic. Or, you could just try it and see what happens.
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Ok. Here are the Crate schematics.
It is a 1x12" combo, not 2x12" as i said. :BangHead:
http://schematics.number26.org/index.php?dir=crate/V50-112/ (http://schematics.number26.org/index.php?dir=crate/V50-112/)
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i think this is the best schematic. I found the "send" but i cant see the "return".
http://www.blueguitar.org/new/schem/misc_amp/crate_v50.pdf (http://www.blueguitar.org/new/schem/misc_amp/crate_v50.pdf)
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I found the "send" but i cant see the "return".
It's on the output section schematic (http://schematics.number26.org/index.php?dir=crate/V50-112/&file=0024347_revA00-SCH%20%28OUTPUT%29.pdf).
Your friend can probably get away with less signal fed to the Crate, but go ahead and swap the resistors for the 4.7kΩ and the 1kΩ pot as I said earlier.
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Ok. Thanks!!!
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You need a volume pot.
The Return in the Crate has no volume control. You really want the Falcon overloading "slightly" before the Crate; to get the full power of the Crate with the overload tone of the Falcon. We can not know the gains/levels closely enough to calculate; you gotta trim on test.
The Crate Return needs around 0.25V; the Falcon delivers 10V-11V. Neither value is sure within 50%. So we "think" we want 40:1 reduction, with 2:1 guess-width. So 20:1 to 80:1, which may as well be 20:1 to zero.
If the other amp is adjacent (3-foot cable) the impedance is not critical. If 30 feet away we need impedance under 10K.
Grab a 10K pot. To cut-down 20:1 to a 10k pot we need another 190K from pot-top to speaker out. 220K is nearest standard value and probably fine considering overall uncertainty. If 220K still leaves you wanting "more", then 180K even 150K.
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Thanks all you!!!!