Mark IIB Loss of Gain after recap

[MK IIB]

Hi folks.
I had my Mark IIb Fully recapped,bias caps too and after getting the amp back there is a loss of gain.I have tried swapping tubes about.There is no loss of volume just gain.With the lead drive at 10 I get only slight singing leads but nothing like what it should be.With a strat(Single coils)it's just slightly overdriven.No singing sustain "Boogie" leads?
The tech did some work on the power supply board as there had been a bias mod done that he said wasn't too clever so he replaced the bias pot and fixed some wiring there too.I noticed that he's removed a couple of caps around the power supply/Ground switch.He's removed an orange drop and the round shaped red cap that sits right at the ground switch?
Any idea what his reason for removing these caps would be?He also changed out the 470uF 16v relay power supply cap and replaced with a larger blue one.Not sure why as there was no problem with the channel switching?
Here are some pics.
Power Supply BEFORE:

AFTER:

Amp BEFORE:

AFTER:

 

[McBarry]

Hi Paul, got yr email - will reply here...
The red and orange caps near ground lift switch are the ground lift cap and a MOV. Neither are a biggie, although the MOV can theoretically help protect amp in a sudden current inrush - eg a nearby lightning strike on the mains.

The PSU and preamp boards look very tidy work - congrats to yr tech..!!! Nice work on the bias area.

re lack of gain - if volume is OK but gain is lacking that says preamp to me..

We can frig around for days trying to email diagnose, but it needs to be on a bench with a signal chased through and seeing what is happening (or not happening...) at each stage.

Ascertain is it one or both channels lacking in gain.. Are the FX send/return jacks clean and functioning well.. are the 12ax7's all OK...

Some diagnostic detective work will help your tech. WRITE STUFF DOWN too..

Hope this helps a bit, but ultimately the gain will need bench diagnosing - AFTER U exclude basics like tubes, fx jacks, etc etc..

 

[MK IIB]

Thanks a bunch Dave.
The thing is the amp's back home.
All was tested during the service and amp was said to be functioning as it should but it had more gain before.
What would be the reasoning behind taking those caps out?
I'll be gigging over the weekend so I'll get a chance to open the amp up a bit more and see how it sounds but certainly at bedroom levels there seems to be a lack of gain(Only in the lead chanel)

 

[stokes]

I would guess,and its only a guess,as McBarry said the amp should be on the bench to know for sure,that the lack of gain could be weak preamp tubes.When you recap an older amp your power supply gets "stiffer" and voltages will likely come up a bit.Tubes that may have been on the edge before the cap job may have been pushed over the line with the improved power supply.You say it is most noticeable in the lead channel,so I would start with V1 and V3.The reason he changed the 470/16v cap is because its an electro,when doing a cap job all electro's should be changed.

 

[BlackBoxy]

your tech did a great job, at least electronics components need burn in time, so leave it burn a lil, and as said , improving PS will get stiffer voltages and curent!

your amp is like a brand new babe , so lets gets some gigs on it !

 

[MK IIB]

Thanks very much guys for your help.
It could well be the preamp tubes stokes as I didn't bother replacing all the preamp tubes as they weren't showing any signs of wear.No microphonics or any noise at all.They are old USA Fender badged "Fender special design" 7025 which I think are GE or Sylvania?
I only replaced V1.I did a little juggling about with the tubes and it improved the gain a bit.There's no loss of volume just actual gain/overdrive on the lead channel.
I'll get some new preamp tubes and see how it goes as the tech said everything was fine.He also said it could be the tubes.

 

[stokes]

Just a thought.Where do you have the Volume 1 knob set?

 

[MK IIB]

Just a thought.Where do you have the Volume 1 knob set?

Hi stokes.I've had it betwen 8-10 for as much drive on the lead channel as possible.
Things got better on the drive channel with some tube juggling and there is more gain available now!
Could you tell me if the following scenario is normal please?
When I have the gain boost engaged the tone controls on the clean channel are rendered ineffective?I can turn the bass and mid controlls all the way up and down and it makes no difference at all? The treble control makes a slight differnce only at about 8-10 but the rest nothing.
Turn off gain boost and the tone controls respond as they should.
So when the gain boost is on the tone controls are unresponsive.
Is this how the amp should function?
Thanks very much.

 

[stokes]

Yes,if I remember correctly the gain boost lifts the ground from the tone stack which gives it the increase in gain,it also puts a cap in the ground circuit that boosts the low frequency some.If changing V1 helped your gain issue,changing V3 and/or V4 will help further.Half of V3 and half of V4 are your lead drive tubes.

 

[MK IIB]

Well I've just gigged with the amp and I must say I'm somewhat dissapointed.
It just doesn't sound all that great.The lead channel is "thin" and a bit fizzy sounding.No fat Boogie leads.
I had volume 1 at 10 and lead drive at 9-10 also.Master was at about 3 and lead master about 5.
Even with volume 1 at 10 there was no break up at all on the clean channel!This was with a strat plus with Lace Sensor pick ups.
I suppose the only thing I can try now is getting a complete set of new preamp tubes and seeing if that helps although the current GE 7025/12AX7's even though they're old are not noisy at all.No microphonics or anything.
I've heard how this amp used to sound and was hoping/expecting it would be as good but it seems not.
Could it be the new STR 440's?
When I last heard it it still had the old Sylvania 6L6's in it.

 

[McBarry]

I get yr disappointment, esp after recent work..
And it doesn't help that the work in your pics actually looks nice quality and finess stuff - eg - coating the voltage lines on voltage selector..
That's fiddly stuff and seems to indicate someone taking pride in their work..!!

However, seems you've done most of what can be done and it def sounds like it's running on 3 cylinders, metaphorically..

Really is bench-time to chase a signal and see where it's disappearing or not getting amplified as expected.. Sounds MORE than tubes alone.

In a side email MkIIB mentioned gain boost reducing effectiveness of tonestack (yeah Stokes, ground lift off mid pot, and either a 0.005 cap +/- with a 330k resistor too). I'm wondering about huge signal loss here, but it's only heresay - bench, sig-gen, CRO will nail it double-quick..

Love to hear the outcome..
Even more love to hear your elation when wailing away into sonic bliss..
Dave

 

[stokes]

I doubt the gain boost is "sucking" the signal.A tone stack when fully grounded,or fully "in" circuit,will deplete the signal that is the purpose of the tube immediately following the tone stack,to bring the signal level back up to usable level.
I still think your problem is tube related,specifically V1,V3&4.I would also check the bias on those power tubes,assuming the power tubes are both good,and fairly closely matched.Pretty much every Boogie amp I did any work on,I would add the bias pot for free and every amp benefitted from a little current boost over the stock setting.New tubes rated by Mesa to be safe in a particular amp can range from 12ma to 25ma's or so.I find most tubes in a Boogie sound best at 30 to 40ma's or so.You already have the bias pot,if I did the work on that amp I would re-set the bias for you at no cost if you came back to me with the complaints you describe.Call your tech,tell him to try closer to 70% dissipation on those tubes.

 

[MK IIB]

Thanks very much for your input and the advice guys,I really appreciate it.
Yeah I'm gonna call my tech and tell him what's happening (or not)with the amp and point out the things that have been touched on here!
The 440's were new but I've yet to try all new preamp tubes because these ones are "working" ie no noise or anything but could it be they might just be weak?
Either way I'm gonna take it back and ask him to check the bias etc.
Thanks again folks.

 

[stokes]

Just FYI,the 30ma's to 40ma's and the 70% dissipation are only rough guides.I find with different tubes you have to tweak the bias while checking the clean and lead modes,sometimes you can set it on the high side and the clean is awesome,with just a bit of break up when pushed but then the lead gets a little "hairy" or "ratty" sounding on the distortion,so you have to back off some.A bit of a balancing act.
As for the preamp tubes,yes I am thinking the ones I pointed out are just weak,causing a lack of strong gain as you describe.

 

[MK IIB]

This is definitely more than tubes and it's driving me nuts.
I haven't been able to get this back to the tech who did the cap job yet(transport and things)but he thought that everything was working as it should and that new preamp tubes would help the lead channel(or lack of)
I'm hoping to get it over during the week.
The lead channel just sounds so thin it's unbelievable.I've tried a full set of new mesa preamp tubes and it just doesn't sound good at all.
There is no sustain or thick creamy lead sound just a thin fuzzy/buzzy lead sound and I'm having to have volume 1 up full and lead drive up full to get a half decent amount of drive but it's a weak sound.
With lead drive at 6 and voulume 1 at 10 it's just a slightly broken up sound.
The lead channel sounds like a cheap SS amp.Seriously!
MESA requested some close up pics of the amp to see if anything jumps out at them but said all looks stock?
It's just so frustrating the money I've spent on it and how bad it sounds.
I gigged with a Fender Blues Deluxe the othe day the clean sound blows the Boogie away?Crisp with a nice warm bottom end.
The tone controls don't seem all that responsive either?Not a lot of bass and the mid control doesn't affect or shape the tone that much at all?Treble has to be up near full.
Other than tubes and the bias being adjusted is there anything obvious that could cause such problems?
The thing is I've heard how this amp can and used to sound.
Thankyou folks.

 

[BlackBoxy]

hi, i know new PS caps can makes the amp sounds thin improving headroom especially drive lead chanel, it will need 100h to burn in to get back lovely fat sustain tones!

i would sugest you to let your amp on along the day for a week (you turn it off on the evening), as i did on my Mark IV to get back the magic boogie tone !

get in back the old tubes too , if it doesn't have any microphonics!

 

[MK IIB]

I've had some success with this
I had tried various new preamp tubes with no change and yesterday I put one of the old original GE 12ax7's in V3.I also had never used the effects loop before,only ever plugging right into the front of the amp.
I also set my pedal board up in the effects loop and instantly it had more gain,even on the clean channel but more importantly the lead channel is sounding like nice with more gain and sustain and harmonic overtones.
I had tried plugging and unplugging a cable into the effects send and return jacks with no change but for some reason yesterday it all came together?
I don't know if it's an intermittent thing,if it's a combination of the current preamp tubes or what but it's sounding a LOT better and that's just at bedroom levels.
I couldn't stop playing it yesterday.
Here's hoping it continues.
P.s I will keep playing it everday to burn in the caps to see if it improves it even more.Did you also loose gain after having the amp recapped?

 

[stokes]

hi, i know new PS caps can makes the amp sounds thin improving headroom especially drive lead chanel, it will need 100h to burn in to get back lovely fat sustain tones!

i would sugest you to let your amp on along the day for a week (you turn it off on the evening), as i did on my Mark IV to get back the magic boogie tone !

get in back the old tubes too , if it doesn't have any microphonics!

I have re-capped hundreds of amps and never found this to be the case,in a Boogie or any other brand amp.Any time I've re-capped an amp the improvement was instant,nothing to "burn in".Leaving an amp on all day everyday for a week is not good to do.

 

[dodger916]

hi, i know new PS caps can makes the amp sounds thin improving headroom especially drive lead chanel, it will need 100h to burn in to get back lovely fat sustain tones!

i would sugest you to let your amp on along the day for a week (you turn it off on the evening), as i did on my Mark IV to get back the magic boogie tone !

get in back the old tubes too , if it doesn't have any microphonics!

I have re-capped hundreds of amps and never found this to be the case,in a Boogie or any other brand amp.Any time I've re-capped an amp the improvement was instant,nothing to "burn in".Leaving an amp on all day everyday for a week is not good to do.

+1. In my experience, new power supply/filter caps increased bottom end. Some mojo may be lost as new caps can make an amp more "hifi" sounding, but I'm not sure I'd call it thin.

 

[Don]

I also set my pedal board up in the effects loop and instantly it had more gain,even on the clean channel but more importantly the lead channel is sounding like nice with more gain and sustain and harmonic overtones.
I had tried plugging and unplugging a cable into the effects send and return jacks with no change but for some reason yesterday it all came together?

Clean the loop jacks with Deoxit. This is a very common problem that can show up unexpectedly and intermittently and can cause the issues that you're having. Cleaning the jacks usually takes care of it.