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installing reverb unit to Bassman...

4/4/1998 7:51 AM
Xavier Cho installing reverb unit to Bassman...
It's my first post here...  
Well, I've just finished some crude 20-watt amp for a practice, (indeed, it was way to dear for a mere "practice", thou...^^;) and  
it turned great help reading those articles you wrote...  
Anyway, I decided to build another tube amp for a "practical" use. I suppose Fender Bassman with reverb unit will fit my purpose.  
I've found all two chems, but not sure how to  
combine them... (You know, I'm a novice...) And some guy told me there should be some springs between output trans and 12AX7 but I'm  
fairly sceptical about it. Isn't it using feedback rather than mechanical process like spring reverbs do to generate reverb effect?  
And most of all, I'd be very appreciated if anyone knows how to install reverb unit to Bassman without all those extra transformers and tubes. In sum, all I wanna know is "How to combine reverb and Bassman at the possible minimum cost".  
And now, a word from our sponsors:

4/5/1998 7:38 AM

Xavier, the only other way, that is easy is a series passive effects loop installed ahead of the driver tube and then use an outboard digital reverb device.  
Feedback and reverb are not quite the same thing.  
The springs you are refering to are very important to the sound of good reverb. Most good reverbs use springs or plates or some mechanical device to generate the delay and and signal distortion.  
Unfortunatly, they work with a small signal and then kill the signal by a tremendous amount through the reverb unit.  
The left over reverberated signal will need serious amplification after the reverb tank.  
This does require another source of amplification that a bassman does not have. It could be SS or tube.  
Fender used one 12AX7 and 1 12AT7 for this,  
but, it can be done with a single 12AT7.  
It will be weaker, less deep and not as good sounding as a stock Fender setup, however.  
For cost svings and heartache.... use a digital reverb or delay in an FX loop or, a delay/chorus- whatever, stomp box on the front end of the input for a very simple solution.  
4/6/1998 2:13 AM
Steve A.

    Amen! I think that Fender really screwed up the designs of their amps when they decided to add reverb. You have all of that gain in the preamp that is lost passing through that 3.3M resistor and 10pf cap... When you add in the cost for a DIYer to add additional tubes, a reverb transformer and a reverb tank, its a no-brainer- go with an FX loop as you suggested!  
Steve Ahola
4/6/1998 7:08 AM
J Fletcher
If you mix the dry and rev signals at the driver input instead of the stage before,you don't need a 3.3 meg resistor,you can use something way lower,100k or so.The value of the resistor for the rev signal at this point can be trial and errored,but somewhere in the range of 470k to 1 meg has worked for me.Nothing sounds quite the same as spring rev ,and adding the circuit to an amp is not that difficult or expensive,compared to adding an effects loop and an outboard rev.Depends on what level of difficulty you're comfortable with.
4/6/1998 9:04 AM

Hi J,  
I agree with you about the sound, J!  
There is something very magical about the complex sound of tube reverb from a real set of springs.  
But, I thought the question was how to get a reverb in a finished homebrew Bassman amp with the least amount of hassle.  
Maybe my answer was too simplistic.  
I don't know if the cost was a serious consideration.  
I'm sure an outboard digitial reverb FX loop system would ultimatly cost more then building a tank circuit and mixing it in the amp.  
But, you agree that the cost to install a generic  
$10.00 FX loop, consisting of 1 1/2 feet of shielded cable, a .2uF/600v cap, one 1/4" shorting jack and one standard 1/4" jack is pretty compelling to a novice homebuilder.  
I noticed Mark was using a similar topolgy as you suggets on some Matchless amps for the reverb but mixing (or inserting) the wet signal back into the phase inverter on the ground grid side! Cool.  
By the way are you saying the wet signal has a series resistance of 470K to 1M to the 100K resistor for level control and the dry / wet signal is then mixed across the 100K resistor? No cap needed.  
Please restate that for me and my dense skull.  
I don't think I ever have done it that way but it sounds like I should check it out.  
I find that the 3M3 and 10pF is fine for most Fender amps but if you have a chance, try out 2M with a 120pF/500v silver Mica cap and see if the amp doesn't bark with a little more authority.  
This, coupled with some different value tone caps and moving the dry channel to the reverb section, is one of my fav hot rod Fender mods.  
Of course not everyone likes what I like, just most everyone! : )  
4/6/1998 10:10 AM
J Fletcher
I made the assumption that an effects loop has a driver at the send,and an active buffer at the return.I have to confess to never having installed one!!However I have removed several,and the customers were quite happy with the results.I don't have an opinion one way or another re:loops,personally I don't use effects other than amp rev and trem,so I don't need the loop.If anybody asks me to install one,I refer them to somebody else who knows these matters.About the reverb mixing at the driver,what I've done on my Princeton Rev,among other things,is send the dry signal to the driver input through a 100k resistor,instead of sending it to the mixer stage,through the 3m3 resistor.The rev signal return is standard Fender,except that the 2nd stage of return,after the rev pot,no longer has the dry signal.The signal then goes from the 2nd rev recovery stage,through a .02 cap and a 470k resistor,to the input of the driver.So the input of the driver is the junction of a 100k dry signal resistor,a 470k wet signal resistor,and a 1000pf coupling cap that goes to the grid,which is returned to ground through a 1M resistor,as in the standard long tailed phase inverter.Values are from memory for the most part,but the idea is to get rid of the extra tube stage in the dry signal path.Why?Because a friend said to me that the rev amps don't sound as good as the no rev Fenders,so I attempted to get back to a simpler dry signal chain.I don't know if it sounds better or not,but if you want to get rid of the evil tone sucking 3M3 resistor,it's an option.
4/6/1998 6:41 PM
Steve A.

    What you suggest sounds great, but wouldn't it put the wet signal out of phase with the dry signal? Or does that not make any difference because the reverb signal is so different from the dry signal? In fact having it out of phase might even make it sound better...  
    As for someone having to buy an fx processor to add reverb, I was just going on the rather shaky assumption that everyone already has an old fx processor or two sitting around gathering dust... But tube reverb definitely has its charm- how about adding a standalone 3 knob Fender reverb to a bassman (like the VibroKing does)?  
Steve Ahola

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