How to make my new Plexi clone louder

[Gaz]

I finished a 50 watt Plexi type amp that sounds beautiful, but just can't keep up with a full band volume wise. I'm used to amps with cascaded gain stages (JCM 800) rather than the paralleled first two stages in the Plexi, and should've realized that this amp would have less volume overall. I would cascade the first two stages, but I really don't want to lose the bright/dark dual volume control so...

I've been brainstorming:

- Remove the cathode follower and FMV tonstack, replacing it with a normal gain stage and trying a low/no insertion loss tone control (i.e. Garnet, 18 watt, ????)

- Add an extra gain stage for 'gain recovery' after the tone stack.

- Cascade the first two stages and figure out another way to fake bright and dark volume controls.

Any other ideas or things to watch out for when trying these mods would be so greatly helpful. Even pointing me towards non Marshall/Fender schematics with unique gain staging would be helpful, although I've already looked at many.

Please, if you can help it, no 'Get a different drummer', or 'Maybe something's wrong? my Plexi is really loud!' comments.

Thanks,

Alex

[Chuck H]

I'm used to amps with cascaded gain stages (JCM 800) rather than the paralleled first two stages in the Plexi, and should've realized that this amp would have less volume overall.


...Please, if you can help it, no 'Get a different drummer', or 'Maybe something's wrong? my Plexi is really loud!' comments.

Cascaded preamp stages have nothing at all to do with an amps volume. If your amp is working correctly as it is now then cascading the preamp stages will not make it louder, only dirtier. Once the power tubes are clipping there is no more volume to be had. More preamp stages won't change this. If you typically have enough volume with a 50 watt amp, and this 50 watt amp isn't cutting it through the same speakers, then it's not putting out 50 watts. And if it's a 50 watt amp and it's not putting out 50 watts then, well, maybe something's... OOoops

Chuck

[bob p]

Please, if you can help it, no 'Get a different drummer', or 'Maybe something's wrong? my Plexi is really loud!' comments.

Did you try turning it up to 11?

Sorry Alex, I just couldn't resist.

[Gaz]

Thanks a lot for the response, Chuck. In that case, understanding of cascading gain stages is grossly misinformed.

To be more specific, I built an AX84 October. It really does sound 'right', but it is not comparable in volume to other 50 watt amps. Is there a way to test wattage without a scope? That would probably the next step, I think.

[Chuck H]

Measure the AC voltage at the speaker. I suppose a steady input signal would be ideal but I've done it with a guitar. Turn the amp up until it just starts clipping and is as loud as it's going to get. Square your measurement and divide that number by the speaker load impedance. Easy peazy.

Chuck

[Gaz]

Thanks again.

So riddle me this: Why would one 50 watt amp be louder than another? Incorrect power ratings? My 100 Marshall JCM 800 is definitely louder than My Sovtek Mig 100. The deal?

[Chuck H]

Mostly because the monikers "100" and "50" are generalizations. The world is full of 50 watt amps that are actually putting out 32 watts or some other amount than 50 watts. If the amp has two big bottles (EL34, 6L6, etc) then it's a "50".

The other issues are speaker efficiency and even configuration (1x12 combo will not be as loud as a 4x12 closed cab), OT efficiency and if the amp is being run into the correct impedance.

Chuck

[Gaz]

Okay, one last clarfication. Here's the quote from The Ultimate Tone that got me on this whole 'I can make it louder if...' trip.

From the "More Distortion via Gain Increase" section:

"There is still a wealth of untapped gain in the fourth stage, which is currently a buffer direct-coupled to the anode of the third stage. this circuit can easily be rewired as a common cathode amplifier, with another 40db of boost possible..."

40db is a measurement of volume, not distortion, obviously. This is seemingly a contradiction, which is now flummoxing me.

If it isn't obvious, I really want this amp to be louder.

[Gaz]

Ok, I just realized something obvious - that the quote is from the "More Distortion via gain increase" section.

Still, why the measurement in decibels? And why all this talk of 'low insertion loss tone controls' and 'gain recovery'? This is all about distortion? I thought it was a concern of volume.

I feel a paradigm shift a comin' my way...

Thanks for any responses.

[black_labb]

more db means there is more signal coming out the other end of the tube in question. if the circuit after the extra gain cant handle the stronger signal then its not more volume, but more distortion from parts of the circuit that are distorting.

if the amp is putting out the correct wattage, i would suspect the tone of the amp (something draining the frequencies that are important) or an inefficient speaker setup. could also be the way you equalise at band. what sounds good alone often doesnt cut through the mixat a band practice.
if its just not loud enough with 50w of tube power and relatively efficient speakers, turn the hell down!!!

[Chuck H]

That 40db of gain results in more over drive for the PI input and nothing more. A tube can only put out what it can put out regardless of how much EXTRA gain there is at it's input. Since your amp should already have enough drive to put the PI well into a square wave, and that is all the voltage it can put out, the extra db of gain generated prior to the PI by changing the cathode follower to a common gain stage will only drive the PI more square. But not higher in output. If "The Ultimate Tone" for you is more distortion then this is a good idea.

Gain is gain. But that doesn't always mean more output from the FOLLOWING amplification stage. If that stage is already putting out all it can the extra gain behind it only results in more overdrive.

Provided your amp is set up correctly, the circuit is capable of driving the PI and power tubes into a fully square wave. There is no more output to be had by cascading more stages prior to that event. Only more squareness to the waveform.

Chuck

[Gaz]

OK, I believe you all, and thanks for the clarification. This was a concept I originally understood but tricked myself into believing otherwise.

I will measure the wattage, but I'd love to think that I just miswired something. Would there be any situation that any of you can think where an amp in a new build would sound perfect besides being too quiet?

The amp is loud, but it gets lost with the drummer bashing away. Everything I've read about Plexi's makes it sound like you'd have to plug your ears before you can get it to distort, which is why I'm actually hoping something is ayre.

FYI, the bias is correct as well as plate voltages. The only thing fishy I have noticed is that the volume slightly drops when the treble is turned past noon.

And black labb, thanks for the hint, but we can't turn down the drums - you know that!

[Chuck H]

FYI, the bias is correct as well as plate voltages. The only thing fishy I have noticed is that the volume slightly drops when the treble is turned past noon.

Aha... You could have a parasitic oscillation. These usually present when treble frequencies are increased. The amp is working quite hard to create an oscillating frequency that is above the range of human hearing. this reduces the available watts that could be used to amplify frequencies in the audio spectrum.

Layout, lead dress and/or grounding are usually responsible for this condition. It can be a real bitch to track down unfortunately.

What does your presence control do?

Chuck

[black_labb]

good thought chuck
the fact that the treble controls it suggests that the feedback issue stems before the tone stack, so look there first.

if you have a scope it could help alot in tracking it down. look for parts from later in the circuit near components or wires from before the tone stack.

[Chuck H]

good thought chuck
the fact that the treble controls it suggests that the feedback issue stems before the tone stack, so look there first.

if you have a scope it could help alot in tracking it down. look for parts from later in the circuit near components or wires from before the tone stack.

But there's a rub. Whatever is CAUSING the feedback isn't necisarrily in that area. And using a scope will also only reveal the effected circuit, not the cause.

Long leads, unshielded input, OT leads inside the chassis that are too close to preamp leads and/or not twisted together, shared ground points that shouldn't be, layout fo paux like having the output jacks close to the preamp, NFB lead too long and/or too close to preamp leads, these are some of the things which cause parasitic oscillations in guitar amps.

But my first thought depends on what the presence control does.

Chuck