Trying to modify my VTM120 and well now it looks like no power is going to the amp

[Guiary]

Hello all,


I'm new here and I was wondering if I could score some guidance on my situation? I've replaced R1 and R2 on the preamp board to 220K/1M resistors and changed R27 to 82K.... Prior to that I swapped out the brite cap to a 56K resistor but it worked after that mod.

After the latest two I'm not getting any power to the amp I have yet to see if I blew a fuse or 4 but just from a quick check there are two on the board which are not blown and another two (including the one on the back of the amp) which covered in white so I can't see if they're blown. I may just replace those tubes first and then go through my soldering work and make sure it's all good still. I'm almost positive that the ribbons are the right direction from when I pulled them off....

Any suggestions?!?

Thanks,

Gary

PS when I just tried to discharge the capacitors there was no spark nothing... Am I safe to assume that it's a fuse that's blown?

Thanks again.

[Enzo]

Hi, welcome to the forum.

Let me get this straight. The amp appears dead, no power. SOme of the fuses are opaque so you can't see into them. SO you didn't test them?

Ceramic fuses work just like the glass ones, you can replace them with glass ones.

Pull the fuse out of its holder and measure it for continuity with an ohm meter.

Fuses usually blow for a reason, so if a new one blows, find out why. Power tubes are always suspect.

[Guiary]

Hahaha it was a screw up on my part! When I flipped over the amp the power (A/C) cable got caught under it, so with a little tug the cable came free without damage.... so I thought. Upon further inspection it looks like one of the wires in the power cable was cut. Little patchup and we're running with full power!

Thanks,

Gary

[Gtr_tech]

Sure is quite a flurry of ppl modding those VTMs. I don't know what they're trying to accomplish but they're great amps as they are. The only mod I'd suggest is adding some pwr supply decoupling or just more filtering in the front end. Lower freq's from the guitar can modulate the pwr supply a little. You can demonstrate this by hitting an open string and wiggling the low "E" up and down quite a bit (works best with a humbucker). The signal will exibit a tremolo effect from the subsonic freq introduced by the string wiggling.
Changing the coupling caps between gain stages to .01s is a good idea as well.

[Amp Kat]

It's just the Peavey sound I think that people are trying to mod and give them more gais as it seems in this case.They use so many frequency shaping networks that it's hard to get the Peavey out of it. The hardest thing to do is turning it into another type amp but it seems to be a common practise.

[Guiary]

Only thing I was really trying to do is tighten up the amp, give it more gain, give it a bit of low end umpf, and bring out the mid range a little by more clearing up the sound....

And so far I've achieved all of these except I want tighter! and I think that will come with a Mercury Magnetics Output Transformer and Choke, atleast it did wonders to my 5150 Also I really really want a tube swap!!! I'm using some JJ 6L6's and tungs sol in V1, JJ V2, unlabled who knows what V3, and a sovtek 12AX7WC in V4.... What I want is SED Winged C 6L6's Mullard RI V1, Pentas in the rest.

Oh and just ordered a new head cab for it too! It better fit

[Gtr_tech]

Only thing I was really trying to do is tighten up the amp, give it more gain, give it a bit of low end umpf, and bring out the mid range a little by more clearing up the sound....

And so far I've achieved all of these except I want tighter! and I think that will come with a Mercury Magnetics Output Transformer and Choke, atleast it did wonders to my 5150 Also I really really want a tube swap!!! I'm using some JJ 6L6's and tungs sol in V1, JJ V2, unlabled who knows what V3, and a sovtek 12AX7WC in V4.... What I want is SED Winged C 6L6's Mullard RI V1, Pentas in the rest.

Oh and just ordered a new head cab for it too! It better fit

For tightening up response, see my post above. No choke will get rid of that modulation. The preamp is pretty hot to begin with....its not a 5150 so you should not expect it to perform like one since there's only 3 stages to work with. Thats what I like about them....they retain clarity while gained out amps get mushy.

YMMV....do what you want with it.

[Guiary]

I'll post clips when it's done (minus the tranny and choke), which it pretty much is. It's at the exact tone I was wanting out of it but now I'm having problems with the high gain input, low gain is fine but when plugged into high gain it sounds like there is something loose.

Trial and error right?

[Gtr_tech]

Trial and error right?

Not so much. It sounds like you're driving a grid too hard and blasting a stage into cutoff. Make sure the low jack's contacts are clean and making good contact since signal goes through it on its way to stage 2.

[slacker190]

What do you recommend to add for power supply decoupling or increase the filtering? I dont know what exactly the coupling cap is, but i do know my way around electronics decently to preform any mods.

[Gtr_tech]

What do you recommend to add for power supply decoupling or increase the filtering? I dont know what exactly the coupling cap is, but i do know my way around electronics decently to preform any mods.

My favorite approach to this is to add a 10k (at least 1/2w, preferably 1w) in series with the plate load R on each gain stage, the 10k connects to the V rail. Where the 2 resistors meet, put in a 22u 350-450v cap to gnd. That gives the wiggle somewhere to go and isolates the pwr rail some from the AC plate signal.

Looking at the print for that chassis, there's a 22u at the first gain stage, and 2 (isolated via 10k R) 22u at the cathode follower. You could get away with decoupling stages 2 and 3 and that would more than likely cure the modulation problem.

Some amps don't need it...some do. Typically Mar$hall JCM800s won't need it. All those I've modded the front end in so far have been wiggle free as long as the pwr supply caps are in good health. CArvin X amps definitely need it. The Crate VC30 I just finished up mods on didn't need it. It just depends on the design....you can usually tell by looking at the schemo....too many gain stages connected to the same pwr supply node without isolation or stiff filtering *will* cause wiggle on the V rail.