'75 Peavey Session 400
Peavey LTD
LTD Rear view
Power Supply Filter Capacitor Update
New power supply filter capacitors with high quality film cap's bypassing them. This is the first and most important mod for the Session 400 or LTD. Mainly because electrolytic capacitors go bad over time. Actually this should be called general maintainence. The stock value was 6000uF/50v. It's been recommended to increase that value. On one of mine I used some 10,000uF/65v. The ones shown here are 8200uF/100v. The larger value (in uF) cap's will provide a bit more reserve impulse power that comes in handy with loud punchy/bassy parts. The high quality film bypass caps are also very key in this section. The electrolytics smooth out the DC power supply but only so well. The high frequency ripple and noise from the power supply and wall A/C can pass thru electolytic caps, leaking garbage into the amplifier circuit. It's sort of like a 2-way speaker. The big cap smoothes out the bulk of the power, the low end. The little film cap cleans up the high frequency junk. What's really cool in this amp especially is how much the transistors quiet down when they aren't fed all the power supply junk. The difference in noise floor is huge compared to the stock setup. I think this is a must for anyone recording with the Session. It's already a real quiet amp but with this cap mod you wont even think the amp is on when sitting next to it. The film bypass cap's also really help when you are on a circuit with lots of neon, compressors, dimmers, fluorescent lights, etc. Like any bar gig. The little film caps I used here to bypass the big electrolytic ones are .47uF/450v. Really any film cap from .1 on up to 5uF rated at 50v or higher would do.
Signal and Tone Capacitors
Here you can see the preamp mod's (upgrades). I call them upgrades because I'm not changing anything in the amp design at all. The Session 400 EQ is so perfectly voiced for steel that I wouldn't want to alter anything. What I've done though is improved the quality of just a few of the capacitors through which the audio signal passes. Imagine looking thru a piece of glass, then add another, and so on. Pretty soon your view is less clear. Since capacitors are usually required in an audio path to prevent any DC from getting thru (audio being AC), the cleaner the capacitor, the more pure the end product is. For cost and size reasons, corporations have to draw the line on parts costs and decicide how good is good enough. So what happens is that in a few stages (5) here in the Session 400 or LTD, one section of the preamp passes the signal on to the next stage. Each interstage pathway is coupled with a capacitor to keep DC from entering the next stage. These cap's need to be relatively big in value so as to carry the widest bandwidth of audio necessary. Since the values are large, the cost issues have helped choose the use of cheaper electrolytic capacitors. When film cap's get that big, they start to cost a whole lot more than electrolytics, and they are physically big (see photo). This happens with every company. Peavey actually leans toward the higher end of the spectrum with regards to component quality. Their longevity is a testament to that.
The smaller cap's indicated were originally ceramic disks. These aren't big interstage cap's, but are smaller tone capacitors in the EQ section. Ceramic disks are not known for their warmth and tone to put it nicely. It's easy to find good film cap's at these values. For the mod I recommend, there are two .02uF, and two .01uF caps in the tone section, and one .047uF for the reverb return. You can do more but these seem to be the most critical.
Here's another angle where you can see the replaced reverb return cap and some of the big coupling caps. The reverb cap will clean up the fidelity of the reverb tank's signal. I replaced 5 electrolytic 2.2uF caps with the big blue film ones in this picture. Keep the voltages on all these caps at 50v or higher.
Original 2uF Coupling Capactors
Here is one of the original 2uF coupling caps. This one in particular is at the output stage of the preamp. I've upgraded it also since the photo was taken. These little 2.2uF electrolytic caps are very worthy of replacement. The amp design really gets to breathe and do its thing well when these caps are upgraded to film types.
Also notice all the little transistors and the total lack of IC's. This is an all discrete amplifier. Today you'll only find very high end audiophile gear made this way. It's a delicate design to build thus too expensive to build for the big market. In it's day it was really the only option because the audio IC chip was just developing. Modern high-end IC's do sound great, like the Burr-Brown chips found in the Peavey upgrade kits for their post-Session 400/LTD amps. But, there is a real warmth and sweetness to these old transistors in a discrete circuit used for a guitar amp.
This is in no way an official website. It's just for fun. If anyone has cool photos or more information or corrections to my info, please drop me an email at bradsarno@earthlink.net . I want to thank Bob Mason, Scotty's, Ken Fox, Mike Brown, DeWitt Scott Jr., Peavey, Curly Chalker, and Buddy Emmons and everyone else for their info and/or involvement with the development of this very cool steel amp. This amp is a recent discovery for me. I'll try to keep this page updated as more geeky technical info comes in.
Thanks,
Brad Sarno
Blue Jade Audio Mastering
St. Louis, MO
