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Topic: Question for tube amp electronics experts |
Tommy Boswell
From:
Virginia, USA
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Posted 20 Jul 2016 6:26 pm |
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| My Fender Bassman 70 has a line out jack. How easy would it be for an amp tech to modify that to be a power amp input? So I could plug in an external preamp, and just use the power section of the Bassman? |
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Lane Gray
From:
Topeka, KS
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Posted 21 Jul 2016 3:15 am
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It depends where they pull the signal from. Many amps get their line out by putting a big honking resistor in line with the power amp output, so it's in parallel with the speakers, but with such high resistance that it doesn't taste real power.
The schematic is here: http://schematicheaven.net/fenderamps/fender_bassman70.pdf but I am not knowledgeable enough to say yes or no. One side of it comes from just before the phase inverter (a handy place for a power amp in), but the other side comes from the speaker side of the output tranny (oops, too late).
_________________
2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Mark Fowler
From:
Minnesota, USA
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Posted 21 Jul 2016 6:07 am
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| I would just put in an interrupt style effects loop (preamp out) (power amp in) jacks on back of amp between the master volume and phase inverter input. |
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Michael Brebes
From:
Northridge CA
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Posted 21 Jul 2016 7:03 am
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The Line Out is tapped off the speaker output. The only way would be to bring the signal into the input of the phase inverter circuit.
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Michael Brebes
Instrument/amp/ pickup repair
MSA D10 Classic/Rickenbacher B6/
Dickerson MOTS/Dobro D32 Hawaiian/
Goldtone Paul Beard Reso
Mesa Boogie Studio Pre/Hafler 3000
RP1/MPX100 |
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Mark Fowler
From:
Minnesota, USA
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Posted 22 Jul 2016 5:47 am
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For power amp in use a Switchcraft 12A shorting jack between Master volume and phase inverter input.
When nothing is plugged into the 12A jack the signal from the Master Volume goes through the switching terminal to the tip onto the phase inverter.
When plug is inserted to the switching jack it breaks the preamp signal and plug source signal goes to phase inverter entrance giving you a power amp.
Mark |
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Tommy Boswell
From:
Virginia, USA
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Posted 22 Jul 2016 8:08 am
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| Thanks Mark, you make it sound easy. I'll run it by an amp technician. |
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Tim Marcus
From:
San Francisco, CA
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Posted 23 Jul 2016 1:44 pm
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What Mark says will definitely work.
My advice is: if you go that route where you are relying on a jack closure, I would make sure and use a very high quality jack. The Mil spec switchcraft are good, and have a strong shorting mechanism
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Milkmansound.com |
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Len Amaral
From:
Rehoboth,MA 02769
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Posted 24 Jul 2016 7:19 am
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| I agree with Mark. I have been thinking about doing the same thing with my Fender PA-135 as it has the same Power 4 6L6 as a Fender Twin. |
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