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Found out how to give a marshall a lot of bass

4/15/1999 1:59 PM
j burt
Found out how to give a marshall a lot of bass
on my '78 1959 head i put a .033uF cap on the 100k feedback resistor and woah!! i didnt know i could get that much bass out of this amp!! it sounds good too! i put a 100uf cap over the 2.7k input resistor, i changed the slope resistor to 47k and that's about it!i had the slope resistor at 56k but i thought it would be too much with the cap on the feedback resistor. my bass pot actually does something now
4/15/1999 2:43 PM

Well good for you! I'm gald you got something you finially like outta that amp!! :-)  
Do you have caps on the other cathodes?  
4/16/1999 12:32 AM

it sounds so fat! it doesnt have just less treble, ive got about 8 more notches of bass to play with! it actually sounds like the voice of the angels now! is this kind of mod bad for the amp, it doesnt seem like it would be.
4/16/1999 11:30 AM

I don't know if it's bad or not myself but Dave brought a good point about driving the output section. It may become unstable so be careful when you are cranking it just in case.  
There's other ways to get more bass out of the amp that wouldn't make it unstable. It's up to you. If you like what you have and it's not "freaking out" then run with it!  
4/16/1999 12:06 PM

well, just to be sure im going to experiment with lower values of caps there. i meant to get a .0033uF and put it there but the guy gave me a .033uF cap instead. so, it sounds fat, but it could get a little skinnier and still make me wondering about something- the 2 caps on the plates of the PI tube are .047 and stock is .022. should i change it back to stock? somebody tried to turn this amp into a fender at one point.
4/15/1999 11:16 PM
Dave Stork

By putting that cap across the feedback resistor, you increased the negative feedback at higher frequencies, which means less treble... which sounds like more bass.  
I'm reluctant to mess around with the time constants of feedback loops, myself, since doing so can cause instability when the output section is driven into clipping. It used to be common in the hi-fi world to add caps across feedback resistors to attempt to flatten frequency response by compensating for output transformer LF rolloffs. But to the best of my knowledge, this practice fell out of favor in later years.
4/16/1999 5:35 PM

I'm with Dave.. you can get yourself in a heap o'trouble.. and there are better ways to do this anyhow, to wit:  
If you want more bass, get it back from the pre-amp where it's all rolled off.. Look at the 1986 circuit and work towards that idea.
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