CC Resister drift in old Fenders and marshalls

[Tone Lover]

Has anyone taken the time to record the upwards resister drift in any old amps presumably nice sounding old Tweed fender types . That you would share your results with us. or even if you don't want to put on the internet you might share with me . I want to build a 5B6-A bassman and I know what the schematic says but we all know that's not what the actual values are after the upwards carbon comp. drift . Just throwing this out there .
Thanks Bill

[Leo_Gnardo]

Has anyone taken the time to record the upwards resister drift in any old amps presumably nice sounding old Tweed fender types . That you would share your results with us. or even if you don't want to put on the internet you might share with me . I want to build a 5B6-A bassman and I know what the schematic says but we all know that's not what the actual values are after the upwards carbon comp. drift .

Having repaired easily well over a thousand old Fenders, I have noticed a trend, but it's only recorded in my head so... take it for what it may be worth. Fenders from say '55 thru '65, not awfully drifty, maybe 10% and mostly upward. Same LOOKING CC R's '65 on, drift starts at 7 or 8 % and I've seen up to 150 - 160 KOhms measured on 100K ohm (plate) resistors, 2K2 on what should be 1K5 cathode R's & similar upward drifts of 40 to 60% on others. - - - WAS this a general trend, OR did the smarty pants CBS engineers choose a resistor that was cheaper and tended to drift. Pick whatever choice, that's what I find.

Very early Fenders say pre '55, I dont see too often. So for your 5B6, could pick 10 or 20% over spec value resistors & try that. I'm sure some modders & scratch builders have tried this angle before but never heard a report on results. One thing I've found for sure, in the standard 60's tone stacks, if that "contour" 100K resistor drifts upward, the sound tends to be "hard" as less signal is sent to the mid/low part of the circuit.

[cbass]

So when do you consider a resistor bad?when it drifts past its tolerance?
Or if it sounds ok leave it?

[Leo_Gnardo]

So when do you consider a resistor bad?when it drifts past its tolerance?
Or if it sounds ok leave it?

G'Mornin' CBass, up and at 'em!

I've found a correlation between drifted and noisy. If it's drifted beyond say 10% out it goes. If not noisy yet, in anticipation.

In the output drive circuit often find the 100K and 82K off by 10% or more, and won't pass a balanced looking sine wave at clip. Swap 'em out. On the bias side, 220K is the highest I'd go and drift counted in, say 240K. It's good to have reliable resistors in this location, goes without saying. I've reduced value to 180K all the way down to 100K with no complaints.

Resistors riding atop output tube sockets are often roasted out of tolerance. Swap 'em out. Also don't want resistors feeding bias to pins 5 to open up, even for an instant. CC resistors in this "stopper" function are supposed to be the best. Marshall used 5K6 resistors there, so exact value may not be too critical, just that they're matched & not noisy.

For those experimenting with off-values, select new reliable resistors, not old drifted ones. Unless you like the sound of a crackly old amp. Sometimes that IS the charm... cue "radio guitar" intro to "Wish You Were Here."

And of course if you're very happy with the sound, might consider not messing with it at all.

[Structo]

This is something I have wondered about as well.

Say a friend has a Pro Reverb that sounds like the best amp ever.

Then you open it up and start measuring and just about every part is out of tolerance.

As mentioned the CC resistors tend to drift upwards, especially where there is high voltage and heat.
Bad electrolytic caps and leaky coupling caps.

So if we clone the amp, should we use the values on the schematic or the measured values of the actual amp?

[Blackburn]

Just using NOS ABs will get you some of the way there anyway, as they are usually off spec to begin with. I don't think I ever ordered any ABs and got them within 10% of the value they were supposed to be, unless they are very small values. The higher the value, the farther off they are, usually. Didn't bother me much because I was thinking the same way you guys are. Why do those old tweeds sound so damn good? For old 50s tweed Fender stuff I'll likely use ABs just to keep that mojo workin.

[Richie]

I've worked on a lot of old amps, fenders to whatever. Its really hard to answer that question. I see more drift where they go up in value. Along the lines of what Leo mentioned. I've also seen old amps that are very much in tolerance and very close to the value.

So you never know what you will see. And it also doesn't seem to matter if they are 1/2 or 1w. A ballpark figure, i'd say you'll see a 5 to 15% reading in value.
This could be in a NOS never used resistor. Sometimes I think maybe the brand,to where they were stored/climate,to where they are in a circuit etc. If an amp was played, or just stored. Usually you see the amp will also show other signs if it was in a humid or damp location,to a dry one. You may see the higher value plate resistors will be off more than say a bypass resistor value. And usually those are ones that may be located to a heat source,tube socket,etc.

The real thing is as was mentioned, if they are way off, or if they are noisey and causing problems. Popping, rustling sounds. And again I have seen old amps with no problems, to later ones with problems. So you just never know. Then you may have good resistors, and a conductive board,dirty sockets or tube pins can make similar noises. Meaning when someone hears that noise, they blame it on the CC resistors.

[Firestorm]

This is something I have wondered about as well.

Say a friend has a Pro Reverb that sounds like the best amp ever.

Then you open it up and start measuring and just about every part is out of tolerance.

As mentioned the CC resistors tend to drift upwards, especially where there is high voltage and heat.
Bad electrolytic caps and leaky coupling caps.

So if we clone the amp, should we use the values on the schematic or the measured values of the actual amp?

Every once in awhile, you hear about the amazing Super/Twin/Plexi/Vox that is in huge demand for recording. If I ever get the chance, I'd measure every single thing and try to suss out the reason for the magic. To clone an amp like that, you'd use the drifted values wouldn't you? Except maybe the 5% parts in the PI or the grid loads.

[renshen1957]

Just using NOS ABs will get you some of the way there anyway, as they are usually off spec to begin with. I don't think I ever ordered any ABs and got them within 10% of the value they were supposed to be, unless they are very small values. The higher the value, the farther off they are, usually. Didn't bother me much because I was thinking the same way you guys are. Why do those old tweeds sound so damn good? For old 50s tweed Fender stuff I'll likely use ABs just to keep that mojo workin.

Hi,

An old trick from the Audiophile community when AB resistors were the vogue. Bake the CC resistor in the oven remove the moisture. Coat the resistors with shellac to water proof.

The baking removes the moisture, and returns the resistor back to its original resistance if drifted. (I have yet to attempt this procedure and measure the resistance to verify). The shellac seals the resistor.

If I was buying a custom made resistors, I would purchase Carbon Comp resistor from RCD (same distortion characteristic as the old ABs) and would have these hermetically sealed similar to capacitors.

Best regards,

Steve

[Blackburn]

Just using NOS ABs will get you some of the way there anyway, as they are usually off spec to begin with. I don't think I ever ordered any ABs and got them within 10% of the value they were supposed to be, unless they are very small values. The higher the value, the farther off they are, usually. Didn't bother me much because I was thinking the same way you guys are. Why do those old tweeds sound so damn good? For old 50s tweed Fender stuff I'll likely use ABs just to keep that mojo workin.

Hi,

An old trick from the Audiophile community when AB resistors were the vogue. Bake the CC resistor in the oven remove the moisture. Coat the resistors with shellac to water proof.

The baking removes the moisture, and returns the resistor back to its original resistance if drifted. (I have yet to attempt this procedure and measure the resistance to verify). The shellac seals the resistor.

If I was buying a custom made resistors, I would purchase Carbon Comp resistor from RCD (same distortion characteristic as the old ABs) and would have these hermetically sealed similar to capacitors.

Best regards,

Steve

That's a really interesting idea. It would be great to try it and post the results. I am curious about the shellac treatment, though. Is it flammable only in liquid form and not when dry?

[renshen1957]

This is something I have wondered about as well.

Say a friend has a Pro Reverb that sounds like the best amp ever.

Then you open it up and start measuring and just about every part is out of tolerance.

As mentioned the CC resistors tend to drift upwards, especially where there is high voltage and heat.
Bad electrolytic caps and leaky coupling caps.

So if we clone the amp, should we use the values on the schematic or the measured values of the actual amp?

Every once in awhile, you hear about the amazing Super/Twin/Plexi/Vox that is in huge demand for recording. If I ever get the chance, I'd measure every single thing and try to suss out the reason for the magic. To clone an amp like that, you'd use the drifted values wouldn't you? Except maybe the 5% parts in the PI or the grid loads.

Hi,

You would still be in competition, with "Leo Fender's Ghost" (or Jim Marshall's Ghost, or Jennings Ghost). To my experience the "Ghost amps" still smoke their modern equivalents.

Usually you can get close to a vintage amp.

Problem is you would be attempting to build a half century old+ amp with new modern/improved parts. The components might not react the same. Xicon CC resistors for example do not have the same properties as the old Stackpole, AB, etc. NOS resistor you could source.

Electrolytic Caps are more problematic. E-Caps have changed greatly since the Golden Era of Tube Guitar Amps. The ESR is lower, the industry the parts are specified are different (Computer Monitors) from back in the day. The Atom Caps by Sprague have an exterior size that's similar, but no way are these any way the same Caps electrically compared to an ancient Astron MiniMite in a Tweed. Or behave the same. Unfortunately, E-caps don't sit on the shelf uncharged like a NOS tube. Kevin O'Connor had a method to dial the sound of a leaky E-Cap in The Ultimate Tone Volume 1 to simulate an old tired amp.

Speakers aren't much of a problem, there are companies that make an Alnico Jensen equivalent (but not the Modern Jensen) or a Vintage sound Celestion (but not Celestion).

Everyone has their idea as to a signal cap equivalents fir an Astron, Blue Molded, Mullard/Phillips Mustard, etc. Orange Drops were said to be the bees knees in Tweeds and Black Face amp, yet some techs rail against old amps being "orange dropped." Others claim Mallorys get the job done. Sozo made a name or a reputation for themselves for Mullards, and some criticism, too.

Clark Amplification when they sold parts (maybe they still do so on Ebay, I haven't checked recently) claimed at one time that NOS Russian Wax Paper and Foil or was it a PIO cap nailed the the Tweed Sound; now they use Jupiter Red capacitors. I haven't tried these yet, but these are Tin Foil and Film caps.

The original Astrons were Aluminum foil and Polyester/Mylar in an encapsulated (molded) body; a silica-filled, clay-like material, probably injection molded epoxy. The drab yellow/mustard color is throughout and the molding is quite thick.

One unusual feature is the end where the lead comes out. This appears to be a dab of sealant, but dissection reveals it to be a tapered bead, the color and transparency of cobalt glass. It may in fact be a glass or plastic bead. The structure of the molding indicates that the bead was already present at the time the encapsulant was molded. The bead is probably intended to act as strain-releif rather than a seal. It is harder than the molded compound and would tend to protect it from cracking as the part is worked in installation.

Did I mention Astrons, even most NOS Astrons, leak. How to simulate a leaking cap? I guess there are methods to simulate leaky signal caps.

They are some builders to their own ears rightly think they nailed the sound, but some one will pull out a 5F6-A with some years and too many miles on it, in need of a recap, and it will blow the competition away.

This always reminds me of antiquing furniture, to gave the appearance of something that it's not. YMMV.

By the same token, I have worked on a 1959 5F6-A which was recapped (E-Caps by Atom) with replacement vintage Alnico speakers (the serial numbers didn't match) and a new MM transformer. The sound was very sweet. The transformer wasn't paper wound (MM uses plastic).

I guess, Leo's Ghost is more powerful than I thought.

Best regards

Steve

[gingertube]

Sorry to blow off the pixie dust.

Every old amp I've worked on has sounded better after the drifted high CC resistors have been replaced with decent modern metal films.

Yeah.. I know their (CC resistors) resistance has a small voltage dependency which means they introduce a small amount of 2nd harmonic distortion when used as anode loads but my experience has been that this is trivial compared to the distortion from the tube itself, and particularly trivial when compared with the NOT small amount of noise they introduce.

Cheers,
Ian

[renshen1957]

Just using NOS ABs will get you some of the way there anyway, as they are usually off spec to begin with. I don't think I ever ordered any ABs and got them within 10% of the value they were supposed to be, unless they are very small values. The higher the value, the farther off they are, usually. Didn't bother me much because I was thinking the same way you guys are. Why do those old tweeds sound so damn good? For old 50s tweed Fender stuff I'll likely use ABs just to keep that mojo workin.

Hi,

An old trick from the Audiophile community when AB resistors were the vogue. Bake the CC resistor in the oven remove the moisture. Coat the resistors with shellac to water proof.

The baking removes the moisture, and returns the resistor back to its original resistance if drifted. (I have yet to attempt this procedure and measure the resistance to verify). The shellac seals the resistor.

If I was buying a custom made resistors, I would purchase Carbon Comp resistor from RCD (same distortion characteristic as the old ABs) and would have these hermetically sealed similar to capacitors.

Best regards,

Steve

That's a really interesting idea. It would be great to try it and post the results. I am curious about the shellac treatment, though. Is it flammable only in liquid form and not when dry?

Shellac can burn, but it's not highly flammable, (carbon composition resistor can burn, too, but not highly flammable).

Hi Blackburn,

Shellac dissolved in alcohol, very inflammable. Shellac flakes being organic will burn.

Shellac was one of the ingredients of sealing wax; I don't know if it still is. A stick of sealing wax doesn't burst into flames if you apply a match to it, but it will burn to a limited extent.

This I found at a Hi Fi parts retailer, quite some time ago.


"The best method is to place the resistor/s on a baking tray set, your oven to 80 degrees centigrade and leave in the oven for 8 hours. The secret is to remove the moisture slowly if you rush it it will become noisy in circuit. Once cooked seal the body of the component in Shellac (preferably the flakes) to stop any further water absorption. At the end of this procedure you will have a much lower noise, more closely tolerance resistors. Special thanks to Colin Cornish for this excellent tip."

I haven't attempted this, so I do not how well it works, but someone over at the MetroAmp forum, did:

"Re: AB Carbon Comp resistor readings higher than nominal
Postby leonard.gulyas » Sun Aug 19, 2012 5:17 am

I thought I let you know I went ahead with the procedure on hificollective.co.uk.

Here are the result after the procedure (Please note that I added some more resistors. So once again the legend: resistor nominal values in first, measured values in second, new values in third after baked in an oven for 8 hours at 80C/176F and sealed in shellack):

33k Ohm 5% - 35.55k 35.62k
470 Ohm 5% - 500 501
56k Ohm 5% - 60.2k 60.4k
68k Ohm 5% - 70.5k; 72.7k; 73.0k; 73.6k; 74.7k 71.4k; 72.1k; 72.3k; 72.7k; 73.8k
470k Ohm 5% - 481k; 486k; 487k; 507k; 508k 488k; 493k; 494k; 513k; 513k
100k Ohm 10% - 111.2k; 112.0k; 116.7k 111.1k; 112.4k; 116.7k
1M Ohm 5% - 1.118M; 1.131M; 1.159M; 1.161M 1.120M; 1.130M; 1.158M; 1.160M
820 Ohm 5% - 817; 841 820; 843
10k Ohm 5% - 10.2k 10.32k

So we can call the process pretty much useless but maybe the shellack seal will prevent the drifting of the resistor values in the future.


Amtrans AMRG resistor from Japan have black anodized aluminum case and epoxy filled, but are Carbon film resistors. The green fire proof coating on KOA Speer (Kiwame) power carbon film resistors, and the Epoxy coated Metal Films all address an environment issue, but has anyone though of applying this technology to Carbon Comp?

Best regards,

Steve

[Structo]

Were the resistors you baked Allen Bradley CC resistors?

I kind of expected bigger changes before and after.

[renshen1957]

Were the resistors you baked Allen Bradley CC resistors?

I kind of expected bigger changes before and after.

The person who baked the resistors (not me) did bake AB CC resistors.

Does this prove that change a resistor back to its non drifted value is Hi Fi horse feathers? No it doesn't, but until a number of parties prove otherwise and it should stand that it does not correct drifted values as until it is proven once and for all. A number of other people would have to perform the same experiment, to decide the matter one way or the other, or reach inconclusive results.

I will bake and Shellac CC resistors with my next build if only to reduce the moisture content until I can afford to purchase CC resistors encapsulated against moisture.

But my day (and night) job keeps getting in the way of my building time.

Best regards,

Steve