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I built the 18 watt plexi kit & installed the vrm
the voltages looks ok (I think?)
& It sounds ok but the bass is flubby so I have it turned down to 4 or less & the it sounds Mushy & looses treble when the VRM is turned down
It sounds best when the VRM is turned up & sounds great using it clean & with pedals but I want this amp to sound great with nothing & to get any great crunch the amp is way too loud to crank up
So I'm wondering if there is anybody else with this problem & what can I do to solve this ?
At about 5 on the VRM I get this noise that goes away at about 7 (strange ?)
I did some trouble shooting looking for some bad solder joints & reheated the joints
here's the voltages
B+371
V1
1 150
3 .98
6 152
8 .99
V2
1 176
3 1.65
6 271
8 180
V3
1 189
2 38.2
3 85.8
6 173
7 40
8 85
V4
3 10.4
7 351
9 347
V5
3 10.4
7 373
9 350
V6
1 310
3 374
7 307

The 2 things I did were 1, build an attenuator and 2, cascade the channels and install a PPI master volume. You do not need both however, you could just do one or the other.

The attenuator I built is very simple and cost less than $50 in parts. It is what you want if you want great cranked plexi tones out of the single TMB channel.

The cascade and PPIMV also works great but gives you a different sound. More like that of 80s high gain amps. It also has less clean headroom and is a little less dynamic. Check out "Plexi Brown" in the Resources section (viewtopic.php?f=12&t=3007) for how I set up the cascade.

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Les

http://www.soundclick.com/unimind

So the vrm that I installed don't solve my Issues ,I on the verge of buying or building one of those L Pad atteunator's
should I leave the vrm in or take it out ?

You could just leave it in for now. If nothing else you could use it to just knock the B+ down just a little. At 374 yours seems a tad on the high side. The higher the plate voltage, the cleaner the sound. Lower plate voltage equals more clipping and compression. That is also why they say you should reduce the master volume as you reduce the VRM.

I still don't fully understand the whole VRM thing. I would think that once you got down past a certain level it would sound horible as on the negative swing of the signal it would be so compressed it would be almost non-existant.

_________________
Les

http://www.soundclick.com/unimind

Forgot to mention this earlier. One of the biggest contributors to flubby bass at higher gains is the V1A coupling cap. Change it to .0022uf and it will really bring bass under control.

_________________
Les

http://www.soundclick.com/unimind

IMO that's a really extreme measure, and to me it kills a bit too much bass. Also consider that the original Marshall Plexi amps that this preamp comes from didn't use that low of a cap value, so why should we have to use this Vox value here? 0.01uF or 0.0047uF sounds a bit more natural to my ears and doesn't kill as much gain.

The other thing to look at is the tone stack, since the original Marshall tone stack came from the Fender Bassman and was intended to work best in the bass guitar's frequency range. If you reduce the Mid cap in the tone stack from 0.02uF to 0.01uF it pulls the bass control more into the guitar frequency range. Then changing the Bass pot down to a 500k or even 250k will give a much more usable sweep range on the bass control.

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Great tones come in small glass jars!

Yeah. I would agree that .0022 is the extreme low end value but still gives good tight bass (with bass at 10) when at full gain. At lower gain settings it can sound a little thin. I actually have .0047 on my normal channel. Gives about 25% more bass, just enough to thicken the sound more without getting whooly. Now the other thing about the .0022 is I think it goes better when cascading the channels. I now cascade the "bright" ch. V1A into the "normal" ch. V1B and so I have very little bass in the first 2 gain stages. Then with the .0047 on the normal ch brings the bass response back in. This really helps keep it tight and not flubby at high gain. I have also added a "fat" switch (330uf bypass on V2A) to bring a little bass back when not cascading. Again just enough to thicken it up a bit. +1 I was going to mention this as well, as you had helped me with that over a year ago, but I had to go and just posted what I had written at the time. I found that this tone stack mod was a little more subtle and even with that I still found that I would have my bass set around 4-5 to keep it from getting flubby at higher gain. It does however give a good all around bass response in the amp. Turn up the bass to around 8 for lower gain and turn it down to around 4 for higher gain.

So, there are a few tricks to "tuning" the bass response. For higher gain, what I have found is it is better to knock the bass down in the early gain stages and let it back in later stages. This will keep things nice and tight. It will really come down to how you use your amp and what sounds good to your ears.

_________________
Les

http://www.soundclick.com/unimind

Before you go making changes to the signal path I’d sort out your VRM issue first.

With a properly installed VRM it will be better than a PPIMV and attenuator hands down.

First I’d confirm all your wiring is correct, reflow your joints, verify component values. Next I would turn the amp on and measure the scaled B+ as your turn the pot from min to max. Make sure the control is working and voltage is actually increasing/decreasing.

Pics of your amp and the VRM specifically would help.

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http://pdfelectronics.com

Ok I have got an L-pad Attenuator & it really don't help the tone ,so I will take some pics & post soon

What I think I've read is the amp sounds great when loud and not so great (flubby) when turned down using VRM. And also, an L-Pad attenuator didnt help or make any difference to the flubby tone at low volumes.

Is that correct?

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

No I just think it sounds OK at best

I could be wrong

OK looks like the 220k and 100 k res are reversed on the vrm board.

One other thing to me (could be the picture) looks like the preamp ground wire has broken off the rail

Another thing is you swapped c8 .022uf and r31 8k2 on the layout. Looking at the schematic I do believe that might cause an issue going to pin 2 on v5 as it is the cap that's connected not the resistor. First cap and resistor after the 16uf cap from the left hand side of the board

+1

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http://pdfelectronics.com

Got to it before I added the other note

Now were talking

Just checked the vrm ,The purple wire goes middle prong on the pot (I know it looks deceiving in the pic but all 3 wires are connected same colors on both ends )

The c8 .0022uf cap & the r31 resistor was wrong .thanks so much it sounds pretty dam good now
the vrm sounds good ,even turned down a little ,Next is to work taming some of the noise ,the VRM creates a buzz/hum abound 7 till about 9

ok did u fix the resistors on the vrm board and check the preamp ground wire at the buss

Yes,
Swap those two resistors and fix the VRM pot wiring.

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http://pdfelectronics.com