Mark 1 Loop Question (before real loops)

[kdove]

Actually, the original Mark 1 (like my '78 60w/100w) didn't really have real loops. In fact, mine did not work with a reverb device in the loop. I think the IN and OUT shown in the attached picture are probably just taps between the preamp and poweramp. I was wondering what were they really designed for ... slaving two power amps off of one preamp maybe?

Also: Can you use a volume pedal at this point as a master volume like you can in regular loop amps now? How about a clean boost at that spot?

It's this rig ...

 

[jonarobb]

On my 77' Mark I the bottom jack is a padded line out coming off the output tranny/speaker jacks. This works in conjunction with the slave control(pot).

The top jack on mine is a dedicated preamp signal that is taken before the phase inverter. It will shunt the power section to ground but you still need to make sure your speaker is attached. I use the this jack often to harvest the Mark I's preamp signal and run it out to a seperate power amp, and then from the power amp into a custom 2x12".

 

[igfraso]

Jonarobb, kdove:
do you use the switchable Gain Boost?
I know it sounds better if you use the guitar volume to change the sound rather than the Gain Boost but I am wondering if you have made a good use of it.
Thanks and regards.

 

[jonarobb]

Jonarobb, kdove:
do you use the switchable Gain Boost?
I know it sounds better if you use the guitar volume to change the sound rather than the Gain Boost but I am wondering if you have made a good use of it.
Thanks and regards.

I don't use the Gain Boost. I use my Mark I for it's clean tone. The Gain Boost is handy though if you're looking for a channel switching type solution. Be warned though that not all Mark I's have the Gain Boost wired to the 1/4 jack underneath the chassis. I've posted about this before. The jack underneath the chassis has been wired several different ways for the Mark I based on custom orders.

For instance, I have an early Mark I in the shop right now, serial # 16, and the jack underneath is for the preamp out/power amp in.

It's easy enough to wire the Gain Boost to be footswitchable, and again, this is a sensible solution for those looking to make the Mark I have 2 distinct tones on demand.

The Gain Boost feature on the Mark I is nothing more than taking the tone stack out of the circuit. This gives quite a boost in the way the guitar signal hits the third tube stage. There's quite a jump in output but it can be sweet if used sensibly.

 

[igfraso]

Nice info! I didn't know it was removing the tone stack from the circuit.

My '77 Mark I (signed by RCS) is wired for switchable EQ and Gain Boost. I sent it to Mike Bendinelli and he suggested to mod it and moving the EQ after the master in order to make it more usable (the same way Keith Richards had his).
Regards

 

[kdove]

Mine is wired for the switchable gain boost but for 30 years it's been far too over the top for me. I'm actually just wanting to add a bit more volume to the 2nd clean channel. The first channel is fine.

 

[jonarobb]

Mine is wired for the switchable gain boost but for 30 years it's been far too over the top for me. I'm actually just wanting to add a bit more volume to the 2nd clean channel. The first channel is fine.

Just simply lift the 220k resistor to ground coming off of input 2. It's not a grid load resistor, it's just a pad. You can even make it switchable along with the regular gain boost thus giving you 3 distinct voices on Input 2 alone....

 

[kdove]

Mine is wired for the switchable gain boost but for 30 years it's been far too over the top for me. I'm actually just wanting to add a bit more volume to the 2nd clean channel. The first channel is fine.

Just simply lift the 220k resistor to ground coming off of input 2. It's not a grid load resistor, it's just a pad. You can even make it switchable along with the regular gain boost thus giving you 3 distinct voices on Input 2 alone....

Will that affect the input 1 sound? Since the stack is in series with the 2nd stack?

On my 77' Mark I the bottom jack is a padded line out coming off the output tranny/speaker jacks. This works in conjunction with the slave control(pot).

The top jack on mine is a dedicated preamp signal that is taken before the phase inverter. It will shunt the power section to ground but you still need to make sure your speaker is attached. I use the this jack often to harvest the Mark I's preamp signal and run it out to a seperate power amp, and then from the power amp into a custom 2x12".

So have you used a 25k volume pedal to tap off the power here? Would that work? It works on another amp that I have with a true loop.

 

[igfraso]

kdove:
have you tried using the footswitchable Gain Boost on channel 2?
Regards

 

[kdove]

kdove:
have you tried using the footswitchable Gain Boost on channel 2?
Regards

Yes ... in fact ... my perfect solution is a footswitchable channel 1 "As-Is" with a channel 2 with a clean gain boost (specifcally the Xotic RC Boost) that is loud enough to compete.

Now the only issue is ... they are not really separate channels. The input 1 is in series with the input 2. You can't turn one off and use the other one. Not using the input 1 is not an option ... it is what the Mark 1 is ... without input 1 it's just a basic fender. That's OK but not what I want to do.

The original Mark 1 came with a footswitch that is pretty specific (see related post) that I think is not a true A/B switch. My guess is currently that it is a Input 2 is always on then A+B adds the input 2 and they are both going. That's my guess anyway since no regular A/B switch has worked with my rig. I think the Mark 1 was the only one done this way before amp switching became common place.

Now the other thing is that I don't really want the boosted input 2 to affect the input 1 too much either. The reason I ask about the boost in the loop is that it would not effect the input 1 tone at all. So one solution would be a bit of master volume reduction in the loop while input 1 is running ... then remove it when the clean channel is going. I've also had some luck running the Xotic RC boost in a true loop ... so maybe the boost running in the loop while input 2 is running.