Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dinkotom on June 16, 2015, 10:21:38 am
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Hi, can anything be done to this PI to make the overdrive sound smoother? I've been reading about cathodyne PI a lot lately, but the more I read, the more I'm confused (eternal noob :BangHead: ). I already put a 1M grid stopper between PI grid and plate of the previous stage, not quite sure if I can hear a difference. Should I try to lower the B+ voltage for PI (don't remember where I read that it helps it make smoother) or something else? There are no voltages shown in this schematic, my readings are:
B+ for PI (B node in schematic):320V
PI:
Plate:209V
Grid (with 1M stopper):99V
Cathode:110V
Previous stage:
Plate:109V
Cathode:0.9V
Schematic:
http://4.bp.blogspot.com/-r_TK-MCP5Xs/UMCLLLrTxDI/AAAAAAAAAOI/IYP_peBX1K0/s1600/magnatone+260+2.JPG (http://4.bp.blogspot.com/-r_TK-MCP5Xs/UMCLLLrTxDI/AAAAAAAAAOI/IYP_peBX1K0/s1600/magnatone+260+2.JPG)
Any thoughts welcome.
Thanks, Dinko.
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Try clipping in a 100 pf cap across the plate load resistors. If it seems to help, increase to maybe 200 or so, or until it starts to noticeably roll off the treble. Sometimes all you need is a 47 - 100 pf to smooth things out.
You can also try clipping in the caps from plate to cathode on your preamp tubes, (or across the plate resistors) that could be where some hash, or harshness, is coming from.
Merlin advises against the cap across plate resistor; I've never noticed a problem myself, but I don't build high gain stuff, so it's more forgiving.
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John thanks, will try that