Schematic-related question from a beginner...

[Brinkman]

Hey guys.

So I'm building a few kits right now and ordering various types of resistors for them based on the specific placement of the resistor (putting mills wirewound in the B+ circuit, Kiwame carbon comps as grid-stoppers, etc...) and I was advised to use a certain type of resistor for "resistors in the signal's path". This has me slightly confused.

For instance: I have an input signal that runs through various pots before meeting an interstage capacitor (100nF), followed by a grid leak resistor (330Kohm) then a grid-stopper resistor (220ohm). The cathode resistor is 680ohm. Since the leak and cathode resistor are connected to ground, are they still considered in the signal's pathway? If so, when is a resistor not considered in the signal's pathway?

Here is the partial schematic in question to help anyone answer my question.

Thanks,
Brinkman

[TomMcNally]

Well actually ... ALL resistors in an amplifier are
really in the signal path. I think the value of using
boutique resistors is probably not all that important.

The only thing I've heard is that the old style
"carbon composition" resistors are good as
plate resistors in preamp stages. Take a look
at one of the diytube manuals available here in
the forum, and check the Mouser parts list -
they are nice quality and inexpensive and
work well. Handmade Electronics has a nice
assortment and reasonable prices on all of the
common types and values used in tube amps.
http://www.hndme.com

... tom

[EWBrown]

Old (and I mean 30-50 years old) CC resistors may not have their original resistance values, sometimes far exceeding their tolerance range.

These resistors can (and do) absorb humidity from the air, and any other stuff floating around (those who live near the ocean take note). The phenolic bodies aren't all that "waterproof" ore humidity proof and spending decades socked away in some dark and dank cellar, attic or garage can take its tool. Measure first, then install...

Your circuit looks almost like the standard "symmetrical" SRPP design, but normally R308 and 310 would be the exact same value and there would be no R 309. Given all that, there are around 4 or 5 variations of teh basic SRPP cirucit, with the "Aikido" being the most complex (and sophisticated).

Also a 470K resistor should connect across the output connectors to ground, in order to prevent any residual DC voltage on the output coupling cap from getting into the power amp or next stage.

/ed B in NH

[Brinkman]

Well actually ... ALL resistors in an amplifier are
really in the signal path.... The only thing I've heard is that the old style
"carbon composition" resistors are good as
plate resistors in preamp stages.... tom

I kinda assumed the first half of what you've stated to actually be the case. As for carbon comp, carbon film, etc..., I was curious about using vishay nudes for the "direct signal path resistors" (or whatever that would mean). It would seem that on this level, things are fairly subjective. For every one person I can find who insists on carbon comp for this or carbon film for that, there's someone else who'll state it doesn't matter and that if you want to spend money, spend it on precision and quality.

I'm still curious about using boutique resistors, though I may not be able to justify the expense on some of them.

Best,
Brinkman

[kheper]

In applications where power losses are high,
carbon film resistors, if their insulation is not
made of fire resistant materials, can ignite.

I would not use wirewounds outside of the
power supply, because most are inductive.

[erichayes]

Metal film resistors have the best temperature coefficient of all non-wirewound types. As tube stuff tends, by nature, to have a wide operating temperature range, metal films are ideal for plate and grid leak applications--and, if they can handle the current, cathode.

Metal oxides can handle up to five watts and have the next best Kt. They work well in phase inverters as plate and cathode resistors using medium mu tubes such as 12AU7s or 12BH7s.

Carbon film resistors are cheap, but have such lousy Kt that they're not even spec'd. They're OK for solid state work but should be avoided in tube gear unless they're the only option.

Carbon composition resistors have a slightly better Kt , but have, as Ed mentioned, a tendency to drift in value over time. Unlike carbon films, however, that drift is exacerbated by heat. Comps have been around since radio was invented, and have developed their own folklore. One legend (which I adhere to out of superstition) is that they make the best control grid stopper resistors. Another (true) legend is that they can get noisy over time. For me, that rules them out as plate or cathode resistors.

I have no opinion of wirewound resistors in the signal path, as I've never used them in signal paths (save for the occasional output tube cathode resistor in a guitar amp--hardly an objective test bed).

Back in the '60s and '70s, Marantz's ad agency bragged about Marantz using 1% resistors in its amplifiers and preamps because they were such precision pieces of equipment. It was impressive copy to the lay person. The truth was that it was the only tolerance metal film resistors came in.

Buying boutique resistors and caps, to me, is analogous to buying underwear at Neiman-Marcus.

[Brinkman]

There's some great posts here and I appreciate all this input.
I can't really ask for a nail-in-the-coffin verdict, but I do feel like I'm on the right track now.

I notice no one here is refuting the claims made by Kiwame resistor vendors that Kiwame carbon comp resistors' values don't change over time (drift). Have they even been on the market long enough for this claim to be verified?

As it stands, I'm building both an op-amp-toting phono preamplifier and a SRPP-design tube line-stage (both are Hagerman kits). My impression is that if I want to splurge on components (other than caps, opamps, and tubes), I should consider wirewound resistors for the power supplies, Kiwame carbon comp for the grid-stoppers, possibly vishay nudes for key resistor applications (adjacent to tubes/opamps) and PRP metal films anywhere else.

Other endeavors I'm considering are tantalum input/output resistors and bypass caps for key power supply caps and wired across opamps. However, I may need some additional demystification before I'd be willing attempt such procedures.

Thanks again!

[cartoonweirdo]

The things no one has asked here are
A: what are you building?
and
B: what is you budget?

In the event this is a guitar amp I actualy do think the carbon comp/film sound better (e.g. trashier and less acurate). If I remember right carbon comp/film resistors were always a compromise in hi-fi gear due to cost and/or availablity.

If you have a load of cash to spend it is worth the time to expiriment with boutique caps and resistors.

If you have some cash to spend you might try the following first:

Good sockets: I have been FLOORED at the difference between crappy old cheap ones and new expensive ones

Good tubes: Duh!

Silver wire: Isn't the conductivity of the wire one of the limiting factors in the amp?

A DC heater supply: Worked wonders for me after I got the ground in the right place (NOT with anything else)

If you have no cash to spend get the cheap stuff and worry about design and layout

[Brinkman]

The things no one has asked here are
A: what are you building?
and
B: what is you budget?

Well, no one asked directly, but to answer your questions, I'm building a "Ripper" phono pre-amp and a "Clarinet" tube line-stage for my home stereo system. Both are Hagerman kits (silk-screened pc-board deals). So the whole design is already pre-determined, so all I have to do is fret over the little details.

I have no idea what my budget is, as I'm assembling both on the installment plan. In other words, I contemplate my next step, decide on a course of action, wait until I've accumulated the proper funds, and when I'm ready, I spend it. I repeat this sequence until my project(s) is/are completed.

Then, next kit.

[TomMcNally]

The following comments are general ... not aimed at Brinkman.

If I were going to build a circuit board only kit, I would initially
go with the parts the designer specified. If after assembling
it, and hearing it work, I might tweak a little. In an amplifier,
the first place to go is the coupling capacitors ... spend a few
bucks on some boutique caps, and maybe they will sound
better, maybe not.

Designers of kits aren't always going for the lowest cost,
like commercial designers of low budget stuff.

If you read the messages here on the forums, the folks
with the most problems and who have the hardest time
getting their amplifiers working, are the guys who don't
even build the amp with the suggested parts, but immediately
change the capacitors, the tubes and the transformers,
even though they've never built anything before.

Don't try to out guess the designer !

[erichayes]

Amen, Tom.

[Brinkman]

If you read the messages here on the forums, the folks
with the most problems and who have the hardest time
getting their amplifiers working, are the guys who don't
even build the amp with the suggested parts, but immediately
change the capacitors, the tubes and the transformers,
even though they've never built anything before.

Like I said in my first post... just asking about some resistors....