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Topic: Adding reverb to a tweed 4x10 Bassman |
J Fletcher
From:
London,Ont,Canada
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Posted 16 Feb 2017 9:06 am |
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Some may be interested in this. Of course these days you wouldn't do this to an original 50's Bassman , but if you are building your own , like I have done , here's one way. Requires one extra tube , a 6SN7 or a 6FQ7 which is the 9 pin equivalent.
You have to sacrifice one of the channels , or else add another 12ax7.
Split the signal post volume control , one path goes to the 270k resistor to the input of the next 12ax7 , this is the dry signal , same as the original circuit. I will call this grid , point X.
The other path goes to the grid of one of the 6SN7 triodes which is used as a voltage amplifier to drive the 2nd 6SN7 triode. The 2nd 6SN7 triode drives the reverb pan via a transformer , same as Fender would use. Cathode R 2k2 ohm , bypassed with a 25uF cap. Grid R 1 meg.
I used these values to set up the 6SN7 voltage amplifier , 100k plate R , 2k2ohm cathode R, 4.7uF cathode R bypass cap and a 470pf coupling cap to the driver stage. This coupling cap can be mounted on the tube socket , from the plate of the voltage amp , to the grid of the rev driver triode. You need this voltage amp to get the rev signal hot enough.
The reverb pan (Accutronics 4AB3C1B)output signal is returned to the other half of V1 which originally was a input channel , the channel I sacrificed for the reverb add.
This 12ax7 is configured exactly as a reverb recovery stage found in a Twin Rev. 100k plate R , 1k5 ohm grid R bypassed with a 25uF cap. Use a 3000pf cap to couple the output of this triode to a 100k linear pot , and then return the wiper of this pot through a 270 k resistor to point X. This is where the dry and reverb signals combine.
Reverb signal is pre EQ in this set up.
Jerry
Last edited by J Fletcher on 18 Feb 2017 11:12 am; edited 1 time in total |
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Jim Sliff
From:
Lawndale California, USA
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Posted 17 Feb 2017 2:15 pm
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I'm interested. I built a 5E7 Bandmaster a while back - it'd be an interesting mod (I don't use Fender-tyupe reverb as it only works on high frequencies - but others use that amp often). I'd like to know what the other components are...and the reverb tank part number, as they vary in input and output impedance.
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional |
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J Fletcher
From:
London,Ont,Canada
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Posted 18 Feb 2017 7:12 am
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Hi Jim
The rev circuit would work in the 5E7 amp as well.
I'll open up the amp and get the component values, etc.
I used a Fender reverb transformer , or equivalent , and an Accutronics pan , same pan as a Fender Twin Rev , 4 ohm input , 200 ohm output. These are DC ohms. Will check on the actual part number . |
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J Fletcher
From:
London,Ont,Canada
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Posted 18 Feb 2017 11:16 am
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| Have edited my original post to include more component values. This would have been easier if I had just posted a schematic , but I would have had to know how to do that...Jerry |
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Ken Fox
From:
Nashville GA USA
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J Fletcher
From:
London,Ont,Canada
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Posted 26 Feb 2017 1:03 pm
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Ken , I have seen remote reverb only amps , made by Supro and Ampeg from the 60's. Same idea as the Mojo product , taps the dry signal from your amp's speaker.
Once upon a time I had a Guild amp which used a similar idea , except the reverb only speaker was built into the cabinet. |
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