Peavey VTM Mods Lounge

[aaronsg71]

Ty!

[aaronsg71]

Just cut it right out and that's it?

[aaronsg71]

I believe my VTM 60 is stock and I just ran a Danelectro EQ pedal in the fx loop and wow I found a great Marshall tone with it! I'm running it through my 4x12 8ohm cab loaded with Swamp Thangs and damn wow it kills!!!!

[the_shrews]

I recommend making it switchable, or at the very least, desoldering it vs. just cutting it out. But yes, you simply remove it.

 

These amps are great! I'm still waiting to hear about the bias resistors I'm having trouble with.

[aaronsg71]

Thx a ton man! Can't wait for any updates!

[3rdof3]

I think I just bought your amp!  The heater fuse was bad, so it wouldn't output any sound.  I replaced the fuse, gave it a test run, and I'm on to modding it!

 

It has the same caps that look added on the control board. So unless it was something they did at the factory, I think it used to be yours.

 

I'll be turning it into a clone of a Bogner modified JCM800.  Think Alice in Chains "Dirt" tones.

[3rdof3]

I just got a 120 with a bad heater fuse.  Replaced tested and it's good to go!

Now I'm performing the bias mod, adding a choke, and developing a new tone mod to emulate a Bogner modified JCM800 a'la Alice in Chains "Dirt" tones.

Since it is already pretty close to a JCM, it will be a snap.  PM me if you want the schematic when I'm done. Should be about a week.

[3rdof3]

Sorry for the double post ...  Thought I was replying to an OP.

[the_shrews]

With two of these heads, I finally got around to experimenting on one of them; here's what happened:

ATTENTION: I can solder.  That's my experience level; please do your own research!

Peavey VTM 120 Bright Cap mod - switchable

This mod made the most effect to my sound and was also the easiest.  In a pinch I would say cut the cap out and you'd be good to go.  I chose to make mine switchabloe so I could hear the differnece, but I like it too much without, so I'll never want it "on."

Basically it cuts the "harshness" way down and allows for more control with the high pot.

Here's what I did:

I initally removed what I thought was C2 on the control board, but in fact was not the right cap.  Thanks to other users in this and other threads, I found the ACTUAL C2 and made it switchable.  I chose to mount the switch inside the chasis, near the back.  Of course, I can't switch it without opening the back of the amp, but that's not an issue for me since I pretty-much "set it and forget it."

• One side of C2 removed with one lead soldered to the cap and the other end soldered where the cap was removed.

• The leads run to a switch.  When the switch is engaged, the cap is in the circuit, when switched the other way the cap is taken out of the circuit.

• And here's the switch mounted in the head.

 

Peavey VTM 120 Choke mod - switchable

DO NOT WORK ON POWER BOARD WITHOUT DISCHARGING CAPS AND WITHOUT FULL UNDERSTANDING OF THE WORK TO BE PERFORMED; THIS IS A PARTIAL EXPLANATION OF WORK I DID AT MY OWN RISK.

I used a Mercury Magnetics MAR-100C and made it switchable with R5 on the power supply board. Like the bright cap, I wanted it switchable (which is not necessary, but I wanted to be able to A/B them).  Also like the bright cap, I chose to mount the switch inside the head.  

Overall the choke made a little difference. The amp does have a minimal amout more sustain and low clarity.  That said, it doesn't necessarily "beef" it up.  It just basically tightens it up and cleans it up.  I'm not 100\% sold on this mod, though am glad I did it.  If cost is an issue, you can probably skip this and be fine in any live situation where you're turned up above 4.

• I mounted the choke next to the power transformer and let them share a hole for the wiring

• My rudimentary trace-cutting and soldering job (only the trace on the left of this photo was cut). Here you can somewhat see how the switch is wired.

• Switch mounted near the back of the head next to hole for the bias pot (up next)

 

Peavey VTM 120 Bias mod

DO NOT WORK ON POWER BOARD WITHOUT DISCHARGING CAPS AND WITHOUT FULL UNDERSTANDING OF THE WORK TO BE PERFORMED; THIS IS A PARTIAL EXPLANATION OF WORK I DID AT MY OWN RISK.

I found the bias mod to be the most challenging with little to no effect on the sound.  Of course, I did not swap tube types, which I believe is 90\% of why this mod would be necessary.  If you plan to use 6L6GCs, tube stores are able to sell you a set of tubes with a known bias point where you can simply order what rating you need and skip the hassle of this mod.  Since I'm going for a much more "mean" than vintage sound, I fall into that category and on my next VTM I doubt I'll do this mod. 

(for anyone wondering what the tube bais sould be on the VTM 120, I came up with just over 35mA @ 60\% dissipation. I figured this out using my plate voltage reading of 527v without tubes and cross referenced the chart for 6L6GCs near the bottom of the page here here.)

For this mod I followed 3rdof3's directions as indicated in their Flickr images here (note: I found the trace to CR1 also needed to be cut, which was not marked in the photo).  Also, I may be wrong, but the trace cutting and jumping is only necessary for EL34s, which I realized after I started.  Still, I am happy to have done it, should I want to try them in the future.

IF I did this again I would probably use a 25k pot and play around with different resistors to get a larger sweep and a lower starting point.  But again, I am using 6L6GCs and have not experimented with other tubes.

• Cut traces and added jumpers; I marked what I had cut with sharpie so as to keep up with. Also, note that I increased the screen grid resistors from 100ohms to 470ohms (at the reccomendation of an amp tech in reference to the way "today's 6L6GC tubes" are manufactured. I believe I'd have to go to 1000ohms if I want to try EL34s.)

• More trace cutting and jumping.  You can see where I mistakenly used an insulated wire on to jump 1 and 8 on V1.

• R8 and R9 are located under C6 and C7 on the power board, which are the two blue caps at the bottom of this photo.

• C6 removed showing R8 and R9

• One side of R8 removed and run to 47k bias pot; R9 replaced with 2k7

• Caps and resistors resoldered and hot glued in place

• 2nd lead from 47k pot run back to circuit

• back side of pot mounted in head

• bias pot mounted in head and marked for 35mA

Side note: I am fortuate enough to live in Memphis where Tube Depot is located.  As such, I visited them and was advised that for what I play, the 6L6GCs at 60\% would not make a huge differnece from brand to brand since they would be running cold enough to avoid much break-up.  All things considered, I settled on 4 Sovtek 6L6GC WXT+ for the power section.  And I bought two new Tung Sol 12AX7s for v1 and v2 in the pre. (for v3 I have something generic and v4 is a JJ). They sound great. 

 

Plugs for bias pot and choke switch.

In order to remove the power board after adding the choke and bias mods, I needed to install plugs for the repsective components.  

• You'll see the choke leads on the left and bias leads on the right.

 

• Bias plug with leads to pot on left, choke plug with leads to switch on right

 

Last but not least...

VTM 120 chasis mod

I did this for weight more than aestetics, otherwise I would have taken more steps to perfect the enclosure. It being spray-painted with knicks and dings is fine with me, and it probably about 10lbs lighter. I pulled some of the hardware off the old chasis and bought new corners from parts express. Some of the old hardware screws were too long, so I replaced those as needed.

 

Overall, I am VERY happy with all my mods and my new-ish amp.  Though the bias, choke, and chasis mods made a minimal difference, I am glad I did them as I had no other way to know what their effect would be.  If I could only do one, it'd be the bright cap.  Honestly, though, having traveresed the learning curve with these, it's likely I would do them again faster and with more confidence.

[3rdof3]

Here's the schematic for the Bogner JCM mod I just finished.  Clips to come soon!

 

In addition to this, I replaced almost every cap and resistor with higher quality devices, performed the bias mod, added a choke, brought the grid resistors down from 47k to 5k6, and the screen resistors up from 100 to 1k. 

 

http://www.flickr.com/photos/3rdof3/10229508883/in/set-72157626053220084

[hypnolobster]

Is it just me, or am I really confused?

Everybody talks about the bright cap mod, and it being the cap behind the pre knob. That looks an awful lot like a resistor to me, or did some evil jerk decide to make rounded axial caps?

What's the disc cap next to it, then?

 

Edit: I think I'm beginning to understand. The little blue axial cap is the real bright cap, and the brown disc cap is actually the treble peaking cap?Looks like I might as well make them both switchable.

Now I have to try and understand the ~40 different versions of the bias mod.

[Mr.scary96]

I have a stock one. Can't wait to read up on the mods for it (its a fine amp stock but I want to see if a mod will do it justice). Got it yesterday for $200 in working condition

[the_shrews]

@hypnolobster: I made the same mistake with the bright cap. The C1 does make a difference, but only slight. Making them both switchable is a great idea.

 

The bias mod is not a big deal IMHO. You can just get some hot tubes if you want it hotter. The plate voltage is about 520 so you can work with a tube supplier to get a set that's where you want them. I guess if you want to use something other than 6l6gc then you may want to do it. Personally, keeping them about 60% and letting the preamp tubes do the distortion worked for me. My understanding is that you don't need to cut the traces during the bias mod if you're gonna keep 6l6gc tubes in there.

[3rdof3]

Do any of you smart guys know why there is a 47R (R28 on the Preamp Board) ground reference resistor?  I've never seen one in another amp and am curious if it's necessary.

[3rdof3]

Here's a sample of the VTM I modified.  Adj bias, bright, gain 1, and gain 2 switches, and I put it in a new head shell as well.  I modified the circuitry to sound more like a Bogner.

 

 

[doomedfuzz]

Oh man, I have like a gazillion pages to read now! I have two 80s Butchers and maybe a VTM120 is hitting the stable tomorrow. Peace and thanks!