Help with Mesa mark V CH2 mod

[Tahoebrian5]

I've never really been happy with channel 2 and deciding to see what can be done. I'd like to get a bogner ecstacy blue type lead tone.. I know the mesa circuit is a long way from a bogner but thought I'd throw that out to give a idea. Currently it sounds a bit low on overdrive / gain. I have an RFT in V2 and that did help a bit. V2 is the only tube that is specific to CH2 only. I was considering lowering the cathode resistor value to bump up the gain a bit as a starting point. Please let me know if any one has any ideas to get closer to the sweetness of the bogner blue compressed liquid lead sound. I've read that the bogner preamps run a lower B+... Not sure if this would help without the rest of the bogner circuit. Thanks in advance.
Ohh, there is a Mesa mark V schematic posted in this forum... Thought I would skip reposting it but if anyone thinks I should let me know and ill get it in this thread


Edit: Link to thread with schematic.. http://music-electronics-forum.com/t15431/

[Tahoebrian5]

Also I thought I would mention that half of V2 is not being used. This might open up some possibilities.

[Tahoebrian5]

Studying the Mesa schematic some more it appears to me that V2a is pretty close to center biased with 334v, 100k anode R, and 1.5k grid R. Then the signal goes into the tone stack and on to another center biased stage. The sound is close to a light crunch which makes sense. I'm thinking of adding the unused portion of V2 as another gain stage. I could either cold or hot bias the stage to get more distortion or I could use it as a cathode follower to drive the tone stack harder. As I'm still learning here I'm not sure how option 2 would affect the tone. I could change the bias on V1a to cold and then hot bias v2a to get the alternating affect. Anyone care to comment?

[Chuck H]

IMHestO you're trying to make toast from tortillas! If I interpret the schematic correctly there is a channel3 on that amp that cascades seven gain stages! Why not use that channel for "compressed liquid lead sound"?

That amp isn't a good mod platform. Well, other than the simple stuff like clipping bleeders and changing cap and/or resistor values. If you try altering the number of stages on one of that amps channels you're going to run into switching problems, possibly stability problems and you could damage the tender board with too much invasive surgery. I really think you should reconsider your position on this amp.

[Tahoebrian5]

I agree that it's pretty tight in there and I am going to try just tweaking values for now instead of adding V2b. As far as it not being a good platform.. Maybe your right but I got it for dirt cheap, and I am guessing most channel switchers out there aren't much better but maybe I'm wrong. For now I think I'm planning on increasing R19 to cold bias the stage.. Maybe to 4k or so, and decrease or remove R8 to hit the stage a bit harder and make up for lost gain. If I understand correctly this will decrease the input impedance of this stage though. Would it be better to increase the value of R10 to increase signal strength, maybe both?

[Chuck H]

Since you got the amp "dirt cheap" you know what I would turn it into?!? Money! Then go to your nearest Guitar Center and make a fat down payment on a Bogner Ecstacy

It's true enough that none of the modern channel switching PCB amps are good mod platforms. But that doesn't mean they're all equally good to mod. It means they're all equally bad to mod!!! As far as what's possible you'd be looking at a lot of work and possible snarky problems for very little headway or gratification in the end.

[Tahoebrian5]

I'm very tempted to do that, in fact I happen to know where I can get a 101b for about 2k. I'm really curious though if I can make this thing sound better. Ahhh, what to do!

[screamersusa]

Bieng a modder at heart I must confess there are times its not worth it. This is probably one of those times.
Go play the bogner and make sure its your baby. if not then try the bogner blue pedal with the mesa.
Trying to find one amp that does almost everything you want is always a challenge

[Tahoebrian5]

You're both probably right but I'm still curious what experienced amp builders would suggest for V2a other than ditch it and get a new amp

[Chuck H]

You're both probably right but I'm still curious what experienced amp builders would suggest for V2a other than ditch it and get a new amp

I would suggest that V2A already has two switching circuits in place for the cathode and operates appropriately for the stage!!! Raising the cathode resistor value here will decrease gain but probably NOT alter much else because there simply isn't enough drive this early in the amp to take advantage of any asymmetrical clipping. Not to mention the hassle of working around the switching circuits to idealize frequency response. On a Mesa board!?! Not my amp, but I would let sleeping DOGS lie.

[Tahoebrian5]

Thank you for responding Chuck, In the interest of helping me understand can you tell me what is wrong with my analysis here... I was led to believe that after the input gain stage a 200m volt signal from a guitar will roughly speaking be amplified by a factor of 30 so will be around 6 volts p to p. I was suggesting removing or lowering the 220k resistor in series with the signal chain prior to V2a. With also running the gain pot near max it seems like there would be plenty of signal to clip. Now the rest of the gain stages in the CH2 signal path appear to be make up gain stages for the tone stack, reverb, and eq. So the way I'm seeing it, V2a is the only true overdrive stage in CH2. Im sure you are right but it would be helpfull to know where have I gone wrong in my circuit analysis.

[Chuck H]

I'm following you. And yes, you could increase gain by reducing or lowering that resistor. There is still gain following that stage. The tone stack doesn't use it all up down to unity. And it appears there is one more stage after that, though as you say, it is also padded by features. Please forgive that no one else is grilling over the schematic with great intensity to discover the nuances of this complicated circuit. You are, after all, the only one who thinks modding it is worth a try.

Something to consider is that it usually doesn't sound as good overdriving one stage hard as it does to overdrive two stages less. There is a point of diminishing returns when it comes to how much clipping sounds good for a given circuit. But if you don't mind the work I don't suppose there's much harm in trying it.

[Tahoebrian5]

Thanks again Chuck, more than anything the learning process here is very valuable to me. I'm really looking forward to receiving Merlin's book and maybe my questions won't be so dumb after reading it.. Well probably not.

I think at this point I will just change out that single 220k resistor and hope the additional gain will help the channel become more usable to me. One resistor on the Mesa board shouldn't be too bad. Hmmm, maybe ill just parallel another 220k.

In retrospect, I should not have put "Mesa" in the thread title and maybe would have more interest! "Help with mystery amp needed"

[Chuck H]

Parallel a pot across it and dial in the amount of gain that sounds best. Then remove the pot and measure. Parallel a resistor of the nearest value.

I've modded Mesa's with the tacked in, parallel resistors before. It's not ideal but it should be durable enough and you won't need to lift the board.

[screamersusa]

From out of left field here........
Well If your gonna do it anyway.... perhaps changing the 220k to a 330k, and the other cap (2.2k?) to a 3.3k will
give you the Engl gain trick. I've done it it PV valvekings and TGraxx and just did it in my GXT100. You'll probably need
to lower any decoupling cap before the tube to cut some lows. The tube will saturate faster. I like it because I don't set my
gain/distortion too high and can let my fingers dynamics dictate the response while getting an articulate aggressive tone.
It'll sing too (if enough gain is getting there first) but you have to get used to it, not just dime it.
Hopefully it wont affect anything after that if it's cascaded with other tubes for other channels.