09-19-2025, 07:51 AM
My Magnavox Quad 6V6 amp (and my brother in law's) had PM Dynamic speakers.
Other than the sockets and sand resistors, looking at the schematic, I think that the problems are as much from what plugs into the amp as the amp itself. The most interesting part of this is the combination of S7, R6, R7 and R16. This has to affect the feedback circuit. What is S7 labeled as? There is an adjustable wirewound resistor next to the power cord. Did this replace S7 on a previous or future version as opposed to the schematic? Maybe the pot has to be "exercised". they are not the most reliable things, especially if they handle any amount of current. I give you an RCA convergence board and rest my case.
In addition, there are important parts that are not in the amp chassis but are in what I think is a control chassis. Not only are the volume, treble and bass controls on this subchassis, the cap coupling and grid leak resistor for the 1st AF amp also on this subchassis. To use this amp as a monoblock or guitar amp, this has to be addressed.
In your pic there are several caps, one an electrolytic (Power supply filter or cathode bypass for the output tubes?) that attach to the ground lug of the input RCA Jack. There was a theory of establishing a "Mecca" ground or ground bus where all "power" grounds (B+ return, output tube cathodes, electrolytic cap grounds, etc.) at either one point or on a very thick "Bus Bar" that attached to chassis at only one point. The have current carrying connections were grouped as close together as possible and kept away from the input. Only one connection was made to the chassis.
Addressing these points as well as replacement of the sandy resistors, old caps, arced sockets, etc. should provide a stable amp.
Other than the sockets and sand resistors, looking at the schematic, I think that the problems are as much from what plugs into the amp as the amp itself. The most interesting part of this is the combination of S7, R6, R7 and R16. This has to affect the feedback circuit. What is S7 labeled as? There is an adjustable wirewound resistor next to the power cord. Did this replace S7 on a previous or future version as opposed to the schematic? Maybe the pot has to be "exercised". they are not the most reliable things, especially if they handle any amount of current. I give you an RCA convergence board and rest my case.
In addition, there are important parts that are not in the amp chassis but are in what I think is a control chassis. Not only are the volume, treble and bass controls on this subchassis, the cap coupling and grid leak resistor for the 1st AF amp also on this subchassis. To use this amp as a monoblock or guitar amp, this has to be addressed.
In your pic there are several caps, one an electrolytic (Power supply filter or cathode bypass for the output tubes?) that attach to the ground lug of the input RCA Jack. There was a theory of establishing a "Mecca" ground or ground bus where all "power" grounds (B+ return, output tube cathodes, electrolytic cap grounds, etc.) at either one point or on a very thick "Bus Bar" that attached to chassis at only one point. The have current carrying connections were grouped as close together as possible and kept away from the input. Only one connection was made to the chassis.
Addressing these points as well as replacement of the sandy resistors, old caps, arced sockets, etc. should provide a stable amp.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Best Regards,
MrFixr55
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