One-wire mod SF Bassman 100

[kniqht152]

Well guys I got an update for you here.

Turns out it was a bad tube after all. The mod works great and I'm super excited to start messing with settings and preamp tubes. Thank you all for your help it is greatly appreciated.

On a side not I feel like its time to replace the filter caps anyone know of any good websites where I could order some?

[Tom Phillips]

...On a side not I feel like its time to replace the filter caps anyone know of any good websites where I could order some?

Some places only sell to established shops. One outlet that will sell to all is Antique Electronic Supply LLC

[kniqht152]

Thank you for that link Mr. Phillips.

Ola gentlemen/gentlewomen,

I'm waiting till I get paid later today to do some tube shopping and stuff. However this leaves me with free time which is bad cause once you start modding it's very hard to stop.

Question for you, in the Bassman both V1b and V2b share the same cathode resister and bypass cap. I don't beleive it is outside my skill level to seperate them. What kind of benifits would this give me in a cascading enviroment such as this one? Is it worth the effort?

[Tom Phillips]

There is no down side to separating the shared cathode components so I would just do it since it's so simple and inexpensive. The only con is that you need to devise a clean, neat & reliable mounting technique since there are no extra eyelets available. On the plus side you can experiment with different bypass cap values for each stage. Search for discussions about using lower value cathode bypass caps. Especially on the first stage of your pre-amp.
Cheers,
Tom

[pdf64]

Well spotted, it's bad practice for stages in series to share cathode components.
Bypass caps always have some ESR and that can create positive feedback paths, leading to oscillation.
Pete.

[kniqht152]

Yeaya, looks like I do know a thing or two.

I've been messing with settings and it seems like the whole thing is really mid scooped. I noticed that the mid pot on the normal channel is a 10k pot. Would changing it to a 250k or 500k solve my issue here?

Or

I've read about how changing the tone controls to be cathode follower instead like the marshalls would get rid of that mid scoopyness.

In all actuality I'll prolly just replace the pot but just for future reference what would converting to cathode follower tone stack do for me? What can I expect from a mod like that? What should I watch out for?

Thanks for the info guys!

[Tom Phillips]

Remember that, with your "one wire mod" configuration, the signal is passing through two tone stacks in series. Any mid scoop is therefore exaggerated. It appears that you are moving into a more sophisticated mod. If you are going to take this path to avoid buying a gain pedal you will need to spend some time and money for both knowledge and materials. If you are having fun that's good and that's what this board is about so I don't mean that to discourage you. There is good information in the Kevin O'connor TUT books. You can also locate (Via Google) and download Duncan's Tone Stack Calculator. It will show you the effect of various tone stack changes. This is just so you can get a feeling for what does what. Listening is the final test.

[kniqht152]

Hey guys I'm back! and now with more issues!

So it was working fine, thanks to all you lovely gentlemen, and naturally I decided to try and mess with it some. So now I've GREATLY modified the original preamp circuit and my problem is with a guitar plugged in I get a very intermittent, quiet, grainy, kinda like a busted radio sound out of it when I strum hard. When I strum normally I get no sound out of it at all. In my opinion it seems like a bad solder joint but I went through it all with my bro and tested for continuity via multimeter to find that it all checks out. Tried different preamp tubes and that doesn't seem to be the case. To be honest I'm not exactly sure what the it could be.

Here is a schematic of what the preamp section of the bassman looks like so far, everything else is pretty much stock.



Anybody have any ideas or suggestions? Other then re-soldering everything I'm not sure what to do.

Thanks again.

[pdf64]

The grid of V2a needs a ground reference, eg another 470k resistor adding to ground, at the node of the existing 470k and 220k resistors in the grid circuit.

[kniqht152]

That did the trick! Thanks much pdf, figures it'd be something simple like a missing resistor. Happy 4th everyone!

[kniqht152]

Hola!

Amps still working fine. Does anyone know how to implement a presence control? I understand/think its basically a pot with a cap on the negative feedback loop but to be honest I don't know what part of the circuit that is on the default bassman schematic.

help?