Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on June 25, 2013, 01:07:31 pm
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Major preamp fizz and gain restructuring
This is what i have done to my XXX. Sounds 85% better. Still the EQ rc network sorta sucks but good tone can be found after fiddling for a while. The EQ response is like all or nothing and the sweet spot is some where in between. I cant bring myself to dig into the rc network and reconfigure to a normal tone stack. Maybe someday if i still have it. I have too much else going on right now to devote to this amp. Still much better after making the following preamp and power board changes:
Change R11 & R7 150K Plates to 100K
Remove C19 & C20 Cathode Caps
Change R23 68K Slope to 100K
Remove V2 Diodes D1 & D2 / Removes Early Clipping / Smoother / May use resistor to subdue slight channel pop between Ultra & Crunch
Change LTPI Resistors R108 & R111 to 1Meg / Buss R106
Change Screen Resistors R101-R102 R117-R118 to 1K 5 Watts
Remove CR-100 & CR-103
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I was looking at the old thread on this tonestack - it is an usual one, considering the increase in parts count, I wonder why it was preferred over the standard TMB stack, anyhow, its frequency response is more like a HF shelving EQ, with a relative narrow peak at ~1.5k, which does not allow for much flexibility IMO, so you will probably have better luck with another type of tonestack...
(https://dl.dropboxusercontent.com/u/1326040/Peavey%20TS.jpg)
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so you will probably have better luck with another type of tonestack...
I have been piecing info together. From what i have read the preference is to take the XXX and put into an oven @ 375" for 45 minutes. Baste well! Just kidding*
The preference is to take the XXX and make changes that the JSX uses. Leaving the XXX EQ as it is. I did this last night.
Clean Channel sounds Fenderish nice and not as mutted or dark. This morning after 11:00 am i will make noise and see if i have accomplished taming the gain in the Ultra & Crunch channels
Why would i bother doing all of this? The challenge. PRR (Divide & Conquer)
Update: Fantastic usable gain and tone
PM me for the complete mod info
I ripped off the old covering and used rough blonde tolex yesterday. It took 6 hours to finish. I think it looks super fine*
Wish i could find 5/16 cream chicken head knobs
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Pretty Cool! I am a fan of James Brown the deseigner of most of Peavey's tube line and saw on his website that the XXX was still his fav amp of all he has made.I thought it was the worst and least usable and love the Coupe amps he did with Kustom but would take a valveking or Classic over the XXX anyday,hell even a 5150!
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When i first got this XXX i noticed right away that there was clipping diode action going on. It sucked and was just plain chainsaw and bee's. After making the preamp & PI mods it is a completely different amp. Has good JCM 800 tones now and the gain is under control without runaway squeeling. I hated it in the begining. Im happy with it now as are others that have played it.
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Wish i could find 5/16 cream chicken head knobs
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If you can get away with 7/8 x 5/8 Here:
http://www.smallbearelec.com/servlet/Categories?category=Knobs%3AChicken-Heads (http://www.smallbearelec.com/servlet/Categories?category=Knobs%3AChicken-Heads)
What is the diameter of the pot shafts? 6mm or 1/4 inch?
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Hi,
I'm aware this is a very old Thread. Is @plexi50 still on Board here.
I've done a few of the described Tweaks an still some questions about, particular at the Tone Stack.
Greetz