Throwing metal film resistors in Fender amps

[Mick Bailey]

Wil,
My advice on resistors.
Carbon Composition resistors are always very bad. These resistors are noisy and drift badly in value with age . They should NEVER be used in the build of a new amplifier and you will most likely not see them except in amplifiers built before 1970 (unless an "dodgy" builder was using cheap old parts to cut costs).

ASIDE: "Dodgy" is the english common expression for disreputable, some one likely to do things the most inexpensive way at the expense of reliablity. We make jokes about "the Dodgy Brothers" motor mechanics or the "Dodgy Brothers" plumbers and the like, so "dodgy" is anything a little (or a lot) suspect.

I've got a recent hand wired AC30 with its factory carbon comps - wouldn't call it dodgy. I also get a few Dr Z amps and don't consider them to be dodgy either. Then there's Victoria, Clark and all the rest. They seem pretty well built to me and sound just fine.

[soundguruman]

Wil,
My advice on resistors.
Carbon Composition resistors are always very bad. These resistors are noisy and drift badly in value with age . They should NEVER be used in the build of a new amplifier and you will most likely not see them except in amplifiers built before 1970 (unless an "dodgy" builder was using cheap old parts to cut costs).

ASIDE: "Dodgy" is the english common expression for disreputable, some one likely to do things the most inexpensive way at the expense of reliablity. We make jokes about "the Dodgy Brothers" motor mechanics or the "Dodgy Brothers" plumbers and the like, so "dodgy" is anything a little (or a lot) suspect.

Carbon Film resistors generate more noise than metal films. The amount of noise is generally proportional to the current through the resistor. The resistors which are in the signal path and have most current though them will be the anode load resistors and the cathode bias resistors. The cathode bias resistors will generally have a bypass capacitor which should shunt the noise, so concentrate on anode load resistors as the usual source of noise.

The highest source of noise (from a resistor) in most amplifiers is the grid stop resistor (usually 68K) on the input tube. This is one of the things that is addressed in Merlin's "Designing Tube Preamps for Guitar and Bass" book. His suggestion (which I agree with) was to reduce that grid stop resistor (from the typical 68K) to 10K and keep the same high frequency roll off (from this resistor + Miller capacitance at the tube grid) by adding an additional capacitor from grid to 0V on that input tube. I add 100pF but about 470pF would be required to keep the original high frequency roll off.

Common sense tells us that when looking for noise problems we should concentrate on the sections which have the highest amount of gain following them, that is, the input stage and the reverb recovery stage.

For new builds I use ONLY metal film resistors.

Is there any sonic benefit from using Carbon Film resistors? The answer is maybe - Carbon Film resistors have a slight voltage dependency to their resistance value. In the particular case of anode load resistors their resistance changes with the signal as the total voltage across the resistor changes with signal swing. This will impart some second harmonic distiortion (in addition to what is generated by the tube itself). This is a low level effect but those with "golden ears" MAY notice it. It is unfortunate that Carbon Films as anode load resistors is about the worst place to use them with respect to noise.

These are my opinions, I do not expect that everyone will agree.

Cheers,
Ian

A. for one thing, there are millions of leftover carbon resistors, now obsolete, and lots of vendors who need to sell them all.
They will tell you anything to get rid of them.

B. There is really no excuse why all new products should not be built with metal film or bulk metal foil.
The new resistors are superior in every way.

C. Bulk Metal Foil is the Future of audio. It won't be long before these are the accepted standard. Nobody will use carbon anymore.

D. Took these guys long enough.
I have been crying about using low noise parts since the 1970s. We started by building with military grade parts. When NO-body else realized the need.
At first, people reacted like I was crazy. NOW, it's "state of the art."

E. In the near future, TUBE guitar amps will have noise specs as good as Hi Fi equipment.
Hum, buzz, hiss, oscillation and ringing, etc...will be completely eliminated. And it's LONG overdue.

It was the Aerospace industry that pushed the development of lower noise components forward.
If you can build a Satellite Dish amp (LNA) with almost no noise at all...
Then you can build a TUBE Guitar Amp with almost no noise at all, TOO.

Now, let's get on with it.
You may have thought that tube technology "has all been done before."
I disagree. We have only just started, I think.

[kg]

i prefer good wirewounds myself.

[soundguruman]

i prefer good wirewounds myself.

Noise wise, wire wound is almost as good as metal film, and certainly better than carbon.
And WW very reliable, the most reliable.

[Mick Bailey]

Noise wise, wire wound is almost as good as metal film, and certainly better than carbon.
And WW very reliable, the most reliable.

It's the other way round - wirewound is the quietest.

[kg]

It's the other way round - wirewound is the quietest.

that has been my experience.

[soundmasterg]

And the OD 225P Polyester because I read that they are the closest to those old blue fender caps.

Plus, I'm quite influenced by all the hype and mojo, haha.

I wouldn't say the 225P's are that close to the old blue molded mallory caps that Fender used in their blackface stuff. The handmade Sozo blues are probably closer...at least to my ears, and they are marketed that way too. I've used them in a couple amps for people and they sound spot on, but sound is subjective as everyone knows so YMMV. Fender used some blue caps in the early to mid silverface stuff also so you have to specify which old blue fender caps you are talking about. These were excellent caps too btw....the bad ones were the brown terds that were used in some late blackface/early silverface stuff. They were lousy caps from the beginning.

Mojo and hype are fine, but its best to try before you form an opinion.

[soundmasterg]

And the OD 225P Polyester because I read that they are the closest to those old blue fender caps.

Plus, I'm quite influenced by all the hype and mojo, haha.

I wouldn't say the 225P's are that close to the old blue molded mallory caps that Fender used in their blackface stuff. The handmade Sozo blues are probably closer...at least to my ears, and they are marketed that way too. I've used them in a couple amps for people and they sound spot on, but sound is subjective as everyone knows so YMMV. Fender used some blue caps in the early to mid silverface stuff also so you have to specify which old blue fender caps you are talking about. These were excellent caps too btw....the bad ones were the brown terds that were used in some late blackface/early silverface stuff. They were lousy caps from the beginning.

Mojo and hype are fine, but its best to try before you form an opinion.

I'll add that I've used carbon comp, carbon film, metal oxide, metal film, and wirewound in amps. They range in noise from loudest to quietest as I have listed them. RG has noted on his site about the distortion benefits of carbon comp when used in certain places...and metal oxide has some advantages in a power supply location.....aside from that metal film is probably an overwhelming choice for most today. I like using carbon comp for plate resistors for phase inverters or down the line in the preamp sometimes. I've built amps with all carbon comp for customers as they have requested on occasion, but I prefer metal film for most applications myself. They are quiet and work and sound just fine. If I could find some good wirewounds to use for more applications I would try those too. For now I just use whatever I have to try to use up stock but once I am done with carbon film and carbon comp, I won't buy anymore except what I need for repairs.

Greg

[soundmasterg]

i prefer good wirewounds myself.

What brands do you like to use Ken? The standard NTE stuff at any of the local stores that still stock stuff, or got some hot tips for some to find at Mouser or Digikey or elsewhere?

Greg

[Alf]

the bad ones were the brown terds

What caps do you mean , can you link me to a picture ?

Tia

[Tom Phillips]

...the bad ones were the brown terds that were used in some late blackface/early silverface stuff...

I agree that they "look" bad and their appearance certainly doesn't inspire confidence. In what way do you find them to "be" bad? Photo Attached. (Edit: From a '68 Twin Reverb)

[Alf]

Thanks for the link Tom , now I know what you're talking about.
Those caps, I seldom come across. Are they so much inferior to, for example , the blue moulded ones ?

[soundmasterg]

Thanks for the link Tom , now I know what you're talking about.
Those caps, I seldom come across. Are they so much inferior to, for example , the blue moulded ones ?

The brown ones were bad caps from the beginning...very high ESR, very high L (parasitic inductance), terrible insulation resistance, usually drift off value...according to Dan Torres on page 56 of his book Inside Tube Amps. I know he isn't the best authority out there with many things, but in this case he is spot on....the brown caps used in the vintage Fenders and shown above in the linked pic are pretty crappy. The blue molded ones according to Torres are listed as having medium L, ESR as being fair, very high insulation resistance, usually close to specified value. He says they deaden the circuit compared to some modern caps....I assume he is comparing them with polypropylene here with this statement. The later blue caps with white lettering found in many Silverface Fenders are listed as having moderate ESR, medium L, real good insulation resistance, and fairly close to specified value.

I've done some testing with a Sprague Tel-Ohmike that I have on some of these caps and modern ones and what Torres said about those older caps is pretty true. The brown ones are pretty lousy. I will add that the brown ones with the white line to mark the outside foil that are often found in Sunns, Silvertones, etc....these ones aren't all that great either.

Greg

[Wil]

I wouldn't say the 225P's are that close to the old blue molded mallory caps that Fender used in their blackface stuff. The handmade Sozo blues are probably closer...at least to my ears, and they are marketed that way too. I've used them in a couple amps for people and they sound spot on, but sound is subjective as everyone knows so YMMV. Fender used some blue caps in the early to mid silverface stuff also so you have to specify which old blue fender caps you are talking about. These were excellent caps too btw....the bad ones were the brown terds that were used in some late blackface/early silverface stuff. They were lousy caps from the beginning.

Mojo and hype are fine, but its best to try before you form an opinion.



I'll add that I've used carbon comp, carbon film, metal oxide, metal film, and wirewound in amps. They range in noise from loudest to quietest as I have listed them. RG has noted on his site about the distortion benefits of carbon comp when used in certain places...and metal oxide has some advantages in a power supply location.....aside from that metal film is probably an overwhelming choice for most today. I like using carbon comp for plate resistors for phase inverters or down the line in the preamp sometimes. I've built amps with all carbon comp for customers as they have requested on occasion, but I prefer metal film for most applications myself. They are quiet and work and sound just fine. If I could find some good wirewounds to use for more applications I would try those too. For now I just use whatever I have to try to use up stock but once I am done with carbon film and carbon comp, I won't buy anymore except what I need for repairs.

Greg

Hello Greg !
Thanks for the answer. What do you recommand for the vibrato channel tone stack caps, then ? The blue molded Sozo are out of my budget and out of my range.
What would you choose between : Jupiter, Mallory 150s, OD 225P, 715 or 716, Sozo Mustard, Cornell Dubilier, Roederstein MKT1813, ... (see all caps at https://www.tube-town.net/ttstore/in...apacitors.html or Capacitors :: Passive Components :: Electronic Parts :: Banzai Music GmbH)
And do you think my small modification for a stereo jack footswitch might work ?

[soundmasterg]

Hello Greg !
Thanks for the answer. What do you recommand for the vibrato channel tone stack caps, then ? The blue molded Sozo are out of my budget and out of my range.
What would you choose between : Jupiter, Mallory 150s, OD 225P, 715 or 716, Sozo Mustard, Cornell Dubilier, Roederstein MKT1813, ... (see all caps at https://www.tube-town.net/ttstore/in...apacitors.html or Capacitors :: Passive Components :: Electronic Parts :: Banzai Music GmbH)
And do you think my small modification for a stereo jack footswitch might work ?

Well the Blue Sozos would be my choice for a modern cap replacement in a blackface Fender type build since they are made to emulate the old blue molded Mallorys.....they are cheaper than the Jupiters....I've used them in a few rebuilds of vintage Fenders and they sound very much like the original caps.

Mallory 150's are good caps and would sound fine....likewise the Sozo Mustards....and a lot of others. I would stay away from 715 or 716P as they are polypropylene caps. The originals were polyester, and any polyester cap will get you closer to that vintage sound than polypropylene. The 225P are film and foil polyester. The Sozo Mustard are also film and foil polyester. The M150's are metallized polyester.

A good thing to do is to mix and match....so say Sozo blues for most of the vibrato channel but use OD 225P's in the phase inverter or something like that. There is a difference in sound between various caps, but it is more the difference between the dialectric type and the way it is made rather than the brand per se. So for instance a film and foil polyester will sound a bit different than a metallized polyester, and they will both sound different that polypropylene. Make a decision and pick something and you can always change it out later...caps are cheap relatively speaking. You will notice more difference in sound by changing speakers or tubes than you will be changing caps. The main thing is to get something in place that is good quality and all of the brands you listed above are that.

Greg