Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ed_Chambley on September 26, 2011, 07:28:32 pm

Title: JTM 45 Build Resistor Question
Post by: Ed_Chambley on September 26, 2011, 07:28:32 pm

I am using a layout from mojo and this is my first build using their kit.  It is different than Metro Kits.  I am wanting to substitute metal film caps everywhere noise can be caused by CC's.  On the pin 5 of the EL34, it shows a 5.1k resistor, but hoffman does not offer a 5.1 in metal film.  Closest is a 5.6, which I did order.  This goes to the 220k bias resistors.  Will the difference in value cause any problems? 

Also, where would be the best place in the build to use CC as opposed to metal film.  As I have read CC are more warmer and vintage sounding.   I don't know if there is any truth to this as I have substituted metal for CC before, and did not notice any difference, but that was for simple repairs to get the amp going for bandmates and friends.

Title: Re: JTM 45 Build Resistor Question
Post by: sluckey on September 26, 2011, 07:46:03 pm

Quote

Will the difference in value cause any problems?

No. The original did not have any grid stopper resistors.

I'll bet that if metal film resistors had been available and cheap when the original amp was built, it would have 100% metal film.

Title: Re: JTM 45 Build Resistor Question
Post by: stingray_65 on September 26, 2011, 11:05:58 pm

Hey ED,

CC resistors change more with heat and age than metal film.

I believe what many of us hear as a vintage sound has more to do with a higher B+ rail (because of higher voltages at the wall) and the drift of components like CC resistors and caps.

It's been said numerous times here that changing a value of a component will alter the tone more significantly than changing brands of components.

I try to select components for durability rather than tone. After an amp is built, I can mod and tweak it to get what I want if I need to.

Probably the BIGGEST tone enchantment is the musician!

Just my opinions. take em or leave em

Ray

Title: Re: JTM 45 Build Resistor Question
Post by: Iannone on September 27, 2011, 09:50:54 am

Most objects have their resistance increase as they get warmer.  Metal film resistors increase resistance, but not very much, as they get warmer.  Carbon Comps, on the other hand, decrease in resistance as they get warmer.

So when folks talk about the warmth or very gentle compression of CC's, this is the physics of what they are talking about.  Of course, this characteristic of CC's is only going to show itself when the resistor is dissipating enough heat to cause a meaningful increase in heat, e.g., plate load resistors. 

Title: Re: JTM 45 Build Resistor Question
Post by: spacelabstudio on September 27, 2011, 09:14:24 pm

I really enjoyed this article:

http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm (http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm)

Chris

Title: Re: JTM 45 Build Resistor Question
Post by: phsyconoodler on October 04, 2011, 03:30:20 pm

Quote:" I believe what many of us hear as a vintage sound has more to do with a higher B+ rail (because of higher voltages at the wall) and the drift of components like CC resistors and caps."

I don't really think so.I find that old iron transformers and the way they were interleaved and using paper separators has far more to do with 'vintage sound'.
  And old loose speakers.Drifted resistors? Nah!

Title: Re: JTM 45 Build Resistor Question
Post by: stingray_65 on October 04, 2011, 11:29:20 pm

Quote from: phsyconoodler on October 04, 2011, 03:30:20 pm

  And old loose speakers.

I will agree 100% on that!

Speaker choice has such a dramatic affect on the amps voicing.

There really is no way to age a speaker 30-40 or 50 yrs artificially.

Ray