Best type of dropping string resistors to use?

[Mike_J]

I was wondering whether there is a tonal difference between the type of dropping string resistor you use.  Does it make a difference whether they are metal oxide or wire wound or is there something better?  Does anyone have a brand they prefer?

Thanks
Mike

[sluckey]

I would not expect to hear any tonal difference between different types of B+ dropping resistors. I like the three watt metal oxide resistors Hoffman sells. They are flameproof. If I need a value that Doug does not supply I use 2 Watt CCs from my huge stash.

Sometimes you may need a higher wattage resistor. It all depends on the particular application.

[HotBluePlates]

I was wondering whether there is a tonal difference between the type of dropping string resistor you use.

I can't say I've ever heard a tonal difference due to dropping resistor type. The "best type" is the one that yields the supply voltage you need and delivers sufficient filtering/isolation (meaning, the one that's the proper resistance).

It is my opinion after a long while messing with this stuff that tonal change/improvement is found by changing a circuit type or parts values; the intense focus on a type of part appears to come from those who don't know much about how the circuit work (i.e., the ones who want to "mod an amp" but don't know enough about the amps to make a real circuit change) or from those who are marketing to the first group.

To be fair, there are differences in the characteristics of various parts, and some can have a tonal impact. I'm simply saying my opinion is that the tonal change due to a part-type change is maybe a couple-percent, compared to a tonal change from a circuit change giving a 20-100% difference.

Example:
In a recent build, I used metal film resistors almost exclusively. It wasn't a "Tone!" choice, but because metal film has the lowest self-noise of all types except wirewound (but you can't readily get those above a certain resistance).
I used Solen polypropylene filter caps; some say they have a "sound" but I chose to use them where I could because they will never wear out and need replacement like an electrolytic cap. In one part of the amp I used a multi-section can cap because it didn't have the space to fit the very-large Solen caps.
For power supply dropping resistors, I used wirewound (for the low-noise reason). I didn't used the white cement-block types, and chose some black Dale wirewounds instead. Why? Mostly because they look prettier (at least I'm honest about the reason!).
I used some no-name coupling caps because they were cheap and had a polyester dielectric, which I thought would mimic 50's-60's coupling caps close-enough.
I used some expensive pots; in most locations their only benefit was that they're completely sealed and they feel nicer when turning knobs, but in a couple locations (bias adjustment and balance) the expensive type was the best part because they were screwdriver-adjustable and include a lock to prevent accidental changes.
Throughout I used some strips I made which were a single-row of turrets. I could have used terminal strips (would've been cheaper & faster), but I don't like the look of terminal strips as much (another cosmetic choice), and because a part change/repair is a little messier with terminal strips because of how the lead is wrapped instead of sitting in a hole.
I used Teflon-insulated wire in the amp, but the main reason was that the plastic insulation won't melt when you solder (plus I got it cheap, and in an array of colors for color-coding during wiring).
I was advised to use particular transformers and speaker for the amp, which turned out to be critical (in my opinion). For the output transformer I used a type which is "too good" for a guitar amp (rated for full power 30Hz-30kHz & bigger than the average 100w guitar amp OT for a 25w amp) but the choice likely contributed to the quality of the extreme-clean the amp gets. The speaker was also a full-range hi-fi type with high sensitivity which translated the amp's relatively low power into a lot of volume in the room.

I think you'll see a few things were chosen for a tonal contribution, but most parts were chosen for other characteristics (even if that was just "I like the way it looks"). I've arrived at my attitude towards parts types because I'd gone the route of using expensive tweak-parts (once including coupling caps costing me $20-30 each and resistors at $1-2 each) and found the resulting amps didn't sound materially better than if I'd used much cheaper parts.

But that's just my opinion.

[Mike_J]

I was wondering whether there is a tonal difference between the type of dropping string resistor you use.

Your reply reminded me I have four of the Solen 22uF @ 630V poly caps that I have never used because they are too big for the chassis I used in the past that needed 20 uF electrolytics.  However, I think they will work perfectly in my upcoming Sluckey inspired JTM45/800 build on the JTM45 side.  The chassis will be 3" deep and as long as I don't get too carried away with extra bells and whistles the caps should fit.  I too like the look of the Dale black wirewounds.  Didn't know the wirewounds are quieter. 

I have learned the value of using metal film resistors too.  My Sluckey inspired Plexi/800 has metal films on the plates and most of the cathodes in the amp.  I am still tweaking it but it is so quiet you can't hear it at all unless a guitar with single coils gets plugged in it.  On all my amps, except the early Fender tweed amp clones with the CCs in them, I tweak the amps until I can not tell they are on with all controls dimed from more than one foot away from the speaker/s with nothing plugged in.  Getting them this quiet also requires proper wiring technique.  Doug's boards did more to help my wiring technique than anything else has.

I too like to use Teflon wire so I won't melt it which I did many times with the plastic wire.  The amp does not look like a quality build when half of your wires are wrapped with electrical tape here and there to cover the mistakes made with a soldering iron.

I would like to learn more about the potentiometers you used.  I am always concerned that solder will drip into a pot.  That would be a hard problem to diagnose for me.

I personally like the old blue caps from the blackface era and the mustard caps Marshall used.  I used a lot of the mustard caps in the Sluckey inspired Plexi/800 amp because I had them and they have very low dc leakage rates compared to some new caps I tested.  I used the Orange Drop 6PS caps in the Dumble sections of the amp because that is what Dumble used in the originals and they are inexpensive.  I agree though these parts have a very small impact on the tone compared to say speaker selection.  If I used the 6PS caps all around the change may not be noticeable.

[Mike_J]

I would not expect to hear any tonal difference between different types of B+ dropping resistors. I like the three watt metal oxide resistors Hoffman sells. They are flameproof. If I need a value that Doug does not supply I use 2 Watt CCs from my huge stash.

Sometimes you may need a higher wattage resistor. It all depends on the particular application.

I have been working on creating a stash of the metal oxide resistors for a while now.  Never seem to have the correct value when I need it though.  That is why I bounce back and forth between ordering exactly what I need when I need it versus having everything on stock so it can be pulled out of a bin.  That never works though because I don't reorder the parts I most commonly use to replace what I have already used leaving me with a ton of parts I will probably never use.  This case is different because I don't know what values I will need so I have to order a lot of parts I will never use so there is no other option except to buy a lot of expensive 2 watt potentiometers.

Thanks
Mike

[shooter]

you could build a "test" box with meters, clips n pots, then measure n install fixed.  that way you only buy the pots once.

[HotBluePlates]

I personally like the old blue caps from the blackface era and the mustard caps Marshall used. ...  I used the Orange Drop 6PS caps in the Dumble sections of the amp because that is what Dumble used in the originals and they are inexpensive.  ...

I'm not certain, but believe all 3 cap types you mentioned use a polyester dielectric, probably in a film & foil construction.

Capacitors can be divided into classes largely based on the type of dielectric used as the insulator between the conductive plates of the cap. Different dielectrics (and to some extent, the construction method used) have varying characteristics, the most important of which amount of capacitance per unit of volume. The dielectric can range from air (like in a radio or oscillator tuning cap, where big metal plates are spaced a fixed distance from each other) but have a capacitance measured in only a few picofarads, through to the molecules-thick oxide layer in an aluminum electrolytic cap where physically-huge caps can measure a Farad.

In a guitar amp, parts in the pF range are generally ceramic or mica dielectric (or polystyrene if someone spent a bunch of money aftermarket for exotic caps). If you tried to make a cap out of these materials only as big as 0.01uF, they'd have to be physically-huge (I know; I've got a GR 1419A which uses styrene caps inside, and it takes up a lot of real estate for the 0.1uF decade). Polypropylene has a higher dielectric constant, so packs more 0.01uF's for the same physical space. But polyester can be smaller still for the same capacitance, as you'll know if you compare the (polypropylene) 715P/716P-series of Sprague orange drops to the (polyester) 225P or 6PS orange drops.

Size can be reduced further if you don't build a cap by taking 2 sheets of metal foil and placing a sheet of plastic between them. Metallized caps (often metallized polyester) are formed by sputtering/spraying metal directly onto the plastic film. There's a tiny air layer that exists when using discrete foil sheets that goes away when you make a metallized cap, and the result is the conductive plates move closer together which raises capacitance and the size for a given capacitance drops.

Aluminum electrolytic caps take this to an extreme by having the electrolyte paste act as one of the two plates. Apply a voltage of the proper polarity and an oxide layer grows on the other, actual metal aluminum, plate. So the distance between plates gets very small and capacitance goes up dramatically compared to plastic film (as you'd know again comparing an electrolytic cap to a Solen cap).

As a general rule, going to a dielectric that yields more uF's incurs some kind of performance degradation, though this can get quickly over-hyped when sitting around talking about it. And in many cases, the performance changes happen in frequencies way above the audio range, or can be measured by lab equipment but not heard. I hear a slight loss of highs in polyester caps that I don't hear in polypropylene caps, but either one could work fine in a guitar amp.

I would like to learn more about the potentiometers you used.

Do an internet search for "RV4 pot" and they'll turn up. That's the series-number prefix for the military part number for that family of pots, and a number of companies make them. Be careful jumping on cheap ones on eBay without researching the part number convention; they're typically linear taper in values you'd expect for your volume control (where you need an audio/log taper).

As for solder drip into a pot, proper soldering (tip size, wattage/heat, solder thickness, soldering technique) will ensure that doesn't happen. An unsealed pot would probably take years-decades for a problem to show up due to dust/gunk entering the opening unless you play in Middle Eastern dust storms. And the downside of a sealed pot is you can't spray cleaner into it to clean it.

[Mike_J]

I personally like the old blue caps from the blackface era and the mustard caps Marshall used. ...  I used the Orange Drop 6PS caps in the Dumble sections of the amp because that is what Dumble used in the originals and they are inexpensive.  ...

I'm not certain, but believe all 3 cap types you mentioned use a polyester dielectric, probably in a film & foil construction.

 I hear a slight loss of highs in polyester caps that I don't hear in polypropylene caps, but either one could work fine in a guitar amp.

I would like to learn more about the potentiometers you used.

Do an internet search for "RV4 pot" and they'll turn up. That's the series-number prefix for the military part number for that family of pots, and a number of companies make them. Be careful jumping on cheap ones on eBay without researching the part number convention; they're typically linear taper in values you'd expect for your volume control (where you need an audio/log taper).

As for solder drip into a pot, proper soldering (tip size, wattage/heat, solder thickness, soldering technique) will ensure that doesn't happen. An unsealed pot would probably take years-decades for a problem to show up due to dust/gunk entering the opening unless you play in Middle Eastern dust storms. And the downside of a sealed pot is you can't spray cleaner into it to clean it.

HBP

I am pretty sure the reason I like the polyesters is because my first build was a 5f6a which can be harsh on the high end.  A polypropylene in that application is harsher than a polyester in my opinion.  The harshness is largely attributable to new speakers that need time to break in.  However in a darker amp the polypropylenes would probably work better to brighten the amp up a bit.

I agree that proper soldering technique will prevent a solder problem in an open potentiometer.  To the best of my knowledge I never got any solder in a potentiometer but that doesn't stop me from being a little neurotic about it. 

I buy a lot of parts from a distributor that sells the Precision Electronics Corporation Series K pots that are made to RV4 specs.  They cost nine times more than the 16mm pots I used in my Sluckey inspired Plexi/800 build but are two watts and I have no doubt they are much more precise than the Alpha pots are.  I am glad I asked you the question because I am going to place an order for some today for my voltage system to adjust the voltage string on the Plexi/800 build.  Given their cost I probably should give some thought to where I can use them in other builds so I don't have a $50 one time use voltage system taking up space after I have finished tweaking the Plexi/800.

Thanks
Mike

[sluckey]

so I don't have a $50 one time use voltage system taking up space after I have finished tweaking the Plexi/800.

The values you need will not be used very often on an amp circuit. Maybe a mid pot or presence pot or bias pot.

[Ken Moon]

Make sure you check the shaft length when buying the RV4- style pots (also sold as 53C series, and by Allesandro (audio taper only/labeled as Allesandro)).

I bought some from eBay, a couple from Allesandro, and a couple from Newark the first time I used them, and ended up with 3 different shaft lengths.

I like a 1/2" long shaft best (with a .090 thick chassis + faceplate), but the Allesandros, which are 3/8, will work.  Or, if you want, you can cut the shaft off to the proper length.

[Mike_J]

so I don't have a $50 one time use voltage system taking up space after I have finished tweaking the Plexi/800.

The values you need will not be used very often on an amp circuit. Maybe a mid pot or presence pot or bias pot.

Sluckey

My initial analysis shows the only values that would likely have value to me in a build are 25KL and 100KL, values which are not perfect.  I would prefer audio in both positions for the voltage system.  Any suggestions?

Thanks
Mike

[Mike_J]

Make sure you check the shaft length when buying the RV4- style pots (also sold as 53C series, and by Allesandro (audio taper only/labeled as Allesandro)).

I bought some from eBay, a couple from Allesandro, and a couple from Newark the first time I used them, and ended up with 3 different shaft lengths.

I like a 1/2" long shaft best (with a .090 thick chassis + faceplate), but the Allesandros, which are 3/8, will work.  Or, if you want, you can cut the shaft off to the proper length.

Ken

The PECs have a 3/8" shaft length.  While I agree a 1/2" shaft would be preferred for faceplate flexibility, the 3/8" shaft will work fine in my application.

Thanks
Mike

[sluckey]

Any suggestions?

Yes. F'getaboutit. Get a good set of 3 watt fixed resistors like I already suggested.

Let your ears, not your voltmeter, tell you when the amp is done.

[Mike_J]

Any suggestions?

Yes. F'getaboutit. Get a good set of 3 watt fixed resistors like I already suggested.

Let your ears, not your voltmeter, tell you when the amp is done.

Okay, message received.  When someone I trust tells me the same thing ten times in succession eventually I catch on albeit grudgingly at times.

Thanks
Mike