Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mike_J on December 16, 2020, 12:26:32 pm
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Years ago I purchase a lot of 1/2 watt Allen Bradley carbon comp resistors in the Fender values. Since then I have learned that metal films do reduce hiss and crackling to a noticeable extent and have used them pretty much exclusively. I bought the AB's over twenty years ago because they supposedly have some magic in them. I am going through all my builds on one by one basis to try to perfect them to the extent I can.
I am pretty certain every resistor related to V1 should be metal film. No sense in introducing noise early and amplifying it down the line as far as I can see but am willing to listen to comments to the contrary. There is information on the internet that says CCs are useful as plate resistors where there is a significant DC voltage drop. Anybody have experience with this and suggestions for anywhere else to place them in a Fender Bassman 5f6a build? I am leaning towards trying the 100K resistor on V2 and the 82K and 100K resistors on V3. Any other places that might make a difference anyone is aware of?
Thanks
Mike
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I would replace ALL half watt CCs with MFs and all 1 watt CCs with metal oxide.
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I would replace ALL half watt CCs with MFs and all 1 watt CCs with metal oxide.
Definitely metal oxide three watts on the power rail including the 470r resistors feeding the screen grid on socket 4 of the power tube. Metal films make for a much quieter amp versus the CCs. Don't have the crackle and much less hiss. Just looking to see if anyone has experimented with a combination of the two they liked from a sound point of view and not just a traditionalist point of view.
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I'll use em if i don't have metal oxide in the particular value. never on the first stage and never in the power supply though. don't think i have the ears to distinguish between the types and frankly too lazy to swap just for funsies
i use metal film for the 1M and 33k/68k on the input and mostly metal oxide everywhere else (they often come with nice hefty leads which helps with point-to-point/eyelet board builds)
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Thank you for replying. It is my guess from the lack of enthusiasm for the CCs that I am going to keep using metal films everywhere. Does make for a quiet amp which to me is very important.
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+1 with Sluckey. I have replaced many Plate resistors (Nominal 100k) in the lat 40 years, which have seriously drifted ie +50% or so. And the 470R in Fenders 100% or so drift and occasionally cracked in half but held in place by the soldered ends :icon_biggrin:
I suggest if you want reliability and quietness them MF and MO is the go, at the expense of some Mojo :laugh: but is it guitar/amp magic or just organic noise? (Would Leo have used MF/MO if the PRICE was right?)
I did get a stash of Philips CC 1W 5% about 40 years ago, then a huge stash of Beyshlag Carbon Film 1W and 2% which were nice to use. More recently I decided to stock only MF/MO resistors. MF for small Signal and Metal Oxide for 1W, 2W and 3W. I purchase all MF resistors in Magazines of 1000 units and common values for Plate and Screen MO in Quantities of 100s From a national supplier. For me it is cost effective and saves waiting for the next order as these components are not readily available at the "local" store.
Consider having a look at the following Link, may get tedious but the author has some good points. [Christmas is round the corner and if its a bit cool outside, this article with a few glasses of port and a little dark chocolate and of course the warm open fire, could be just the go before attacking "Vt = SQRT(4kTBR)"]
:laugh: :laugh:
https://www.aikenamps.com/index.php/resistor-types-does-it-matter
Kind regards
Mirek
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+1 with Sluckey. I have replaced many Plate resistors (Nominal 100k) in the lat 40 years, which have seriously drifted ie +50% or so. And the 470R in Fenders 100% or so drift and occasionally cracked in half but held in place by the soldered ends :icon_biggrin:
I suggest if you want reliability and quietness them MF and MO is the go, at the expense of some Mojo :laugh: but is it guitar/amp magic or just organic noise? (Would Leo have used MF/MO if the PRICE was right?)
I did get a stash of Philips CC 1W 5% about 40 years ago, then a huge stash of Beyshlag Carbon Film 1W and 2% which were nice to use. More recently I decided to stock only MF/MO resistors. MF for small Signal and Metal Oxide for 1W, 2W and 3W. I purchase all MF resistors in Magazines of 1000 units and common values for Plate and Screen MO in Quantities of 100s From a national supplier. For me it is cost effective and saves waiting for the next order as these components are not readily available at the "local" store.
Consider having a look at the following Link, may get tedious but the author has some good points. [Christmas is round the corner and if its a bit cool outside, this article with a few glasses of port and a little dark chocolate and of course the warm open fire, could be just the go before attacking "Vt = SQRT(4kTBR)"]
:laugh: :laugh:
https://www.aikenamps.com/index.php/resistor-types-does-it-matter (https://www.aikenamps.com/index.php/resistor-types-does-it-matter)
Kind regards
Mirek
Thanks good information. Interesting that the smallest resistors that eliminate radio interference in an amp should be used to reduce noise. Don't know how much to trust that though because different areas have better radio reception. What is good for my area may not work in a big city is my guess. Probably best to stick with tried and true Fender 68K values. Otherwise it sounds like MF/MO and bigger wattage is usually better.
Understand Vt=SQRT(4kTBR) about as well as the Gaussian probability density function. Somethings I am happy not knowing. Just a general idea of what makes an amp quiet without causing significant impact on tone is about as much as I am capable of knowing at this point in my life. Have builds with MF/MOs exclusively and they sound good but was wondering if I was missing anything. I think the question has been answered to my satisfaction.
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Understand Vt=SQRT(4kTBR) about as well as the Gaussian probability density function.
WOOF yup. The important parts.. warmer the part and the higher the resistance the more noise. Also, the material. Theres more space for microscopic bits of gas in carbon comp than in the metals.. the gas prevents electron flow symmetry, increasing electrical resistivity and therefore, the contact resistivity. Really, its just a number. Metal restricts current less to achieve the same resistance, therefore less heat introduced, and less noise. Contact resistivity is really dependent on the overall geometry (ahem.. manufacturing) variations thats why its not the same for all resistors.. much like why they don't all measure the same exact resistance but have a tolerance.
FWIW Merlin suggests 10K for those input grid stoppers with an additional cap to ground. 68k isnt perfect.. but certainly good enough.
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re:
WOOF yup. The important parts.. warmer the part....
For a moment, vampwizard you had me worried ie I thought the Port mixed with 4kTBR went down the wrong way, too big of a gulp whilst sitting in your favourite chair in front of the fire :laugh:
Even with laser trimmed MF precision resistors you have a tolerance. You either fork out big bucks for individual precision resistors or buy a thousand same value MFs and a precision LCR meter and spend hours measuring/grading your own down to 3 decimal places.
Kind regards
Mirek
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lol i worry myself sometimes. every once in a while gotta dust off that college degree. :laugh:
fun times, great thread.