Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on April 24, 2018, 11:44:31 am
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What would be the optimal point for the effect loop in AB165,
before PI or one stage earlier?
http://el34world.com/charts/Schematics/files/fender/Fender_Bassman-AB165-schematic.pdf
/Leevi
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That AB165 is a nest of interacting negative feedback loops. Cutting into it means a LOT of re-engineering and tone-tweaking.
Aren't there other big Blackfaces you can work from?
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Unless you're paying me BIG $'s, I put all my effects loops before the input jack, works great :icon_biggrin:
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But then again, it does have that unused triode that would be great as a FX loop recovery stage.
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Thanks for your replies PRR and shooter.
I fully agree with your comments and wouldn't add a loop to my AB165.
Anyway it's a question about how to connect a keyboard to Bassman and I think the needed
input in this case is "power amp in" (Return).
/Leevi
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Referring to the schematic... Disconnect the left side of that .1µF cap that feeds into the PI. Reconnect that end of the cap to the return jack. Now connect another .1µF cap between the send jack and pin 6 of V2.
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Referring to the schematic... Disconnect the left side of that .1µF cap that feeds into the PI. Reconnect that end of the cap to the return jack. Now connect another .1µF cap between the send jack and pin 6 of V2.
Thanks sluckey, so you prefer to place it just before PI.
/Leevi
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Referring to the schematic... Disconnect the left side of that .1µF cap that feeds into the PI. Reconnect that end of the cap to the return jack. Now connect another .1µF cap between the send jack and pin 6 of V2.
Bear in mind that the power amp global NFB loop is unusual, configured as an inverting amp in that model, gain = Rf/Rin.
Rf = 47k + Xc, Rin = V2 output impedance + Xc
If V2 is removed from circuit by the use of the return jack suggested above, then the power amp characteristics will be altered, and will depend on the output impedance of whatever is plugged into the return jack.
http://schems.com/schematicheaven.net/manu/fender/bassman_ab165_schem.pdf
The amp has 4 input jacks already, why aren't any of those deemed suitable for this application / why would a return jack be 'better'.
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> The amp has 4 input jacks already, why aren't any of those deemed suitable
They have high gain and heavy tone control.
A "keyboard" typically has a higher level output, and shouldn't normally want the heavy EQ we like on naked strings.
Of course a pad from the keyboard, and careful tone-knob settings, WILL work.
I did not go back to see what Sluckey suggests. My recollection is that the power amp (driver+final) works at several Volts input and also in this model has the NFB injected AT the driver input. But hey, give it a try.
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Anyway it's a question about how to connect a keyboard to Bassman and I think the needed
input in this case is "power amp in" (Return).
If you only want a Power Amp In, I think it can easily be done.
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Attached is an idea for a loop with a passive Send and an active Return that utilizes V2A. The output impedance of V2B is low enough to drive a long cable, but the signal is too large at that point. The low output impedance allows V2B to easily drive the voltage divider for the low impedance Send. V2A should have a gain of 5 plus some change while presenting an output impedance that is similar to V2B so that the NFB loop ratio remains about the same.
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Thank you 2deaf for your ideas. The buffered alternative is optimal since the
extra amplification stage can be used for that. I also like extra switches for the effect loop since
the switch of the jack is not that robust in long run.
/Leevi