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Bias balance AND adjust mod in a Bassman 135?

2/29/2004 10:13 PM
Carlo Pipitone
Bias balance AND adjust mod in a Bassman 135?
I have once turned the bias balance circuit of my SFVR into the mixed balance & adjust circuit, which I do love.  
Now I have a Bassman 135 on the bench. It has a bias balance pot on the back of the chassis (as opposed to the usual under-the-chassis location of most BF and SF amps).  
May I perform the same mod to this amp?  
May I simply follow the same procedure used for the SFVR?  
Further, ther is a bad burning residue (like a coalish black stain) on the chassis exactly beneath the same pot. What could have happened in that part of the circuit?  
3/1/2004 4:48 PM
Le Basseur
Hi Carlo,  
The location of the burned-out pot suggests that this isn't actually the bias pot,but the hum pot.However,I might be wrong,but I recall that Bassman 135 has the hum pot in that location,as well as Bassman Ten.Please check where goes the wiring from that pot for being sure that I didn't mislead you involuntarily!:)  
Now,let's take it one at the time:  
1.The "mixed balance &adjust" mod suits perfectly on a 135,as on every PPP final stage susceptible of not having a closely-matched quad of output tubes.Assuming the owner of the Bassman 135 is a bass player,he'll fall in love once again with his amp after having each output tube individually biased.And yes,the procedure is the same as in the SFVR case.  
2.If I was right and the rear-pannel pot is the hum one,the burned marks are a living proof of a big mess in the output stage (most probably,a dead tube or even a pair wich caused a major imbalance in the circuit).Even if the present tubes are new,please do the following:  
a.Take a look at the screen grid resistors found on each output tube's socket and measure them.They should be 470 Ohm/1W Allan-Bradley's.If any of them is shorted or burned,replace it with the same value/wattage.Also,please read the threads on screen grid resistors found elsewhere on this forum,just to see what would be your alternative option about this issue.  
b.Please check the entire bias circuit (caps,resistors).Sa above,if you find a bad component,replace it.  
c.About the hum pot itself ;  
it's most probably that due to the above-mentioned imbalance because of a dead tube,this pot was also partially destroyed."Partially" means that even if the pot looks in one piece,the coalish black stain indicates that the carbon layer inside the pot was burned out,and this happened right at the point where the wiper was set to stay.Unfortunately,the 100 Ohms hum pots in Fenders are the average carbon-layer type,not the wirewound ones found in other pieces of equipment. In other words,even if all the other components are OK or you replaced the bad ones,It's unlikely to get the "smallest hum point" with that pot.If you care for the original configuration,replace it with a new one.  
If not,just go for the "2x47 Ohms/0,5W" approach as seen on other Fender amplifiers and not only.This way,you'll have a safe bet about any possible accident wich may occur in the heater circuit and also the intended function (getting a simetrical grounded reference on the filaments)will be fulfilled.Don't worry too much about the Fender's original use of a hum pot in that place:these pots were originally intended for use in circuits involving another "hard-to-tame" types of tubes,or in RF domain where hum is a REAL issue to deal with.Strictly speaking about the filaments,using the 12A*7 family and any of the current output tubes in a PP configuration isn't a  
big deal in controlling or minimizing the (thermal induced) hum.  
d.You asked what other mods/improvements could you do on the Bassman you got on the shelf.This is an extended story,and please take a read on the different threads on this forum.Take your time and you'll find a lot of great ideas to apply in your amp.  
As a rule of thumb,you'll want to look at the HV caps (if dry or leaking,replace them),at the cathode caps (idem),at the signal caps (replace them if they show any weakness with anything what suits your taste),at the anode resistors (replace them if they're too "hissy" or if they're drifting),and last but not least,the integrity of the internal wiring.  
HTH,and good luck!
3/2/2004 9:38 AM
Carlo Pipitone

Thanks a lot buddy! :)  
I'm gonna overhaul this amp as appropriate - I've already done this job on a few bf and sf amps.  
A few points:  
The location of the burned-out pot suggests that this isn't actually the bias pot, but the hum pot  
This amp has two pots on it's back (please go back to my other thread in the General Discussion forum): one is labelled HUM BALANCE, the other OUTPUT TUBES MATCHING.  
If I was right and the rear-pannel pot is the hum one...  
Yes, the burned marks are on the chassis close to the hum balance pot.  
just go for the "2x47 Ohms/0,5W" approach...  
This pot's wiring (apparently stock) looks quite different from the schematics. I have troubles in interpreting the schem correctly, and in othe words I don't know how to replace the pot with 2x47ohms resitors. Will you please send me your e-mail address, so I can contact you and stop boring the other Ampagers?  
The "mixed balance & adjust" mod suits perfectly on a 135...  
Good! But I'm thinking to do the "BF mod" and reverse the bias circuit to BF specs, i.e. plain bias adjust (I'm not the owner of this amp, and I don't want to give the owner one more pot that he doesn't know how to use exactly...). This "SF to BF" bias mod is safe in such amp, isn't it?
3/1/2004 6:01 PM
Le Basseur Oh,one more thing...
...that I forgot about improving:please consider a replacement of those 4 HT diodes with modern,hi-speed (around 20 ns) ones. :)
3/2/2004 9:41 AM
Carlo Pipitone

The schem says "1A 800V" diodes.  
What is the reference (commercial) name/abbreviation for the new diodes you suggest?  

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