Lowering voltage on preamp plates for fatter sound

[balokker]

This is the power section of a Mesa Nomad 100 head. Ideas for lower the voltage on C to something around 250v? Looking for a fatter distorted tone. Compared to some other amps, the distortion tone of this head has a hard bright clean edge to it that I'm trying to fix.

[Chuck H]

Without looking at the whole schem I'll speculate that the node marked "C" is powering the PI. If you target the resistors circled you'll be in the preamp (not sure why Mesa didn't designate that last node). Mesa uses the series resistor arrangement to allow the use of resistors with a lower voltage and power rating. You can use single resistors here (and most amp companies do) but be sure that whether you go with the series arrangement or single replacement units that you observe safe margins for voltage and current. Metal oxide resistors typically have a high enough voltage rating for replacing the series string with a single component.

You can download Duncan PSUD2 (available on the Duncan amps site for free). It would allow you to mock up a similar enough power supply to allow for predictable value adjustments. If this is beyond your skill level at this time I'll mock it up. I'll need the full schematic for that.

[balokker]

Thanks Chuck!
So the resistors you circled are part of a voltage divider that's dropping the voltage on B, C and D?
If so then lowering those R values would get me there.

Channel 2 looks to have 3 stages run off C, one stage D.

Looks like B is supplying the PI.

here is the full schem:
http://www.thetubestore.com/lib/thet...-Schematic.pdf

[mozz]

Not really a voltage divider, just voltage dropping resistors. You would increase their value to lower the voltages.

[Chuck H]

That's a strange power supply in that it has two "nodes" unused. No amp company squanders parts expense and board complexity for an unused circuit. The PI and the screen share a node with no filtering in between (never seen that). Another node is feeding three preamp stages in series (a less than ideal practice) on channel 1 and on channel 2/3 that same node is feeding the first and third stages but a later node is used on the triode in between (not unheard of, but a little odd). It's all a little suspect because Mesa is known for putting errors in schematics intentionally. I can mock something up from what it shows, but it doesn't matter if it's a lie. I'll TRY to get an idea of some values to change and report back.

[balokker]

So the two 1.5k resistors and the three 5.6ks would influence the output at node C? Raising their value would drop C?
Would the ten 365 ohm resistors combined play a role in affecting C?

[Chuck H]

It's the other way around. Sorta:



The power rail goes the opposite direction of the signal chain. I added a numerical order to wrap your head around. Notice that changes to earlier nodes supply resistance also affects all those downstream.

I don't believe the schem. I've never seen a PI powered from a screen node with no filtering in between. I've never seen a superfluous cap in the middle of a string of resistors (constitutes a node). I've never seen an unused node at the end of an HV rail in a production amp. And I've never seen three series gain stages powered from the same node in a distortion preamp. All of this constitutes poor design and a pro outfit like Mesa wouldn't do it. They DO, however, have falsehoods in some of their schematics. I can't know this is one of them without having the amp on the bench, but I think it is.

EDIT: Oh yeah! You would increase the value of the resistors to decrease the voltages. ie: increase the two 1.5k resistors following C (4) to decrease the voltage to D and the last (unused?) node (5 and 6). Try a pair of 5k at 2W or 3W. Metal oxide preferred, wire wound ok.

[g1]

I don't believe the schem. I've never seen a PI powered from a screen node with no filtering in between. I've never seen a superfluous cap in the middle of a string of resistors (constitutes a node). I've never seen an unused node at the end of an HV rail in a production amp. And I've never seen three series gain stages powered from the same node in a distortion preamp. All of this constitutes poor design and a pro outfit like Mesa wouldn't do it. They DO, however, have falsehoods in some of their schematics. I can't know this is one of them without having the amp on the bench, but I think it is.

This should be considered with any mesa product for repair or modification. Unless you are familiar with a particular model, you need to verify that the unit matches the schematic.
In my experience, no other company has as much variance between published schematics and actual production units. Whether it is deliberate or not is beside the point, verification of schematics is critical.

[balokker]

It's the other way around. Sorta:

[ATTACH=CONFIG]41039[/ATTACH]

The power rail goes the opposite direction of the signal chain. I added a numerical order to wrap your head around. Notice that changes to earlier nodes supply resistance also affects all those downstream.

I don't believe the schem. I've never seen a PI powered from a screen node with no filtering in between. I've never seen a superfluous cap in the middle of a string of resistors (constitutes a node). I've never seen an unused node at the end of an HV rail in a production amp. And I've never seen three series gain stages powered from the same node in a distortion preamp. All of this constitutes poor design and a pro outfit like Mesa wouldn't do it. They DO, however, have falsehoods in some of their schematics. I can't know this is one of them without having the amp on the bench, but I think it is.

EDIT: Oh yeah! You would increase the value of the resistors to decrease the voltages. ie: increase the two 1.5k resistors following C (4) to decrease the voltage to D and the last (unused?) node (5 and 6). Try a pair of 5k at 2W or 3W. Metal oxide preferred, wire wound ok.

Thanks Chuck
There are five 365 ohm 1/2 watt resistors (I believe those common mesa 5 band resistors are 1/2 watt?) totaling 1.8k leading to node 4 (C) where we have 412 volts. I'll swap in a couple of 4k 2 watters in there for starters to what values I get for C and what sound changes I get.

[Pedro Vecino]

And I've never seen three series gain stages powered from the same node in a distortion preamp.

F and Express series. They use a single node for the entire preamp including PI.
It´s a different concept associated with other circuit details. Its purpose is to get a different reaction in overdrive. A soft sound, with compression and a special character.
Nomad in this regard are similar. Sound soft, with low definition at high levels of gain (handle higher overdrive levels). Clean channel has a lot of capacity in lows, and channel 3 with a high gain setting too. Definition and good transitions in gain controls (good balanced) is what need that amp because the latter are baffling.

[Chuck H]

I think one of the Fender HR series designs also has like phase circuits sharing a filter. One hitch with the concept is that filters chosen for production amps can often be shorter lived than the ones in the good old days. With like phase circuits on the same filter, a filter showing even a little increase in ESR can cause a positive feedback loop. I know some of the HR amps started having stability problems in just a few years. Replacing the filters is the cursory solution. I guess it's alright if it gives something back in tone. I don't think I'll be designing an amp like that anytime.

[pdf64]

The Fender 75 has a whole 4 cascading CC stage pre-amp fed from one 20uF HT node; plus the reverb recovery and a CF!
http://bmamps.com/Schematics/fender/75_Schematics.pdf
There's a fair amount of bass cut and other attenuation between stages.

[Chuck H]

Ok!!! Ok... Take it out, it hurts!

I'll concede the like phase cascade stages. But what about the PI being fed unfiltered from the screen node or that unallocated node, complete with series dropping resistors and a cap at the end of the rail? I'd have to see it to believe it.

[balokker]

What do those 10uf caps do exactly to create a node?

[The Dude]

I'm not sure if I completely understand your question, but it's the resistors dropping the voltage in combination with the additional filtering that creates another "node" or power tap.