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Re: Peavey Classic 30 mods for playing harmonica?


 :
12/30/2004 7:23 PM
Mr. Droopy-Drawers
Re: Peavey Classic 30 mods for playing harmonica?
You won't get this mod anywhere else, so you can thank me later. If you are using a crystal mic, ceramic, or even piezzo, try this. To get the full frequency response, a VERY high input impedance is required. These mics work 'OK' in a guitar amplifier, but to get the 'best' bass response, and more gain, you need to raise the input impedance to about 10Meg. You need to reconfigure the first stage to contact bias. Feed the input jack to a 47K resistor in series, then a .05uF capacitor. The capacitor goes to the 12AX7 grid, and the grid goes to ground through a 10Meg resistor. Ground the cathode of that first 12AX7 stage, and you are done. I also raise the Plate load to 220K, and the results are very satisfactory.
 
1/5/2005 1:01 PM
Rick Erickson

quote:
"You won't get this mod anywhere else"
 
 
So you invented grid-contact bias? Thanks for sharing... ;) btw - It also works well for guitars with low-moderate output pickups.  
 
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1/5/2005 4:19 PM
Mr. Droopy-Drawers

I wish I could take credit. It doesn't matter; Randall Smith probably beat me to the patent rights ;).  
I always look for ways to do 'thinking' mods, rather than say taking a Marshall and just slamming a 12AX7 in front, while calling myself clever. Grid contact bias works, and I am always surprised no one seems to espouse the values of the circuit. Every time I see a question about mods for an amplifier used by a harp player, I really wonder why no one else seems to bring that up. So I would like to take the credit, but as I say, I'm not that clever.
 
1/6/2005 8:32 AM
daz
what would that mod do.......
.....for a C30 used for guitar only? I use a strat with a hot humbucker in the bridge and 2 vintage type (van zandt) singles in the mid and rear. i like full frequesncy vintagy tones, but i also like high gain grind as should be apparent by my use of a hot bridge hummer. From your description it sounds like it would sound fuller with the singles, but that the humbucker might then overload the input to the point where it begins to fart out. Am i on the mark here or would it still be a good idea to try this? I generally use the distorto chan only and clean up with the guit vol, and i only use the humbucker for solos or hard rockin ryhthms.
 
1/6/2005 3:40 PM
Rick Erickson

quote:
"for a C30 used for guitar only?"
 
 
The grid-contact circuit tends to compress a bit more than the cathode-biased (common-cathode) circuit. The only way to know if it will work for you is to try it.  
You can always put it back stock if you don't like it.  
 
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1/6/2005 4:10 PM
daz
The only way to know if it will work for you is to try it.  
 
Well, yea. But i had to ask, and if you've ever taken a C30 apart you know why ! ;)
 
1/6/2005 3:46 PM
Rick Erickson
Re: Peavey Classic 30 mods for playing harmonica?
Mr. D-D,  
 
It's funny - I was reading this thread and thinking about grid-contact bias when I saw you had beaten me to it. Thats' why I reacted the way I did - more out of surprise and a bit of "Oh Yeah?" :-)  
Some other "Harp Mods" involve removing some High Freqs by use of small caps (either from the signal path to ground or across a plate resistor) to reduce feedback at higher levels and if you want a dirty lower level sound you can unballance the p.i. circuit by adding small (820 ohm) resistor on one cathode only. This sounds terrible for guitar but seems to work pretty well for harp.  
 
RE
 

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