How much will a re-cap affect the tone/feel of an amp?

massless

My Rev G rectifier has a bit of a hum that I suspect is probably down to old power caps (tried a valve swap. But I absolutely do not want the amp to change in feel or tone as its absolutely killer.

After I recapped an 80's Carvin X100B it lost some of the feel I liked and became a bit harsh and stiff so ended up selling it.

Do different amps react differently to a recap?
Can a tech recap an amp to reduce hum but keep the measured values the same (as opposed to making it 'to spec' - or is this a contradiction)?

octatonic

I had an amazing sounding '62 Fender Bassman head back in the 90's- it had a lovely softness to the midrange that was awesome for funk and rock.
Me being young and inexperienced, I was talked into recapping which changed it into a hard and sharp sounding amp.
I was gutted and got rid of it.

I ended up buying another one in the mid 00's that had that lovely midrange of my old one.

massless

Sounds like my experience exactly. Hard, clinical, uninspiring.

Problem is I really would like to get rid of the hum.

ecc83

How exactly doe the hum show itself? If the amp hums with no input and all controls at minimum then yes, likely to be power supply hum, ripple. Could of course be a hummy PI valve? Could be way out of balance power valves (tho fixing that could change the tone as well! Guitar amps can be a comedy of errors. )

If the main HT cap IS borked then you HAVE to get it changed or one day it could spew conductive crap all over the chassis. If you get it done make sure the tech does not slam in a bigger one on the false premise that "more is always better"!

Tricky! You need The Man...IC! I say ICBM!

Dave.

massless

Amp hums with no input but requires master volume to be up. Hum level tracks the master volume.

i did another swap out of the preamp valves and managed to reduce it, V2 was noisy. It's not gone completely and is worse when the loop is active. Tried swapping v4 (loop valve) but no alternative valve reduces it.

it seems to be multiple things in the preamp section since when pulling the PI the amp is quiet (im assuming all you hear in this situation is the power valves since they have no input).

ecc83

That sounds like poor heater lead dressing or maybe the amp does not have a centre tapped heater supply? If it is centre tapped you could try the old dodge of jacking the tap up a few volts, maybe tens of volts positive. If not CT'ed fit a 100 Ohm humdinger pot, you can connect the wiper of that to +ve as well.

None of that is going to do Jack to the basic sound.

Dave.

ICBM

Even if it's a filter cap that's causing the hum it depends which one, you don't necessarily have to change them all - the feel and tone is more affected by the ones in the power stage, but hum which is controlled by the master volume must be coming from the preamp which will be from a cap further down the chain.

It's unlikely to be lead dress on a Mesa because the preamps are entirely PCB-built, and the rest of the wiring looms are tightly tied so they can't move - if it didn't hum when it was new it won't be the cause now .

Mesa Dual Rec…

ecc83

Cannot see any twisted heater wires in there IC. Is it perhaps DC heated?

Dave.

ICBM

ecc83 said:

Cannot see any twisted heater wires in there IC. Is it perhaps DC heated?

They're AC-heated via PCB traces. If you look closely you can see the line of preamp valve sockets - left to right parallel with the choke. The easiest to see is the second from the left, with a cap and two different-sized resistors in the middle of the circle of pins - you should be able to see pins 4 and 5 linked, and pin 9 in the middle of the shielding plane - its trace is on the other side.

Cirrus

octatonic said:

I had an amazing sounding '62 Fender Bassman head back in the 90's- it had a lovely softness to the midrange that was awesome for funk and rock.
Me being young and inexperienced, I was talked into recapping which changed it into a hard and sharp sounding amp.
I was gutted and got rid of it.

I ended up buying another one in the mid 00's that had that lovely midrange of my old one.

Were the replacement caps the same value?

ecc83

Cirrus said:

octatonic said:

I had an amazing sounding '62 Fender Bassman head back in the 90's- it had a lovely softness to the midrange that was awesome for funk and rock.
Me being young and inexperienced, I was talked into recapping which changed it into a hard and sharp sounding amp.
I was gutted and got rid of it.

I ended up buying another one in the mid 00's that had that lovely midrange of my old one.

Were the replacement caps the same value?

Electrolytic capacitors of yesteryear had a vast tolerance. +80 to -20%. Modern caps are better, +- 20% or so therefore you could have an old amp with caps on the lower tolerance and a tech replaces them with THE SAME NOMINNAL VALUE but they are top tolerance and so nearly half as big again!

Now I have NO idea how a 22mfd cap might change the sound/feel of an amp compared to 16mfds but it might! This is just one of a long list of things gitampwise that are waiting for some enterprising U grad to do as a thesis!

A thought occurs? You can fit a bigger, better res' cap but insert some resistance in the rectifier circuit. Best of many, you get lower ripple, less stress on a valve rect and the power traff plus you keep your sag!

Shoot! You could waste yer life messing with these things!

Dave.

massless

ICBM said:

Even if it's a filter cap that's causing the hum it depends which one, you don't necessarily have to change them all - the feel and tone is more affected by the ones in the power stage, but hum which is controlled by the master volume must be coming from the preamp which will be from a cap further down the chain.

It's unlikely to be lead dress on a Mesa because the preamps are entirely PCB-built, and the rest of the wiring looms are tightly tied so they can't move - if it didn't hum when it was new it won't be the cause now .

Mesa Dual Rec…

Thanks for the info ICBM. That confirms what I suspected. What are the four big 30uF caps, they're not in the PSU? AFAICT apart from the orange ones (I assume they're more to do with signal?) in your image there's the two 4700uF caps top right, the two 47uF caps bottom left and C31 just above them. If I started replacing them with the aim to replace the ones most likely to be the  cause of the hum first what order should I do it?

Here's mine:

massless

Also I read over at Grailtone that the Export models (which mine is) should have R332 added and R282 removed. I can see that R332 is empty, but don't know where R282 is). Is that info correct? It corrects for 50 vs 60hz apparently:

http://forum.grailtone.com/viewtopic.php?f=24&t=52723&sid=7dd58edf11f43ae383b09ff9fcdb5791#p377375

ICBM

The four big blue caps are the preamp filters - I would probably start by replacing them from right (furthest down the B+ chain) to left - although all of them could well need doing. The two main power stage filters are the large 220uF caps at the other end of the amp, don't replace those.

The two small blue caps are the bias filters - don't change those, the hum isn't coming from the power section. The medium-size purple one replacing the smaller original orange/black cap is for the low-voltage power supply for the LDR switching - also not going to be the problem.

All the other orange caps are signal caps and shouldn't be replaced unless the fail (extremely rare).

The R332/R282 change is the other way round according to the schematic - export 50Hz models should have R282 but not R332. I think R282 is the right-hand one of the group of three below and to the right of the R332 space, partly hidden.

Looking at the temperature rating on the preamp filter caps that are right next to two of the power valves is slightly worrying - 65ºC Max. I stuck a thermal probe into my Trem-o-verb once and measured 80º after it had been on for about two hours… that might explain why they're on the way out!

massless

Great diagnosis and explanation ICBM, thanks. I'll replace the 4 preamp filter caps, and with higher temp ratings.

Will this change the sound/feel at all?

ICBM

massless said:

Will this change the sound/feel at all?

I shouldn't think so - but if you have the time and want to be ultra-careful, I would replace each in turn and try the amp, listening to see if the hum has stopped or the sound has changed. Mark each cap so you know which is which if you want to put any of them back and you've identified any of them as being the cause - it may be only one, or it may be all of them.

You can safely discharge the caps each time by simply turning the power off with the standby still set to 'play' and waiting a minute or two - they will all discharge through the divider resistors on the main cap pair. Leave the standby in 'play' until you're ready to power up again.

Many techs will find this too fussy and would just replace them all and be done with it! But if you're doing it yourself rather than paying someone else and you are really concerned about changing the sound of the amp, it's probably worth doing it the long way. The worst case is that you end up replacing all four anyway.

massless

Thanks. I've replaced caps before so know the drill about discharging (usually leave it 10mins to be sure, then short them to be doubly so). Changing them one at a time sounds like a plan. thanks!

ecc83

Up to you of course but I would not bother to go for 105C caps. You might have a problem getting then in. Modern 85C caps are going to last 30 years at least, will we still have/can afford valves by then?

ICBM. You have oft remarked that Blackstar amps are "stiff". I had put this down to the generous filter caps used but then I learn that Mesa uses 220mfds!

Funny old things Amps...Innit!

Dave.

massless

I was thinking of getting 85C ones

ICBM

ecc83 said:

ICBM. You have oft remarked that Blackstar amps are "stiff". I had put this down to the generous filter caps used but then I learn that Mesa uses 220mfds!

They're stacked though, so actually only 110.

Mesas also seem to have very 'poor' transformer regulation, which I think has a lot to do with it. There's a significant change the the HT voltage when altering the power valve bias - which I know they don't make adjustable normally! But I did . I can't remember the exact figures but it was much more than I was expecting.

ecc83

ICBM said:

ecc83 said:

ICBM. You have oft remarked that Blackstar amps are "stiff". I had put this down to the generous filter caps used but then I learn that Mesa uses 220mfds!

They're stacked though, so actually only 110.

Mesas also seem to have very 'poor' transformer regulation, which I think has a lot to do with it. There's a significant change the the HT voltage when altering the power valve bias - which I know they don't make adjustable normally! But I did . I can't remember the exact figures but it was much more than I was expecting.

The B's use stacked 220s in the higher power amps. Don't have all the schems in head but for sure the A100 is 2 450V 220s in series (had one in the wrong way round just the once..Messy!) .

The saggy Mesa traff could be the result of running it on 50Hz? Does it get hotter than you would expect?

Dave.