Tips for increasing bass crunch

[islandamp]

I have a homemade Plexi that has flabby, loose, bass and I,d like to make it more of a crunchy style bass. What types of mods affect the bass area only and add that type sound?

[jimipage]

Is this an exact clone or a one-off? Post a schematic and we can figure this out.

[John_P_WI]

This is a loaded question. Almost everything effects the bass from the type of pickups (and their distance from the strings) to the wiring of the speakers and everything between.

A few obvious that I'll throw out:

What is the value of the first stage grid stop R?

Does the first pre amp tube share a chathode resistor / cap? What is the value of the cap? If they are split, change the cathode cap to something like .67u to 2.2u. If they are not split, you can split them and change / add Rk accordingly.

Is there a 500 pf bypass cap on the mixing resistor after the volume?

What is the slope r value / treble cap value?

What is the PI input cap value?

What are the values of the PI coupling caps to the output tubes.

What are the output tubes? What are their grid block R's?

What is the feedback Resitor and ratio at the PI. What tap is the feedback taken from (4, 8 ohm etc).

Are you running a PPIMV type master?

Is the presence control wired correctly?

What is your first filtering stage after the diodes? 32u? 50u? You can increase this to 100 or more if you don't have a tube rectifier.

Are the transformers sized accordingly?

What type and size of speakers? Open or closed back. Are they wired parallel or series?

Just a few ideas to get the discussion started...

Take care,

John

[islandamp]

Its pretty much an exact Plexi clone. I,ve got 415v B+ and pins2 and 7 are internally jumped. Other than that its basically a stock board. Just doesn't have that nice Blackfoot type crunch on the bass. It does have a monsterous hum at volumes over 3 that I can't find that could be affecting the bass, but its sounds flabby and loose on the E string.

[briane]

I agree you need to post a schemtic, theres a lot of plexis out there from the huss 6v6, to the 1987, 1986, etc.....

And john's right on, its not usually just 1 singular component, rather the combination of the whole.

Ive built the huss 6v6v plexi (basically a 1987 with 6v6's), and while I have not powered it up in a bit, I recall no issues with weak bass. Its an awesome marshall clone.

You should compare your build with marks schematic, as see where the differences are. Sounds like you have a few more hours of voicing to do, so dont get discouraged, and keep with it. Sometimes I find it really helps if I put a build on the shelf for a few days and then come back to it with new eyes.

I always keep in mind that every build needs a tremendous amount of voicing work, and even when I believe I am working off a tried and tested schematic this is always the case. These things are always one-offs, and yes it would be easier to just make a visit to guitar center. But then you dont learn anything, do you?

[islandamp]

Its basically a stock 1987 plexi circuit with an Avatar 4x12 celestion cab hooked to it. Stock Ceriatone board except a switchable shared/serarate v1 cathode and a Larmar master volume. v1 pins 2 and 7 are jumped and a standard 68k input resitor. It hums loudly after 3 o clock and has a high pitched squeal after 3.5. I will find and fix the problem, but what I,m after is a little tighter bass crunch. Could be the problem causing the squeal and hum is zapping the bass, but right now its plenty strong at light distortion, but turns to mush with the gains up. Frustrating because the mids and upper frequencies sound great.

[John_P_WI]

Check the phase of your output transformer if you are getting squeal. You might have to swap the transformer wires on the power tubes. This certainly will affect the bass. Try disconnecting the neg feedback first to see if the squeal goes away.

John

[tjauernig]

make sure your grounding scheme is good. all preamp related grounds tied to the input jack, and all power section grounds, PT centertap, fils, etc tied to the same point. This usually works well to get rid of hum issues.

[keithrick]

+1 on the above suggestions.

Regarding grounds, don't forget the to ground the speaker jack! I used the isolated jacks and chased a hum for a few days myself before I figured it out.

If you can post some pics.

[Bob-I]

I'll bet the oscillation is robbing power supply energy causing the bass to be starved. Fix that before you move on.

Start with a chopsitck and jiggle the wires until you hear a change in the oscillation. There's you're most likely problem spot. Re-dress the wires.

If you can post some pictures it might help.

[ScottW]

A combination of loose bass plus hum points to bad filter caps...worth a look.

[Phil_S]

Is there a bottom plate?

[islandamp]

I re routed the o.t. wires and made some changes last night and its still the same. I notice that I have an unused turret that touches absolutely nothing but if I tap it with a meter lead, it pops and crackles. Strange, huh. I can also ground the signal at the treble cap and the amps dead silent, however, grounding pin 2 or 7 at v1 I still have the problem, so its between v1 and v2 ????

[Bob-I]

I re routed the o.t. wires and made some changes last night and its still the same. I notice that I have an unused turret that touches absolutely nothing but if I tap it with a meter lead, it pops and crackles.

Very strange. Sounds like it has more to do with capacitance of the meter lead than the turret.

I can also ground the signal at the treble cap and the amps dead silent, however, grounding pin 2 or 7 at v1 I still have the problem, so its between v1 and v2 ????

If this is the schematic I'm thinking of, V1 is strictly a gain stage where V2 is a gain stage into a cathode follower. The tone stack and the treble control follow this so I'd say your problem is at the input, not V2. It's possible that the problem is with the volume controls between V1 and V2, but I'd say you all but eleminated that with the grounding of the grids test.