I recently rebuilt a PL700B for a friend, this BLOG is a result of that. It has been years since I rebuilt my own PL700Bs (before the age of internet BLOGS) so had forgotten most of what I rediscovered with this rebuild.
I always enjoy seeing the inside of an amp where all the wiring seems nicely organized and neatly laced with ty wraps. It looks great but when you peel back the covers, it is often counter to achieving the best signal integrity. Usually a couple of simple modifications make a huge difference in the noise floor an amp can achieve.
The primary rule is keep noise sources, especially AC noise sources away from amplifier inputs and input wiring, even if shielded. The amp circuit design has great power supply rejection ratio for defending noise signals from coming in the back door but has no defense against noise signals coming in the front door, after all, amps are designed to amplify these. If an input absolutely has to be near a noise source, keep any wire crossing at 90 degrees to the noise source and keep it to a single cross. The original lacing of the PL700B violated this rule in several areas.
The second rule is always use a single point ground. It can be vexing at times to get a single point ground scheme right but once you do it yields great benefits and usually looks the cleanest too. The original ground scheme used in the 700B violated this rule in several areas.
The 700B is somewhat unique in that the AC comes in on the right hand side of the amp (when looking from the front panel side), goes through a fuse and to a convenience outlet and then proceeds to run from right to left to go to the main power transformer. Likewise the rectified DC B+ and B- wires run from the left side of the amp to the fuses on the right hand side and then back to the left again to the application point on the back wall and the control board. This is hardly ideal and leads to many parallel runs of AC and DC.
With those 2 rules in mind, tackle the AC wiring in the 700B first. The best way to route the AC line in the 700B is to bring it from the fuseholder directly up to the left of the fuseholders and the convenience outlet, to the left of the output binding posts and the input selection switch and the input jacks but to the right of the last column of output transistors. Keep the AC wiring nested as close to the chassis metal wall as possible. At the top of the chassis, make a bend to the left and run the AC across the top of the chassis under the lip flange in the chassis. Get rid of the wire run to the two thermal switches that are mounted on the chassis back wall that run from left to right as well and re-route this to the top of the chassis as well. At each of the thermal switches, make a 90 degree bend straight down to the thermal switch and back up again to the top of the chassis. Tyrap these closely together to keep these needed AC runs away from the other wiring and transistors. Join the wire run that goes to the front panel switch over at the left hand transformer side of the chassis and run these to the front panel switch nested closely to the left wall of the chassis. Form these AC wires so that they do not come near the left channel volume pot or shielded wiring, it is very possible to do. I jacket these wires to keep them together and manageable. Unlace the 2 green low voltage AC wires that power the light board on the front panel and route them along with the AC line wiring discussed prior. These wires are long enough to route this way and it keeps them well away from any signal wiring. This rewiring should leave the bottom edge of the amp chassis devoid of any AC wiring. On the top edge of the chassis, lace only the AC wires together and lace the shielded input wires together and separate from the AC wiring runs. Push the AC wires into the upper inside bend of the chassis sheet metal, these should be the only wires in this area. Neaten up and re-lace the left to right DC voltages and ground wires that are now the only wires left in the bottom section of the chassis. Inspect your work and visually confirm that there are no AC wires present in any of the DC or signal wiring anymore.
I always enjoy seeing the inside of an amp where all the wiring seems nicely organized and neatly laced with ty wraps. It looks great but when you peel back the covers, it is often counter to achieving the best signal integrity. Usually a couple of simple modifications make a huge difference in the noise floor an amp can achieve.
The primary rule is keep noise sources, especially AC noise sources away from amplifier inputs and input wiring, even if shielded. The amp circuit design has great power supply rejection ratio for defending noise signals from coming in the back door but has no defense against noise signals coming in the front door, after all, amps are designed to amplify these. If an input absolutely has to be near a noise source, keep any wire crossing at 90 degrees to the noise source and keep it to a single cross. The original lacing of the PL700B violated this rule in several areas.
The second rule is always use a single point ground. It can be vexing at times to get a single point ground scheme right but once you do it yields great benefits and usually looks the cleanest too. The original ground scheme used in the 700B violated this rule in several areas.
The 700B is somewhat unique in that the AC comes in on the right hand side of the amp (when looking from the front panel side), goes through a fuse and to a convenience outlet and then proceeds to run from right to left to go to the main power transformer. Likewise the rectified DC B+ and B- wires run from the left side of the amp to the fuses on the right hand side and then back to the left again to the application point on the back wall and the control board. This is hardly ideal and leads to many parallel runs of AC and DC.
With those 2 rules in mind, tackle the AC wiring in the 700B first. The best way to route the AC line in the 700B is to bring it from the fuseholder directly up to the left of the fuseholders and the convenience outlet, to the left of the output binding posts and the input selection switch and the input jacks but to the right of the last column of output transistors. Keep the AC wiring nested as close to the chassis metal wall as possible. At the top of the chassis, make a bend to the left and run the AC across the top of the chassis under the lip flange in the chassis. Get rid of the wire run to the two thermal switches that are mounted on the chassis back wall that run from left to right as well and re-route this to the top of the chassis as well. At each of the thermal switches, make a 90 degree bend straight down to the thermal switch and back up again to the top of the chassis. Tyrap these closely together to keep these needed AC runs away from the other wiring and transistors. Join the wire run that goes to the front panel switch over at the left hand transformer side of the chassis and run these to the front panel switch nested closely to the left wall of the chassis. Form these AC wires so that they do not come near the left channel volume pot or shielded wiring, it is very possible to do. I jacket these wires to keep them together and manageable. Unlace the 2 green low voltage AC wires that power the light board on the front panel and route them along with the AC line wiring discussed prior. These wires are long enough to route this way and it keeps them well away from any signal wiring. This rewiring should leave the bottom edge of the amp chassis devoid of any AC wiring. On the top edge of the chassis, lace only the AC wires together and lace the shielded input wires together and separate from the AC wiring runs. Push the AC wires into the upper inside bend of the chassis sheet metal, these should be the only wires in this area. Neaten up and re-lace the left to right DC voltages and ground wires that are now the only wires left in the bottom section of the chassis. Inspect your work and visually confirm that there are no AC wires present in any of the DC or signal wiring anymore.
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