Adding effects loop to Marshall-esque amp

[Matador]

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?

 

[PRR]

> drive requirements of the phase splitter.

Over 4 Volts. This (and the high-high Z pot at Send) is a poor place to insert most effects.

 

[Bo Deadly]

Adding another tube could cause supply to drop a little too much and affect stability. Perhaps a step-up transformer? Something 1:10 into the Master pot would be 10K:1M. Like Hammond 107X. Having the Master pot in there is probably important. Not sure about send though. Maybe just get two 107X. At $23 USD each it's not the cheapest solution but I think it would make up for it in simplicity and in the usual transformer benefits.

 

[Matador]

> drive requirements of the phase splitter.

Over 4 Volts. This (and the high-high Z pot at Send) is a poor place to insert most effects.

4v peak or peak-to-peak?

Not sure what is meant by "the high high Z pot at send"?

 

[PRR]

> what is meant by "the high high Z pot at send"?

What is the wiper impedance of a 1Meg pot half up?

The amp is 100 Watts. The NFB tap is on 16 Ohms. So 40 Volts RMS. The NFB loop is like 47k:4.3k, so power stage gain is less than 10. The input of the power amp needs like 4V (rms).

The amp is 100 Watts. I can't see that another 0.3 Watt tube is going to invite much sag or instability.

 

[Matador]

I see, that makes sense.  So if I want to be able to connect something spitting out -10 dbv into the return I need to plan for 20-30dB of gain (perhaps less with a hot preamp) prior to the phase splitter, which is exactly what Kurt's buffered send/return provides.

Looks like the best bet is to add another tube!

 

[CJ]

just copy what has been done already. just about every amp made these days has some type of loop, 

they can get pretty elaborate, or simple, just a matter of how complicated you want to get,

those amps are crazy clean, so some type of pedal is needed for ZZ Top,

why not just use the stop box after the guitar?

or put a loop in the guitar like Garcia?

 

[Matador]

just copy what has been done already. just about every amp made these days has some type of loop, 

they can get pretty elaborate, or simple, just a matter of how complicated you want to get,

those amps are crazy clean, so some type of pedal is needed for ZZ Top,

why not just use the stop box after the guitar?

or put a loop in the guitar like Garcia?

My use case isn't really to insert effects, it's to tap in to the power amp.  I have a slew of rack preamps that I use between guitar and bass so it would be nice to have a single tube power amp to use across all of them.

 

[Heikki]

Looks like the best bet is to add another tube!

Or even better bet is to add few transistors. This plan works well and it's very simple.
http://tube.ghr.fi/bossa/bossa.sch.svg

 

[fazer]

Or even better bet is to add few transistors. This plan works well and it's very simple.
http://tube.ghr.fi/bossa/bossa.sch.svg

That is stunning work Heikki.