In 1967 Ampeg was began using circuit boards in their amplifiers instead of the eyelet board construction they had used since the 50s. The advantage was simple; the circuit boards helped cut down the assembly cost. Ampeg amplifiers were generally more expensive than their fender counterparts and any savings in production costs could help them close that price gap.
When they changed over to circuit boards they also started using an integrated tone stack module. Ampeg was using the Baxandall tone circuit, which again was more expensive as it used used more components than the tone circuit fender championed. However,by using the PEC6470000 integrated tone stack module that combined 4 capacitors and 2 resistors into one 7 pin package Ampeg was able to reduce the parts count and make the construction of their amplifiers marginally less expensive.
The down side is that the internal components of the integrated circuit are not high quality. the capacitors are ceramic type, which have high distortion characteristics, and impart a sharp abrasive quality to the high frequencies and poor low frequency imaging. Even the ultra high (or bright) caps were ceramic type.
Shown above is a cropped image of just the tone stack from the v4. however, the circuit is the same for all of ampegs amplifiers. The part values were obtained from a b25 schematic that used the PEC6470000 which had all of the internal values labeled. Notice that there are 7 nodes numbered 1-7.
The obvious solution is to replace all of the components with the same value, but higher quality components. This is a very simple modification, if you could even call it a modification (we aren’t actually changing the circuit).
Here we see the PEC6470000 on the control circuit board from the v4. These are always mounted on the control circuit board. This is true fro all ampeg amplifiers made 1967 and later. B15, B25,GV22, etc…
This is a picture of the backside of the board. circled in resolder the solder connections for the tone stack wafer. Notice that they are labeled 1-7 just like the schematic. The each number indeed coincides with a node on the schematic. This makes our job easy.
With the PEC6470000 removed the board looks like this. There is plenty of room to mount the new components. Notice c103 and c104 to the right of the vacant tone stack. These caps are in the ultra high circuit for channel 2. C101 and 102 are just a little further down, and those are for channel 1. In the v4 you can boost or cut the highs using the ultra high switch, always replace these capacitors too, as the original ceramic capa are super harsh. I replace all of these with 250pf 500v silver mica capacitors. These have a smoother sound and the larger value gives a more pleasing high boost, not so shrill. You might also notice that I upped the value for c102/103 and decreased the value of c101/104. There is good reason for this. I always found that the high boost only boosted the highest of highs whilst the high cut cut just about everything but the lowest of lows. Either way it wasn’t useful. Experiment with the values, Both 250uf or 470uf sound great there. Same with c101/104 the .001uf caps that come stock make the high cut a muffled mess. I never used it. Ever. Truth be told, I still don’t, but It at least doesn’t seem out of the realm of possibility.
***Note*** the ultra high capacitors do not appear in all ampeg amplifiers that use the PEC tone wafer. The capacitor reference numbers I list above correspond ONLY to the V4/VT22/V2
Here the board is repopulated with the new parts. Care must be taken to ensure that the leads don’t short to one another or to the potentiometer. So shrink wrap them, or organize them a different way. The caps in the photo were just organized to show general placement and to provide a quick pictoriel diagram wrap which cap goes where.
Then reassemble your amp and hear it sound better than it ever has before!
I intend to make a small 7 pin circuit board that has all the components on it already, which would make for an easy install. Stay tuned for that.
****UPDATE*****
The replacement PEC6470000 module is ready!
here is a photo of the new module. They are available on eBay: here
