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PostPosted: Mon Aug 22, 2016 6:49 am 
Bottle Rocket

Joined: Mon Aug 22, 2016 5:51 am
Posts: 2
Hey guys! Long time reader - first time posting!
I have a big problem with my new/old mesa!

Long story-short: 5-6 years ago I met a guy in the guitar shop that I worked in and he wanted to test some new cabs with his head - .50 Cal (EL84). Never heard of that model before at that time. As we started testing it trough various 4x12s I suddenly fell in love with it (I played - he changed cabs). The best clean tone I have played through! (mind you we had all sorts of Mesa, Fender, Marshall amps in stock and I played them all). And the drive - MY GOD - my favorite two sounds almost blended together - The Mark mid sweetness and the punch on some Brit amps. As we started discussing how sweet the amp is he admitted that he needs more gain. MORE GAIN?! And I play mostly metal - it has plenty of gain IMHO!

So he told me if he finds more suitable amp for him he can sell it to me for dirt cheap. At the time I didn't have the money. Later that year I contacted him and he said he is no longer selling it. Few months passed by - he calls me with an offer again - guess what - I couldn't afford it again... And this vicious circle continues for 4 more years.

Couple of weeks ago I called him asking what happened to the amp - he sold it to another guy. I contacted the new owner and asked him if he would sell it to me. Lo and behold - two days later I got the amp after all this time.

I plugged it and started playing - DAMN - that sweet, sweet clean sound again. I switch to the drive ... WTF - the drive sounds nothing like before! I quickly opened my tube drawer and put fresh JJs both front and back (12at7 on V4 for the Reverb as stated by Mesa). Clean sounded even better, but the drive had the same "weak, almost like mild fuzz tone". Basically - doesn't sound healthy at all. I put back the old tubes and suddenly there was weird noise and cracks on the clean channel and the weak tone on the drive became even weaker.Another problem - It was overheating and the PCB already had some damage from this heat problem, but nothing dangerous so far. So I fitted a fan to extract the heat, but thats a bandage on a 50 cal riffle wound (pun intended :P ).

I called the original owner and asked him for the full history of the amp. Guess what - just before he sold it to that other dude, he found some "tech" guy and modded it to have 'more gain' with optocoupler between the input and V1. Looking at the PCB there are some things that should't be there IMHO. I have a degree in computer electronics and some basic understanding. Correct me if I'm wrong, but you cannot add gain with just changing LDRs and possibly he ruined a perfectly good amp.

I forgot to mention - the amp is converted with a step-down transformer so it can be used in EU. So naturally I thought this was the problem. I brushed off the dust on my multimeter and started checking voltages - everything is fine. (Helping me with everything is the best amp guru/tech in my country - Garland Amps).

The next test was to put my rack preamp from Garland (based on Mesa) into the return of the Caliber - NO PROBLEMS. Pristine sound from the power section of the amp. Unfortunately I can't test the other way around at this moment (preamp of the Caliber into my BE100) but I think I found the source of the problem - the "MORE GAIN" mod he did on the preamp.

Thank you and take care!

Last edited by dann.valentino on Sun Feb 12, 2017 11:32 am, edited 1 time in total.

PostPosted: Tue Aug 23, 2016 5:06 pm 
Bottle Rocket

Joined: Tue Aug 23, 2016 4:47 pm
Posts: 2
Hello - just a thought: check mains transformer has all secondary connections made; sounds crazy but I worked on a studio 22 & all dc voltages across the board were low...!
Took me a while but traced problem back to mains tx which had one of the red, sec leads blanked off (on cct diagram it's R1 or R2 - both of the red leads feeding the rectifier diodes D2-D5; only one set of diodes was effectively connected...!!)....amp worked fine but was even better/louder obviously after connecting all sec tx leads - plus, all dc voltages now very close to schematic values.
May not be the fault that you're experiencing but worth a look eh...?!
Cheers, Paul
P.S.: the transformer for USA can be converted to UK voltage by connecting the primaries in series instead of parallel (connect mains neon across half primary to protect it from over-volts)... :-)

PostPosted: Sun Feb 12, 2017 12:38 pm 
Bottle Rocket

Joined: Mon Aug 22, 2016 5:51 am
Posts: 2
@Cordwangle - I looked around - everything you mentioned is fine.
Also I canceled the order for new PT just to investigate the problem further. As for connecting the primaries in series - not possible in my case.

As for the strange sound on the drive channel.. We have been modding in and out almost every aspect of the preamp concerning gain, character and feel, but with no avail. When the Volume knob (Gain) is below 8 Wah pedals and boosters in front of the amp sound weird, almost like overly clipping the signal. Harmonics and pinch harmonics sound weak, undefined and somewhat strange (like a fuzz pedal is used). The only way to make this thing disappear is bumping the gain on 10 and turning up the treble above 8 as well. Maxing the gain makes it fine for super saturated solo work, but its too gainy for rhythm and turning up the treble on max makes the gain super harsh and unpleasant.

Also leaves the clean channel unusable. With called for a mod that enables me to to have two separate gain knobs for the channels.
Also - the strange thing is that on the clean channel I am able to get perfect pinch harmonics and wahs and boosters sound totally fine on every possible setting. With makes me think there is something wrong when the drive channel kicks in (in particular something around V1)

So the mods we did so far:
-Added the two separate gain knobs for the two channels
-Added separate Reverb knobs for the two channels
-Added a foot switchable Solo Master (Recto style)
-A cooling fan - makes a big difference (using the 6.3VAC for the heaters)
-Shielded every possible signal cable to eliminate possible noise
-Raised the Preamp plate voltages from 240V to 300V for more headroom. Quite a nice thing to do actually - I love the turnout. Thinking of bumping it a bit more - something like 320/340V. (the hope was that the drive channel is hitting the next stages beyond the swing point of the tubes... Sure lowered the clipping and added more gain - as in strength of the signal, not distortion - but DID NOT fix the initial problem.)
-Changed some Cathode Bypass Caps to fatten up the sound a bit - still - not the reason for the problem.
-Made the FX Loop a little bit more FXs friendly (before it was clipping the non +4dB FXs)

In my first post I mentioned I tested my external Preamp from Garland Amps in the return of the 50 cal - Superb sound from the EL84s!!
I was able to test the opposite not long ago - the preamp of the Mesa .50 Cal into the power amp of my BE100 - That showed me the problem IS the preamp of the 50 Cal. (Checked this with various sets of tubes to be sure.)

After all this testing and swapping parts for few months and the problem still persisting, I'm thinking couple of things: (I ask for ideas before touching because the PCB is very old and the heat from my iron is basically ruining it!)
1. Something is leaking or a resistor/cap is bad.
2. Can it be that the amp just sounds that way? I Hope not!
3. As far as my understanding goes - the signal is degrading somewhere and possibly losing strength before V2A and the tone stack. That's why maxing out the gain knob on the drive channel is "fixing" that problem.
4. The tone stack is not working correctly?
5. Can it be the step-down transformer? If the Main PT is made to work with 60Hz and it sees 50Hz (Europe is 230V/50Hz mains) can this be the problem? Making the harmonics uneven? Probably not since everything is fine on the clean channel..All the people I talked to about this say is not the problem - 100%.

Any other ideas? What can make wah and drive pedals sound like a pig is dying? :D
I love that amp and Its the perfect bed for my rack-based rig I'm working on right now. So I wont get rid of it! :D

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