Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rosincore on January 27, 2016, 08:16:16 am
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I was approached to "look this over" and see if it needs anything. What a golden oldie, it sounds fine but I pulled the chassis out for a look see. Darn near all original. Someone replaced the resistors it looks like but didn't put them into the eyelets, just laid them on and soldered them. I am going to de-solder joints and install them properly. There is a few new metal film resistors in there also and I will be putting carbon comps back in it. Attached are a few pics to start, more as this proceeds. Notice the blue caps still in place, they are not passing DC so I am letting them there.
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... There is a few new metal film resistors in there also and I will be putting carbon comps back in it. ...
Metal film or metal oxide? Which resistor/circuit locations?
I ask because there were originally big carbon comps in the power supply, but some techs would swap those out with flameproof metal oxide resistors. The reason? Because with a significant-enough short-circuit, the carbon comps actually could go up in flames. Whether that is a concern for you, I don't know.
Other places, there could be a circuit location very sensitive to noise. A metal film resistor will give the lowest self-noise when you need a high resistance value (wirewound is lowest-noise overall, but may not be available in a needed value or may present unwanted inductance). So someone may have just been fixing a noise issue in the amp...
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The 470 in the bias section and the 2.2 M in the tremolo section are blue metal film. The power supply has been changed and I will let them alone also at your recommendation. I will keep what I take out handy just in case anything develops. Thank you for the info !! :thumbsup:
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The 470 in the bias section and the 2.2 M in the tremolo section are blue metal film. ...
There is nothing to be gained by installing carbon comp for those 2 resistors.
The 470Ω resistor in the bias circuit will likely be more stable with temperature changes if you use metal oxide or metal film; carbon comp drifts more with temperature change. The impact would probably be a very small shift in bias voltage ("vey small" meaning there but not enough to matter).
The 2.2MΩ in the trem circuit simply passes raw bias voltage to the oscillator tube to shut it off if you don't have the trem footswitch plugged in and connecting hot-to-ground at the jack (the footswitch is therefore needed to activate the trem oscillator). So there's no tonal contribution by this 2.2MΩ resistor, no matter what you use. You might change it for aesthetic reasons ("I just like the look/smell of old carbon comps in my vintage Fenders; they're authentic...").
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Agree nothing gained electrically , just pure asthetics.
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Good deal! Make it like the old days!! :icon_biggrin:
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A look under the cap pan of this oldie..
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Cabinet tube sheet still on it.
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A shot of the entire chassis. This amp sounds great and has the usual real deep reverb and the trem works great as well. This is before I resoldered the resistors. An classic amp such as this deserves to leave it looking good so I will .....