Matador
Well-known member
I am in need of a 100W power amp, and I was thinking of modifying a Sovtek Mig-100B that I already have.
The amp is loosely based on the JCM-800 / Fender Bassman circuit topology, however doesn't have an effects loop. I'm figuring if I add an effects loop to the amp, then I can plug in to the return to utilize the power amp section (as is commonly done with amps with effects loops).
Here is the schematic:
A common recommendation is to provide a tap from the middle lug of the master volume that is post-tone stack, then inject the return signal into the cap that drives the phase splitter.
The simplest mod would be to add a shorting 1/4" jack that breaks the connection between the master volume pot and the phase splitter when a plug is inserted, then all I need to add is a high-ish value resistor between the cap and ground to provide a DC reference point for the input (and to establish the input impedance). Perhaps a 100K? This means all I need is the jack and a single resistor to utilize the power amp.
However there is also a full send/return circuit, which needs a new tube:
This would allow me to inject non-line level signals into the return jack, adjust their level (and apply gain), and also to utilize the preamp into other external power amps.
What is unclear to me is the drive requirements of the phase splitter. If I send a normal line-level effect (between 0.5V and 1V PTP) into the jack, can I drive the power output to full level without additional gain?
The amp is loosely based on the JCM-800 / Fender Bassman circuit topology, however doesn't have an effects loop. I'm figuring if I add an effects loop to the amp, then I can plug in to the return to utilize the power amp section (as is commonly done with amps with effects loops).
Here is the schematic:
A common recommendation is to provide a tap from the middle lug of the master volume that is post-tone stack, then inject the return signal into the cap that drives the phase splitter.
The simplest mod would be to add a shorting 1/4" jack that breaks the connection between the master volume pot and the phase splitter when a plug is inserted, then all I need to add is a high-ish value resistor between the cap and ground to provide a DC reference point for the input (and to establish the input impedance). Perhaps a 100K? This means all I need is the jack and a single resistor to utilize the power amp.
However there is also a full send/return circuit, which needs a new tube:
![fx_loop.gif](https://proxy.imagearchive.com/921/921e125c0976b01390d8604dd1abe124.gif)
This would allow me to inject non-line level signals into the return jack, adjust their level (and apply gain), and also to utilize the preamp into other external power amps.
What is unclear to me is the drive requirements of the phase splitter. If I send a normal line-level effect (between 0.5V and 1V PTP) into the jack, can I drive the power output to full level without additional gain?