Peavey Rockmaster pre mods

[Enzo]

PV will send you any schematic you ask for.

[Guitarist]

Just wanted to say, do some mods but keep them to a minimum. I had a Rockmaster. It is one of the best lead tones I've heard from a preamp. (Hopefully you don't have the relay clicking issue).
I spent a lot of time tweaking it but it is GREAT stock. The clean channel was dull and lifeless which was a big negative. Maybe a 12ay7 tube would've helped.
Consider yourself lucky to have one.

[Amp Kat]

The Rockmaster preamp is the exact same circuit as the XXX and JSX preamp wise.

[tedmich]

The Rockmaster preamp is the exact same circuit as the XXX and JSX preamp wise.

not EXACTLY...the RM has no 2nF/2.2M/2nF fat switch, also a 220k/220k voltage divider into the 2nd gain stage (JSX has 220k/56k)

Clean tone stack differences:
RM->JSX
56k->100k........................resistor parallel to treble pot,
47nF->22nF........................cap to mid pot
50k->10k........................mid pot

2nd gain stage on differences:
RM-> JSX
270pF-> 470pF........................cap across crunch/ultra vol pots
yes->no........................presence after crunch/ultra vol pots
150k->100k........................second gain stage plate resistor
2.2k->1.5k........................V2b cathode bypass resistor
47k(sw)->none........................V2b cathode bypass cap series resistor

Also added to JSX: parallel 470k/470pF into V2a, 1M pot on anti-parallel diode noise gate, 22nF->47k coupling from V2b (58k->22nF in RM) FX loop, etc etc

it really is an evolution!

[rockon1]

Sounds like it might be a good idea for me to check out the JSX preamp schematics to see if I can learn something from there since it's supposed to be one of the smoother peaveys.

Guess that all depends on tastes. I owned the JSX and after trying to get it "right" with speaker swaps and tubes I gave up short of modding it. It always sounded weak/thin compared to my trusty Ultra or Rockmaster.

Ended up loading my Rockmaster with all GE 5751's and than threw away a bit of gain but smoothed it out nicely. Bob

[tedmich]

I would have put the JSX on a Portcity or Mesa Thiele, never thin then...

[rockon1]

I would have put the JSX on a Portcity or Mesa Thiele, never thin then...

Cant explain it. I tried it with various cabs and TONS of different speakers. I couldnt bond with it. It was actually annoying that my Ultra 112 always sounded fuller and warmer. I personally think they tweaked it(XXX) in the wrong direction. Bob

[tedmich]

Cant explain it. I tried it with various cabs and TONS of different speakers. I couldnt bond with it. It was actually annoying that my Ultra 112 always sounded fuller and warmer. I personally think they tweaked it(XXX) in the wrong direction. Bob

well having cheaper taste us its own reward....enjoy!

[rockon1]

well having cheaper taste us its own reward....enjoy!

Well I prefer "less expensive" tastes lol! The build quality between the Ultra's and the JSX is virtually identical. After ditching my JSX I picked up an Ultra Plus head for a rediculous 299$ IMO to compliment my Ultra 112. Bob

[silentrage]

Can anyone here help me out a bit with my rockmaster mod?
I tried a whole bunch of mods to see what differences they'd make, including:


C4 from .047uF to .47uF
C5 from 820pf to 2200pf
C17 from .47uF to 1uF
These are supposed to allow more lowend through.


R33 from 22kohm to 10kohm
R34 from 10kohm to 5kohm
These raised the preamp's voltage from 290v to 320v

R3, R13, R32 from 100k to 200k
These are supposed to cause harder clipping

After this the bass got too muddy so I restored the caps back to stock with the exception of C5.
Also played around with lowering R20 and R24, which seem to be V2's plate resistors, it made the distortion smoother but was too loose.

I did turn it on to hear how it sounds after each part swap, but somewhere in the process I noticed the bass was getting out of hand.
I remember before the mods with the boost pushed in the bass is kinda big and loose, and when pulled it's tight but a bit hollow. Now I seem to have made the pulled bass as big and loose as the "pushed-in" bass, and the current pushed-in bass is just overwhelming, flubby and not very playable.

Also there seems to be barely any distortion now withthe knob pushed in, and about 3-4 times the distortion with it pulled, I don't remember the difference being this big.

I've since restored each part to stock to see if it helps, but strangely enough I couldn't get the stock sound.

Right now I just have the R3,13,33,34 and C5 mods in place, the rest are all back to stock, and I've ran out of ideas.

Any help appreciated, the schematic is here.
http://www.schematicheaven.com/newam...ter_preamp.pdf

[tedmich]

Bold stuff!

Those are some fairly big changes; the B+ is 10% higher and the 100k->200k jump for the plate load resistors can give an increase in gain of about 30% per stage. The C5 upped 3 fold, C4 10 fold (!). Did you do just the C17->1uF and C14->0.0022uf mods that were suggested?

I find recording sound clips under semi controlled conditions (same guitar/cable/settings) can be very useful for "A->B" tests, especially when the mod takes a few days. Parallel resistors can go on the 200k resistors pretty easily to check if 150 (or 100k) sounds better (with the lower gain). The "zener trick" can drop the B+ incrementally (maybe back to stock).

I'd tweak coupling caps first with small changes, then cathode bypass caps, then (maybe!) plate load resistors. The PV Ultra series amps are extreme gain designs and the values are quite sensitive to small changes (on the edge-> over the edge). Check the JSX plate load resistors, cath bypass caps and coupling caps, incremental changes are safer and PV has found they work better (with much experimentation undoubtedly).

It can be definitely be fixed (by putting it back to stock) and MAY be slightly improved; with small changes, probably ones already tested by PV on later amps, and possibly by 3rd party moders.

I bet your getting better at soldering at the very least, congrats!

[silentrage]

Hey, thanks, I think "Small changes" is a good advice. I just followed what heath from harmony central told me at first without any idea of what to expect really.
I went with big changes because I thought they'd show me what each component did in very obvious ways for example the V1 plate resistor really affected how much gain and how hard the clipping was. I put 200k resistors on the plate load positions so I could solder in parallel resistors to lower the values without havin to remove things all the time.

I'll definitely experiment with small changes from now on, I guess I was just getting frustrated after not getting the results I wanted.

I'll also be getting some switches so I can flip parts in and out while playing to hear the difference better.

Which ones are the cathode bypass resistors and what changes can I expect by swapping different values here? Could you explain to me what exactly the coupling caps and cathode bypass resistors do? I used to know this from reading that valve wizard article but I'm on trip(took the preamp and parts with me, lol) and can't download it. Actually if you could send me a copy of it I'd greatly appreciate that too, hehe.

If I recall correctly, from the valve wizard guide, larger caps allow more low frequency through while smaller ones only let high end through, and resistors snub the high end, but the thing is I'm not sure WHERE in the preamp should I be changing these values.

Any idea where the extra bass might be coming from? I mean at one point I swapped everything back to stock except C5 and it was still too bassy, you think 820 > 2200pf in C5 could make this much difference? If so I'll really have to order a lot more parts and go in much smaller increments.

Edit: Oh yeah, shocked myself 3 times and grabbed the business end of a soldering iron during this fiasco too. Lesson 1: Don't touch the chassis while poking around inside an amp. Lesson 2: Don't grab the business end of a SOLDERING IRON!

[tedmich]

Edit: Oh yeah, shocked myself 3 times and grabbed the business end of a soldering iron during this fiasco too. Lesson 1: Don't touch the chassis while poking around inside an amp. Lesson 2: Don't grab the business end of a SOLDERING IRON!

read more, poke (and hurt) less!

Tube amp building and use safety

SafetyTips
(RA knows his, and other's, stuff)

www.geofex.com/tubeampfaq/tube_amp.htm
(RG's place!)
The Blue Guitar

AX84.com - The Cooperative Tube Guitar Amp Project

18 Watt Community Center

Sloclone Forums • Index page

[Joey Voltage]

Good Lawrd!,

I have done several mods to the Rockmaster/Ultra circuit. They are ugly as well, and weigh a TON!... mostly in the headshell. The two biggest objectives to modding in that circuit, as far as I'm concerned are these:

1) Get rid of the cocked Wah tone these things inherently have
2) make the mass amount of gain and distortion these things have more useable.

This circuit is inherently good sounding, it just needs a little love. If you find the culprits of both of these things you will be much happier with it I Feel.

[silentrage]

Good Lawrd!,

I have done several mods to the Rockmaster/Ultra circuit. They are ugly as well, and weigh a TON!... mostly in the headshell. The two biggest objectives to modding in that circuit, as far as I'm concerned are these:

1) Get rid of the cocked Wah tone these things inherently have
2) make the mass amount of gain and distortion these things have more useable.

This circuit is inherently good sounding, it just needs a little love. If you find the culprits of both of these things you will be much happier with it I Feel.

That's exactly what I'm tryin to do, but could you point me in the right direction? Where in the circuit should I tweak to get rid of that "wah"?