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Thank you CrustyBooger for your input. I certainly don't want to fiddle with things and end up with a bad sounding amp, yes I want to understand why it sounds like it does, and if some reversible minor modifications can be done to explore new sounds, who knows, maybe I'll do some tests in the...

I am still learning so I really am unsure, but what I understand from the article below is the two resistor determines the feedback voltage, which appears to be the important factor given how the tube acts as a voltage amplifier. But it also says that the transformer tap is important in the...

Thank you "CrustyBugger" for the precious history details and your input on my amp! Helps me understand it more and why it is that way, and how different it potentially is from other variations of the Mark 1. I really like the presence control as it is so I do not think I am going to play with...

I have just ordered replacement cap for the preamp board 60uF 350V cap, along with resistors to put 100k and 1.5k instead of the 82k and diodes. In order to reach the minimum amount for the order I also added a second cap, higher value (100uF 350V) thinking I could maybe use it as an...

On my Mark 1 (03-1978) volume 1 has no effect when connected into input 2. And I have the more drastic master volume curve.

I added some details in my schematics revision, I left the original above, below are the updated ones. What I changed: - "Mojo" wire going from pin 3 of one of the power tubes, through a 220k resistor, and wraps tied around the 3 signal wires on grids of stages 1-2-3. Not sure how that adds to...

The first times I tried to connect a cable directly in the send-return, this is exactly what I tried, I thought I'd either get more gain or more aggressive sound but I did not notice much change except for the higher volume. But I have to say I did not play often with that setting, I will try...

Yes the EQ is the GE-7. I knew it was far from the best but it was pretty cheap used available locally so I thought I'd give it a try and see if I like the results of tweaking the tone with an EQ in the FX loop. While searching before buying the GE-7 I did see the Mesa EQ pedal and liked the...

Glad I could help, were you able to get the values you were after for the phase inverter? I think I got that covered in the revision of the schematic. If there is no note, it is because the one indicated was correct. Thank you for the answers, I will have to teach myself on cathode follower...

Thanks James for the feedback about that big cap on the preamp board. I'll try to find a replacement along these lines. For the pots, I don't have a good enough multimeter to measure (does not measure high enough resistance). Do you usually get a good reading of the pots while they are in...

Ok so yesterday I spent some time tracing the preamp circuit (compared to the available drawings of the Mark I) and highlighted the differences. Also I traced my effect loop mod and researched the components a bit. The yellow caps are Philips Mepolesco series (sometime known as Philips...

I would certainly not describe mine as bright, I always thought it tended to be dark. Of course with high presence settings or bright knob pulled it can become bright, but not in a good way, it is not the place where the amp needs to be to have a balanced sound. To me it seems for a certain...

Ok, the big filter ones as well as the bias ones are now replaced so that's good. I will look for a replacement part, but if I replace it I will be careful with the old one and try to keep it, in case it affects the tone in a way I don't like. I thought a decoupling cap would have the signal go...

Now regarding the decoupling cap 60uF 350V, do you guys think it also needs to be replaced, or does it have an easy life and should just stay there? If I were to replace it, what impact does different voltage and capacitance values have on tone and operation, should I just keep looking to find...

Thank you James for your observations and link! That was another interesting read for me. That A2283 certainly puts a pretty clear spot on the timeline. Fred

Thanks Mark for clearing it up. I love learning more about my amp, in fact after trying to discuss the amp's components with you I realized I needed to learn a bit more about how tube amps work, so I hope eventually I'll be able to discus the circuits a bit more, and understand your suggestions...

Just to add to the list, mine is a Mark I dated 3-78 with a green sharpie on the chassis, with serial # A1747. Thanks for taking the time to do some Boogie archeology! :) It is in fact not a perfect science but still very interesting to know and learn about how our amps were manufactured. Fred

For future reference I am copying another link where there is information on converting for 12AX7 instead of Fetron (on an MK II of same date as my MK I) so I don't lose the link :) https://boogieforum.com/threads/first-boogie-first-post.46303/ The dates brings another question to my mind. My...

Thanks for the information on the fx loop. In mine the pot does not do anything when nothing is connected to the FX loop, so I was a bit puzzled to read the fx loop mod document above, I thought they meant that the installation of the mod affected the tone at all times because always in circuit...

Very interesting information as I have an effect loop installed on my Mark 1, and I always wondered if it was done by Mesa or not. Some of the components used for the FX loop mod look quite different from the ones used on the remaining of the amp, so at first I was thinking not Mesa, but I'm not...