Anybody ever attempt to recreate the sound of a well aged amp by replacing caps and resistors with others that have differing values?
A stupid question really, as I'm sure any true amp building fanatic has experimented endlessly on such things.
How do you go about deciding which values to alternate in order to acquire certain tones? (I feel like this is a question that can't be answered quickly)
It's what fender customshop does for signature guitars
The problem with using off spec parts is electrolytic caps this usually means a dead cap or leaking... Resistors not as important but sometimes noise is a factor as the carbon drifts.
I'm assuming you are referring to exceeding the 20% drift.
joel_ostrom wrote:Anybody ever attempt to recreate the sound of a well aged amp by replacing caps and resistors with others that have differing values?
A stupid question really, as I'm sure any true amp building fanatic has experimented endlessly on such things.
How do you go about deciding which values to alternate in order to acquire certain tones? (I feel like this is a question that can't be answered quickly)
Two options:
Go to school for electrical engineering or put in countless hours and dollars building/repairing/modifying amps, gaining the experience for yourself.
I do believe many of the old classic amps have each aged individually and you can't really fabricate that with newer, lesser quality components... That's not to say we can't build some great stuff with what's out there now.
Best bet is to source some NOS product and go from there.
To me a primary part of an aged amp's tone is an old used OT. Unfortunately also a more costly item than caps or resistors.
I try to use old OTs, old speakers with original cones, old tubes, old signal caps and resistors, and old pots in my builds, which are mostly Dumble circuits these days. Since original Dumbles have drift free 1% metal film resistors in the key places I haven't had to guess at resistor drift.
However in my reading it seems that carbon comp resistors usually drift upwards 15-20% of nominal after 30 to 40 years or so. Not always, though, which has to do with improving or not improving the amp's tone. For example, I had occasion to change a 100K plate resistor in the PI of a '77 Marshall 2204 head as it had drifted up to 118K. The 82K PI plate resistor had not drifted at all from nominal and this created a serious imbalance in the PI that was noticeably improved after the change.
Signal caps in my experience can absorb water vapor into the dielectric over the years which can increase their values 10 to 50%. This is with difilms and mylar dielectric typical of old paper & polyester/100% polyester orange drops and the earlier brown drops (choco drops). I have never seen any old ones below nominal value.
joel_ostrom wrote:Anybody ever attempt to recreate the sound of a well aged amp by replacing caps and resistors with others that have differing values?
How can you be sure that the values of whatever you choose won't keep drifting in a bad way?
As to the rest of your question, that is much longer and greater discussion...
According to published data, capacitance does increase in film caps under high humidity conditions, from 1-5% depending upon the dielectric. But this is reversible. I don't know what would cause changes of 50%, but plastics are certainly known to break down over time.
I suppose you could do it if you're cloning an old amp (read nothing, measure everything); but it'll be different if you're just making a generic "old amp". Which components should have drifted? Every deviation is just as likely to make it sound worse.
Aleks, I know it sounds wrong, but I have dried out over a hundred old orange drops and chocodrops that had spent 30-40 years in non-A/C storage in Houston, a very humid climate, and that gets them back in spec about 90% of the time. It takes about 72 hours at 225 deg F in a convection oven to do it.
I measured them before and after drying them out and there is no doubt that the values do come down quite dramatically in most cases.
Heat must break down some form of crystalization in there? I leave amps in the bright summer sunshine for a couple days before I try to reform the electrolytics.
I wonder if the PVCs get chalky in there.. PVC piping from the same period gets very brittle, UV or not.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
Larry (novosibir) has said before that he will take old cap cans such as the ones used in Marshall's and submerge them up to the top in very hot water to dissolve the crystals in the electrolyte.
He says that even NOS caps that haven't been used, just sitting on a shelf can benefit from this.
He does this in preparation of reforming them.
Of course this probably wouldn't help a cap that is all dried up.
