Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jordan86 on June 15, 2020, 04:40:42 pm
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I rewired my Princeton for the 275v HV tap on the PT, instead of the 325v tap. Was curious how the feel and gain would respond. I noticed it is doing that weird crackle/distortion again, that goes away when I pull the 12AT7 (reverb driver). I know many of you have given some very helpful advice to me directly. I've also searched a few other threads here related to this and see there are two courses of action. I know both will tame the gain going into the reverb but curious what the difference would be tonally or maybe effectively.
Two options...
1) Pull the 25uf bypass cap on the cathode of the 12AT7. Leaving just the 2.2k resistor.
2) Install a 10K grid stopper going into pin 7 grid of the 12AT7
Based on the ampbooks.com bypass cap calculator, Option 1 would give me about an 7-8dB drop in gain, with real no change in EQ. But that would not tame the grid, if that is where things are clipping, correct? I have no scopes or fancy tools to determine if that is the case. I guess I could do both and it wouldn't necessarily hurt?
Thanks in advance!
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My crackling distortion problems have always been caused by intermittent electrical contact (my error)--never by a design flaw in the original proven circuit (Leo's error). With the amp running and my best hand behind my back I use a wooden chopstick to wiggle every wire starting in the reverb circuit. Listen for the crackle to be duplicated when the bad connection is moved. If that doesn't work, I wiggle each jack and listen some more. Then I wiggle terminals on the pots because I have had a pot wiper making poor contact inside the pot case as a crackle cause. In your case, I would be especially suspicious of intermittent grounding of the reverb jacks (fixed by connecting all three jack ground terminals with one wire), bad trace on the reverb pot, or bad connection on the reverb tube sockets. Good luck and stay persistent.
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I rewired my Princeton for the 275v HV tap on the PT, instead of the 325v tap. ... I noticed it is doing that weird crackle/distortion again, that goes away when I pull the 12AT7 (reverb driver). ...
My crackling distortion problems have always been caused by intermittent electrical contact (my error)--never by a design flaw ...
As much as I want to believe otherwise, the same has been true for every not-right sound in an amp I built: I have a wiring, soldering, or part-value error somewhere.
Hoffman's Law: "If it was wired right, it would be working now." :wink:
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> Install a 10K grid stopper going into pin 7 grid of the 12AT7
100k. Do not delay. Let us know.