One-wire mod SF Bassman 100

[kniqht152]

Hi guys I'm new here but I come with questions. Also if this is the wrong thread for this kinda stuff then I apologize.

I play mostly pop-punk-rock and I've been trying to squeeze gain out of my old modded SF Bassman 100 and nothing I do is really hitting the mark.
Now I like the way it sounds tonally but I just need to add more bite to it, I mainly use the bass instrument input (input 1) and I was wondering if there was some form of "one-wire" mod which could cascade the other two triodes from the normal channel to the tail end of the bass.

Here is a link to the regular schematic http://www.drtube.com/schematics/fender/bassman100.gif

I understand that I can just jumper the two inputs to have them run in parallel but in my opinion that only complicates the sound, what I'm really looking for is a hi-gain cascading distortion ya know?

If anyone could help me out with knowledge here I would be immensely grateful.

Thanks.

[Jazz P Bass]

IMHO your 'best' bet is to find a gain pedal that you like.
The preamp is pretty much doing all it can.
Can't get blood from a stone.
Or something like that.

[kniqht152]

Thanks for the reply and I appreciate the honesty. I've heard this suggestion before and it is definitely on the table in fact I have a couple ideas for pedals too , however I'm a pretty poor bloke and I'm some what handy with a soldering iron. That being said I plan to beat this stone until it does bleed. How many licks does it take to get to the center of a tootsie pop? I will find out.

Using the very popular search function on the forum I found these two links that look very promising.
http://satamax.free.fr/bandmastermod.jpg
http://music-electronics-forum.com/t4118/

I figure doing something along those lines would prolly work out. Any one got any advice or suggestions? Are there any known pitfalls that I'm overlooking?

[kniqht152]

Would this work guys??




If I went with something like that could I still use input 4 safely? Would I be better off disconnecting it completely?

[Bruce / Mission Amps]

The one posted by satamax.free.fr. will give you so much additional gain and massive subsequent preamp distortion, you might dislike it with it cracked open very far at all.
But, it will work and it at least has tone controls you can roll the bass off and a volume control after the first gain stage, which in these amps is running flat out, balls to the walls before the tone-volume control section.
You could then mess with less gainy preamp tubes to get the sound you are looking for.

[kniqht152]

Bruce,

Good point with the gain, I never even considered the possibility of to much distortion. Would it be better then to wire the bass channel from pin 6 of v1b to pin 7 of v2b with some cap and resistor? That way its only using 1 added gain stage. I figure that might be more manageable.

[kniqht152]

Mods can someone move this thread to the "mods & tweaks" section, I just figured out its much more appropriate there.

[pdf64]

Either of those methods would work (there are dozens of options and no one 'correct' method); you could fit a pot into the hole for jack socket #3, to get additional control of the overdrive level.
Input 4 will still work, you could use an external AB box to switch between them (may get a bit hissy though).
Experiment and have fun, you are very unlikely to damage anything when messing around in the preamp.
Apart from yourself that is, so maintain appropriate safety precautions when messing with open amp chassis.

Regards the original schematic, I noticed that the input designation arrangement looked unusual, in that the #1 and 3 inputs are the padded ones, with #2 and 4 being the unattenuated ones, which seems to be the opposite way around to normal. Do any other Fenders have the same arrangement?
Pete.

[Tom Phillips]

...Regards the original schematic, I noticed that the input designation arrangement looked unusual, in that the #1 and 3 inputs are the padded ones, with #2 and 4 being the unattenuated ones, which seems to be the opposite way around to normal. Do any other Fenders have the same arrangement?
Pete.

I have previously seen that on the Bassman 100 schematic. I think that it's a drawing mistake on the schematic. The actual panel makings for the inputs are labeled 1,2 - 1,2 not 1,2,3,4 as shown on the schematic and the actual wiring is set up with attenuation on both #2 inputs per traditional Fender practice. Looks like the "New Guy" made the Fender schematic drawing.

[kniqht152]

Thanks for the input guys,

pdf64, I will definitely give that a shot later on.

Tom is correct as far as the input thing goes, I made a little change to my schematic here in an effort to save the input jack(please give me a shout-out if you see something wrong).

I'm new at guitar amp circuitry and I'm wondering here, if v1b is in series with the guitar inputs, what is preventing the guitar player from being shocked? is there just not enough voltage going through the preamp at that point or do the input resistors prevent that from happening?

[Tom Phillips]

… what I'm really looking for is a hi-gain cascading ...

That pretty much requires at least one additional added triode stage. If you were just looking for a little more kick then there are other simple options.

… Are there any known pitfalls that I'm overlooking?

There is the potential to cause instability / internal feedback because of the re-routed wiring which brings the amplified signal back to the input stage of the amp. Good lead dress and shielded wires can help prevent this problem. The sloppy wiring of the silverface amps contributes to this potential problem.

… I made a little change to my schematic here in an effort to save the input jack (please give me a shout-out if you see something wrong)…

The mod you show here is creating a voltage divider with you added 68 kΩ forming the top half and 34 kΩ (from the equivalent resistance of the input circuit with the shorting jack switch closed) forming the bottom half. That attenuates the signal 66%. There will also be some additional signal attenuation because the plate of V1B will be loaded down more than usual. This attenuation may actually be a good thing because the potential gain of the cascaded channels is so high. I think you are already aware that your approach is not the usual way that a high gain channel would be implemented. We are limited by the “one wire mod” request.

… if v1b is in series with the guitar inputs, what is preventing the guitar player from being shocked? is there just not enough voltage going through the preamp at that point or do the input resistors prevent that from happening?

The resistors are not there to provide protection. There is no high voltage at the input jack because the grid of the tube is essentially at zero volts. Shorts from the high voltage on the plate to the grid are obviously extremely rare and have not been a problem or the design would have been changed.

[kniqht152]

Well here is an update guys.

I soldered it all up according to the schematics I posted up earlier and the connections seems solid and what not. Plugged the amp in and heard a small pop first thing.
Second thing I noticed is that the 2nd tube (tung sol 12ax7) is glowing a very cool bright purple color, thats not normal although very cool.
With the volume around 1 for the bass instrument channel I heard a nice distorted cleanish guitar however when I rolled the volume up around 2 all lights went out on the 2nd tube and I got no volume for guitar at all no matter how I set the volume knob. I don't think the tube is blown because as soon as I shut the amp off and repeat the process the same thing happens over and over. It seems to me like I might be over-volting the tube? Does that make sense?

Lets pretend I know a thing or two here, the 2nd tube is overvolted because of the little resistance between gain stages. What I need to do is either wire in a pot or a 470k resistor like the marshall's have between their gain stages.

Any suggestions guys? I appreciate your help, discharging caps and reading schematics is basically the extent of my knowledge here.

I think you are already aware that your approach is not the usual way that a high gain channel would be implemented. We are limited by the “one wire mod” request.

Yea it's not normal I figured that out pretty quick, please don't be limited by the "one wire mod" request I just figured it would be easiest to implement. My only real requirements are that the amp works, no holes need to be cut into the chasis, and that any mods are easily reversible just in case I screw something up. I play in a band regularly and if I need an amp for practice I gotta get it up and running you know. If you can modify the schematic above to show a better way to steal a gain stage please do.

Thanks for the help guys.

[g1]

I don't think the tube is blown because as soon as I shut the amp off and repeat the process the same thing happens over and over. It seems to me like I might be over-volting the tube? Does that make sense?

No it does not make sense. You can't rule out the tube unless you try another one, or try that tube in another amp. Right now, it seems the tube is the most likely suspect.

[pdf64]

Some voltage measurements are needed to work out what's going on; are you competent to do that (ie safely)?
Possible issues are parasitic oscillation (as mentioned previously) or Vdc getting to the 1st V2 grid.
Pete.

[kniqht152]

Some voltage measurements are needed to work out what's going on; are you competent to do that (ie safely)?

Negative, In all actuality I could probably but I'm not comfortable with the idea of an open chasis being plugged in. Electricity scares the crap outa me which is ironic considering I am an I.T. guy and I work with electronics everyday.

I understand that one way people deal with parasitic oscillation is to have risistors soldered directly to the tube socket. Which is where my 68k resister is at.You guys are the experts tho.

The plan for today is to run back to the practice space after work and just try another tube to see if the same thing happens. I'll let you know whats up then and we'll take it from there.

Thanks for the replys guys!