72-74 Bassman 10

TragicTravisty · Post by **TragicTravisty** » 19 Aug 2008, 03:50

pawn shop find

300$. it sounds great. the 2nd is chimey, and the 1st channel is FAT. ive been using the 2nd channel for cleans, and the 1st channel for dirty sounds. since the 2nd channel can be set up to sound similar to the bass channel, im wondering if there are any simple mods i can do to it to make the bass channel higher gain? not too much, just a little extra kick. anybody got a schematic?

AL · Post by AL » 19 Aug 2008, 13:59

http://www.schematicheaven.com/fenderam ... _schem.pdf

NIce score. I'm not sure which channels you're talking about - the schem lists them as Normal and Studio.

Standard gain mod would be to raise the 100K plate load resistor - probably no higher than around 220K but you'll need to experiment. You could also make a variable NFB which would give the entire amp a little more gain. There are about a million more things you can do with that amp.

Here is a page of standard mods that may help. Be careful !!!

http://www.tone-lizard.com/Mods_and_Odds.htm

AL

TragicTravisty · Post by **TragicTravisty** » 19 Aug 2008, 16:38

thank you very much! i was thinking to start with adding two pushpulls, one to put the normal(1st) channel in series with the studio(2nd) channel, and another to internally connect the two while they are in parralel. this should give me allot more gain, as well as being able to blend the channels. anythoughts? what wattage push-pull should i use?

AL · Post by AL » 19 Aug 2008, 17:40

You can use both channels by using a short cable to connect channel 2 of the normal to channel 1 of the studio - or vice versa. I'd try that first before adding additional components.

I would think 1/2 watt pot should do the trick. I think Hoffman carries some push/pull.

http://www.hoffmanamps.com/

AL

TragicTravisty · Post by **TragicTravisty** » 20 Aug 2008, 00:39

yea, i tried using a jumper, but the channel not directly plugged into is quieter, and theres a loss of treble. by the way, my data was off. for some reason i confused no-tail logo with tail-logo while i was trying to date it. i emailed fender and they say its a late 75 or early 76. im gonna get 4 push-pulls. one for series-parralel, one for channel strapping, and the other two to bypass the series resistor on the input and raise the resistor from the power supply to 220k on each channel. from your suggested reading, this will really hit the preamp hard.

TragicTravisty · Post by **TragicTravisty** » 20 Aug 2008, 00:44

actually ill simplify things by having one of the inputs on each channel have no series resistor.

AL · Post by AL » 20 Aug 2008, 14:12

TragicTravisty wrote:... and raise the resistor from the power supply to 220k on each channel. from your suggested reading, this will really hit the preamp hard.

Yeah that is going to slam it really hard. If you like gain then that might do the trick for you, but you may wanna start a little lower than that just to be on the safe side.

AL

TragicTravisty · Post by **TragicTravisty** » 20 Aug 2008, 15:22

thats a good point. if i use something like 150k, and then put the two channels in series, id have all the gain i need anyway.

TragicTravisty · Post by **TragicTravisty** » 21 Aug 2008, 03:45

by the way the bass control on the 'normal channel' is either busted or tuned to a frequency so low that there is almost no audible difference as to what position it is... i guess id mod that to deluxe reverb tonestack specs or something....

soulsonic · Post by **soulsonic** » 21 Aug 2008, 05:41

Try removing the .001 cap that's in parallel with the 100K plate load resistor on the first stage of the Normal channel. That thing cuts the highs alot.

See point "A" in the schematic? That's where the cathode of the second stage of the Normal channel connects to the cathode of the second stage of the Studio channel where they share a common cathode resistor. Cut this connection, so that only the Studio channel's cathode is connected to that resistor. Then, give the Normal channel's cathode it's own resistor; you could stick with the same value of 820 with a cap bypassing it in parallel (though I'd use a larger cap, like 25uF). This bumps up the gain of both stages.

You could put a different style of Master volume on it that's after the phase inverter - these kinds usually use a dual pot. This would let you get more apparent gain and distortion by adding the phase inverter stage as part of what you're overdriving when you turn the channel volumes up. As it is now, you'd need to turn the Master up all the way to get that same amount of distortion.

There's alot of stuff that can be done to improve something like this. It would actually be much much much much easier to work on than that Peavey you were talking about.

TragicTravisty · Post by **TragicTravisty** » 21 Aug 2008, 15:57

i never would of thought of allot of those things. i thought this would be easier to improve since i assumed it was non-pcb, and not as unnecessarily complex as the peavey. its also closer to sounding great.

TragicTravisty · Post by **TragicTravisty** » 24 Aug 2008, 03:02

how do i wire that master volume?

soulsonic · Post by **soulsonic** » 28 Aug 2008, 07:50

TragicTravisty wrote:how do i wire that master volume?

It's basically done like this:

Definitely remove those nasty 2200pf caps! They wreck the sound. Adding the 47pf one as show accomplishes pretty much the same thing (making it more stable), but doesn't kill the highs as much.
Make sure all the caps you add are of appropriate voltage ratings. The 47p should be a 500v silver mica. The .1uF ones should be 400v or higher, though you can probably get away with a lower voltage because the main high DC voltage is already being blocked by the existing coupling caps. Any decent quality dual pot should work.

Nyquist5 · Post by **Nyquist5** » 02 Sep 2008, 07:01

I have tried all sorts of mods on mine. Putting the channels in series gives a usable sound but the amp becomes a bit mushy and definetly unstable due to all that gain. The PPI master volume is the best mod to get some crunch out of it. Then you can tweak the values of some of the coupling caps and cathode bypass caps to tailor the sound to your taste and avoid muddiness.

I have modded mine that way and I have been gigging it quite a lot with a TS in front of it (my band plays some kind of stoner rock). A few experienced players I know sold their 4x12 to buy 4x10 because of its tone.

Oh, and I also changed the speakers for Celestion G10L35s.

TragicTravisty · Post by **TragicTravisty** » 02 Sep 2008, 18:49

ive never really understood what "unstable" means when it comes to amps. does it mean a tube's gonna blow?

marshmellow · Post by **marshmellow** » 02 Sep 2008, 19:40

Unstable normally means an amplifier is oscillating in some way. Can be audible, really low frequency or in the HF region. Unfortunately those oscillations can burn out your power tubes because they're running on their limit all the time (the G1 resistors for example prevent this). You would not hear it, but you would see the plates glowing red. And of course you could check with your oscilloscope. Or osciallations can modulate your sound in a not so nice way.

But more likely are oscillations in the preamp, especially in high gain designs, due to layout problems, when two signals that are in phase (phase difference = 360°) are too close together.

Nyquist5 · Post by **Nyquist5** » 03 Sep 2008, 07:09

In practice and in this particular case, the amp started to whistle when the first preamp volume control (now a gain control) was high. As Marshmellow explained, this was due to the original layout not being adapted to the new high gain nature of the new preamp configuration. And changing the whole preamp layout for a not-so-great-sound wasn't an option

Once you have done the master volume mod, try replacing the 12AT7 phase inverter with a 12AX7. It doesn't give more crunch but the tone and response changes a lot.

TragicTravisty · Post by **TragicTravisty** » 22 Nov 2008, 18:06

in the back of the amp, theres an external speaker jack which doesnt disconnect the internal speakers. if i plug an amp into the back of it, to use the bassman as a cab, will i damage anything?

Fuzzer · Post by **Fuzzer** » 22 Nov 2008, 19:26

The external speaker jack is that, just for speakers. You never plug an amp speaker output to another amp's input.

TragicTravisty · Post by **TragicTravisty** » 23 Nov 2008, 00:29